Illuminance: exploring ‘incident light’
Illuminance is the amount of light striking a surface – also known as incident light, where the “incident” is the beam of light actually landing on the surface.
Illuminance is calculated as the density of lumens per unit area and is expressed using footcandles (lumens/square foot) in the United States or the metric version, lux (lumens/square meter), elsewhere. Illuminance is measure using a light meter.
Illuminance is the amount of light striking a surface – also known as incident light, where the “incident” is the beam of light actually landing on the surface.
Lumens are a measure of the quantity of light, referred to as luminous flux or just flux, emitted by a light source. For example, a dinner candle provides about 12 lumens. A 60-watt incandescent bulb provides about 840 lumens.
Footcandles and Lux
A footcandle is a unit of illuminance, or light energy, produced by one lumen hitting a one-square-foot area. It literally means the amount of light on a surface one foot from a standard candle. One lux is equal to one lumen per square meter, so 10 lux is equal to about one footcandle.
For perspective, illuminances produced by daylight cover a broad range, from 150,000 lux on a sunny day in the summer to 1,000 lux on a gray, overcast day in winter. The typical illuminance that results from moonlight is about 0.3 lux.
When considering lighting applications, illuminance is used to measure the amount of light reaching an object such as a desk or wall.
For instance, the illuminance value for a typical home or office is between 30 to 50 footcandles (300 to 500 lux).
The closer to a light source the illuminated area is, the higher the Illuminance value. Horizontal illuminance describes the amount of light landing on a horizontal surface, such a desk, and vertical illuminance describes the illuminance landing on a vertical surface, such as a wall.
Illuminance values for lighting applications depend on how complex the visual tasks are being performed.
So, walking down a hallway range will require an illumination value much lower than a specialized task where visual performance is critically important, such as assembling small pieces of machinery or reading fine print.
Illuminance vs. Luminance
Don’t confuse illuminance with luminance. The former measures the incident of light, luminance measures what leaves, or is reflected from, the surface.
Luminance is also considered the human perception of brightness, or how bright we perceive the light that is reflected from the surface.
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