CA1297700C - Method of calibrating reflectance measuring devices - Google Patents
Method of calibrating reflectance measuring devicesInfo
- Publication number
- CA1297700C CA1297700C CA000538704A CA538704A CA1297700C CA 1297700 C CA1297700 C CA 1297700C CA 000538704 A CA000538704 A CA 000538704A CA 538704 A CA538704 A CA 538704A CA 1297700 C CA1297700 C CA 1297700C
- Authority
- CA
- Canada
- Prior art keywords
- reflectance
- sample
- standard
- light
- measuring
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/47—Scattering, i.e. diffuse reflection
- G01N21/4785—Standardising light scatter apparatus; Standards therefor
Abstract
METHOD OF CALIBRATING REFLECTANCE
MEASURING DEVICES
Abstract Of The Disclosure A method of calibrating reflectance measuring devices, such as spectrophotometers, that incorporate automatic gain or sensitivity setting capability. According to the method, offset reflectance and reflectance of a second-ary reflectance standard for several wavelengths are calculated and stored in a memory of a reflec-tance measuring device. A secondary standard is mounted in the reflectance measuring device and reflectances at a selected wavelengths from the secondary standard are read prior to measurement of the reflectance of the test sample. The stored offset reflectance and secondary reflectance for the operational wavelength are used to calculate true reflectance of the test sample.
MEASURING DEVICES
Abstract Of The Disclosure A method of calibrating reflectance measuring devices, such as spectrophotometers, that incorporate automatic gain or sensitivity setting capability. According to the method, offset reflectance and reflectance of a second-ary reflectance standard for several wavelengths are calculated and stored in a memory of a reflec-tance measuring device. A secondary standard is mounted in the reflectance measuring device and reflectances at a selected wavelengths from the secondary standard are read prior to measurement of the reflectance of the test sample. The stored offset reflectance and secondary reflectance for the operational wavelength are used to calculate true reflectance of the test sample.
Description
~2~ 0'~) METHOD OF CALIBRATING REFLECTANCE
MEASURING DEVICES
FIELD OF THE INVE~TION
The present invention relates to a new and improved method of calibrating reflectance measuring devices and to a method of measuring light reflectance from a test specimen. More particularly, the invention is directed to a method of calibration of, measurement and measurement correction of light reflectance in a light reflectance device that incorporates an automatic gain or sensitivity setting. Light reflectance measurement from a single, secondary reflectance standard, prior to measurement of reflectance from a test sample, enables the cal-culation of accurate true reflectances from the test specimen reflectance measurement.
BACKGROUND OF THE INVENTION AND PRIOR ART
Reflectance ratio is the ratio of the intensity of light reflected from a sample to the intensity of light incident on the sample. It is relatively easy to measure the intensity of light reflected from a sample, however, measuring the light incident on the sample is more diffi-cult. Incident light has been measured by plac-ing a primary reflectance standard in a readhead of a spectrophotometer and, by knowing the true reflectance at the operational wavelength of the MS-14~9 1~29~7~3~
--2-- ~
device, the incident light can be back-calculated from the detected reflectance. ~eflectance of the test sample then is determined from the inten-sity of light reflected from the test sample divided by the intensity of light reflected from the primary reflectance standard multiplied by the known reflectance of the reflectance standard.
This technique assumes that there is no stray light within the reflectance measuring device, but this assumption is wrong and leads to signi-ficant measurement errors.
To calibrate reflectance measuring devices, such as spectrophotometers, it is custom-ary to establish 100% and 0% reflectance values.
To establish these reflectance values, a measure-ment relative to some standard must be made.
The 0% reflectance value has been difficult to obtain, and many procedures and devices operate on the assumption that 0~ reflectance is the value measured with the device turned off. The resulting 0% reflectance value is not correct since it is very difficult to impossible to eli-minate stray light.
In the prior art, recalibration using a primary reflectance standard must be performed before each measurement and the instrument sensi-tivity or gain must be reset. Under these condi-tions, each measurement and reading is dependent on the sensitivity or gain set of the device.
The necessity for the user to place a primary reflectance standard into the device prior to each measurement to reset the instrument sensi-tivity decreases the utility of the device and increases the cost of use. Further, each reflec-tance measuring device must be made to close ~7~70~
tolerances to insure that the stray light for each device is the same. This requires the main-tenance of costly and difficult manufacturing tolerances and strict quality control standards.
Another disadvantage with these devices i5 a need for frequent device recalibration due to constant sensitivity changes.
One approach to calibrating spectro-photometers is disclosed in United States Patent No. 4,029,419. In this system, the spectrophoto-meter is calibrated by using a secondary white standard calibrated against a primary white standard, a black standard to compensate for dark current and internal reflectance from a pressure plate, and a mirror to obtain internal wall reflectance used in a correction factor.
Although this approach does not assume that the stray light is zero, for each use of the spectro-photometer two samples must be read and the instru-ment recalibrated.
A process of compensating for radiationvariances in a spectrophotometer light source is disclosed in United States Patent No. 3,245,305.
The spectrophotometer described in this patent employs two light sources and means for sensing the relative intensities of the radiation from the two sources. The device of U.S. Patent No.
3,245,305 also includes means for altering the intensity of the radiation from one light source.
Additional apparatus is provided to be responsive to the sensing apparatus for automatically chang-ing the intensity of the radiation from one light source to keep the ratio of the intensity of the two sources constant. This device does not mea-sure the incident light directly. Further, the 12~7(~
requirement of a second light source and the attendant electronics increases the complexity and cost of the device and its use.
United States Patent No~ 3,646,331 discloses a method and apparatus for correcting radiation measurment errors in a spectrophoto-meter by digitizing the output of a 100% line input at selected discrete wavelengths. A factor, called a M factor, is computed from the output at each discrete wavelength such that the digi-tized output multiplied by the M factor will give a corrected 100~ output at each wavelength.
These M factors are each stored, and during a sample measurement by the spectrophotometer, each of the stored M factors is synchronously applied to multiply the input signal derived from the signal, thereby generating a corrected output. This apparatus and method completely neglects the 0% reflectance value, whereby a significant, inherent error is introduced into the apparatus and method.
