Thermal camera for infrared imaging

• US 5,994,699 A
• Filed: 11/20/1997
• Issued: 11/30/1999
• Est. Priority Date: 11/25/1996
• Status: Expired due to Fees

• Current Assignee: Nikon Corporation
• Original Assignee: Nikon Corporation
• Inventors: Akagawa, Keiichi
• Primary Examiner(s): Westin, Edward P.
• Assistant Examiner(s): HANIG, RICHARD E
• Application Number:
  US08/975,174
• Time in Patent Office: 740 Days
• Field of Search: 250/332, 250/330, 250/334, 250/338.1, 250/339.02, 327/350
• US Class Current: 250/332
• CPC Class Codes: H04N 23/23 from thermal infrared radiation
• CPC Class Codes: H04N 25/63 applied to dark current
• CPC Class Codes: H04N 25/671 for non-uniformity detection or correction
• CPC Class Codes: H04N 5/33 Transforming infrared radiation cameras or camera modules for generating image signals from infrared radiation H04N23/20; circuitry of SSIS for transforming infrared radiation into image signals H04N25/20

Abstract

Thermographic infrared cameras are disclosed that comprise an infrared image sensor having linearly positioned thermosensing elements. A physical property of each thermosensing element changes in proportion to a corresponding temperature change caused by infrared irradiation incident on the thermosensing element. The infrared image sensor comprises a read device that reads signals from each thermosensing element in an ordered manner. A logarithmic converter receives signals from the infrared image sensor and outputs corresponding logarithmic values of the signals. An image information output device receives the logarithmic values from the logarithmic converter and outputs a corresponding infrared image. Such thermographic infrared cameras can further comprise a reference infrared image sensor. The reference infrared image sensor includes reference thermosensing elements each having a physical property that exhibits a change in value in proportion to a temperature change of the respective thermosensing element. A compensating logarithmic converter outputs a logarithmic value of the ratio of the signal read by the infrared sensor and the signal read by the reference infrared sensor. An image information output device receives the logarithmic values output by the compensating logarithmic converter and outputs corresponding infrared image information.

Claims

1. A thermographic infrared camera, comprising:

(a) a thermal infrared sensor comprising a plurality of thermosensing elements each of which conducting an electrical current, each thermosensing element having a physical property that exhibits a change in value with a change in temperature caused by infrared irradiation on the respective thermosensing element, the thermal infrared sensor reading signals from each thermosensing element, each signal having a parameter that is proportional to the value of the physical property of the respective thermosensing element;

(b) a logarithmic converter that receives the signals read by the thermal infrared sensor and outputs respective logarithmic values of the signals;

(c) an offset correction circuit that receives the respective logarithmic values of the signals from the logarithmic converter and corrects offset in the respective logarithmic values of the signals when infrared irradiation is not being received by the thermosensing elements; and

(d) an image information output device that receives offset corrected logarithmic values of the signals from the offset correction circuit and outputs corresponding infrared image information.

2. The thermographic infrared camera of claim 1, wherein the thermosensing elements are arranged in an array.

3. The thermographic infrared camera of claim 1, wherein electrical current levels that individually flow through the thermosensing elements vary with changes in temperature of the respective thermosensing elements caused by infrared irradiation incident on the respective thermosensing elements.

4. The thermographic infrared camera of claim 3, wherein at least one of the thermosensing elements comprises a bolometer.

5. The thermographic infrared camera of claim 3, wherein the logarithmic converter comprises an analog circuit.

6. The thermographic infrared camera of claim 5, further comprising an A/D converter that receives the logarithmic values from the logarithmic converter and converts the logarithmic values to corresponding digital signals.

7. The thermographic infrared camera of claim 1, further comprising an offset data memory device that receives the respective logarithmic values of the signals from the logarithmic converter and stores correction data for the respective logarithmic values of the signals whenever infrared irradiation is not being received by the thermosensing elements, wherein the offset correction circuit receives the logarithmic values of the signals from the offset data memory device and corrects offset in the signals using the correction data.

8. The thermographic infrared camera of claim 6, further including a reference thermal infrared sensor comprising a plurality of reference thermosensing elements each having a physical property that exhibits a change in value with a change in the surrounding environmental temperature, wherein the reference thermal infrared sensor reads reference signals from each reference thermosensing element, each reference signal having a parametric value that is proportional to the value of a physical property of the respective reference thermosensing element.

