# Overhead glare - tree light mounting height calculation

Discussion in 'Landscape Lighting' started by steveparrott, May 22, 2009.

1. ### steveparrottSponsorfrom Tampa, FLPosts: 1,196

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.

2. ### David GretzmierLawnSite Gold Memberfrom Fayetteville,ARPosts: 3,645

hmm. I've always found glare to hit around the 51.7 degree mark rather than 53. I've tried 55 and I know that it too much. I find if you take the glare angle in decimal form and divide by pi, then subtract your price per fixture, you can find the perfect exposure time for dusk photo's with an SLR. not always, but usually pretty close.

Very funny

4. ### JoeyDLawnSite Silver Memberfrom Escondido, CAPosts: 2,933

my rule is if you can see the lens from the ground you will have glare....

5. ### Lite4LawnSite Gold Memberfrom Indianapolis, INPosts: 3,113

Now Joey, didn't anybody ever tell you not to stand directly under the lights and look up at them?

6. ### The Lighting GeekLawnSite Senior Memberfrom Gold River, CaliforniaPosts: 875

it depends on what he took before he started staring at the lights...

7. ### Pro-ScapesLawnSite Platinum Memberfrom South MississippiPosts: 4,181

I think I am more in tune to looking for glare than my clients would be. Ash is always telling me I am fussing over nothing but if there is a way for me to make it even a little better we do it. That being said fixture selection has a lot to do with glare control. I dont just look to control glare but any hotspots at or near the source. I tend to walk thru some jobs looking for glare and the source. Just a habit I have developed. Not sure if its good or bad

Im sure most of the guys here who really care about the work they do have trimmed a few branches or even small limbs off (with homeowners permission of course) to obtain optimum mounting locations.

9. ### INTEGRA Bespoke LightingLawnSite Platinum Memberfrom Muskoka, Ontario, CanadaPosts: 4,102

Pruning is often necessary to achieve the desired mounting location and effect. We do it when installing and also when maintaining. It is absolutely necessary and not something we really ask permission for. That being said, you have to be careful not to create a mess of a hole in the foliage just to accomodate a fixture, and you obviously cannot damage the tree when you prune. Choosing the right tree up front is critical, as is using 'tree friendly' hardware such as SS hangerbolts, SS wire staples, etc.