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I work in a scientific laboratory and I need to take a picture of a sample that emits low intensity visible light (blue) when an infrared laser (980 nm) is pointed to it.

I am using a filter with an optical density of 8 to cut the infrared light so the camera does not detect it.

I was able to take a good picture of one sample where the blue intensity was quite high. However, every other picture of samples emitting blue (visible with naked eye), appear with white instead of blue.

I exhaustively tried every combination of aperture and exposure time, but I didn't get better results.

Any ideas of how something like this can be done?

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    It sounds like it could be a white balance issue to me. Post what camera you're using; it'll help us help you. – John Apr 16 '15 at 12:11
  • Have you tried manually selecting the white balance? Start at about 5200ºK. If the blue light still appears white, decrease the exposure value (narrower aperture, faster shutter, lower ISO) manually until the exposed pixels are not completely saturated. – Michael C Apr 16 '15 at 22:38
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I am using a filter with an optical density of 8 to cut the infrared light so the camera does not detect it.

You should be using a filter that blocks IR, but passes visible light. Ordinary neutral density filters work more or less the other way around.

However, every other picture of samples emitting blue (visible with naked eye), appear with white instead of blue.

I exhaustively tried every combination of aperture and exposure time, but I didn't get better results.

If the spot is white, it suggests overexposure. I can see three possibilies here:

  1. It can be either because you don't actually filter all the IR out and it registers on the sensor. Good IR blocking filter might possibly help...
  2. It can be that the exposure of the camera makes the blue blown out. You can try different levels of underexposure and white balance. You could also try increasing the ambient light so that the blue is not much brighter than the objects around it.
  3. Or it can be that this particular blue does not work well with the Bayer mask filters. For example, isn't the blue a combination of blue and UV? Isn't the UV overexposing the picture?
  • It was a problem with the intensity of the IR light. I was able to use 2 filters instead of just one, and it worked! Apparently, an optical density of "only" 8 was not enough (and yes, of course I was using one that cut IR and passed visible light). Thanks everyone for all the other comments. They were also useful for other experiences. – cinico May 8 '15 at 17:35
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As John states, the camera may be compensating for the monochromatic light, or it could be saturation, if the point is much brighter than the background.

Have you tried intentionally underexposing, i.e. set compensation -2 or more EV?

Have you shut automatic white balance?

You might also include a print of the spectrum in the photo, lighting the paper a bit dimmer than the blue spot, to serve as a basis for comparison and to prevent automatic white balance from over-compensating the blue.

Hope these suggestions help.

  • You're probably going to need much more than -2 stops compensation if in an automatic exposure mode. – Michael C Apr 16 '15 at 22:29
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I have seen the same problem when photographing a red laser spot, the spot came out white, not red.

In my case it was an exposure problem, the small intense dot on a dark background, the dot was severely overexposed.

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A Bayer sensor has red, green or blue filters in front of the individual sensor elements. A point source will only hit one element. A Bayer array cannot determine color in this case. A Foveon sensor detects the three colors at each sensor element. This should work.

  • A Bayer sensor has either a red, green, or blue filter in front of each individual sensor element. In real world practice a point source will usually still be more than one pixel wide unless the lens is rather exceptional. In this case the light source is not a truly theoretical point source, as it has some minimal width. You are correct that a Foveon type sensor should perform better for this case, but may not be absolutely necessary. – Michael C Apr 16 '15 at 22:39

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