Bicycle Lights

So, we've been having a discussion about the pros\cons of HID lights VS Halogen lights. The consensus is that HID's blueish hue light output doesn't feel warm and doesn't seem to match the spectral response of the human eye. To understand this a bit better I hopped on the internet to do a little bit of investigation into the human visual system.

Here's a plot of the visual spectrum:


I found a few tidbits of information which explain why the blue light of an HID does not match the eye's spectral response. Excerpts below are from http://www.photo.net/photo/edscott/vis00010.htm

"The human visual system has much greater sensitivity in low ambient illumination. The cones contribute little or no sensitivity in this condition. Imaging is primarily accomplished by the rods when illumination levels are very low. The plot that follows shows the spectral sensitivity of the rods. "

The plot above totally explains why the human eye is less sensitive to the blue light of an HID light system. To extrapolate on the plot above, it seems like a lighting system with a green output would be optimally matched the the peak sensitivity of the human eye.

In addition, from the symmetry of the sensitivity plot above, it appears that the eye is just as sensitive to blue-green light (HID) as it is orange-yellow-green light (Halogen). However, in practice we all know that Halogen light system looks "better" or warmer. The only other thing that I can think of to explain this phenomena is related to the frequency content of light. Blue light has a lower wavelength, thus higher frequency, whereas orange-red light has a higher wavelength or lower frequency.

I would reason that low frequency characteristic of a Halogen light would some how produce better contrast and thus look "better" to the human eye. I think this makes sense since, higher frequency information (whether speaking of sound or light) is usually associated with higher resolution. Whereas, lower frequency information is usually associated with contrast.

Dang it, I'm trying to think of a good analogy form the music\sound world, but it's just not coming to me. Maybe I need a few beers or a joint ;-) I know it has something to do with bass being low frequency and needing subwoofers VS high pitch metalic sound being high frequency and needing tweeters. The bottom line is that you can hear music with just a mid-range speaker, just as you can see at night with either and HID or Halogen. However, if you have the proper speaker for the proper frequencies the music will sound better, just as the appropriate spectral output to match the spectral sensitivity of the human eye will give you a better picture at night.

Some food for thought!

Cheers,
Isaias

ADDENDUM: Check out Cannon's website for a good explanation about light, cameras and it's applications: http://www.canon.com/technology/s_labo/light/index.html