luminous_efficacy

  LUMINOUS_EFFICACY - The luminous efficacy (in lm/watt) vs. wavelength  

   VL = LUMINOUS_EFFICACY(WAVELENGTHS)
     or
   VL = LUMINOUS_EFFICACY(WAVELENGTHS, SCOTOPIC)

   Returns the luminous efficacy for the requested WAVELENGTHS (in nm).
   If the second argument SCOTOPIC is provided and it is 1, then the scotopic (dark vision/rod vision)
   values are returned. Otherwise, PHOTOTOPIC vision (light vision) values are provided.

   The luminous efficacy is the conversion factor between radiated light energy in
   watts and the brightness as experienced by the human visual system, which has units of 
   lumens.  

   For example, if you have a 5mW laser beam at 680nm, the luminous flux is 
   lf = (0.005 W) * luminous_efficacy(680) (lm/watt) = 0.0581 lm
   If you have a 5mW laser beam at 630nm, the luminous flux is 
   lf = (0.005 W) * luminous_efficacy(630) (lm/watt) = 0.9050 lm
   The latter is better because 630nm has a stronger overlap with visual perception.

   Note: Values of VL for wavelengths < 380nm and greater than 770nm will be 0 by default.

   Source:  http://hyperphysics.phy-astr.gsu.edu/hbase/vision/efficacy.html
   Their source: Williamson & Cummins, Light and Color in Nature and Art, Wiley, 1983.