9. The thermographic infrared camera of claim 8, wherein the physical property of the reference thermosensing elements is substantially identical to the physical property of the thermosensing elements.

10. The thermographic infrared camera of claim 8, wherein the reference thermosensing elements and the thermosensing elements are situated on the same substrate.

11. The thermographic infrared camera of claim 8, further comprising a compensating logarithmic converter that outputs logarithmic signals, each logarithmic signal being a ratio of the signal read by the respective thermal infrared sensor and the signal read by the respective reference thermal infrared sensor.

12. The thermographic infrared camera of claim 8, wherein the image information output device receives the logarithmic signals from the compensating logarithmic converter and outputs infrared image information corresponding to the logarithmic signals.

13. The thermographic infrared camera of claim 12, wherein the compensating logarithmic converter comprises:

a logarithmic converter that converts signals read by the thermal infrared sensor to corresponding logarithmic values of the signals;

a reference logarithmic converter that converts signals read by the reference thermal infrared sensor to corresponding logarithmic values of the signals; and

a subtraction device that outputs a difference between the logarithmic value produced by the logarithmic converter and the logarithmic value produced by the reference logarithmic converter.

14. A thermographic infrared camera, comprising:

(a) a thermal infrared sensor comprising a plurality of thermosensing elements each of which conducting an electrical current, each thermosensing element having a physical property that exhibits a change in value with a corresponding change in temperature of the respective thermosensing element caused by infrared irradiation incident on the respective thermosensing element, the signals each having a parametric value that varies in proportion to a corresponding change in value of the physical property of the respective thermosensing element;

(b) a reference infrared sensor comprising a plurality of reference thermosensing elements, each reference thermosensing element having a physical property that exhibits a change in value with a corresponding change in temperature, the reference infrared sensor reading reference signals from each reference thermosensing element, the reference signals each having a parametric value that varies in proportion to a corresponding change in the value of the physical property of the respective thermosensing elements;

(c) a compensating logarithmic converter that outputs logarithmic values of a ratio of respective signals read by the thermal infrared sensor and respective signals read by the reference infrared sensor;

(d) an offset correction circuit that receives the logarithmic values of the ratio of respective signals from the compensating logarithmic converter and corrects offset in the logarithmic values of the ratio of respective signals whenever infrared irradiation is not being received by the thermosensing elements; and

(e) an image information output device that receives offset corrected logarithmic values of the ratio of respective signals from the offset correction circuit and outputs infrared image information corresponding to the offset corrected logarithmic values of the ratio of respective signals.

15. The thermographic infrared camera of claim 14, wherein the compensating logarithmic converter comprises:

a logarithmic converter that converts signals read by the thermal infrared sensor to corresponding logarithmic values of the signals;

a reference logarithmic converter that converts signals read by the reference infrared sensor to corresponding logarithmic values of the signals; and

a subtraction device that outputs a difference between the logarithmic value converted by the logarithmic converter and the logarithmic value converted by the reference logarithmic converter.

16. The thermographic infrared camera of claim 14, wherein each reference thermosensing element has a variable physical characteristic that is substantially identical to the physical property of the thermosensing elements.

17. The thermographic infrared camera of claim 14, wherein the reference thermosensing elements and the thermosensing elements are situated on the same substrate.

18. The thermographic infrared camera of claim 14, wherein at least one of the thermosensing elements comprises a bolometer having an electrical resistance that changes in proportion to a corresponding change in the temperature of the bolometer.

19. The thermographic infrared camera of claim 18, wherein at least one of the reference thermosensing elements comprises a bolometer having an electrical resistance that changes in proportion to a corresponding change in the temperature of the bolometer.

20. The thermographic infrared camera of claim 14, further comprising an offset data memory device that receives the logarithmic values of the ratio of respective signals from the compensating logarithmic converter and stores correction data for the logarithmic values of the ratio of respective signals whenever infrared irradiation is not being received by the thermosensing elements, wherein the offset correction circuit receives the logarithmic values of the ratio of respective signals from the offset data memory device and corrects offset in the logarithmic values of the ratio of respective signals.