Illumination Converter

Illuminance is the photometric quantity that measures how much luminous flux (light) falls on a surface per unit area. Unlike luminance, which describes what an observer sees when looking at a surface, illuminance describes what arrives at the surface from light sources. Formally, E = dΦ/dA (illuminance equals differential luminous flux divided by differential area). The SI unit is the lux (lx), defined as one lumen per square meter. Illuminance is the primary quantity used in architectural lighting design, workplace lighting standards, roadway lighting specifications, and photographic exposure control.

The human visual system perceives a vast range of illuminance — from roughly 0.001 lux (moonless starlit sky) to 100,000 lux (direct sunlight on a clear day). This six-order-of-magnitude range requires careful measurement and specification. The eye adapts to these changes through pupil dilation, photopigment bleaching, and rod-to-cone switching at approximately 1–5 lux (the mesopic range). Workplace lighting standards reflect this physiology: tasks requiring fine visual discrimination (surgery, precision inspection) need much higher illuminance than general corridors or rest areas.

The foot-candle (fc) is the US customary unit of illuminance, defined as 1 lumen per square foot. 1 fc = 10.7639 lux. It is standard in US lighting specifications from the Illuminating Engineering Society (IES) — design manuals, commissioning reports, and photometric calculations from US-based consultants routinely use fc. When exporting designs or equipment specifications to metric countries, converting fc to lux is essential. The IES RP-1 standard for office lighting recommends 300–500 lux (28–46 fc) for typical office tasks.

The phot (ph) is the CGS unit of illuminance: 1 ph = 10,000 lux = 1 lm/cm². It appears in older photometric literature, some French and German engineering standards from the pre-SI era, and some military and industrial optical specifications. The nox = 0.001 lux = 1 millilux was proposed for specifying very low illuminance levels in scotopic conditions (moonlight ≈ 0.1 lux, starlight ≈ 0.001 lux).

The meter-candle is a historical synonym for lux (1 meter-candle = 1 lux), derived from the illuminance produced by 1 international candle at a distance of 1 meter. Similarly, centimeter-candle = 10,000 lux (the illuminance from 1 candle at 1 cm distance). These terms appear in older photometric reports, lamp catalogs, and photography guides from the pre-SI era.

The lumen per square foot (lm/ft²) is identical to the foot-candle. Lumen per square centimeter (lm/cm²) equals the phot. These lumen-based expressions are sometimes preferred by engineers who want to make the connection between illuminance and luminous flux explicit in documentation. Candela steradian per square meter (cd·sr/m²) is numerically equal to lux (1 cd·sr = 1 lm), expressing the same quantity from an intensity-angle perspective.

In road and outdoor lighting, illuminance on the road surface (in lux) and luminance (in cd/m²) are both specified. CIE standards for road lighting class M (motorways), C (conflict areas), and P (pedestrian areas) define minimum average and uniformity ratio requirements in lux and foot-candles. Street lighting engineers designing projects for international deployment must convert between lux (European standards) and foot-candles (ANSI/IES standards) constantly.

In photography and cinematography, incident light meters measure illuminance in lux or foot-candles to determine correct camera exposure settings. The Exposure Value (EV) system provides a single number that encodes the combination of aperture and shutter speed needed for a given illuminance. Professional cinematographers use incident meters to ensure consistent illuminance across a scene, and DPs working on international productions must switch between lux and fc depending on equipment and client conventions.

In horticulture and plant science, illuminance in lux is used to specify grow light levels for different plant species. Seedlings require 2,000–4,000 lux; fruiting plants may need 25,000–75,000 lux. Greenhouse designers convert between lux (measurement instruments) and PPFD (photosynthetic photon flux density, in µmol/m²/s) for different light spectrum analyses. This converter supports all 12 illuminance units — lux, meter-candle, centimeter-candle, foot-candle, flame, phot, nox, cd·sr/m², lm/m², lm/cm², lm/ft², and W/cm² — instantly and free.