Lux to foot-candles

Lighting design specifications split along the same regional line as most measurement systems: US lighting codes and older lighting guides often specify illumination in foot-candles, while international standards and most modern light meters read in lux. One foot-candle equals about 10.76 lux, a conversion worth running before comparing a US-published lighting requirement against a lux reading from a light meter.

Film and photography sets carry the same split into a creative context: older American cinematography references and some exposure calculations are still written in foot-candles, while most contemporary digital light meters, and international productions, default to lux, so a cinematographer referencing a classic exposure guide converts between the two before setting up a shot.

A foot-candle is defined as one lumen per square foot, and since one square foot equals exactly 0.09290304 square meters, that fixed geometric ratio is the entire conversion factor between the two units.

fc = lx × 0.09290304

A typical office desk lit to 500 lux converts to 500 × 0.09290304 = 46.45 fc. Overcast outdoor daylight around 1,000 lux converts to 1,000 × 0.09290304 = 92.90 fc. Full midday sunlight around 50,000 lux converts to 50,000 × 0.09290304 = 4,645.15 fc.

Going the other way, multiply foot-candles by 10.76391042: a lighting spec calling for 50 fc converts to 50 × 10.76391042 = 538.20 lux.

Because the conversion factor isn't a round number, exact decimal precision rarely matters in practice: lighting specs are almost always already rounded to one or two significant figures (such as "50 fc" or "500 lx"), so rounding the converted value to the nearest whole number loses nothing a lighting designer would actually use.

The mistake specific to this pair is treating lux and foot-candles as if they were roughly the same number with different names, the way some metric-to-imperial pairs are close to 1:1. They are not — one foot-candle is about 10.76 times larger than one lux, so skipping the conversion entirely, rather than just rounding it, produces an error of more than 10x.