United States Patent No. 4,310,243 discloses a spectrophotometer and a method of simultaneously compensating for the dark current of a photomultiplier tube and the offset of an operational amplifier in the spectrophotometer, so that the output voltage of the operational amplifier is zero volts under dark conditions.
Although this patent discloses a technique for compensating for the dark signal of a detector, it does not account for stray light.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a new and improved method for calibrat-ing reflectance measuring device~.
1;~9~7~
Another object of the present invention is to provide a new and improved method of measur-ing light reflectance from a test sample.
Another object of the present invention is to provide a new and improved method for cali-brating reflectance measuring devices that incor-porate automatic gain or sensitivity setting capability or are otherwise subject to significant sensitivity changes due to time, use and environ-mental conditions.
A further object of the present inven-tion is to provide a new and improved method for calibrating reflectance measuring devices that minimizes error due to stray light.
~ A still further object of the present invention is to provide a new and improved method for calibrating a reflectance measuring device that minimizes the time needed for recalibration.
Another object of the present invention is to provide a new and improved method of cali-brating a reflectance measuring device to correct a detected value without recalibrating the device prior to each measurement.
A further object of the present inven-tion is to provide a new and improved method forcalibrating a ref~ectance measuring device that calculates stray light and is independent of the gain set of the device.
Briefly, the present invention is directed to a new and improved method of cali-brating reflectance measuring instrumentsr such as spectrophotometers, and to a method of measur-ing light reflectance from a test sample. In accordance with one important feature of the present invention, the method of the present ~29770 invention is particularly useful for reflecto-meters that incorporate an automatic gain or sensitivity setting or are otherwise subject to significant sensitivity changes due to time, use and environmental conditions~ A significant problem in reflectance measuring instruments is the need to constantly recalibrate the instrument due to sensitivity changes~. If the instrument is used at several wavelengths and sensitivities vary often, recalibration can be time consuming, and the utility of the instrument ultimately suffers. In the past, measured reflectance has been dependent on the gain set of the instrument.
For example, over the life of these instruments, the light source decays and the electronics change.
To compensate for these fluctuations, and to keep the light reflected off the sample being measured high thereby providing ma~imum resolution, the gain is reset prior to each measurement.
Prior art instruments also fail to consider the affect of stray light on the measure-ments. In spectrophotometry, for e~ample, 100 and 0% reflectance values must be established for correct readings from the instrument. The 0% value is impossible or very difficult to deter-mine due to stray light inherent in all instru-ments. In the past, stray light has been ignored or assumed to be zero resulting in a significant error in the measurements.
The present invention provides a method to avoid dependence of reflectance measurements on spectrophotometer sensitivity or gain setting and to minimize the error by accurate detection or measurement of stray light. The present inven-tion is directed to a calibration process or 70~
method wherein the reflectance of a single, second-ary reflectance standard disposed within the readhead is measured just prior to measurement of an unknown test specimen. A set of two coeffi-cients, determined at each operational wavelength of the reflectance measuring device, is used to calculate reflectance of the unknown test speci-men. A reflectance standard installation or recalibration process, using a set of at least two, and generally three to five, primary reflec-tance calibration standards is used to determine and store the coefficients. The set of primary reflectance standards provides the final calibra-tion of the instrument. The single secondary reflectance sta~dard disposed within the readhead achieves quick, short-term calibration of the instrument.
BRIEF DESCRIPTION OF THE DRAWING
The above and other objects and advan-tages of the present invention will become apparent from the following detailed description of a preferred embodiment of the invention illustrated in the accompanying drawing wherein:
FIG. 1 is a schematic illustration of a readhead for a reflectance measuring instrument such as a spectrophotometer of the type that may be calibrated and used for reflectance measure-ments in accordance with the principles of the present invention; and FIG. 2 is a graphic illustration of a least-squares linear regression line plotted through data collected by reading reflectance from a plurality of primary reflectance standards each with a known reflectance.
125~77C~(~
DESCRIPTION OF THE PREFERRED EMBODIME~TS
_ _ Reflectance measuring instruments, such as spectrophotometers, are used to measure reflectance from samples such as, for example, reagent strips. For reagent strip samples, the instrument reading can be, for example, a measure of light reflectance from one or more constituents in a blood sample, such as glucose. The spectro-photometer is an instrument designed to measure reflectance at a selected wavelength corresponding to a specific color.
In the past, the measurements from reflectance instruments have been dependent on the device sensitivity or gain set of the instru-ment necessitating recalibration prior to eachmeasurement. Usually, recalibration was accom-plished, rather crudely, by ignoring the existence of stray light. This failure to compensate for stray light results in signif~cant errors in reflectance measurements.
The present invention is directed to a method of calibrating reflectance measuring devices, such as spectrophotometers, resulting in measurements that are independent of device sensitivity or gain set of the instrument and minimiæing the error due to the affect of stray light within the readhead of the device. In accordance with an important feature of the present invention, a secondary reflectance standard is disposed within a sample aperture of the readhead of the reflectance measuring appara-tus to obtain a secondary reflectance measurement, while the sample also is disposed near or within the readhead. In accordance with an important feature of the present invention, calibration constants 7~
for stray light (pO) and a secondary reflectance standard (Pc) are determined for each operational wavelength of the instrument and stored in the instrument's memory. Accordingly, in accordance with an important feature of the pre~ent invention, once these constants are stored, the instrument need not be recalibrated prior to each test speci-men measurement since the true reflectance of a test specimen (p5) can be calculated from the measured reflectance (rs) without being affected by instrument sensitivity or gain. Accordingly, in accordance with an important feature of the process of the present invention, primary reflec-tance standards are no longer required to cali-brate the reflectance instrument.
A reflectance measuring device producesa numerical output reading (rs) that is propor-tional to the true sample reflectance (Ps) as viewed by the readhead. This relationship is as follows:
(1.) rs=R (Ps + Po) -In this equation, K is a scaling factor that includes instrument sensitivity or gain. K can be held constant for short periods, typically minutes, but often has long-term variation due to design, ambient conditions, or use. pO
accounts for the affects of stray light, which afflict all reflectance measuring instruments to some degree. Stray light, which cannot be removed by simple dark reading subtraction, affects the output reading (rs) the same as if a small additional offset reflectance, pO~ were present in the readhead along with the sample.
