This effect was generated in Filter Forge, a Photoshop plugin that allows you to create your own filters.
This filter allows you to mask some of the luminance of an image and alter the mask for further control over it.
craig kraellin walters luminance luminosity brightness darkness brilliance shadow mask selection threshold
This filter has earned its author one of our rewards. They are given to the authors of popular filters and include a free copy of Filter Forge.