When using the Displacement Map-effect there are several options to choose from when specifying input channel. I would assume that "Luminance" is the most useful, or at least the most used alternative.
However, I'm trying to figure out what the "Lightness", "Full" and "Half"-options mean (isn't "Lightness" basically the same thing as "Luminance", they at least provide me with identical results. And what do the "Full" and "Half"-options mean?)
Displacement Map is a compound effect that can use one layer to warp (displace) another. First, it maps the pixels from the Displacement Map Layer onto the pixels of the layer it's applied to. Then, for each pixel in the layer it's applied to, and separately for X and Y, it looks at the grayscale value of the corresponding map layer. If that map value is between middle gray and black, it causes an increasingly negative displacement. If that map value is between middle gray and white, it causes an increasingly positive displacement.
The "Use for Horizontal/Vertical Displacement" channel selector lets you control how that grayscale value is calculated.
Luminance uses a weighted average of the red, green, and blue values of each pixel to approximate the perceived visual brightness of a color. It especially emphasizes green and de-emphasizes blue. To try this for yourself, make a few solids -- red, yellow, green, cyan, blue, magenta -- and put them all on-screen at the same time, and note how some appear brighter than others.
Lightness takes the average of whichever channel is brightest, and whichever channel is dimmest. This does not relate to the way you see color.
A picture is worth a thousand words:
Off, half, and full are great for testing, as they override the source image completely. Off returns black for every map pixel (maximum negative displacement), half returns middle gray (no displacement), and full returns white (maximum positive displacement).