During calibration of the instrument several primary reflectance standards with ~2~77{~
different known reflectances are read in the instrument. A data set of the form:
(Pl, rl), (P2, r2) ~ (Pn, rn) results at each wavelength of operation of the instrument. In gathering this data, the primary reflectance standards can be constructed, for example, by loading Plexig].as*with varying mix-tures of BaSO4 and carbon black. The samples then can be measured on a Cary 17 Spectrophoto-meter. These measurements are traceable to theNational Bureau of Standards.
The data from measurements of the pri-mary standards i plotted on a graph as in FIG.
MEASURING DEVICES
FIELD OF THE INVE~TION
The present invention relates to a new and improved method of calibrating reflectance measuring devices and to a method of measuring light reflectance from a test specimen. More particularly, the invention is directed to a method of calibration of, measurement and measurement correction of light reflectance in a light reflectance device that incorporates an automatic gain or sensitivity setting. Light reflectance measurement from a single, secondary reflectance standard, prior to measurement of reflectance from a test sample, enables the cal-culation of accurate true reflectances from the test specimen reflectance measurement.
BACKGROUND OF THE INVENTION AND PRIOR ART
Reflectance ratio is the ratio of the intensity of light reflected from a sample to the intensity of light incident on the sample. It is relatively easy to measure the intensity of light reflected from a sample, however, measuring the light incident on the sample is more diffi-cult. Incident light has been measured by plac-ing a primary reflectance standard in a readhead of a spectrophotometer and, by knowing the true reflectance at the operational wavelength of the MS-14~9 1~29~7~3~
--2-- ~
device, the incident light can be back-calculated from the detected reflectance. ~eflectance of the test sample then is determined from the inten-sity of light reflected from the test sample divided by the intensity of light reflected from the primary reflectance standard multiplied by the known reflectance of the reflectance standard.
This technique assumes that there is no stray light within the reflectance measuring device, but this assumption is wrong and leads to signi-ficant measurement errors.
To calibrate reflectance measuring devices, such as spectrophotometers, it is custom-ary to establish 100% and 0% reflectance values.
To establish these reflectance values, a measure-ment relative to some standard must be made.
The 0% reflectance value has been difficult to obtain, and many procedures and devices operate on the assumption that 0~ reflectance is the value measured with the device turned off. The resulting 0% reflectance value is not correct since it is very difficult to impossible to eli-minate stray light.
In the prior art, recalibration using a primary reflectance standard must be performed before each measurement and the instrument sensi-tivity or gain must be reset. Under these condi-tions, each measurement and reading is dependent on the sensitivity or gain set of the device.
The necessity for the user to place a primary reflectance standard into the device prior to each measurement to reset the instrument sensi-tivity decreases the utility of the device and increases the cost of use. Further, each reflec-tance measuring device must be made to close ~7~70~
tolerances to insure that the stray light for each device is the same. This requires the main-tenance of costly and difficult manufacturing tolerances and strict quality control standards.
Another disadvantage with these devices i5 a need for frequent device recalibration due to constant sensitivity changes.
One approach to calibrating spectro-photometers is disclosed in United States Patent No. 4,029,419. In this system, the spectrophoto-meter is calibrated by using a secondary white standard calibrated against a primary white standard, a black standard to compensate for dark current and internal reflectance from a pressure plate, and a mirror to obtain internal wall reflectance used in a correction factor.
Although this approach does not assume that the stray light is zero, for each use of the spectro-photometer two samples must be read and the instru-ment recalibrated.
A process of compensating for radiationvariances in a spectrophotometer light source is disclosed in United States Patent No. 3,245,305.
The spectrophotometer described in this patent employs two light sources and means for sensing the relative intensities of the radiation from the two sources. The device of U.S. Patent No.
3,245,305 also includes means for altering the intensity of the radiation from one light source.
Additional apparatus is provided to be responsive to the sensing apparatus for automatically chang-ing the intensity of the radiation from one light source to keep the ratio of the intensity of the two sources constant. This device does not mea-sure the incident light directly. Further, the 12~7(~
requirement of a second light source and the attendant electronics increases the complexity and cost of the device and its use.
United States Patent No~ 3,646,331 discloses a method and apparatus for correcting radiation measurment errors in a spectrophoto-meter by digitizing the output of a 100% line input at selected discrete wavelengths. A factor, called a M factor, is computed from the output at each discrete wavelength such that the digi-tized output multiplied by the M factor will give a corrected 100~ output at each wavelength.
These M factors are each stored, and during a sample measurement by the spectrophotometer, each of the stored M factors is synchronously applied to multiply the input signal derived from the signal, thereby generating a corrected output. This apparatus and method completely neglects the 0% reflectance value, whereby a significant, inherent error is introduced into the apparatus and method.
United States Patent No. 4,310,243 discloses a spectrophotometer and a method of simultaneously compensating for the dark current of a photomultiplier tube and the offset of an operational amplifier in the spectrophotometer, so that the output voltage of the operational amplifier is zero volts under dark conditions.
Although this patent discloses a technique for compensating for the dark signal of a detector, it does not account for stray light.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a new and improved method for calibrat-ing reflectance measuring device~.
1;~9~7~
Another object of the present invention is to provide a new and improved method of measur-ing light reflectance from a test sample.
Another object of the present invention is to provide a new and improved method for cali-brating reflectance measuring devices that incor-porate automatic gain or sensitivity setting capability or are otherwise subject to significant sensitivity changes due to time, use and environ-mental conditions.
A further object of the present inven-tion is to provide a new and improved method for calibrating reflectance measuring devices that minimizes error due to stray light.
~ A still further object of the present invention is to provide a new and improved method for calibrating a reflectance measuring device that minimizes the time needed for recalibration.
Another object of the present invention is to provide a new and improved method of cali-brating a reflectance measuring device to correct a detected value without recalibrating the device prior to each measurement.
A further object of the present inven-tion is to provide a new and improved method forcalibrating a ref~ectance measuring device that calculates stray light and is independent of the gain set of the device.
