What is the equation for ENVI's Photographic Stretch routine?

This Help Article discusses the equation for ENVI's photographic stretch routine since there are no references for this routine currently.

This example covers the red band, but it is exactly the same for green and blue:

out_red = in_red/(1+in_red*lambda)
where
lambda=(max-(2*mean)) / (max*mean)

where max and mean are combined for all bands.

If the "lambda" factor is less than 0, then the input is the same as the output.

Review on 12/31/2013 MM