Optical sensor calibration

1. An apparatus comprising:

an optical sensor comprising an optical transmitter and an optical receiver;

a calibration system configured to calibrate the sensor when a measurement taken at the optical receiver passes a test, wherein the measurement is taken while the optical transmitter is on and the apparatus is in use and wherein the calibration system is configured to calibrate the optical sensor by:

generating a compensation value using the measurement;

compensating a reference measurement using the compensation value; and

calibrating the optical sensor using the compensated reference value.

2. An apparatus as claimed in claim 1 , wherein the test comprises a reflector proximity test.

3. An apparatus as claimed in claim 1 , wherein the test comprises a sensor saturation test.

4. An apparatus as claimed in claim 1 , wherein the test comprises a best-in-sample test.

5. An apparatus as claimed in claim 1 , wherein the test comprises comparison of the measurement against a threshold.

6. An apparatus as claimed in claim 5 , wherein the threshold depends upon a contemporaneous measurement taken at the optical receiver of the sensor while the optical transmitter of the sensor is off.

7. An apparatus as claimed in claim 6 , wherein the threshold has a reflector proximity test value or a sensor saturation test value depending upon the value of the contemporaneous measurement.

8. An apparatus as claimed in claim 1 , comprising a memory system configured to store, during production of the apparatus, a reusable reference measurement taken at the optical receiver while the optical transmitter of the sensor is on and to store a default compensation value for initial calibration of the sensor.

9. An apparatus as claimed in claim 8 , wherein the reference measurement is a white point calibration measurement.

10. An apparatus as claimed in claim 8 , wherein the compensation value is a dark calibration value representing a measurement value at the optical receiver when the optical transmitter is on and there are no other optical sources.

11. An apparatus as claimed in claim 1 , wherein the calibration system is configured to re-calibrate the sensor when a scheduled measurement taken at the optical receiver of the sensor, while the optical transmitter is on, passes the test.

12. An apparatus as claimed in claim 11 further comprising a protective screen in an optical path between the optical transmitter and the optical receiver whereby scheduled re-calibration compensates for progressive wear to the protective screen.

13. A method of automatically re-calibrating an optical sensor comprising:

taking a measurement at an optical receiver of the optical sensor while an optical transmitter of the optical sensor is on;

changing calibration of the optical sensor only if the taken measurement passes a test;

generating a compensation value using the measurement;

compensation a reusable reference measurement using the compensation value; and

calibration the sensor using the compensated reference value.

14. A method as claimed in claim 13 , wherein the test comprises comparison of the measurement against a threshold.

15. A method as claimed in claim 14 , wherein the threshold depends upon a contemporaneous measurement taken at the optical receiver of the sensor while the optical transmitter of the sensor is off.

16. A method as claimed in claim 15 , wherein the threshold has a reflector proximity test value or a sensor saturation test value depending upon the value of the contemporaneous measurement.

17. A method as claimed in claim 13 , wherein the calibration system is configured to re-calibrate the sensor when a scheduled measurement taken at the optical receiver of the sensor, while the optical transmitter is on, passes the test.