Briefly, the present invention is directed to a new and improved method of cali-brating reflectance measuring instrumentsr such as spectrophotometers, and to a method of measur-ing light reflectance from a test sample. In accordance with one important feature of the present invention, the method of the present ~29770 invention is particularly useful for reflecto-meters that incorporate an automatic gain or sensitivity setting or are otherwise subject to significant sensitivity changes due to time, use and environmental conditions~ A significant problem in reflectance measuring instruments is the need to constantly recalibrate the instrument due to sensitivity changes~. If the instrument is used at several wavelengths and sensitivities vary often, recalibration can be time consuming, and the utility of the instrument ultimately suffers. In the past, measured reflectance has been dependent on the gain set of the instrument.
For example, over the life of these instruments, the light source decays and the electronics change.
To compensate for these fluctuations, and to keep the light reflected off the sample being measured high thereby providing ma~imum resolution, the gain is reset prior to each measurement.
Prior art instruments also fail to consider the affect of stray light on the measure-ments. In spectrophotometry, for e~ample, 100 and 0% reflectance values must be established for correct readings from the instrument. The 0% value is impossible or very difficult to deter-mine due to stray light inherent in all instru-ments. In the past, stray light has been ignored or assumed to be zero resulting in a significant error in the measurements.
The present invention provides a method to avoid dependence of reflectance measurements on spectrophotometer sensitivity or gain setting and to minimize the error by accurate detection or measurement of stray light. The present inven-tion is directed to a calibration process or 70~
method wherein the reflectance of a single, second-ary reflectance standard disposed within the readhead is measured just prior to measurement of an unknown test specimen. A set of two coeffi-cients, determined at each operational wavelength of the reflectance measuring device, is used to calculate reflectance of the unknown test speci-men. A reflectance standard installation or recalibration process, using a set of at least two, and generally three to five, primary reflec-tance calibration standards is used to determine and store the coefficients. The set of primary reflectance standards provides the final calibra-tion of the instrument. The single secondary reflectance sta~dard disposed within the readhead achieves quick, short-term calibration of the instrument.
BRIEF DESCRIPTION OF THE DRAWING
The above and other objects and advan-tages of the present invention will become apparent from the following detailed description of a preferred embodiment of the invention illustrated in the accompanying drawing wherein:
FIG. 1 is a schematic illustration of a readhead for a reflectance measuring instrument such as a spectrophotometer of the type that may be calibrated and used for reflectance measure-ments in accordance with the principles of the present invention; and FIG. 2 is a graphic illustration of a least-squares linear regression line plotted through data collected by reading reflectance from a plurality of primary reflectance standards each with a known reflectance.
125~77C~(~
DESCRIPTION OF THE PREFERRED EMBODIME~TS
_ _ Reflectance measuring instruments, such as spectrophotometers, are used to measure reflectance from samples such as, for example, reagent strips. For reagent strip samples, the instrument reading can be, for example, a measure of light reflectance from one or more constituents in a blood sample, such as glucose. The spectro-photometer is an instrument designed to measure reflectance at a selected wavelength corresponding to a specific color.
In the past, the measurements from reflectance instruments have been dependent on the device sensitivity or gain set of the instru-ment necessitating recalibration prior to eachmeasurement. Usually, recalibration was accom-plished, rather crudely, by ignoring the existence of stray light. This failure to compensate for stray light results in signif~cant errors in reflectance measurements.
The present invention is directed to a method of calibrating reflectance measuring devices, such as spectrophotometers, resulting in measurements that are independent of device sensitivity or gain set of the instrument and minimiæing the error due to the affect of stray light within the readhead of the device. In accordance with an important feature of the present invention, a secondary reflectance standard is disposed within a sample aperture of the readhead of the reflectance measuring appara-tus to obtain a secondary reflectance measurement, while the sample also is disposed near or within the readhead. In accordance with an important feature of the present invention, calibration constants 7~
for stray light (pO) and a secondary reflectance standard (Pc) are determined for each operational wavelength of the instrument and stored in the instrument's memory. Accordingly, in accordance with an important feature of the pre~ent invention, once these constants are stored, the instrument need not be recalibrated prior to each test speci-men measurement since the true reflectance of a test specimen (p5) can be calculated from the measured reflectance (rs) without being affected by instrument sensitivity or gain. Accordingly, in accordance with an important feature of the process of the present invention, primary reflec-tance standards are no longer required to cali-brate the reflectance instrument.
A reflectance measuring device producesa numerical output reading (rs) that is propor-tional to the true sample reflectance (Ps) as viewed by the readhead. This relationship is as follows:
(1.) rs=R (Ps + Po) -In this equation, K is a scaling factor that includes instrument sensitivity or gain. K can be held constant for short periods, typically minutes, but often has long-term variation due to design, ambient conditions, or use. pO
accounts for the affects of stray light, which afflict all reflectance measuring instruments to some degree. Stray light, which cannot be removed by simple dark reading subtraction, affects the output reading (rs) the same as if a small additional offset reflectance, pO~ were present in the readhead along with the sample.
During calibration of the instrument several primary reflectance standards with ~2~77{~
different known reflectances are read in the instrument. A data set of the form:
(Pl, rl), (P2, r2) ~ (Pn, rn) results at each wavelength of operation of the instrument. In gathering this data, the primary reflectance standards can be constructed, for example, by loading Plexig].as*with varying mix-tures of BaSO4 and carbon black. The samples then can be measured on a Cary 17 Spectrophoto-meter. These measurements are traceable to theNational Bureau of Standards.
The data from measurements of the pri-mary standards i plotted on a graph as in FIG.
2 for truth or actual values (Pl~ P2 ------Pn) ver5us measured values (rl, r2, ....... rn). A
least-squares linear regression line of the form:
(2) rn = m Pn + b = m(Pn ~ b/m) then is calculated for the data from each wave- -length of operation where m is the slope and b is the vertical axis intercept for the line in FIG. 2. K, m and b may be different at each wavelength of operation~
In comparing equations (1) and (2) it is noted that:
m ~ X
and b/m - pO-The reflectance of the secondary reflec-tance ~tandard (Pc) also can be calculated from equation (2): -rc ' m Pc + b and (3) Pc ~ (rc - b)/m * Trade-mark , .. " , ., .
