Lightness, from 0.0 to 1.0. The lightness of a color describes how bright a color is. A value of 0.0 indicates black, and 1.0 indicates white. You can think of this as the intensity of the light behind the filter. As the lightness approaches 0.5, the colors get brighter and appear more saturated, and over 0.5, the colors start to become less saturated and approach white at 1.0.
final double lightness