IR/UV related questions

petrochemist wrote:

You obviously know what you're doing and are giving lots of good advice here.

When you're not shooting in visual darkness you'll probably want to use filters to limit the wavelengths being imaged at any one time. I do this a lot with my full spectrum modified cameras, as well as standard long pass IR filters strongly coloured photographic filters can be used (nearly all of these transmit IR as well as the visual colour).

I've had some luck with extracting R-G-B-NIR using raw exposures from Bayer sensors with no NIR-blocking filter, but just in the past week found this to be way harder using webcam-type cameras that only offer JPEGs (no raws). Basically, the colorspace transformation to YUV and subsequent reduction in color bit-depth really cause problems for multi-spectral reconstruction. Auto white balance applied in camera to the JPEGs also hurts. In summary, RGB-NIR is probably not separable in postprocessing of a JPEG.

... Shooting 'full spectrum' (as your conversion without additional filters) can sometimes give interesting results but typically just looks like the colours are wrong. portraits tend to be among the better subjects for this. Clothes can change colour significantly (especially blacks) & skin will tend to have a little of the infra red glow...

In typical RGB-filtered NIR, chlorophyll looks magenta, water looks black, and most organic dyes are transparent (e.g., the dyes used in "black" glassware and synthetic clothing). There tends to generally be a magenta cast because both red and blue filters tend to pass NIR, but green not so much. Camera white balance may help or make things worse, but if you want to be consistent in postprocessed color, it's best to fix the camera white balance rather than leave it on auto... better still, shoot raw.

You might also want to get yourself a IR flashlight with IR LEDs these can be quite reasonably priced & they'll be much brighter than the TV remote.

Being precise, he means NIR, which most "IR flashlights" really are. You can even cheaply make your own NIR LED light -- just use NIR LEDs around 750-850nm, DO NOT use the ~1500nm IR LEDs intended for optical computer networks.

The wireless remote that comes with many cameras is also an NIR emitter, but not as bright as one might think. TVs and cameras recognize a modulation pattern in the pulsed NIR remote controller light -- so the LEDs don't need to brightly overpower ambient NIR, but just to leave a trace of that pattern.