[Radiance-general] Average luminance calculation

[Jay]

Hi people,
I am trying to calculate contrast for images. I am referring to paper 
(http://diuf.unifr.ch/courses03-04/imaging/simoncelli.pdf). This paper 
suggest that to calculate contrast luminance is first needs to be 
calculated.
I am calculating lumninance using the formula.
L_w = exp[ 1 / N( sum[ log( delta + L_w ( x, y ) ) ] ) ]

Where:

  * L_w - log-average luminance
  * N - number of pixels in the image
  * delta - a small factor to avoid problems with black pixels
  * L_w ( x, y ) - the world space luminance for pixel ( x, y )

where log has base e.

I am using the image from http://www.cacs.louisiana.edu/~cice/lal/. i 
cant get the luminance values mentioned on the website.
Can someone please tell me where am i going wrong.

thanks
Jay