Method and system for photoconductive detector signal correction

• US 5,136,154 A
• Filed: 05/10/1991
• Issued: 08/04/1992
• Est. Priority Date: 05/10/1991
• Status: Expired due to Term

• Current Assignee: MKS Instruments Incorporated
• Original Assignee: Advanced Fuel Research, Inc.
• Inventors: Brouillette, Carl R., Carangelo, Robert M., Hamblen, David G.
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): SHAMI, KHALED
• Application Number:
  US07/698,263
• Time in Patent Office: 452 Days
• Field of Search: 250/214 L, 250/214 C, 250/214 R, 307/311
• US Class Current: 250/214.00L
• CPC Class Codes: G01J 1/44 Electric circuits for command of an exposure part G03B7/02
• CPC Class Codes: G01J 3/453 by correlation of the amplitudes

Abstract

A corrective factor is applied so as to remove anomalous features from the signal generated by a photoconductive detector, and to thereby render the output signal highly linear with respect to the energy of incident, time-varying radiation. The corrective factor may be applied through the use of either digital electronic data processing means or analog circuitry, or through a combination of those effects.

Claims

1. A method for producing a linearized electrical signal that is substantially linearly related to the time-varying intensity of electromagnetic radiation impinging upon a photoconductive detector, comprising the steps:

providing a photoconductive detector for converting incident electromagnetic radiation to generate an electrical signal that is indicative of the energy of the radiation;

applying to said detector a constant bias voltage;

causing electromagnetic radiation to impinge upon said detector for a period of time, so as to generate an electrical signal having a time-varying value X_s that is indicative of the energy of the impinging radiation, the generated signal including electrical distortions which render it nonlinearly related to said impinging radiation energy;

producing a corrective signal having the value (X_s -C) ², wherein C is zero or a numeral constant of like units to X_s ; and

adding at least a fraction of said corrective signal value to a signal having at least the value X_s so as to substantially eliminate said electrical distortions therefrom and thereby produce a substantially linearized output signal.

2. The method of claim 1 wherein said generated and corrective signals are combined in accordance with the relationship:

space="preserve" listing-type="equation">1/a×

[X.sub.s +f(X.sub.s -C).sup.2 ],

wherein "f" is a fractional constant and "a" has a value selected from the group consisting of unity and "f"; and

wherein a specific value "f₀ " is established for the fractional constant "f," at which specific value said electrical distortions are substantially eliminated in the combined signal produced.

3. The method of claim 2 wherein said generated electrical signal is the A.C. portion of a modulated signal, wherein C is zero, and wherein said value of "a" is equal to "f."

4. The method of claim 2 wherein said generated electrical signal constitutes detector-generated voltages, wherein C is a reference voltage, and wherein said value of "a" is unity.

5. The method of claim 2 wherein said impinging radiation constitutes an interferometer beam;

wherein said combined signal is Fourier transformed so as to be produce a transformation signal; and

wherein said transformation signal is monitored as the value of "f" is varied so as to determine said specific value of "f₀."

6. The method of claim 5 wherein said detector is nonresponsive to radiation at frequencies lower than a certain cutoff frequency value, and wherein such monitoring is carried out by a step selected from the group consisting of:

(a) determining that said transformation signal indicates substantially no energy at said lower frequencies;

(b) determining that said transformation signal indicates substantially no absorbance of said impinging radiation at a level below zero; and

(c) varying said impinging radiation to produce at least two different levels of steady state radiation energy, and determining that the amplitude of said transformation signal is free from substantial variation at said different levels of energy produced.

7. The method of claim 2 wherein said combined signal is monitored as the value of "f" and the steady state energy level of said impinging radiation are varied, "f₀ " being thereby established as a specific value of "f" at which the amplitude of said combined signal is free from substantial variation at the different levels of energy produced.

8. The method of claim 3 wherein said generated signal is converted to digital form, and wherein said steps of producing said corrective signal and combining said signals are carried out by an electronic data processing technique.

