Continuing from where Illuminating Light’s Mysteries: Part 1 left off, you may remember that Lumens are a measure of the total “amount” of visible light emitted by a source. The lumen is based the International System of Units measurement derived from luminous flux. Now that we have this number, it’s a good point of reference but if its projectors the classroom or a theatrical lighting setting, having a measurement at the source is only part of the answer.
The key measurement is the amount of light that gets to where its supposed to go. That measurement is luminance and can be measured in: Lux, foot-candles, or foot-Lamberts. Illuminance is a measure of how much luminous flux is spread over a given area. Luminance Flux (measured in lumens) as a measure of the total “amount” of visible light present, and the illuminance as a measure of the intensity of illumination on a surface.
One lux is equal to one lumen per square meter. The larger the area, the dimmer the source will appear. One Foot-Candle is equal to 10.76 lux. This is where is gets tricky when buying a projector or selecting stage fixtures.
Projection Example: If you have a dimly lit room (50 lux or 4.6 foot-candles) a 1,000 lumen projector and a 4.5’ by 8’ screen, the illuminance of the projector on the screen is 298.9 lux and will look pretty good. Turn the lights up in the room to an office level (500 lux) and the projected image on the screen is less than the ambient light level and will look washed out. Or, double the screen size to 9’ by 16’ and the illuminance will be 6.94 lux, so you are going to have to turn out the lights to view the projection.
Stage Lighting Example: When purchasing theatrical fixtures, make sure that the fixture can properly provide the lux desired. Most manufacturers supply a chart that supplies the field angle and beam angle and the available foot-candles and/or lux at a specific distance for their incandescent fixtures. Typically this type of information is not available for most LED fixtures. It has been our observation that LED manufacturers may rate the light output of their fixtures at different distances. Some manufacturers use as little as 2 meters (6.5’) as their reference, while most of the top theatrical manufacturers we’ve checked use 6 meters (20’) as a reference point. The difference in distance will make a significant in the amount of lux available.
In the next Illuminating Light’s Mysteries, we will get into practical applications; we will discuss the color temperatures of different sources and special considerations when mixing lighting fixtures.