RADIANT ENERGY DETECTION APPARATUS

Abstract: A radiant energy detector comprising a silicon photovoltaic detector (24) and a lead sulfide photoconductive detector (22) in a common optical path and each responsive to radiant energy in different wavelength intervals. The silicon detector (24) also serves as a filter for isolating the wavelengths of radiant energy reaching the lead sulfide detector (22).

Description: RADIANT ENERGY DETECTION APPARATUS Background of the Invention 1. • Field of the Invention The present invention relates to the detec- tion of radiant energy and, more particularly, to detectors of radiant energy in different wavelength intervals- The invention is particularly adapted for use in spectrophotometers for detecting light emanating from sample materials in different wavelength intervals of the spectrophotometer operating wavelength range. 2. Description of the Prior Art Spectrophotometers measure sample materials by detecting radiant energy emanating from the sample. Typically, a radiant energy beam impinges on the sample and the detector detects radiant energy issuing from the sample- Depending on the type of spectrophoto¬ meter, the detector measures the amount of radiant energy absorbed, transmitted, scattered, reflected, emitted, or such by the sample. The nature of the detected energy provides quantitative and/or qualita¬ tive information about the sample. Spectrophotometers typically operate in a range of the electromagnetic spectrum extending from the ultraviolet region through the visible and into the infrared region. Such a range includes wavelengths from below 200 nm uυward to 3500 n or more. In order OMPI -2- to measure samples at selected wavelength intervals. - within an overall operating range, spectrophotometers . . .include various filters, .gratings and other optical devices for selecting wavelength intervals within which the sample is to be examined and for varying the selec- ted wavelength interval within the operating wavelength range of the instrument. Obviously, the most versative instruments are those with the widest possible operat¬ ing wavelength range and the ability to select any desired wavelength interval within the operating range. _ Various forms of radiant energy detectors have been employed in spectrophotometers depending on 'the operating wavelength range of the instrument. For example, photomultiplier tubes and silicon photovoltaic detectors have been employed in the ultraviolet region. . Lead sulfide and other photoconductive detectors have been employed in the visible and infrared regions while thermocouples " and pyroelectric devices have been em¬ ployed in the infrared region alone. Unfortunately, no suitable detector exists which responds to radiant energy across the full oper¬ ating wavelength range of spectrophoto etric measure¬ ments indicated above. As a result, the detector be¬ comes a limiting factor in the design of a spectro¬ photometer- In view of the restricted detector oper¬ ating range, it is customary in spectrphotometer design to incorporate a plurality of detectors each of which -3- " detects radiant energy in a selected interval of an overall- operating wavelength range. Such multiple .detector arrangements, however, require moving mirrors or other optical elements for directing the radiant energy beam to the appropriate one of the detectors based on the particular wavelength interval to be measured. Moreover, individual optical filters are often required with each detector for isolating the wavelengths of radiant energy passed to each detector and eliminating higher grating orders and other forms of stray light. For example, silicon material whch is relatively transmissive to radiant energy above 1150 nm but relatively opaque to energy therebelow has been used as a blocking filter for a lead sulfide detector to prevent wavelengths below 1150 nm from reaching the detector. Thus while multiple detector spectrophoto¬ meters are capable of operating over a wide wavelength range, they suffer from undue optical and mechanical complexity. Summary of the Invention The present invention resides in an improved radiation detection apparatus which overcomes the draw¬ backs of the prior art. The apparatus is simple in construction and operation and is operable in different wavelength intervals of an overall operating wavelength range. Basically, the invention resides in radiant energy detection apparatus comprising first and second " detectors responsive to radiant energy in. respective- first and second wavelength intervals, the second de¬ tector disposed in a radiant energy path forward of the first detector and being comprised of a material rela¬ tively transparent to radiant energy in the first wavelength interval. With this arrangement the second detector passes radiant energy in the first wavelength interval to the first detector, which responds thereto, and responds itself to radiant energy in the -second wavelength interval. Preferably, the second detector is relatively opaque to radiant energy in the second wavelength interval thereby blocking passage of such energy to the first detector. The second detector thus functions both as a detector of radiant energy in the second wavelength interval and as a filter blocking passage of radiant energy in the first wavelength interval. In the preferred embodiment the second detec¬ tor is a photovoltaic detector comprised of silicon material and the first detector is a photoconductive detector comprised of lead sulfide material. In one form the first and second detectors are disposed indi¬ vidually in the radiant energy beam path- In another embodiment, they are incorporated into a common detec¬ tor assembly. O PI i Brief Description of the Drawings Fig. 1 is an optical diagram of a spectro- ' photometer incorporating radiant energy ' detection ap¬ paratus of the present invention. Fig- 2 is a graphical illustration of detec- : ted radiant energy vs. wavelength for the first and second detectors of the present invention. Fig. 3 is a graphical illustration of radiant energy (transmittance) vs. wavelength for the .silicon detector/filter of the invention. Fig. 4 is a cross-sectional view through detection apparatus of the invention formed in a common detector assembly. Description of the Preferred Embodiment As shown in the drawing for purposes of il¬ lustration and particularly Fig. 1 thereof, the present invention is embodied in an optical system, illustrated as a spectrophotometer 10, incorporating a novel radi¬ ant energy detection apparatus 12 in accordance with the present invention. The elements of the spectro¬ photometer are conventional and include a radiant energy source 14, a' beam dispersing element 16 and a sample receiving compartment 18. Source 14 comprises • one or more individual sources for generating radiant energy beam 20 of a desired wavelength range. Dis¬ persing element 16, such as a diffraction grating, intercepts beam 20 and isolates various wavelength JIEAi OMPl_ " WIPO " _intervals thereof for passage to sample compartment 18 and detection apparatus 12. - The dispersing element.is operative in conventional fashion to scan the.operating wavelength range of the spectrophotometer to pass radiant energy of various selected wavelength-intervals to the sample.* Radiant energy beam 20, after impinging on and interacting with the sample, passes to and is detected by detection apparatus 12. In accordance with a primary aspect of the present invention, detection apparatus 12 comprises first and second detectors 22 and 24, respectively, in the path of radiant energy beam 20 with the second detector 24 forward of the first in the beam path. Detector 22 detects radiant energy in a first wave¬ length interval while detector 24 detects radiant energy in a. second wavelength interval. Moreover, second detector 24 is relatively transparent to radiant energy in the first interval to pass same to the first detector. In addition, detector 24 is relatively opaque to radiant energy in the second wavelength interval to block passage of such energy to the first detector. To the foregoing ends in the preferred em¬ bodiment, detector 24 is a silicon photovoltaic detec¬ tor and detector 22 is a lead sulfide photoconductive detector. Detector 22 includes a respective pair of electrical terminals 26, 28 while detector 24 includes ~a corresponding terminal pair 30, 32. Fig. 2 depicts the radiant energy detection bands for these detectors. Silicon detector 24 responds to radiant energy in the wavelength interval of about 200-1150 nm. Lead sulfide detector 22 responds to energy in the interval of about 700-3500 nm. Fig. 3 illustrates the transmittance of silicon detector 22 as a function of wavelength. Note that the silicon detector is highly transmissive of radiation above about 1150 nm but strongly absorbs radiation below about 1150 nm. The waveforms of Figs. 2-3 together illus¬ trate that in operation the second detector 24 func¬ tions both as a detector in the second wavelength ~ ^~ interval and as a filter for passing radiant energy in the first wavelength interval to the first detector 22 while absorbing and hence blocking the passage of radiant energy in the second wavelength interval to the first detector. In the foregoing arrangement the wavelengths of beam 20 may be varied across the 200-3500 nm and detector apparatus 12 detects such energy without the need for moving mirrors or other beam switching ele¬ ments. Fig. 4 illustrates a second embodiment of the invention with detection apparatus 12 integrated into a common assembly. The apparatus comprises a layered or - * . sandwiched structure of an anti-reflection layer 34, - - -detector 24, transparent insulating layer 36, detector 22, transparent insulator 38, and reflecting backing ox support layer 40. Photovoltaic detector 24 comprises a body of silicon in the form of an inner disc 42 and a thinner, peripheral annular flange 44. Electrical con¬ tacts 26 and.28, of corresponding annular configura¬ tion, make physical and electrical contact with respec¬ tive opposite annular faces of flange 44- This elec¬ trode configuration allows for unobstructed passage of radiant energy beam 20 through silicon inner disc 42. Photoconductive detector 22 comprises a body of lead sulfide in the form of disc 46 to which dia¬ metrically opposed electrodes 30 and 32 make physical and electrical contact- Insulating layers 36 and 38 of silox electrically isolate detector 34 from detector 32 and from reflective layer 40. Various layers of the unitary structure are secured together by suitable adhesive. Anti-reflection layer 34, of magnesium fluoride material, serves to electrically insulate one side of detector 24 and to block entry of stray light into the detector apparatus 12. In operation, radiant energy beam 20 is passed by layer 34 and impinges on silicon detector 24. Radiant energy in the second wavelength interval below 1150 nm is absorbed by and detected by detector 24. Radiant energy in a first wavelength interval above 1150 nm is passed by detector 24 to detector 22 which iTEΛ " , OMH w 170 -9- -responds thereto. Any energy which is not absorbed on the first pass through detector 22 is reflected back to this-detector by reflecting layer 40. It is evident from the foregoing that the present invention provides a simple, convenient detec- tion apparatus responsive to radiant energy over a wide wavelength range which avoids the beam switching, moving mirror and other complex configurations of the prior art. The invention is straightforward in imple¬ mentation and is readily adapted in any application requiring broad band wavelength detector. Moreover, while_ a preferred embodiment has been illustrated and described, various modifications may be made therein without departing from the invention as defined in the appended claims.

