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A lot of christmas lights for example have a low CRI. Therefore its hard to make skin tones look good with these as a light source.

A flash would solve the CRI problem, but also ruin the atmosphere of the shot.

So will a flash with a gel that mimics the original light color improve the CRI of the scene?

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  • \$\begingroup\$ If a flash ruins or not the atmosphere of a scene would depend on the directionality and power. Try using an off-shoe flash with a remote trigger. google.com/… \$\endgroup\$
    – Rafael
    Aug 25, 2022 at 5:29

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Any color bias to a light source results in a lower CRI.

CRI is based on color reproduction compared to full spectrum daylight (midday sun), which a flash is designed to closely approximate. If you absorb a portion of that spectrum with a filter, then it cannot accurately reproduce the colors that depend upon those wavelengths that are now missing.

However, CRI is an average... it is probably possible to have very high accuracy in the majority of colors and one very weak spot, and have a better/equal CRI compared to a light source that doesn't reproduce any colors particularly well.

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  • \$\begingroup\$ Cheap RGB LEDs producing a certain color are not the same as filtered sunlight. Both light sources will result in a colored scene, and the white surfaces can have the same color, but other colors will not look the same. Filtered sunlight has a continuous spectrum (even if not uniform) which produces a response on every color. LEDs have just three wavelength spikes so colors can "hide" from one of these spikes. If the camera's color response fully matched the eyes, you could fully eliminate the sunlight filter in postproduction, but not LEDs. (Interesting: youtu.be/uYbdx4I7STg?t=1135) \$\endgroup\$
    – relatively_random
    Aug 26, 2022 at 7:50
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Do flash gels impact the CRI of the flash?

Not very much.

Why?

Because flash gels don't begin to approach completely blocking any particular wavelengths of light, they merely allow some wavelengths through at a slightly higher transmissive rate than others. And even there, they're still fairly weak.

On the other hand, light sources with complete holes in parts of the visible spectrum will have a much more devastating effect on CRI (Color Rendering Index) than gels that just moderately attenuate some wavelengths of light more than they attenuate other wavelengths of light.

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