This illuminance is flux density on a surface, expressed as lumens per square foot, typically known as a foot-candle. Appropriate foot-candle levels can be found as a function of a particular task. Illuminance as one figure of merit can be estimated or calculated using some basic principles in conjunction with published photometric performance data on a particular lighting system. First, one needs to determine the amount of lighting flux measured in lumens that is produced by a source. System lumens are a product of the rated lumens of the lamp and the relative ballast factor of the ballast designed to operation the lamp.
Ballast factor accounts for the difference in lumens obtained on a reference ballast.
Typical ballast factors can range from .7 to 1.2. The designer also must consider the lumen maintenance of a source, which refer to the light output characteristics over life. This information is typically available to the designer from the lamp manufacturer and can be factored in the calculation. Not all of the lumens from a particular source are available to the designer. All fiber optic systems use some type of optic coupling system, which direct flux into the fiber optic material.