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enter image description hereWhen I'm taking a photo of a LED diode (turned on), it appears to be brighter than it should be and its color is false. For example, red diode looks pink. I have tried changing white balance or use faster exposure, but it didn't help much. Could the diode emit invisible radiation (e.g. infrared), which fools camera's sensor?

On the left photo you can see original picture (LED overexposed). The right photo was took with exposure correction: -2EV (color is still pink, for naked eye it is red).

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  • Add more light falling on the faceplate of the device being photographed (use a lamp or a flash), so the camera adjusts to a faster exposure. Part of the problem is that the led is too bright compared to the faceplate. Lis light can be gelled to use a white balance that renders a better color in the led. – Jahaziel Nov 11 '14 at 22:11
  • Do you know exactly what LED it is? If so, you can determine the frequency and correct back for a known value in post processing. – user13451 Nov 12 '14 at 1:58
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LEDs emit a narrow frequency band of light. Digital cameras detect color by looking at black and white luminance (brightness) sensors that go through a grid of colored filters (red, green and blue). If the frequency of light from the LED is narrow enough and doesn't match up well with the filters on the sensor, you can get odd looking color as the narrow frequency of light is largely filtered out and you only get a little bit of the edge where it bleeds over.

This is encountered a lot in LED stage lighting where you need to get special LED stage lights for ensuring good color when filming with digital cameras. The same principal applies to still cameras and single LEDs though.

As for the brightness, I doubt it is actually coming through as too bright relative to everything else, more likely, your just not seeing how much of a difference in brightness there actually is. While your eyes do have a similar dynamic range (the difference between the darkest and brightest spot) to a good modern camera, your eyes can shift much faster, giving an apparent dynamic range that is much wider. Your brain and eyes do this in the background very quickly without it really being obvious, but you rarely actually look directly in to a light source and if you do, your eyes normally narrow quickly to adjust. Direct emitted light would be considerably brighter than the surrounding reflected light in almost all cases.

  • I think that narrow light from LED not matching sensor would make LED appear too dim, not too bright. – mareko Nov 11 '14 at 20:24
  • @Mareko - It is still an emitted light source and if it is near enough in frequency, there is still going to be bleed. It could have been much brighter if it was full spectrum. I suppose I should have covered how our eyes naturally adjust very quickly shifting between light intensities though. I'll add a bit more to the answer on that. – AJ Henderson Nov 11 '14 at 20:31
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Most likely reason is the diode is overexposed. That would cause (the red channel) to be blown and the red color will appear faded. Decrease the exposure. Depending on what else is in the shot, the white balance could also be off. You can test that by including a white or grey card in the shot and checking and correcting in post.

  • After decreasing exposure the color is still not right. – mareko Nov 11 '14 at 19:28
  • Is it pink, as you first described? Can you open in an editor and use a color picker and let us know what the RGB values are? Or a sample image? – MikeW Nov 11 '14 at 19:33
  • I have added a photo to the question. – mareko Nov 11 '14 at 20:01
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    There is a bit of blue, so the color is possibly not correct, if that LED is truly pure red. But looking at the RGB values (255,7,138), the red (255) is blown. – MikeW Nov 11 '14 at 20:55
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    Well you have something red which is much brighter than the rest of the image, so if you expose for the rest of the image, the red bit is overexposed and you'll get pink instead of red. You could add some ambient light, or else correct the color in post. – MikeW Nov 11 '14 at 21:30
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There are two problems here: exposure and white balance. I assume the markings on the device are supposed to be white: they're not in the images you post, which indicates a white-balance problem.

Here's why overexposure messes up colours. If you think about it, you already know that overexposing can mess up colours since, if you massively overexpose, every colour turns to white. If you think a bit more, you'll probably also realise that, since massive overexposure completely destroys colour, overexposing by a smaller amount should damage the colours to a lesser extent.

Here's how. Suppose that the true value of the colour of the light is two parts red to one part green (which would be a sort of orangey yellow, but it will do for illustration). Now, let's consider some different exposures of the same colour. For the first exposure, imagine that the red component registers at 60% intensity, which means that the green will be at 30% and the colour will be recorded properly. Now, imagine taking another photo with twice the exposure. That means the green will be at 60% intensity and the red should be at 120% intensity but, of course, it can't be. It'll be at 100%, so the colour won't be recorded correctly: instead of two parts red to one part green, you'll get two parts red to 1.2 parts green and the colour will look a bit too yellow. If you expose for three times as long as the original photo, you'll end up with the red at 100% intensity and the green at 90%, which is a very yellow two parts red to 1.8 parts green. A common situation where this happens is in photographs that include blue sky on a bright day: if you notice that the sky looks too cyan-green in your photograph, it's because the blue channel maxed out so what you see has too much red and, especially, green in it.

Unfortunately, even if your camera has a highlight alert, it probably won't be triggered by just one colour hitting maximum intensity. The only way to avoid overexposure messing with your colours is to set the histogram to show all three colour channels (red, green and blue) and to check that none of the graphs is bunched up at the right-hand side, which would indicate a colour maxing out.

  • Markings are in fact yellow/gold. – mareko Nov 12 '14 at 12:44
  • @mareko Oh, OK. But they're not yellow/gold in the photograph either (and don't look over-exposed), so the white balance is wrong. – David Richerby Nov 12 '14 at 15:16

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