Relative Luminance from RGB color.

relativelumance.js

// from http://en.wikipedia.org/wiki/Luminance_(relative)
// R, G, B are integers between 0 and 255
function relativeLuminance(R, G, B) {
 
  var Y = (0.2126 * R + 0.7152 * G + 0.0722 * B) / 255;
 
  return Y;
}