9. The method of claim 8 wherein said specific value "f₀ " is established by an electronic data processing technique.

10. The method of claim 3 wherein said A.C. signal portion is an interferogram.

11. The method of claim 4 wherein said steps of producing said corrective signal and combining said signals are carried out by analog electronic circuitry.

12. The method of claim 11 wherein said specific value "f₀, " and said reference voltage, are established by such circuitry.

13. The method of claim 1 wherein said impinging electromagnetic radiation is spectral radiation in the infrared region.

14. A system for detecting electromagnetic radiation and producing a linearized electrical output signal that is substantially linearly related to the time-varying intensity of the incident radiation, comprising:

a photoconductive detector for converting incident electromagnetic radiation to generate an electrical signal having a time-varying value X_s that is indicative of the energy of such radiation, said detector having an associated bias circuit for applying a constant bias voltage thereto, and having characteristics that tend to introduce electrical distortions which render the detector-generated signal nonlinearly related to the energy of the impinging radiation;

means for producing a corrective signal having the value (X_s -C)², wherein C is zero or a numeral constant of like units to X_s ; and

means for adding at least a fraction of such a corrective signal to a signal having at least the value X_s, so as to produce such an output signal.

15. The system of claim 14 wherein said means for adding combines the generated and corrective signals in accordance with the relationship:

space="preserve" listing-type="equation">X.sub.s +f(X.sub.s -C).sup.2,

in which "f" is a fractional constant; and

wherein said system includes means for establishing a specific value "f₀ " of said constant "f," at which specific value the electrical distortions are substantially eliminated in the output signal produced.

16. The system of claim 15 wherein said means for producing, said means for adding, and said means for establishing comprise analog electronic circuitry to which said detector is connected for opertion upon the voltage values of the electrical signal generated by said detector, X_s of said relationship representing such voltage values;

and wherein means is provided for generating and applying to said circuit a reference voltage, said C term of said relationship being a constant having the value of such reference voltage.

17. The system of claim 16 wherein said means for establishing comprises a variable voltage divider.

18. The system of claim 17 wherein said means for producing and said means for adding comprise and integrated ciurcuit to which said voltage divider is operatively connected.

19. The system of claim 18 wherein said integrated circuit functions in accordance with the transfer equation:

space="preserve" listing-type="equation">V.sub.0 =A [(x.sub.1 -x.sub.2)(y.sub.1 -y.sub.2)/B+(z.sub.2 -z.sub.1)],

wherein V₀ is the output voltage from said integrated circuit, A is its amplification gain, B is a constant-value voltage term, and X₁, X₂, y₁, y₂, Z₁, and Z₂, are variable voltage values.

20. The system of claim 14 wherein the generated electrical signal is the A.C. portion of a modulated signal;

wherein said means for adding combines the generated and corrective signals in accordance with the relationship;

space="preserve" listing-type="equation">X.sub.s /f+(X.sub.s).sup.2,

in which X_s is the A.C. signal portion and "f" is a fractional constant;

wherein said system includes means for establishing a specific value "f₀ " of said constant "f," at which specific value the electrical distortions are substantially eliminated in the output signal produced; and

wherein said means for producing, said means for adding, and said means for establishing comprise electronic data processing means.

21. The system of claim 14 wherein said detector is a mercury cadmium telluride device.

22. The system of claim 14 wherein said system additionally includes a source of electromagnetic radiation operatively disposed to project a beam upon said detector.

23. The system of claim 22 wherein said system additionally includes interferometer means, operatively disposed in the beam path between said source and said detector.

24. The system of claim 23 wherein said A.C. signal portion is an interferogram.

25. The system of claim 23 wherein said electromagnetic radiation is spectral radiation, and is in the infrared region of the spectrum.

26. The system of claim 25 wherein said system comprises a Fourier transform spectrometer.

27. The system of claim 22 wherein said system additionally includes chopper means, operatively disposed in the beam path between said source and said detector, for modulating the amplitude of a beam projected along said path.