Each pixel has two values, which are luminance and chrominance. That portion of the NTSC color television signal which contains the luminance or brightness information. In other words, brightness is the perception elicited by the luminance of a visual target. The formal relationship of EV to luminance or illuminance has limitations. The timing of the luminance signal must allow for this. The lightness correlate in CIELAB is calculated using the cube root of the relative luminance. The lighter (higher luminance) color is spread into the darker. They are also noted for subtle modulations of colour and luminance. It is not necessarily proportional to luminance. In this case, the Y value is known as the relative luminance. Contrasts in colour alone (without changes in luminance) are rarely triggers for PSE. Retinal damage can occur when the eye is exposed to high luminance. This line is the line of zero luminance, and is called the alychne. The drawback, however, is that there is no upper limit on luminance levels. "F" is the luminance level adaptation factor.