~ 377C~
In this manner, b, m, Pc and pO are calculated for each wavelength of instrument operation.
The constants Pc and pO for each wavelength are stored in the memory oE the reflectance measuring instrument for future use.
In accordance with an important feature of the present invention, Ps~ true sample reflec-tance, is computed independent of K by reading the light reflectance from a stable, relatively highly reflective secondary standard just prior to the measurement of each unknown test specimen.
When the secondary standard reflectance (Pc) is read:
rc = K (Pc + Po) or, solving for K:
least-squares linear regression line of the form:
(2) rn = m Pn + b = m(Pn ~ b/m) then is calculated for the data from each wave- -length of operation where m is the slope and b is the vertical axis intercept for the line in FIG. 2. K, m and b may be different at each wavelength of operation~
In comparing equations (1) and (2) it is noted that:
m ~ X
and b/m - pO-The reflectance of the secondary reflec-tance ~tandard (Pc) also can be calculated from equation (2): -rc ' m Pc + b and (3) Pc ~ (rc - b)/m * Trade-mark , .. " , ., .
~ 377C~
In this manner, b, m, Pc and pO are calculated for each wavelength of instrument operation.
The constants Pc and pO for each wavelength are stored in the memory oE the reflectance measuring instrument for future use.
In accordance with an important feature of the present invention, Ps~ true sample reflec-tance, is computed independent of K by reading the light reflectance from a stable, relatively highly reflective secondary standard just prior to the measurement of each unknown test specimen.
When the secondary standard reflectance (Pc) is read:
rc = K (Pc + Po) or, solving for K:
(4) K = rc/(Pc + Po) where Pc is the reflectance (previously calculated and stored) of the secondary standard.
~ Rearranging equation (1), the following is obtained:
~ Rearranging equation (1), the following is obtained:
(5) Ps = (rs/K) - Po-In order to eliminate K, equation (4) is substi-tuted into equation (5):
Ps = (rs (Pc + PO) /rc) - Po (6) = Pc (rs/rc) + pO (rS/rc - 1).
The reflectance of an unknown test specimen can be calculated using equation (6) from readings of the sample (rs) and the secondary standard reflectance (rc) and from the appropriate stored values of Pc and pO. This calculation is independent of spectrophotometer sensitivity (part of K) as long as K is stable during the reflectance readings of the secondary standard and the unknown sample.
12~77Q~
In equation (6), if the stray light were assumed to be zero as is done in the prior art, pO would be zero resulting in a potentially significant reflectance error equal to PO (rs/rc - 1).
This error is greatest at small sample reflectances where rS is much smaller than rc.
The process as described provides two constants (Pc and pO) that once determined for each wavelength, can be stored in an erasable memory of each instrument. The instruments then can be used to read the reflectance from the secondary standard (rc~ followed by a reading of the reflectance from the test specimen (rs).
With this information, the electronics of the instrument make the calcula~ion of equation (6) to determine quickly and accurately tbe true reflectance of the test specimen (Ps) This measurement is independent of the gain (K) of the instrument and minimizes the error due to stray light. Independence from the gain allows the instrument to be operated by a lay person, thereby reducing the cost of operation. The minimization of the error due to stray light increases the accuracy of the reading by the in~trument thereby improving diagnosis.
To more fully understand the present invention, reference is directed to FIG. 1. In FIG. 1 a readhead is schematically iLlustrated and generally designated by the reference numeral 10. Readhead 10 is, for example, of the type used in spectrophotometers and is described more fully in U.S. Patent No. 4,659,229, issued April 2l, 1987, assigned to the assignee of the i 35 present inven~ion.
` . ! ~
. ~ .
~ 770~) Readhead 10 includes a hemispherical upper wall 12 and a conical lower wall 14. Mounted in the readhead 10 is a broad spectrum light source 16, preferably a Xenon flash lamp. A sample aperture 18 is provided in the bottom of readhead 10 for se~arate positioning of the secondary reflectance standard (not shown) and a test specimen (not shown) that is to be measured. A second aperture 20 is provided in the upper end of readhead 10 with first and second lenses 22 and 24 positioned in second aperture 20. First lens 22 is focused on the test specimen and second lens 24 is focused on the inside wall of readhead 10.
In accordance with the present inven-tion, the secondary standard first is disposed within the aperture 18 in a predetermined posi-tion within the readhead 10 with the test speci-men also within the readhead 10 in a predetermined position to obtain a secondary standard reflec-tance measurement rc. Shortly thereafter, thetest specimen is disposed within the aperture 18 of readhead 10 and the secondary standard disposed in a predetermined position within the readhead 10 to obtain a test specimen reflectance measure-ment rs. During these measurements, light fromlight source 16 reflects from the inside of read-head 10 and impinges on the secondary reflectance standard and test sample from all directions.
The light reflected from the secondary reflectance standard and test sample is focused by lens 22 onto a first detector or photodiode 26. Refl~c-tance detected by photodiode 26 is measured by the instrument in accordance with known electron-ics to provide reflectance readings (rs and rc) proportional, respectively, to the true reflectance " ~2~70~) of the test specimen (Ps) and the true reflectance of the secondary standard (Pc) The second lens 24 focuses light reflected from the inside of readhead lO onto a second detector or photodiode 28. The reading obtained from the second detector 28 is used to correct the reflectances rs and rc measured at photodiode 26 to compensate for short term variations in light from light source 16.
An interference filter 30 is disposed between the lenses 22 and 24 and the detectors 26 and 28 to allow only light of preselected wavelength to reach the detectors 26 and 28. A reflectance measuring instrument, such as a spectrophotometer (not shown) including the readhead lO also includes electronics and a memory as well known in the art capable of performing the calculations described herein.
The instrument first reads a reflectance at a preselected operational wavelength from the secondary standard (rc) allowing the instrument to compensate for long term drifts in the instru-ment by automatically changing the sensitivity or gain. The test specimen then is positioned in aperture 18 and the secondary standard re-positioned to another preselected position withinthe readhead 10 for a measurement of the reflec~
tance from the test sample (rs). The process of the present invention enables much faster and more accurate measurements than prior art methods since it is not necessary to calibrate the instru-ment ~rior to each measurement or reading.
