• Hey Elif maybe this helps, I just came across it while studying. Maybe you are trying to determine beam spread?

## What is illuminance?

Illuminance is "light arriving at a surface, expressed in lumens per unit area." In other words, illuminance is the general idea behind footcandles and lux. One lumen per square foot equals one footcandle, while one lumen per square meter equals one lux. Both of these units measure an area's illuminance, just as both miles and kilometers measure distance.

## What is beam angle?

Beam angle is defined as the angle of light emitted from a lamp. The angle is measured between two directions for which the light intensity (candlepower) is 50 percent of maximum intensity. The wider the beam angle, the less intense the light. A PAR 38 has a relatively narrow beam angle, while an A19 has a wide angle.

If beam angle is the angle lumens are emitted from a light source, beam spread is the range of the surface illuminated, expressed in distance. To calculate beam spread, multiply beam angle by 0.18 by distance of surface from light source. Beam spread does not account for field spread –– the outmost area illuminated by a light source.

## What is center beam candlepower?

Center beam candlepower is the intensity of light at the center of a reflector lamp beam (expressed in candelas). This is an important measurement when you're aiming lighting in a specific direction, as it measures the concentration of the light to the center of the beam. Flashlights have a high CBCP while fluorescent tubes do not.

• Hi Alexander, thanks so much for chiming in! In fact, beam angle and spread are the reasons why I am refusing to accept FC=L/Area a a legitimate approach. Why I need this information is basically to be able to calculate the foot candle on a works surface, right? Since I post this I have been through multiple resources to get the bottom of it and in my opinion MEEB has done the best job at explaining it. I was not convinced with the formula FC = Lumen/Area, cause it didn't make sense mathematically or physically... Think about it, How come the power of light fixture attached to a ceiling can be assumed equal from light source through the floor on every horizontal surface? In other words if I had a surface at 4 feet AFF and 2'-6" AFF, the one closer to the fixture/ceiling would have more light on it than the one further right? So MEEB comes to my support and says FC=L/SF is just an assumption of the footcandle amount on the surface, not exact -gives you an average idea! To be able to calculate the amount of actual FC on a work surface requires a very complicated math aka computing. However, another much closer calculation can be achieved by the formula of FC= Lumen/ square of vertical distance and if the point light source is angled than formula becomes FC=L/D2 x cos (angle) . Also not being perfectly accurate, this formula is better at including the distance from the light source to the surface and the angle of light fixture, hence the result is more accurate.

• I see what you're saying now about squaring the distance you are correct there. Check out this PDF http://www.rsltg.com/images/Math-All.pdf I think you'll see alot of what you've been saying represented graphically. page 11 shows the FC=candlepower/ distance^2. page 13 shows using Trig to solve if the light surface is flat and pg 15 has the angled cosign adjustment.