CA2389367A1

CA2389367A1 - Concentric spectrometer with mitigation of internal specular reflections

Info

Publication number: CA2389367A1
Application number: CA002389367A
Authority: CA
Inventors: Perry A. Palumbo
Original assignee: Individual
Current assignee: Hach Co
Priority date: 1999-12-01
Filing date: 2000-11-13
Publication date: 2001-06-07
Anticipated expiration: 2020-11-13
Also published as: EP1236025A1; EP1236025B1; JP4611591B2; DE60010322T2; JP2003515733A; US6538736B1; WO2001040746A1; CA2389367C; DE60010322D1

Abstract

Concentric spectrometers are plagued with internal reflections due to the inherent nature of more than one optical surface possessing a common center of curvature. Reflections from optical surfaces arise when there is a difference or change in the refractive index of the media in which an optical beam or ray of a given wavelength is propagating. Internal reflections in concentric optical systems can produce a myriad of undesirable optical phenomenon at the image plane such as multiple images of an object, interference fringes, and stray light. As a result a loss in contrast or detection limit arise from such phenomenon in which light or detectable radiation that impinges on the image plane does not add to the formation of the intended image, (stray light). The present invention produces high quality images without the optical phenomenon(s) that arise from internal reflections by removing the reflected radiation from propagating through the optical system.

Claims

1. A concentric spectrometer which mitigates internal specular reflections, said spectrometer comprising:
(a) an optical axis; and (b) a radial distance perpendicular to said optical axis;
(c) an object aperture located substantially at said radial distance from optical axis;
(d) an image aperture located substantially at said radial distance from optical axis;
(e) a plurality of non-planar optical surfaces having centers of curvature which are substantially common on said optical axis; wherein said plurality of non-planar optical surfaces includes at least one grating surface which is aplanatic for zero order of diffraction; wherein said plurality of non-planar optical surfaces includes at least one convex refracting surface; and (f) a plurality of planar optical surfaces substantially coincident with the centers of curvatures of said plurality of optical surfaces and are substantially perpendicular to said optical axis.

2. The spectrometer of claim 1 further comprising an aperture mask positioned at said grating surface.

3. The spectrometer of claim 1 further comprising a plurality of light absorbing baffles between said refracting surface and said grating surface.

4. The spectrometer of claim 1 further comprising at least one planar reflecting optical surface positioned in the ray path at an oblique angle relative to said optical axis.

5. The spectrometer of claim 1 wherein at least one of said surfaces is substantially coincident with said object aperture.

6. The spectrometer of claim 1 wherein at least one of said planar optical surfaces is substantially coincident with said image aperture.

7. The spectrometer of claim 1 wherein said object aperture includes one or more apertures through which polychromatic light enters said spectrometer.

8. The spectrometer of claim 1 wherein images of said object aperture are formed within said image aperture spatially dispersed by wavelength along the axis of dispersion substantially perpendicular to the grooves of said grating surface.

9. The spectrometer of claim 1 wherein one said planar optical surface and one said non-planar optical surface form a piano-convex lens of index of refraction higher than of that media that separates the convex surface of the lens and said grating surface.

10. The spectrometer of claim 9 wherein the convex lens surface of said piano-convex lens and said grating surface are spatially separated along said optical axis to advantage of best image formation of diffracted object within said image aperture at said radial distance from said optical axis.

11. The spectrometer of claim 9 wherein said planar surface of said piano-convex lens may include either object aperture or image aperture or both.

12. The spectrometer of claim 1 wherein the radii of said radial distance is sufficient to cause light diffracted from said grating surface to reflect from specular surface of said refracting surface to exclude impingement on said grating surface.

13. The spectrometer of claim 12 wherein said radial distance substantially includes said object aperture and said image aperture and zero order of diffraction.

14. The spectrometer of claim 2 wherein said aperture mask prevents the propagation of light beyond the clear aperture of said grating surface.

15. The spectrometer of claim 7 further comprising an aperture mask for preventing the propagation of diffracted light reflected from the specular surface of said convex surface of said piano-convex lens.

16. The spectrometer of claim 14 wherein said aperture mask does not impede the propagation light that would contribute to the formation of spectra at said image aperture.

17. The spectrometer of claim 1 wherein the non-planar optical surfaces are substantially spherical.

18. The spectrometer of claim 3 wherein said light absorbing baffles absorb light which does not contribute to the formation of spectra by impingement on said baffles no less than two times.

19. The spectrometer of claim 10 further comprising light absorbing baffles which absorb diffracted light reflected from the specular surface of said convex surface of plano-convex lens.

20. The spectrometer of claim 19 wherein said light absorbing baffles absorb light beyond the clear aperture of said grating surface.