Tuesday 13 August 2013

DSLR infrared sensor response

Camera manufacturers tend to be coy about releasing the spectral response of their sensors: they're (logically) more concerned about how the camera as a whole performs than how the sensor might deal with infrared (or UV).

A peek behind that curtain has come courtesy of Christian Mauer of the University of Applied Sciences in Cologne. His thesis Measurement of the spectral response of digital cameras with a set of interference filters (January 2009) is interesting in its own right but if you work your way down to Appendix A page 81 (section A.1.7) you will find the spectral response of a Canon EOS 450D without its IR blocking filter. It shows the usual bump in blue response just beyond 650 nm so that the respective responses of the red, green and blue channels at 800 nm are around 20%, 30% and 40% of the peak green response (at 540 nm) respectively. All channels come together at 20% at 850 nm and the response is monochromatic beyond that.

These figures are actually showing the spectral response of the sensor plus its Beyer filtering, since the underlying sensor is monochromatic and should have a smooth response curve peaking at around 600 nm and slowly dying away to over 1000 nm. However, this explains the strange colours when you take infrared images through a 720 nm filter and why ones through an 850 nm or longer filter are colourless.