Power meter with background subtraction

• US 5,376,783 A
• Filed: 04/24/1994
• Issued: 12/27/1994
• Est. Priority Date: 09/16/1992
• Status: Expired due to Term

• Current Assignee: Ophir Optronics Ltd. (MKS Instruments Incorporated)
• Original Assignee: Ophir Optronics Ltd. (MKS Instruments Incorporated)
• Inventors: Halperin, Ben-Zion, Greenfield, Ephraim, Vecht, Jacob J.
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): Lee, John R.
• Application Number:
  US08/231,884
• Time in Patent Office: 247 Days
• Field of Search: 250/208.2, 250/214 AL, 250/214 B, 250/214 C, 250/214 R, 250/205, 356/218, 356/221, 356/222, 356/226
• US Class Current: 250/208.2
• CPC Class Codes: G01J 1/4204 with determination of ambient light solar light G01J2001/4266
• CPC Class Codes: G01J 1/4228 arrangements with two or more detectors, e.g. for sensitivity compensation
• CPC Class Codes: G01J 1/44 Electric circuits for command of an exposure part G03B7/02

Abstract

A technique for performing background suppression in photodiode detectors is disclosed. A first detector measures the light emitted by the source to be measured and the background light, while a second detector simultaneously measures the background light only. The value measured by the second detector is subtracted from the value measured by the first detector, and the difference represents the portion of the first detector'"'"'s measured value which is attributable to the source to be measured.

Claims

1. A background insensitive light detector for measuring the intensity of light emitted from a source in the presence of background light, comprising:

first and second detectors for simultaneously measuring the sum of the light incident thereon emitted by a source and background light and incident background light only, respectively, and producing an output signal having an amplitude proportional thereto,

electronic circuit means connected to said first and second detectors for determining the difference between the amplitudes of said output signals of said first and second detectors and producing a third signal the amplitude of which is representative of the intensity of the light emitted by said source, and

means for displaying said amplitude of said third signal for grounding a visual indication of the intensity of the light emitted by said source.

2. The light detector according to claim 1, wherein said first detector comprises a first photodiode, and said second detector comprises a second photodiode.

3. The light detector according to claim 2, wherein the output signals of said first and second detectors are current signals.

4. The light detector according to claim 3, wherein said first photodiode and said second photodiode are connected in parallel, and the cathode of each photodiode is connected to the anode of the other photodiode.

5. The light detector according to claim 2, wherein said first detector further comprises a first amplifier electrically connected to said first photodiode for amplifying current signals produced by said first photodiode and said second detector further comprises a second amplifier electrically connected to said second photodiode for amplifying current signals produced by said second photodiode.

6. The detector of claim 5, wherein the cathodes of said first and second photodiodes are connected together and to a source of ground potential,

wherein the anodes of said first and second photodiodes are respectively connected to an input terminal of said first and second amplifiers, the output terminals of which comprise first and second nodes, respectively.

wherein said first output signal is a voltage measured between said first node and ground, said second output signal is a voltage measured between said second node and ground, and said third signal is a voltage measured between said first node and said second node.

7. The light detector according to claim 2, wherein said detector further comprises a variable gain amplifier connected in series with said first photodiode, and wherein said series-connected first photodiode and said variable gain amplifier is connected in parallel with said second photodiode.

8. The light detector according to claim 1, wherein said electronic circuitry means includes subtraction circuitry which receives said output signals from said first and second detectors and produces said third signal the amplitude of which is proportional to the difference between said output signals.

9. The light detector according to claim 1, wherein said electronic circuit means includes compensation circuitry for adjusting the amplitude of at least one of the output signals of said first and second detectors to compensate for differences in sensitivity in said first and second detectors.

10. The light detector according to claim 1, wherein said electronic circuit means includes matching circuitry receiving the signal of one of said first and second detectors and providing a matched signal equal to the signal which the other detection would have provided if it has been exposed to the light incident on the one detector, and

subtraction circuitry which receives the matched signal and the output signal of the detector and produces said third signal the amplitude of which is proportional to the difference between the matched signal and the output signal of the other detector.

11. The light detector according to claim 10, wherein said matching circuitry comprises a variable gain amplifier receiving the signal from one of the first and second detectors and providing a matched signal, and

subtraction circuitry responsive to the matched signal and the output signal from at least one of the first and second detectors, and producing an electrical signal proportional to the differences between the matched signal and the output signal from at least one of the first and second detectors.

