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Joseph Tese

Color Rendition Quality of LEDs and how it specifically affects objects

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This might be a noobish question, but it is asked to better understand how color rendition quality of LEDs (and I guess others as well) affect the luminance/quality of color values on objects.

For example, I've always assumed that because of lacking R9 (and similar) values, skin tones will not show up "well". What specifically does it result in? A literal lack of color information resulting in lower saturation? Digital pixelation? Lower luminance for certain values?

Let's say I want to photograph an object or talent knowing I want to convert it to B&W in post. If I use the worst kind of LEDs imaginable (missing Red values, spiked greens, etc) to light talent, will there be lower or brighter areas depending on the colors on the object/talent? OR - does light (Regardless of color quality) illuminate the object without bias, and when converting to B&W, it makes the original light source quality irrelevant.

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For the most part, I don't think those variations are bad enough to affect luminance for conversion to b&w -- it's not like there is no exposure in the red channel, for example, where skin tones mainly exist.  Depending on the fixture, some odd secondary colors may fall between the spikes and become a darker shade shifted in one direction.

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Seconded. Most modern LED-based lighting made for film and TV work is more than competent enough to avoid issues with monochrome conversions, even the cheap stuff.

To answer the question specifically, if a light lacks (say) red and is used to illuminate an object that contains lots of red, that object will look darker and less red. Skin looks essentially whitish grey without red blood flowing beneath it (notice this next time you pull off a bit of dry skin around a fingernail.)

So, people lit with red-lacking light look dark and grey. The more saturated the colour, and the more lacking the LED in that colour, the darker and less colourful the object looks. Grass is pretty green, so a green-lacking light might make it look very dark and very grey.

There's a comparison picture that shows what happens when more complex colours get involved. It shows a woman in a silk dress that looks teal under good quality light, but which simply looks blue under (very bad) LED light because the LED lacks green; the teal silk just reflects the blue. And it also lacks red; notice how unsaturated her skin looks in the image on the right, and this is someone with a pretty rich, warm skin tone to begin with. The issue is visible in the colour chart; notice that the bottom half of the red chip looks dark and unsaturated compared to the top.

rsn_metamerism.jpg

All this was a common problem with early LEDs, which were routinely absolutely terrible, but as I say it's largely a solved problem in 2020. 

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