[Chimera-users] limits of transparency?

Hi, I'm making my protein ribbons partially transparent by changing the alpha value in color editor. I tried a=0.2 and a=0.05, and I found that after raytracing, there is no difference between these two. Isn't a=0.05 supposed to be more transparent? Is this a bug or something is wrong with some settings in chimera that I don't know? Thanks a lot first! Cheers! Luyuan Zhang