Originally Posted By: chevrofreak
This is something that was learned when LED flashlights started getting popular. Guys were taking them into caves, and on the wet cave floors they couldn't really see much contrast, so they couldn't see the dangers that lay ahead.
It has something to do with light spectrum and the way light refracts off wet surfaces from what I understand. They found that more red light (white LED's tend to be mostly blue and yellow) allowed more contrast to be seen in the wet surface of the rock.
I actually prefer halogen head lamps for this reason. I've driven with a number of HID lamps and the color contrast is far inferior to halogen. It's called the Color Rendering Index. Halogen bulbs are 100, which is the best on the scale. Zero is the worst, and the CRI of sodium-vapor lamps (street lamps) is zero. Objects appear monochromatic under that type of light. Daniel Stern's website claims the CRI of automotive HIDs is somewhere around 70.
They are bright, but my eyes fatigue when driving with them, for two reasons. First is the CRI problem. Everything IN the beam seems monochromatic, with little contrast. Secondly, you have so much bright foreground light and so little light far away that your eyes tend to focus more on the foreground light, which makes seeing things at a distance more difficult.
I have always preferred a very bright halogen light source to an HID one, and will probably continue to do so until they can improve HID's CRI.