Claims: - - _What * is claimed is: 1. . Radiant energy detection apparatus responsive to radiant energy in first and second wavelength inter¬ vals comprising: a " first detector disposed in a path of the radiant energy and responsive to radiant energy in the first wavelength interval; and a second detector disposed in the radiant energy path forward of the first detector and respon¬ sive to radiant energy in the second wavelength inter¬ val, the second detector including a material relative¬ ly transparent to radiant energy in the first wave¬ length interval whereby the second detector passes radiant energy in the-first wavelength interval to the first detector and responds to radiant energy in the second wavelength interval. 2. The apparatus of claim 1 wherein the second detector is a photovoltaic detector comprised of sili¬ con material. 3. The apparatus of claim 1 or 2 wherein the first detector is a photoconductive detector comprised of lead sulfide material. 4. The apparatus of claim 1 or 2 wherein the material of the second.detector, is relatively opaque to radiant energy in the second wavelength interval there¬ by blocking passage of such radiant energy to- the first detector. 5. The apparatus of claim 3 wherein the first and second detectors are included in common detector assembly.

Inventors: KAYE WILBUR I, KAYE, WILBUR, I.
Applicants: BECKMAN INSTRUMENTS INC, BECKMAN INSTRUMENTS, INC.
Application year: 1982
Main classification: H01J40
IPC codes: G01J1, G01N21, H01L25