The apparatus and method of the present invention also are convenient to the customer since a lay person easily can use the instrument.
The costs of purchasing the apparatus of the 77(3~
present invention to obtain reflectance measure-ments are much lower than the prior art since primary standards are not required and need not be maintained. The present invention also is more accurate than the prior art processes since stray light is calculated and incorporated into the reflectance measurement calculations.
Many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention can be practiced other than as specifically described in the above description.
Ps = (rs (Pc + PO) /rc) - Po (6) = Pc (rs/rc) + pO (rS/rc - 1).
The reflectance of an unknown test specimen can be calculated using equation (6) from readings of the sample (rs) and the secondary standard reflectance (rc) and from the appropriate stored values of Pc and pO. This calculation is independent of spectrophotometer sensitivity (part of K) as long as K is stable during the reflectance readings of the secondary standard and the unknown sample.
12~77Q~
In equation (6), if the stray light were assumed to be zero as is done in the prior art, pO would be zero resulting in a potentially significant reflectance error equal to PO (rs/rc - 1).
This error is greatest at small sample reflectances where rS is much smaller than rc.
The process as described provides two constants (Pc and pO) that once determined for each wavelength, can be stored in an erasable memory of each instrument. The instruments then can be used to read the reflectance from the secondary standard (rc~ followed by a reading of the reflectance from the test specimen (rs).
With this information, the electronics of the instrument make the calcula~ion of equation (6) to determine quickly and accurately tbe true reflectance of the test specimen (Ps) This measurement is independent of the gain (K) of the instrument and minimizes the error due to stray light. Independence from the gain allows the instrument to be operated by a lay person, thereby reducing the cost of operation. The minimization of the error due to stray light increases the accuracy of the reading by the in~trument thereby improving diagnosis.
To more fully understand the present invention, reference is directed to FIG. 1. In FIG. 1 a readhead is schematically iLlustrated and generally designated by the reference numeral 10. Readhead 10 is, for example, of the type used in spectrophotometers and is described more fully in U.S. Patent No. 4,659,229, issued April 2l, 1987, assigned to the assignee of the i 35 present inven~ion.
` . ! ~
. ~ .
~ 770~) Readhead 10 includes a hemispherical upper wall 12 and a conical lower wall 14. Mounted in the readhead 10 is a broad spectrum light source 16, preferably a Xenon flash lamp. A sample aperture 18 is provided in the bottom of readhead 10 for se~arate positioning of the secondary reflectance standard (not shown) and a test specimen (not shown) that is to be measured. A second aperture 20 is provided in the upper end of readhead 10 with first and second lenses 22 and 24 positioned in second aperture 20. First lens 22 is focused on the test specimen and second lens 24 is focused on the inside wall of readhead 10.
In accordance with the present inven-tion, the secondary standard first is disposed within the aperture 18 in a predetermined posi-tion within the readhead 10 with the test speci-men also within the readhead 10 in a predetermined position to obtain a secondary standard reflec-tance measurement rc. Shortly thereafter, thetest specimen is disposed within the aperture 18 of readhead 10 and the secondary standard disposed in a predetermined position within the readhead 10 to obtain a test specimen reflectance measure-ment rs. During these measurements, light fromlight source 16 reflects from the inside of read-head 10 and impinges on the secondary reflectance standard and test sample from all directions.
The light reflected from the secondary reflectance standard and test sample is focused by lens 22 onto a first detector or photodiode 26. Refl~c-tance detected by photodiode 26 is measured by the instrument in accordance with known electron-ics to provide reflectance readings (rs and rc) proportional, respectively, to the true reflectance " ~2~70~) of the test specimen (Ps) and the true reflectance of the secondary standard (Pc) The second lens 24 focuses light reflected from the inside of readhead lO onto a second detector or photodiode 28. The reading obtained from the second detector 28 is used to correct the reflectances rs and rc measured at photodiode 26 to compensate for short term variations in light from light source 16.
An interference filter 30 is disposed between the lenses 22 and 24 and the detectors 26 and 28 to allow only light of preselected wavelength to reach the detectors 26 and 28. A reflectance measuring instrument, such as a spectrophotometer (not shown) including the readhead lO also includes electronics and a memory as well known in the art capable of performing the calculations described herein.
The instrument first reads a reflectance at a preselected operational wavelength from the secondary standard (rc) allowing the instrument to compensate for long term drifts in the instru-ment by automatically changing the sensitivity or gain. The test specimen then is positioned in aperture 18 and the secondary standard re-positioned to another preselected position withinthe readhead 10 for a measurement of the reflec~
tance from the test sample (rs). The process of the present invention enables much faster and more accurate measurements than prior art methods since it is not necessary to calibrate the instru-ment ~rior to each measurement or reading.
The apparatus and method of the present invention also are convenient to the customer since a lay person easily can use the instrument.
The costs of purchasing the apparatus of the 77(3~
present invention to obtain reflectance measure-ments are much lower than the prior art since primary standards are not required and need not be maintained. The present invention also is more accurate than the prior art processes since stray light is calculated and incorporated into the reflectance measurement calculations.
Many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention can be practiced other than as specifically described in the above description.
