Light-Dark Ratio is a way to measure how light or dark someone is.
Light-Dark Ratio has been around for a while. It was very importantly used in The Reincarnation (Battle of the Swords 2) to determine who had a 50-50, because only those with a 50-50 ratio could wield the Reincarnation.
Light-Dark Ratio (often called LDR) simply measures using the left side as the percent of light versus the right side being darkness. For example, if you were 67% light, your LDR would be 67-33.
To determine this ratio, you have to use a special device invented by Rief and Embless called a LDR Reader.