Puzzler...

All but one of you got the correct answer to last week's puzzler: the reason why one lighting technology is more pleasing to the eye than another is the smoothness of the intensity of light it emits over the visible spectrum. Incandescent bulbs have a very smooth spectrum curve, with the very best of them (high-temperature halogen) emitting a spectrum that closely resembles that of the sun.

The two technologies that offer the best hope for energy saving (compact fluorescent and white LED) have emission spectrums that are very “peaky”. For example, a white LED typically emits 90% or more of its light in either two or three very narrow bandwidths. If an object that is illuminated by such a lamp happens to be colored such that it doesn't match the colors emitted by the lamp, then it will appear to be a different color than it would have in sunlight (or incandescent light). If you've ever seen the peculiar way things look under low-pressure sodium street lamps (the yellow ones), then you've seen an extreme example of this. CFLs and LEDs aren't quite that bad, but to anyone who appreciates colors they can be very annoying. Perhaps even worse is a different phenomenon, often called “harshness”. This has to do with the opposite situation: when an object illuminated by CFL or LED light happens to be a color that matches the peak emission. Such an object has a peculiarly high intensity, almost as though it were glowing on its own. In reality, the object is reflecting more light than it would under sunlight of the same average brightness – another effect of the “peaky” emission spectrum – and our perceptual system interprets this as that almost-self-glowing effect.

Manufacturers of CFLs and LEDs are working to improve this discontinuous spectrum problem. In the case of CFLs, that means better phosphors and compromises on efficiency. There are multiple technologies being used and developed for LEDs, and I think in the long term the best LEDs are likely to be superior to the best CFLs (this is not true today, however).

This week's puzzler is back to history. As usual, no fair googling before you answer!

America has had a series of violent episodes in its schools, and many Americans believe the incidents are escalating both in severity and in frequency. That's actually a challenging thing to determine, as our population has increased rapidly throughout our history, and the number of schools (and the number of children in those schools) has varied greatly. This complicates analysis and comparison. But at least one data point is very easy: the worst incident of school violence in American history left 38 students dead. What year did this incident occur in?