Infrared, digital cameras and purple fringes

[tomczak]

While I was aware of digital cameras capturing light wavelengths outside of visible range, I haven't paid attention much. Below is a chance picture of a IR comm port of a laptop - invisible to a naked eye. It is rendered by the camera as bright magenta. I don't know what the wavelengths are, but probably near infrared.

Would this be a similar wavelength range to other natural sources (e.g. light reflected off foilage) and what consequence it has on standard landscape photography? I have to say that I haven't really seen any real difference.

The magenta seems to be quite similar to the tint of colour fringes - could it be argued that the fringes have anything to do with IR (but what's the physics of it...)?

Here is link describing IR sensitivity of sensors:

http://www.lrc.rpi.edu/programs/nlpip/t ... ectral.asp

[Dieter Mayr]

Maciej,

The most common IR-Leds have a wavelength of 950nm, the next common have 880nm.
When it was freshly issued, the Leica M8 had problems with Infrared because if its missing AA and weak IR-filter.
Problems were reported when capturing black textiles, they appeared purple.
I can not remember there (Leica User Forum) were discussed problems with landscapes.
As far i know the IR-Sensitivity is so low, that it does not take effect on bright colors, just highly IR-reflective dark colors can show effects.

Dieter

[couman]

Many amateur photographers prepare their B&W images by simply desaturating the color versions leaving 24-bit encoding. If I increase the saturation of those files, the result is frequently a purple coloration. If I convert from 24-bit to 8-bit, and then back to 24-bit, and attempt to saturate, there is no purple. Is the purple due to residual weak IR signal that is not affected by the desaturation process?

[Dieter Mayr]

Bob

It's a interesting effect, but it for sure has nothing to do with infrared.
I get a purple image even with the following workflow:
Taking a24 bit color image
Desaturating to 0% (Amount -100%, Preserve: neither, CS: HSV)
Converting to 8 bit
Converting back to 24 bit
As soon the Saturation slider goes above 0% the image gets purple, (Settings in Saturation are the same as above).
I suppose it's a side effect of how Saturation works internally, maybe Jonathan or Kiril can enlighten this.

[couman]

Thanks, Dieter.

Off hand, I don't see any practical use of this phenomenon. I was curious though because I often *think* I see residual color in B&W images that I see on the web. Indeed, conversion to 8-bit removes the apparition.

[Dieter Mayr]

Bob

Maybe these images have a Profile embedded and the cast comes from this ?
PWP and PS desaturate completely, means all points have a value R=G=B, so it should not make any difference if it's 24 or 8 bit.
Can be other programs don't, or the desaturation is not done completely by the user, or maybe it is a desired color-cast.
But if it desaturated completely, means R=B=G there is no "hidden" information left, whatsoever.
The only difference betwenn 8 and 24 bit is, that 24 bit images can have a color profile, while 8 bit can't in PWP, so it can make a visual difference in not CM-able applications.

[jsachs]

If you boost the saturation is such a way that totally desaturated pixels (S = 0) become even slightly saturated (S > 0), then they will pick up a red cast. In HSV or HSL, neutral grays have an undefined hue which is arbitrarily set to 0 (red), so it you force these pixels to have a non-zero saturation they will be red. I don't know if this is what is happening or not in this case.

[Dieter Mayr]

Jonathan

It gives a pure Magenta, a pure white (R= G = B = 255) gives R=255, G=0, B= 255
All other greys turn also to R=B, G=0.

[jsachs]

If you use preserve Neither, PWP is mapping pixels with zero saturation and undefined hue to non-zero saturation so so color is introduced. I would have expected red but evidently the default color is magenta and not red. If you switch to preserve Low or Low and High the problem goes away. Adding saturation to black and white image is really an undefined operation since is requires assigning some color to a pixel that has none.