In this lesson you will learn how to convert simply RGB/CMYK to an accurate grayscale image, rather than just going from RGB/CMYK straight to greyscale which will not be such an accurate conversion.

Origins of Lab Color
In 1931, the Commission Internationale d'Eclairage (CIE) came up with a color model that displays every color perceived by the human eye. In 1976, this model was updated and refined, in order to create the CIE Lab color system. Unlike RGB colors that are screen-dependent and CMYK colors that vary with printer, ink and paper characteristics, CIE Lab colors are device-independent. Therefore, the visual characteristics of these colors remain consistent on monitors, printers and scanners.
L a b color consists of a luminance or lightness* component (L) and two chromatic components: the a component (from green to red) and the b component (from blue to yellow)

*lightness is the part we will use to create the grayscale.

Original (in this case RGB) image.

Go to the menu. Image/Mode/Lab Colour

Click on Lab Colour

L a b model: A. Luminance =100 (white) B. Green to red component C. Blue to yellow component D. Luminance = 0 (black)

Lab color is the intermediate color model Photoshop uses when converting from one color mode to another

In the Channels Palette you will now see the four channels

Lab-Lightness-a-b insread of Red-Green-Blue

Now go back to the menu and go to: Image/Mode/Grayscale

If you haven't switched it off this menu will now pop up. Click OK

FINISHED RESULT

You now have a quick method for producing reasonable grayscales from RGB or CMYK file formats.