Lumenzia: ✓L

✓L stands for "check luminosity". It actually has the capability to check saturation or hue as well if you hold down <alt/option> or <shift> respectively while clicking on it. In all cases, the purpose is to help visualize specific defects in luminosity, hue, or saturation so that you can more precisely identify the underlying issue and correct each independently. This is particularly helpful for working with composites.