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Some Lights Just Can't Be Graded.


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Every now and then I'll come across someone struggling to grade a shot. Often the problem is a result of a having a light source that has an incomplete or very narrow spectrum.


A lot of cheap LED lights as well as many types of discharge lights such as sodium street lights or neon lights only output light at certain specific wavelength's or only encompass an extremely narrow part of the light spectrum.


These kinds of lights will result in images that will be strongly coloured and next to impossible to colour grade because the narrow bandwidth of the light means the footage only contains a single colour or extremely limited range of colours. You will be able to change that single colour to a new colour, but because there is only one colour everything else in the image will change by a similar amount. It will be next to impossible to pull out  subtle skin tone hues from a face lit by a sodium street light as every part of the face will be the same hue due to the single wavelenght of the light source.


Unfortunately there is no simple fix for this. So, it is something that needs to be considered when shooting under discharge lights. If shooting using S-Log this is one of those times where monitoring and only seeing the S-Log image can be a little dangerous as it may not be obvious that your colour palette is extremely restricted. Monitor via a LUT and it will normally be very obvious.


Another issue I see from time to time is where one of these low quality narrow spectrum lights is spilling into a scene and causing very odd looking, highly saturated highlights. Our eyes will adapt to these lights, often a pool of light from a narrow bandwidth light won't look all that bright to us. But to an electronic sensor that doesn't adapt the same way as our eyes do it might appear very bright and in some situations may cause one of the sensors colour channels to clip resulting is some very odd looking highlights. This is commonly seen where blue LED up-lighters are used to light up a wall. To our eyes the blue might not appear super bright, but to a video camera that intense but narrow wavelength blue light will cause the blue channel to clip. In a lot of cases there isn't much you can do about it (other than turning down the light), so it's something to watch out for. If you have a Sony broadcast camera many have a matrix setting called "Adaptive Matrix" that can be turned on to specifically to deal with this. But if shooting S-Log you won't be able to take advantage of this, so again, look carefully at you images while shooting, preferably via a LUT if you suspect that there may be narrow bandwidth light in your scene.

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Thanks for starting the conversation, yes. I agree with everything you've said. 

A few things that come to mind immediately as I read were the nighttime street lights and those pesky low-quality indoor fluorescents (which are particularly bad). I haven't thought too much about the outdoor stuff because I haven't done much of that work in a paid or high-pressure environment prioritizing the skin tone but now I will certainly make a note of it.

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