Subject: How do they measure blood with light?

Hi - I unfortunately was recently in the hospital with a bout of
pneumonia. I had a small clip put on my finger that had a red light
in it. The doctors said it was to measure the oxygen in my blood.
They also did this occasionally by drawing blood (which really hurt!)
but as I got better they just used the gadget with the light. 
What is this thing and how does it measure my blood? If it measures
my blood why did I still have to get stuck?

Thanks, you guys offer a great service!

Mike

Re: How do they measure blood with light?