Claims (4)
1. A method of calibrating a spectrophotometer for measuring the reflectance of a test sample, said spectrophotometer including a readhead, a broad spectrum light source mounted in said readhead, a first sample aperture, a second aperture, at least one lens in said second aperture, at least one detector for collecting reflected light whereby said detector provides an output signal proportional to the intensity of collected reflected light, a secondary reflectance standard in said readhead and a memory, the method comprising:
detecting the amount of light reflected from at least two primary reflectance standards with different known reflectances for a plurality of wavelengths of operation of said spectrophotometer;
calculating a least-squares linear regression line for said known reflectances and the output signals for said primary reflectance standards at a plurality of wavelengths of operation;
calculating an offset reflectance for stray light for a plurality of wavelengths using the slope and intercept of said least-squares linear regression lines;
detecting for said plurality of wavelengths the amount of light reflected from a secondary reflectance standard with a known reflectance;
calculating the secondary standard reflectance for said plurality of wavelengths of spectrophotometer operation using the slope and intercept of said least-squares linear regression lines; and storing said offset reflectance and said secondary standard reflectance for said plurality of wavelengths in said memory.
detecting the amount of light reflected from at least two primary reflectance standards with different known reflectances for a plurality of wavelengths of operation of said spectrophotometer;
calculating a least-squares linear regression line for said known reflectances and the output signals for said primary reflectance standards at a plurality of wavelengths of operation;
calculating an offset reflectance for stray light for a plurality of wavelengths using the slope and intercept of said least-squares linear regression lines;
detecting for said plurality of wavelengths the amount of light reflected from a secondary reflectance standard with a known reflectance;
calculating the secondary standard reflectance for said plurality of wavelengths of spectrophotometer operation using the slope and intercept of said least-squares linear regression lines; and storing said offset reflectance and said secondary standard reflectance for said plurality of wavelengths in said memory.
2. The method of claim 1 further comprising:
placing a sample of unknown reflectance adjacent said first sample aperture;
measuring the amount of light reflected from said secondary reflectance standard;
measuring the amount of light reflected from said sample; and calculating the reflectance of said sample using the stored reflectance measurements.
placing a sample of unknown reflectance adjacent said first sample aperture;
measuring the amount of light reflected from said secondary reflectance standard;
measuring the amount of light reflected from said sample; and calculating the reflectance of said sample using the stored reflectance measurements.
3. A method for calculating true sample reflectance for a sample as viewed by a readhead of a spectrophometer that incorporates automatic gain or sensitivity setting capability, the steps comprising:
measuring the amount of reflectance of at least two primary reflectance standards having known reflectances for each wavelength of operation of said spectrophotometer;
compiling data including the true reflectance and measured reflectance for each of said primary reflectance standards at each said wavelength;
calculating a least-squares regression line for each said wavelength through said data;
determining the slope m and intercept b for each least-squares regression line;
calculating a stray light constant Po for each wavelength according to:
Po = b/m;
measuring the amount of reflectance rs of a secondary standard reflectance having a known reflectance for a plurality of wavelengths of operation of said spectrophotometer;
calculating the secondary standard reflectance Pc according to:
Pc = (rc-b)/m;
measuring the amount of reflectance Ps according to:
Ps = Pc(rs/rc)+po(rs/rc-l).
measuring the amount of reflectance of at least two primary reflectance standards having known reflectances for each wavelength of operation of said spectrophotometer;
compiling data including the true reflectance and measured reflectance for each of said primary reflectance standards at each said wavelength;
calculating a least-squares regression line for each said wavelength through said data;
determining the slope m and intercept b for each least-squares regression line;
calculating a stray light constant Po for each wavelength according to:
Po = b/m;
measuring the amount of reflectance rs of a secondary standard reflectance having a known reflectance for a plurality of wavelengths of operation of said spectrophotometer;
calculating the secondary standard reflectance Pc according to:
Pc = (rc-b)/m;
measuring the amount of reflectance Ps according to:
Ps = Pc(rs/rc)+po(rs/rc-l).
4. A method of measuring the reflectance of a sample using a spectrophotometer, comprising:
storing a value for offset reflectance at different wavelengths;
storing a value for secondary standard reflectance at different wavelengths;
exposing a secondary reflectance standard to light;
measuring the amount of light reflected from said secondary reflectance standard;
exposing said sample to said light;
measuring the amount of light reflected from said sample; and calculating true sample reflectance using the ratio of the measured amount of light reflected from the sample and from said secondary reflectance standard and the stored values for offset reflectance and the stored secondary standard reflectance.
storing a value for offset reflectance at different wavelengths;
storing a value for secondary standard reflectance at different wavelengths;
exposing a secondary reflectance standard to light;
measuring the amount of light reflected from said secondary reflectance standard;
exposing said sample to said light;
measuring the amount of light reflected from said sample; and calculating true sample reflectance using the ratio of the measured amount of light reflected from the sample and from said secondary reflectance standard and the stored values for offset reflectance and the stored secondary standard reflectance.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/877,169 US4729657A (en) | 1986-06-23 | 1986-06-23 | Method of calibrating reflectance measuring devices |
| US877,169 | 1986-06-23 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1297700C true CA1297700C (en) | 1992-03-24 |
Family
ID=25369402
