What would a sensor (aka phototransistor) "see" when it looks at out skin? I wanted to know this prior to coding to know what to look for - I knew that the blue veins would absorb light differently, but how exactly? 

So I took a modified webcam to take some pictures. I'll use the webcam in another project - it can only "see" IR light now. I removed the IR-blocking filter and replaced it with old analog film (developed) that blocks everything except IR. And I attached some SMD-IR-LEDs as well... somehow the autofocus is not working really good, but you can see the huge difference: the veins look very dark in IR light compared to "normal" light in the visible spectrum. 

Now I know what to look for - it should be a local minimum concerning the amount of reflected light detected by the phototransistor.

When I successfully detected the vein, I'll upload the code and some pics - and a video of this device in action (puncturing my vein)

BTW: I took photos of my fathers arm, this is not mine :-)