More questions about Infra-red
 

Hi all,
Do camera sensors interpret infra-red as red?
I saw a picture taken, which had had it's IR block filter removed, and a -850nm block filter installed, and the images had a red cast. Why is that?
Thanks,
..... john

If a camera has had its IR Blocking Filter removed, but not its Bayer Filter, and an IR Passing Filter installed, the only light to get passed the IR Passing filter is IR , and possibly some red light. The Green and Blue microfilters in the Bayer filter would block the Red/IR light, so the only photoreceptors that would detect any light would be the ones under the Red microfilters. The camera would interpret that as red light on an otherwise black background, because all other wavelengths of light were blocked.

That sounds like what you described.

Hi TCav,
Yes, alright, thanks. I think I understand.
So ....
Does that mean, in modifying a colour digital camera to get a pure NIR image, I would have to remove the IR block filter, the Bayer filter, and install a colour block filter?
..... john
P.S. Where is the Bayer filter in most cameras?

Not quite.

• Yes, you need to remove the IR Blocking Filter.
  Otherwise, IR won't get through to the sensor.
• Yes, you need to install an IR Passing filter (Colour Blocking Filter.)
  Otherwise, all colors of light will pass through, and you'll get a full color image with extra red where the IR is coming from.
• It's your call on removing the Bayer Filter.
  • If you do, with an IR Passing Filter, all the photoreceptors would receive IR Light but the camera would continue to interpret the data as if the Bayer filter were still in place. IR light would be the only light, but since Bayer filters have twice as many green microfilters as red and blue, the camera would produce an image that places a false green tint on all the sources of IR light.
  • If you don't, the only photoreceptors detecting IR would be the ones under the red microfilters of the Bayer filter, so the camera would produce an image that places a false red tint on all the sources of IR light.
  • A red tint is closer to the actual color of IR light than a green tint is.
  • On the other hand, with the Bayer filter in place, 3/4 of the photoreceptors wouldn't receive any light, so exposures will take longer.

If I were going to do this for a serious application, I think I'd choose a camera with a Foveon sensor or a camera with a monochrome sensor. Either would produce a B&W image of an IR scene, instead of an image with a false tint.

See Sigma SD1 and SD15 for Digital Infrared Photography or David English: Infrared Monochrom.

Hi Tcav,
Alright. So, that's a black and white sensor, as opposed to a colour camera with black and white capability?
Thanks,
..... john

Well, the Foveon is a color camera that, with an IR Passing filter installed and the internal IR Blocking filter retracted, will be an IR camera. It can also be used as a conventional camera with the IR Passing filter removed and the IR Blocking filter restored.

A monochrome camera will always be a monochrome camera.

Hi Tcav,
I see now why you said "serious application" These monochrome cameras are expensive. I read this
http://www.digitalphotopro.com/gear/...chrome-capture article, which helped me to understand. But, no, I'm not going to spend this kind of money. If I understand him correctly, he implies that the monochrome setting on a colour camera will produce a monochrome image (qualitatively) but will not quantitatively produce an accurate image, due to the various filters which change the intensity of the light reaching the sensor??
..... john
Thanks,
...... john

The key things are to remove the IR Blocking filter, and install an IR Passing Filter.

Whatever happens after that, you can play with however you want, because all you'll capture is IR. Whether it shows up in the image as white, red, or green, is immaterial, because IR is none of those colors. Whatever color you get will be a false color.

Hi Tcav,
It's my impression that pure NIR images are colourless, i.e. shades of black and white. Is this also false "colour" i.e. just a colourless manifestation of IR, not any 'purer' than the coloured ones?
.... john