Infra red camera calibration
Described herein is a method and apparatus for calibrating an infrared camera at elevated temperatures using a reference surface. The method comprises selecting a normal well-fill condition for pixels in the camera in accordance with normal operating temperatures and stare time ( 30, 32, 34 ), using the normal well-fill condition to calculate a selected stare time/surface temperature combination for which selected non-uniform calibration coefficients are determined ( 36, 38 ), adjusting and re-adjusting the stare time/surface temperature to obtain adjusted and re-adjusted non-uniform calibration coefficients respectively ( 40, 42, 44, 46 ), and determining final non-uniform calibration coefficients for the camera using the selected, adjusted and re-adjusted non-uniform calibration coefficients ( 48 ).
1. A method of calibrating an infrared detector using a temperature-adjustable reference surface within its field of view, the method being characterized by the steps of:
a) controlling the temperature of the reference surface to a first surface temperature and measuring the output of the detector over a first stare time, the first temperature and stare time being selected so as to achieve a predetermined first well-fill (Wactual) of the detector pixels at the lowest possible temperature of the reference surface;
b) processing the output measurements of the detector to obtain a first calibration coefficient (coeff cal1 );
c) adjusting the temperature of the reference surface to a second surface temperature and measuring the output of the detector over a second stare time period (Sactual), the second temperature and stare time (Sactual) being selected so as to achieve a second predetermined well-fill of the detector pixels
d) processing the output measurements of the detector to obtain a second calibration coefficient (coeff cal2 );
e) re-adjusting the temperature of the reference surface to a third surface temperature and measuring the output of the detector over the first stare time period, the third temperature being selected so as to achieve the second predetermined well-fill of the detector pixels over the first stare time period;
f) processing the output measurements of the detector to obtain a third calibration coefficient (coeff cal3 ); and
g) adjusting the first calibration coefficient (coeff cal1 ) on the basis of the second and third calibration coefficients (coeff cal2 , coeff cal3 ) obtained.
2. A method according to claim 1 , wherein the first predetermined well-fill of the pixels is approximately 50%.
3. A method according to claim 1 , wherein the second predetermined well-fill of the pixels is approximately 100%.
4. A method according to claim 1 , wherein step g) comprises determining a final calibration coefficient in accordance with the sum of the first (coeff cal1 ,) and third (coeff cal3 ) calibration coefficients less the second (coeff cal2 ,) calibration coefficient.
5. Apparatus for calibrating an infrared detector, the apparatus comprising:
a temperature-controlled reference surface, the detector being located within the apparatus to view the reference surface;
control means for controlling the temperature of the reference surface and the stare time of the detector; and
processing means for receiving output signals from the detector at first, second and third predetermined detector stare time and reference surface temperature combinations and for producing calibration coefficients corresponding to each of the first, second and third predetermined detector stare time and reference surface temperature combination, and for determining final calibration coefficients for the detector from the calibration coefficients determined for each stare time and reference surface temperature combination, characterized in that
in the first stare time and reference surface temperature combination, the first temperature and stare time are selected so as to achieve a predetermined first well-fill (Wactual) of the detector pixels at the lowest possible temperature of the reference surface;
in the second stare time and reference surface temperature combination, the second temperature and stare time (Sactual) are selected so as to achieve a second predetermined well-fill of the detector pixels; and
in the third stare time and reference surface temperature combination, the third temperature is selected so as to achieve the second
predetermined well-fill of the detector pixels over the first stare time period.
6. Apparatus according to claim 5 , wherein the first predetermined well-fill of the pixels is approximately 50%.
7. Apparatus method according to claim 5 , wherein the second predetermined well-fill of the pixels is approximately 100%.
8. Apparatus according to claim 5 , wherein the final calibration coefficient is determined in accordance with the sum of the first (coeff cal1 ,) and third (coeff cal3 ) calibration coefficients less the second (coeff cal2 ,) calibration coefficient.