I would have tried to add it. I test it on this demo first:
picasaweb.google.com.tw/lh/photo … directlink
It is working well.
Then I test it with the fishes:
picasaweb.google.com.tw/lh/photo … directlink
It seems not the effect I want. I would like to have the ray cast out from the light source (like the first picture). How can I make it / adjust it accordingly ?