http://www.haskell.org/haskellwiki/Numeric_Haskell:_A_Repa_Tutorial#Example:_parallel_image_desaturation

1. for every pixel I first check if blue component is greater than green and red

2. if it IS I use Lightness method (or Average – both are good) … if NOT I use Luminosity method for current pixel.

This algorithm gives very good results. It comes very close to Photoshop’s convert to Grayscale “mystic” algorithm

There appears to be a bug, the weights should add up to 1.0. I think the multiplier for G should be 0.72 rather than 0.71.

As for how B/W film converts to gray, it appears to depend on the exact film chemistry: http://en.wikipedia.org/wiki/Photographic_film#Spectral_sensitivity

In film or a digital sensor there is a spectral sensitivity function that determine which frequencies of light get absorbed/sensitise the emulsion based upon the chemical structure. Through various means this gets converted to either silver halide crystals or in colour film different dye’s are formed (eventually).

You then shine a light through the resultant film and bounce it off a screen, your eyes then view the result, In mathematical terms this is a highly non-linear transformation from scene-to-screen and is a function of many reductions from spectral to n-dimensional ‘records’ all of whose intensity functions are non-linear.

In film the actual ‘colours’ vary mostly due to the stocks used, but there are some effects that vary with intensity of exposure so the camera settings can effect the results though this is minor compared to the stock and development.

Re: the magic numbers, well they are based upon a mixing assumption. They are are saying what linear combination of R, G and B would you perceive as being without colour, i.e. neutral. Depending on the exact colours of R, G and B you will get different proportions. The mixing also assumes that the individual colour channels are linear, else your not really that close to how our eyes behave. What you consider neutral is also heavily effected by the surroundings in which you view, your eyes are able to adjust their own gain functions to compensate for a certain amount of change, thus a piece of paper can look white to you even if the colour of the light shining on it is changed, although with highly coloured light you will still see it as being coloured.

I’ve heard that some portrait photographers use orange filters with black and white film (to remove freckles and spots).