Light attenuation by the human eyelid

I found this abstract on how much light gets through closed eyes:

Light attenuation by the human eyelid
Biological Psychiatry, Volume 39, Issue 1, Pages 22-25
K.Ando, D.Kripke

Abstract

Although it has been reported that light treatment during sleep can modify biological rhythms, the amount of light that is transmitted through the human eyelid has not been established. We evaluated eyelid transmission with a visual threshold response. Estimated light transmission through the eyelids was 0.3% for blue, 0.3% for green, and 5.6% for red light. The eyelid was an effective attenuator and acted as a red-pass filter. Illumination intensity and color balance after passing through the eyelid should be considered in evaluating the effects of light treatments during sleep. http://www.ncbi.nlm.nih.gov/pubmed/8719122

Very helpful information for Procyon developers. Thanks so much. In practice those ratios seem about right. The 'red filter' effect definitely has to be compensated for.