code: function csf (cpd, lum) {
  	var a = 440.0 * Math.pow((1.0 + 0.7 / lum), -0.2);
  	var b = 0.3 * Math.pow((1.0 + 100.0 / lum), 0.15);
  	return csfCache = a * cpd * Math.exp(-b * cpd) * Math.sqrt(1.0 + 0.06 * Math.exp(b * cpd));
  }
docs: computes the contrast sensitivity function (Barten SPIE 1989) given the cycles per degree (cpd) and luminance (lum)