Lighting Requirement Calculator

This is the lumen method, the standard interior lighting calculation, rearranged to solve for lamp count instead of illuminance. In its full form, average illuminance equals lamp count times lumens per lamp times the coefficient of utilization times the light loss factor, divided by floor area. Targets come from task type, not room size: about 10 foot-candles for circulation space such as hallways, 15 to 20 for living rooms and bedrooms, 30 to 50 for kitchens, bathrooms and home offices, and 70 or more for workshops and fine detail work. One foot-candle is one lumen per square foot and equals 10.76 lux, so a 50 foot-candle kitchen target is about 540 lux. The working plane is conventionally 30 inches above the floor, which is counter and desk height. The two correction terms are where most estimates go wrong. The coefficient of utilization captures how much emitted light actually reaches that working plane, falling steeply as surfaces darken, from roughly 0.95 for white walls and ceiling to about 0.55 for dark finishes, because most flux from a ceiling fixture arrives only after one or more reflections. The light loss factor, taken as 0.85 here, accounts for lamp lumen depreciation and dirt accumulation, sizing the design for end-of-maintenance-cycle conditions rather than day one. The height factor reflects inverse-square falloff as the source moves farther from the task. Three limits: this sizes the ambient layer only, it produces an average and says nothing about uniformity, and it does not evaluate glare, for which engineers use a separate unified glare rating.

Before photometric design existed, interior lighting was specified in watts per square foot, a rule that worked only because every lamp in a building was an incandescent of nearly identical efficacy. The Illuminating Engineering Society, founded in New York in 1906, spent its early decades building the measurement infrastructure that made something better possible: standardized photometry, laboratory testing of luminaire light distribution, and studies relating illuminance to visual performance and error rates. The society published the first edition of its Lighting Handbook in 1947, consolidating recommended illuminance levels by task, and those recommendations are the direct ancestors of the foot-candle targets used here. The method itself was reworked as fluorescent luminaires spread through postwar buildings, because different fixtures with the same wattage now threw light in radically different patterns and wattage rules of thumb became meaningless. During the 1960s the zonal cavity method replaced earlier room-index approaches, dividing a space into ceiling, room and floor cavities and deriving the coefficient of utilization from cavity ratios and surface reflectances. The 1970s energy crises then pushed the numbers the other way: recommended office levels that had climbed above 100 foot-candles were cut roughly in half, and the first energy standard covering lighting, ASHRAE 90-75, introduced caps on installed lighting power density. Modern practice sits between those poles, with the Illuminating Engineering Society publishing task-based recommendations in lux through its handbook and recommended practice documents while energy codes limit the watts available.