I have a mesh plane which is a glass object, transmission and alpha all the way up, which means its 100% opaque.

Whenever I use a lighter colour the emission shader goes through the glass, but when I use a dark colour as shown in the example Black, the emission wont shine through, so I lower the alpha to get some emission going through.

Why does the black or dark colours effect the emission luminance once placed over it, since the alpha and transmission is all the way up?

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  • $\begingroup$ Hello :). In Cycles and Eevee, the darker the glass, the less light gets 'through'. Fully black glass won't let any light get through. $\endgroup$ Oct 18, 2020 at 15:04

1 Answer 1


Reframing the idea behind “Alpha” as occlusion should make the idea clearer, where “Transmission” is a degree of filtration.

The 0.0-1.0 range in alpha represents the degree of occlusion from 0% to 100% in a properly encoded image, while the RGB represents the quantity of emission. Something can be completely unoccluding (0.0 alpha, 0%) and emitting (nonzero RGB units), as well as completely occluding (1.0 alpha, 100%) and not emitting (0.0 RGB units).

Transmission represents the ratios of light permitted to pass through the object. 0.0 would indicate that 0% of the given channel is permitted to pass through, while 1.0 would indicate 100%. In this way, Transmission behaves like a light “filter”.

In this way, the RGB values are pulling double duty, both as the emission for the alpha component and as the filtration mechanism for Transmission. It is worth understanding that in different contexts the values present in the three channels can mean different things, and it's not always a "colour" per se.

  • $\begingroup$ Great information, thank you. $\endgroup$ Oct 19, 2020 at 11:40

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