I was reading a new book by the IESNA, "Light and Design: A Guide to Designing Quality Lighting for People and Buildings". It's written like an intro textbook, but has some pretty neat approaches to defining what quality lighting is. One of the topics was direct glare from overhead sources. According to their research, any light projecting from less than a 53 degree overhead angle presents discomforting direct glare into the eyes. This is the important angle to know when installing tree lights - it tells us how high we need to mount the lights. Here's the result of my calculations to determine minimum mounting height to achieve a 53 degree visual angle. ([Distance from tree base to edge of projected light beam] x 1.33) = min. mounting height. (note: built into the equation is recognition that eye height is about 5 ft. - this alters the distance to the edge of the light beam, and final mounting height - the two adjustments roughly cancel each other out.) Feel free to check my math and also comment on practical application of above.