Tuesday, February 21, 2012

[UV, VIS, NIR] Show the camera sensitivity for UV and IR easily

A have often been asked how one could esily see how deep a converted camera body reaches into UV and/or IR. Using a UV sensitive camera and a special transmission grating, this can indeed be done using two different lightsources, a tungsten lamp and a continuous Xenon lamp.

[click on image to see a larger one]


top: unfiltered
middle: Baader UV/IR Cut filter
bottom: 695LP long pass filter

XENON lamp:

top: unfiltered
middle1: Baader UV/IR Cut filter
middle2: Baader U2 filter
bottom: 695LP long pass filter

I had previously made a similar test and used some astronomers software (RSpec) to calibrate the results, so here for reference the Xenon spectra (1 Angstrom = 10 nm):

So, that transmission grating I used was not very well suited as it stops working around 360-370nm already, as it is on a normal glass substrate. Maybe I get a better one and repeat the tests sometime...

Be aware that the result does not reflect the absolute sensitivity of the camera sensor, but if using a Xenon source whose spectra is very close to bright noon sunlight (UV and VIs, less so in IR), gives an indication of what to expect when shooting outdoors.

It also reveals, why from 800-850nm onwards only monochrome results in IR can be expected, as all sensor dyes making up the RGB Bayer pattern get transparent.

Another remark: be aware that these spectra are always taken in a way to get them properly exposed, not too high and not too low, so the brightest parts of the spectra determine what is recorded. If only IR or UV is intended, it would be better to shoot through that UV or IR filter and expose these shots accordingly, as in the UV-IR spectra those parts will easily getting lost as they are several stops beyond the visible usually - in case of UV sometimes 8-10 stops!

Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos