Many people want to check their “values” when doing digital artwork in Photoshop. That is, how bright or contrasty different areas of their image is. A more correct term is the “luminance” of their image.
Different colors are perceived as brighter or darker than others. For example, blue appears darker than yellow, and cyan appears brighter than red. Below is a schematic of fully saturated colors and their corresponding brightness:
I’ve seen numerous suggestions of using a Hue/Saturation adjustment layer, or a Black & White adjustment layer, as well as a suggestion of using a completely black layer with the blending mode set to “Color” or “Saturation”. All of these tricks give an incorrect result (unless you’re working in LAB instead of RGB, which you most likely aren’t).
The correct but cumbersome way is to go to “Image > Mode > Grayscale…” inside Photoshop. Two easier alternatives are outlined below.
Option 1: Proof Colors
A great and easy way described here http://www.artofscholes.com/checkingvalues/, simply involes using Proof Colors.
- Go to View > Proof Setup > Custom…
- Select “Working Gray – Dot Gain 20%” as device to simulate
- Go to View > Proof Colors (Ctrl + Y on Windows, Cmd + Y on macOS)
Option 2: Color Lookup
Another method is to use a Color Lookup. A feature which has been available since Photoshop CS6. This is how to use it:
- Download this 3D LUT: Grayscale_3DLUT.zip
- Extract the zip, and place the Grayscale.3DL file in Photoshop’s preset folder (see below)
- Restart Photoshop
- Create a “Color Lookup” adjustment layer and select Grayscale.3DL
Photoshop presets location on Windows:
C:/Users/[user name]/AppData/Roaming/Adobe/Adobe Photoshop CC 2014/Presets/3DLUTs
Photoshop presets location on macOS:
~/Library/Application Support/Adobe/Adobe Photoshop CC 2014/Presets/3DLUTs/
Replace [user name] with your username and the corresponding Photoshop version. If the 3DLUTs folder doesn’t exist, you can simply create it.
I cannot guarantee that the color lookup table is good enough for high-end production work, but should certainly be good enough for checking your values. The lookup table was made in Photoshop CC 2014 and is based on the grayscale conversion performed by Photoshop.