12. A detector according to claim 1, wherein said means for displaying comprises an oscilloscope type display for displaying said third signal.

13. A detector according to claim 1, wherein said display means is operative to provide an indication of the peak value of said third signal.

14. A method of detecting the level of light emitted by a source, the source being located in a background which is also emitting light, comprising the steps of:

a. supplying the light from the source and the light from the background to a first photodetector, the first photodetector providing a first signal;

b. supplying the light from the background to a second photodetector, the second photodetector providing a second signal;

c. subtracting the second signal from the first signal to form a difference signal indicative of the strength of the source; and

d. visually indicating the measured strength of the source in response to said difference signal.

15. A method according to claim 14 wherein the step of visually indicating includes the step of providing an oscillographic display of the measured strength of the source as a function of time.

16. A method according to claim 14 wherein the step of visually indicating includes the step of visually indicating the peak measured strength of the source.

17. A detector for detecting light from a source to be measured in the presence of background light, comprising:

a. a hand held housing;

b. a first detector, housed in said housing such that it will receive both the light from the source to be measured and the background light, the first detector providing a first signal representative of the light received thereby;

c. a second detector, housed in said housing such that it will simultaneously receive only the background light, the second detector providing a second signal representative of the light received thereby;

d. subtraction circuitry which receives the first and second signals and produces a third signal proportional to the difference between said first and second signals; and

e. display means for providing a visual display in response to said third signal.

18. A detector according to claim 17, wherein said display means comprises an oscilloscope type display for displaying said third signal.

19. A detector according to claim 17, wherein said display means is operative to provide an indication of the peak value of said third signal.

20. A background insensitive light detector for measuring light to be detected in the presence of background light, comprising:

a first detector producing a first signal responsive to the light to be measured and the background light;

a dynamic background light detector producing a second signal responsive only to the background light;

circuitry receiving the first and second signals and producing a third signal representative of the power of the light to be measured; and

a display operative to provide a visual display responsive to the third signal.

21. A detector according to claim 20 wherein the first detector comprises a first diode and the second detector comprises a second diode.

22. A detector according to claim 21 wherein the first and second signals are current signals.

23. A detector according to claim according to claim 22 wherein the first photodiode and the second photodiode are connected in parallel, the cathode of each photodiode being connected to the anode of the other photodiode.

24. A detector according to claim 21 wherein the first detector further comprises a first amplifier and the second detector comprises a second amplifier.

25. A detector according to claim 24 wherein the first signal is a voltage measured between a first node and a common node, the second signal is a voltage measured between a second node and the common node and the third signal is a voltage measured between the first node and the second node.

26. A detector according to claim 21 wherein the first detector further comprises a variable gain amplifier connected in series with the first photodiode and wherein the series-connected first diode and variable gain amplifier is connected in parallel with the second diode,

27. A detector according to claim 20 wherein the circuitry comprises subtraction circuitry which produces the third signal which is a signal proportional to the difference between the first and second signals.

28. A detector according to claim 20 wherein the circuitry further comprises compensation circuitry which adjusts at least one of the first and second signal to compensate for differences in sensitivity between the first and second detectors.

29. A detector according to claim 20 wherein the circuitry further comprises matching circuitry receiving signal of one of the first and second detectors and producing a matched signal equal to the signal which the other detector would have produced if it had been exposed to the light incident on the one detector;

and

subtraction circuitry responsive to the matched signal and the output signal of the other detector and producing the third signal the amplitude of which is proportional to the difference between the matched signal and the output of the other detector.

30. A detector according to claim 29 wherein the matching signal comprises a variable gain amplifier.

31. A detector according to claim 20 wherein the display is operative to provide an indication of the peak value of the third signal.

32. A detector for detecting light from a source to be measured in the presence of background light, comprising:

a. hand held housing;

b. a first detector, housed in the housing such that it will receive both the light from the source to be measured and the background light, the first detector providing a first signal representative of the light received thereby;

c. a second detector, housed in the housing such that it will receive only the background light, the second detector providing a second signal representative of the light received thereby;

d. subtraction circuitry which receives the first and second signals and produces a third signal proportional to the difference between the two signals; and

e. a display operative to provide a visual display responsive to the third signal.

33. A detector according to claim 32 wherein the display is operative to provide an indication of the peak value of the third signal.