CA1108429A

CA1108429A - Spectrophotometer

Info

Publication number: CA1108429A
Application number: CA316,716A
Authority: CA
Inventors: Hans-Erdmann Korth; Franz Schedewei
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1977-12-27
Filing date: 1978-11-23
Publication date: 1981-09-08
Also published as: CH634656A5; DE2758141C2; FR2413644B1; IT1160375B; GB2012067A; FR2413644A1; IT7831080A0; JPS6038644B2; GB2012067B; DE2758141A1; JPS54109890A

Abstract

SPECTROPHOTOMETER
Abstract The spectrophotometer consists of a plate having a linear photo-diode array and an entrance aperture, a lens 8 and a reflecting dis-persing element preferably designed as an echelon grating. The en-trance aperture is in the focal plane of the lens. Since all beams entering through the entrance aperture are directed by the lens in parallel to the optical axis all beams forming any angle of diver-gence, or enclosing any angle within the optical axis, provided these angles are smaller than the aperture angle of the lens, can be measured with the same sensitivity and the same resolution.

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Description

~ 34~9 SPECTROPHOTOMETER
The invention relates to a spectrophotometer with a dispersing reflecting elemen~ and a multiple photodetector array arranged in the evaluation plane.
For testing all sorts of samples, such as textile fibers and fabrics, liquids and a variety of gases, particularly polluted air etc., methods of spectral analysis are applied almost exclusively.
For this purpose, a series of spectrophotometers have been developed, i.e., devices for measuring the spectral light intensity distribut;on of a radiation. These methods permit a quick, safe and simple de-termination of the composition of the samples that have been tested, or the finding of specific components in these samples.
In a monochrometer described, for example, in the literature reference "Classical Methods", Vol. 1, by I. Es~erman, Academic Press, New York and London, 1959, pages 398 to 400, the images of the en-trance gap generated in various colours are successively shifted over an exit gap, or a radiation-sensitive element, respectively, by means of a common movement of a dispersion prism and a plane mirror. Since high resolution devices of that type require a very complex apparatus the devices operating in accordance with this method are very expen-sive, large and susceptible to disturbances. Because they are very sensitive to shock and vibration, frequent and very time-consuming ad-justments are unavoidable. As the intensities for the individual spectral componen-ts are measured not simultaneously but successively by rotating an optical arrangement sensitive to misadjustment, high demands are made on the synchronism of the rotation with the times of the individual measurings. Furthermore, any measurements carried out with devices of the above described type are very time-consuming.

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`` 3L~lg~8~ 9 1 It is the object of the invention to provide a spectrophotometer which with an extremely simple structure, 1QW weight, and small dimen-sions shows a high resolution, high sensitivity, and high working speed.
-- Furthermore, even relatively high deviations of the actual direction from the nominal directions of the radiation to be analyzed are to be without any influence on the measuring precision and the sensitivity.
The very high sensitivity of the spectrophotometer as disclosed by the invention compared with s;milar devices in the prior art is achieved particularly in that between entrance aperture and dispersing reflecting 1~ element a lens, preferably arranged in the immediate vicinity of the : dispersing reflecting element, is provided which directs a beam from the entrance aperture arranged near its focal plane, independently of its angle of divergence, as a bundle of parallel beams onto the dispersing reflecting element. Due to the small distance between lens and disper-sing element, deviations of the direction of the central beam of the diYergent bundle of beams passing through the entrance aperture, from the optical axis oF the device, do not cause any important lateral displacements and no changes of direction of the bundle of beams impinging - on the dispersing reflecting element, so that there are no changes of

2~ position of the entrance aperture images associated to the individual wavelength on the multiple photodetector array. As the lens which - directs the bundle of beams to be analy~ed, independently of its angle of divergence, provided that the angle is within the aperture angle of the lens, and the direction of its central beam, onto the dispersing reflecting element as a bundle of parallel beams, simultaneously provides ~he images of the entrance aperture which are associated with the in-dividual wavelengths on the multi-photodetector array, the number of optical elements required in the spectral photometer as disclnsed in the i;nvention is sm~ller than in all formerly known comparable devices.
GE~-77-~13 - 2 -8 ~ 9 1 Since a particularly small distance between the lens and the dis-persing reflecting element is especially desirable in accordance with the present invention, these two e1ements can be either cemented to each other, or they can even be at a single body, so that the simplicity and the robustness of the device as disclosed by the invention with respect to vibration and shock or contamination are considerably improved as compared with all known spectrophotometers.
As the plate with the entrance aperture can also be rigidly connected to the multiple photodet:ec~or array, the spectrophotometer~
as disclosed by the invention, practically consists of two c~mponents that are relatively easy to make in large-scale production, and are connected to each other, without particularly high demands being made to the mutual alignment~ by a sleeve or a length and width of a few cm only.
The arrangement of the linear multiple photodetector array in the immediate vicinity of the circular, square or rectangular en-trance aperture largely avoids the oblique bundles and the errors - connected therewith which in the known devices of this type are dif-Ficult to prevent.
The inventior. will be described in detail below by means of the figure representing an embodiment oF the invention. The embodiment of the invention schematically represented in the Figure consists of a sample holder l holding a sample 2 to be examined, a lens 20, a plate 4 having a square aperture 3, a multiple photodiode array 5 arransed on that plate in the immediate vicinity of aperture 3 and consisting of photodiodes 6, a lens 8 and a dispersing reflecting element 9 designed as an echelon grating. Plate 4 is connected to ' ,' .