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000538704A Expired - Lifetime CA1297700C (en) | 1986-06-23 | 1987-06-03 | Method of calibrating reflectance measuring devices |
Country Status (6)
| Country | Link |
|---|---|
| US (1) | US4729657A (en) |
| EP (1) | EP0250959B1 (en) |
| JP (1) | JPS635240A (en) |
| AU (1) | AU574287B2 (en) |
| CA (1) | CA1297700C (en) |
| DE (1) | DE3778976D1 (en) |
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| IT1237628B (en) * | 1989-10-03 | 1993-06-12 | Michele Gennaro De | METHOD TO MEASURE THE EFFICIENCY OF A COMBUSTION AND APPARATUS TO IMPLEMENT THE METHOD. |
| US5162874A (en) * | 1990-12-24 | 1992-11-10 | Xerox Corporation | Electrophotographic machine having a method and apparatus for measuring toner density by using diffuse electromagnetic energy |
| US5357336A (en) * | 1991-08-08 | 1994-10-18 | The Dow Chemical Company | Method and apparatus for multivariate characterization of optical instrument response |
| JP3212779B2 (en) * | 1993-11-12 | 2001-09-25 | 富士写真フイルム株式会社 | Method for compensating for differences in the spectrometer of an optical analyzer |
| FI935180A0 (en) * | 1993-11-22 | 1993-11-22 | Rautaruukki Oy | Construction of calibrating devices, calibration devices |
| US5597532A (en) * | 1994-10-20 | 1997-01-28 | Connolly; James | Apparatus for determining substances contained in a body fluid |
| US5695949A (en) | 1995-04-07 | 1997-12-09 | Lxn Corp. | Combined assay for current glucose level and intermediate or long-term glycemic control |
| US6061128A (en) * | 1997-09-04 | 2000-05-09 | Avocet Medical, Inc. | Verification device for optical clinical assay systems |
| US8564780B2 (en) * | 2003-01-16 | 2013-10-22 | Jordan Valley Semiconductors Ltd. | Method and system for using reflectometry below deep ultra-violet (DUV) wavelengths for measuring properties of diffracting or scattering structures on substrate work pieces |
| US20080246951A1 (en) * | 2007-04-09 | 2008-10-09 | Phillip Walsh | Method and system for using reflectometry below deep ultra-violet (DUV) wavelengths for measuring properties of diffracting or scattering structures on substrate work-pieces |
| US7126131B2 (en) * | 2003-01-16 | 2006-10-24 | Metrosol, Inc. | Broad band referencing reflectometer |
| US7511265B2 (en) * | 2004-08-11 | 2009-03-31 | Metrosol, Inc. | Method and apparatus for accurate calibration of a reflectometer by using a relative reflectance measurement |
| US7804059B2 (en) * | 2004-08-11 | 2010-09-28 | Jordan Valley Semiconductors Ltd. | Method and apparatus for accurate calibration of VUV reflectometer |
| US7399975B2 (en) * | 2004-08-11 | 2008-07-15 | Metrosol, Inc. | Method and apparatus for performing highly accurate thin film measurements |
| US7282703B2 (en) * | 2004-08-11 | 2007-10-16 | Metrosol, Inc. | Method and apparatus for accurate calibration of a reflectometer by using a relative reflectance measurement |
| US7663097B2 (en) * | 2004-08-11 | 2010-02-16 | Metrosol, Inc. | Method and apparatus for accurate calibration of a reflectometer by using a relative reflectance measurement |
| AU2005309510B2 (en) * | 2004-11-24 | 2011-05-19 | Idexx Laboratories, Inc. | Reflectometer and associated light source for use in a chemical analyzer |
| US7663757B2 (en) * | 2006-09-27 | 2010-02-16 | Alberta Research Council Inc. | Apparatus and method for obtaining a reflectance property indication of a sample |
| US20080129986A1 (en) * | 2006-11-30 | 2008-06-05 | Phillip Walsh | Method and apparatus for optically measuring periodic structures using orthogonal azimuthal sample orientations |
| JP2009192336A (en) * | 2008-02-14 | 2009-08-27 | Panasonic Corp | Analysis device |
| JP5274031B2 (en) * | 2008-01-25 | 2013-08-28 | パナソニック株式会社 | Analysis method and analyzer |
| JP5020219B2 (en) * | 2008-11-28 | 2012-09-05 | パナソニック株式会社 | Analysis method and analyzer |
| WO2009093453A1 (en) * | 2008-01-25 | 2009-07-30 | Panasonic Corporation | Analysis device and analysis method |
| US20090219537A1 (en) * | 2008-02-28 | 2009-09-03 | Phillip Walsh | Method and apparatus for using multiple relative reflectance measurements to determine properties of a sample using vacuum ultra violet wavelengths |
| US8153987B2 (en) * | 2009-05-22 | 2012-04-10 | Jordan Valley Semiconductors Ltd. | Automated calibration methodology for VUV metrology system |
| JP5540354B2 (en) * | 2010-04-30 | 2014-07-02 | 独立行政法人 宇宙航空研究開発機構 | Method of measuring reflectance and reflection density with calibration function and system for implementing the method |
| US8867041B2 (en) | 2011-01-18 | 2014-10-21 | Jordan Valley Semiconductor Ltd | Optical vacuum ultra-violet wavelength nanoimprint metrology |
| US8565379B2 (en) | 2011-03-14 | 2013-10-22 | Jordan Valley Semiconductors Ltd. | Combining X-ray and VUV analysis of thin film layers |
Family Cites Families (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3646331A (en) * | 1970-09-03 | 1972-02-29 | Kollmorgen Corp | Automatic 100{11 line adjustment of spectrophotometers |
| US3828173A (en) * | 1972-08-01 | 1974-08-06 | Dickey John Corp | Grain analysis computer circuit |
| US3874799A (en) * | 1973-06-01 | 1975-04-01 | Color Control Inc | Method and apparatus for color spectrophotometry |
| US4029419A (en) * | 1975-10-10 | 1977-06-14 | International Business Machines Corporation | Textile color analyzer calibration |
| JPS591977B2 (en) * | 1978-01-25 | 1984-01-14 | 株式会社京都第一科学 | Analysis method using color test paper |
| US4526470A (en) * | 1982-03-05 | 1985-07-02 | Beckman Instruments, Inc. | Stray light measurement and compensation |
| CA1201300A (en) * | 1982-11-01 | 1986-03-04 | Albert Brunsting | Optical readhead |
| CA1201299A (en) * | 1982-11-29 | 1986-03-04 | Albert Brunsting | Optical readhead |
| US4725147A (en) * | 1984-09-17 | 1988-02-16 | Somanetics Corporation | Calibration method and apparatus for optical-response tissue-examination instrument |
| US4659229A (en) * | 1984-10-10 | 1987-04-21 | Miles Laboratories, Inc. | Readhead with reduced height sensitivity |
-
1986
- 1986-06-23 US US06/877,169 patent/US4729657A/en not_active Expired - Lifetime
-
1987
- 1987-06-03 CA CA000538704A patent/CA1297700C/en not_active Expired - Lifetime
- 1987-06-05 AU AU73890/87A patent/AU574287B2/en not_active Ceased
- 1987-06-10 DE DE8787108370T patent/DE3778976D1/en not_active Expired - Fee Related
- 1987-06-10 EP EP87108370A patent/EP0250959B1/en not_active Expired
- 1987-06-22 JP JP62153591A patent/JPS635240A/en active Pending
Also Published As
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|---|---|
| EP0250959A2 (en) | 1988-01-07 |
| AU574287B2 (en) | 1988-06-30 |
| AU7389087A (en) | 1987-12-24 |
| JPS635240A (en) | 1988-01-11 |
| EP0250959A3 (en) | 1989-09-13 |
| EP0250959B1 (en) | 1992-05-13 |
| DE3778976D1 (en) | 1992-06-17 |
| US4729657A (en) | 1988-03-08 |
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