What's the difference between CMYK Black and RGB Black


I know this sounds a stupid question, black is black right?


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It's black right? so why are the CMYK Values so far away from black?

1/26/2011 8:40:00 PM

Accepted Answer

I know this sounds a stupid question, black is black right?

Not really. It all depends on colour model used, ambient light, substrate, and perception. Black is, by definition, no light hitting our eyes. This is very difficult to accomplish. :)

CMYK is a Subtractive Colour Model. It is used in printing because the mixing of the different pigments of ink subtract (absorb) different wavelengths of light.

RGB is an Additive Colour Model. Different wavelengths of light are emitted by whatever technology used by your monitor (nowadays, usually LCD crystals filtering an LED backlight). Technically, it's not additive because the colours do not overlap, but the pixels are small enough that optical mixing causes us to perceive different colours on the screen.

Theoretically, a mixture of pure Cyan, Magenta and Yellow would produce black on the paper, but an actual Black ink is added, basically for two reasons. Firstly, there are always impurities in the CMY colours and the substrate (paper) such that the mixture would be muddy (brownish) and inconsistent. Secondly, 100% of the three inks would be a lot of ink, and cause problems drying/covering in most printing processes.

So, your question boils down to: Why doesn't Photoshop just use 100% Black ink (K) when it converts from RGB values to CMYK values?

Even 0,0,0 RGB on your monitor isn't a "pure" black. In a dark room, you will see that your monitor still glows perceptibly. Likewise, in printing, even 100% Black doesn't absorb all the light hitting the page. So, if we want something to look "really black" we add a bit of Cyan/Magenta/Yellow ink to cover the page more and absorb even more light. In printing we call this Rich Black.

Depending on what colour profile you're using in Photoshop, it takes this into account, and converts the blackest thing it can show on screen to a "richer" printed black. The values of CMYK used are judged to be (by the profile) the most ink that can be used, without causing printing problems, that still looks black.

1/26/2011 9:22:00 PM