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1 dispersing element 9 by a sleeve 10 which at its top is closed by a cover 21.
Sample 2 arranged on sample holder 1 is illuminated by an obliquely impinging polychromatic radiation 11. Instead of oblique-angled il-lumination it is also possible to provide a vertical illumination via a semi-transparent mirror arranged between sample 2 and lens 20 (not shown in the figure.) The radiation which is diffusely scattered or reflected at a point of the surface of sample 2 is focused through lens 20 as a convergent bundle of beams 12 within the entrance aperture 3 in the focal point of lens 8, and exits therefrom as a divergent bundle of ; be~ms in the direction of lens 8. When passing through th;s lens the divergent bundle of beams is transformed into a bundle of parallel beams which at a small angle of incidence impinges onto dispersing reflecting element 9. This is of such a design that the impinging light is diffracted mainly in the direction of the first order of diffraction only. As the direction of a diffraction order depends on the wavelength the first diffraction order is reflected for each wavelength in another direction and imaged through the lens on one respective photodiode 6 associated with a specific wavelength. To simplify the representation, and to better understand these relationships, only one single beam 13 representing the right-hand limitation of the divergent bundle of beams passing through entrance aperture 3 is shown. Beam 13 passing through ; lens 8 is transformed by that lens into a beam that is parallel to the opti;cal axis of this lens~ and at the dispersion reflecting element 9 is . .
diffracted only in the direction of the first order. The directions ~ssociated to the colours red, green, and blue are marked 13R, 13G and :.
13B When passing through lens 8, these beams are deFlected in the - direction of photodiodes 6R, 6G, 6G associated with their colours.
~E9-n ~Q13 - 4 -1 It is quite obvious that the same process takes place with each beam propagating in parallel to the optical axis of lens 8, and im-pinging on the dispersing reflecting element 9 so that the radiation of a specific wavelength reflected from the entire surface of the dispersion element is respectively focused to one specific photodiode 5.
It is quite obYious that the measuring results with a bundle of beams 12 having a sma11er angle of divergence, providing that bundle of beams has the same light flux, are not different, and that the measuring sensitivity is not reduced. If the beam passing through en-trance aperture 3 shows an angle of divergence, as it will generally be observed, which is much smaller than the aperture angle of lens 8 the djrectjon of the central axis of this beam is without influence on the position of the images of the individual colour component on multiple photodiode array 5 and on the measuring sensitivity. The spectrophotometer as disclosed by the invention can therefore be used in connection with a measuring microscope without any specific steps for aligning the optical axis of the spectrophotometers to the optical axis of the measuring microscope being required.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A spectrophotometer having a dispersing reflecting element and a multi-photodetector array arranged in an evaluation plane, an entrance aperture, and a lens arranged between said entrance aperture and said dispersing reflecting element, said lens directing all beams passing through said entrance aperture, in all directions lying within an aperture angle of said lens, as a bundle of parallel beams, onto said dispersing reflecting element, and focusing components of said beams diffracted by said dispersing reflecting element as a function of their respective wave-lengths in different directions, onto portions of said photodetector array associated with said respective wavelengths.

2. A spectrophotometer as claimed in claim 1, wherein said dispersing reflecting element is a reflecting line grating.

3. A spectrophotometer as claimed in claim 1, wherein said dispersing reflecting element is a reflecting echelon grating.

4. A spectrophotometer as claimed in claim 1, wherein said dispersing reflecting element is a dispersion prism with a mirrored back.

5. A spectrophotometer as claimed in claim 2, claim 3 or claim 4, wherein said lens is arranged in the immediate vicinity of said dispersing reflecting element.

6. A spectrophotometer as claimed in claim 2, claim 3 or claim 4 wherein said dispersing reflecting element is cemented to said lens.

7. A spectrophotometer as claimed in claim 2, claim 3 or claim 4, wherein said dispersing reflecting element and said lens are formed in the same body.

8. A spectrophotometer as claimed in claim 2, claim 3 or claim 4, wherein said entrance aperture is arranged in the focal plane of said lens.

9. A spectrophotometer as claimed in claim 1, claim 2 or claim 3, wherein said photodetector array consists of photo-detectors arranged in parallel to the focal plane of said lens in the immediate vicinity of said entrance aperture.

10. A spectrophotometer as claimed in claim 1, claim 2 or claim 3, wherein said entrance aperture has a shape selected from the group comprising circular, square or rectangular.