Customers often ask these two questions: How much light they need for a particular type of jobsite, and how much light output comes from a light tower that is used on site? OSHA (Occupational Safety and Health Administration) takes a bit of the mystery out of part of this by giving general guidelines for lighting in various types of worksites. For example, OSHA says that areas where tasks like excavation and concrete work take place, a minimum of 3 foot-candles of light is required. These guidelines also state that a general construction plant should have 10 foot-candles, and a first aid area or office setting should be at 30 foot-candles minimum.
Ok, that's great, but what are foot candles exactly?
Foot-candles are a unit of light intensity. A fancy definition would be “a unit of illuminance on a surface that is everywhere one foot from a uniform point source of light of one candle and equal to one-lumen-per-square-foot.” A simpler explanation would be “a measure of intensity of light falling on a surface.”
Now, we are getting somewhere. But what about lumens? Lumens are a unit which quantifies the amount of light energy emitted by a light source. To tie foot-candles and lumens together, foot-candles equal the amount of lumens-per-square-feet of area.
Let’s bring this back to light towers. These light sources come in a range of heights, and lights are often adjustable at various angles and orientations. Charts are available for individual lights, and those can be used as a starting point for determining what area can be illuminated and at what levels.
The example below shows the light pattern for the new Terex RL4 vertical mast tower, equipped with round lights. These are positioned at 23.4 ft, which is the max tower height. Measurements are given with the lights at an angle of 5° from horizontal, pointed down.
The red area shows that the RL4 light tower at full height gives an area just shy of 3,400 sq-ft that is at 10 foot-candles. The outside blue portion is at 0.5 foot-candles and has an area approximately 38,000 sq-ft.
Going back to the OSHA standard, this light tower would be a good option to illuminate a construction plant (which required 10 foot-candles) needing light in an area just under 3,400 sq-ft. If the work area was larger, another light tower could be placed nearby to illuminate a greater area.