Illuminance
Measure of incident light per unit area weighted for human vision.
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Illuminance is the total luminous flux incident on a surface per unit area, wavelength-weighted by the luminosity function to correlate with human brightness perception. In SI units, it is measured in lux (lx), which is equivalent to lumens per square metre (lm·m−2). A related quantity, luminous exitance (or luminous emittance), measures the luminous flux per unit area emitted from a surface and is expressed only in lm·m−2, not in lux. In the CGS system, the unit of illuminance is the phot, while a non-metric unit used in photography is the foot-candle. Historically, illuminance was often called "brightness," but this usage is now discouraged because it causes confusion with luminance, which is a different photometric quantity. "Brightness" should be reserved for nonquantitative references to physiological sensations and perceptions of light.
The human eye can perceive a range of illuminance spanning somewhat more than two trillion-fold. Under starlight, at approximately 50 micro lux, the presence of white objects is somewhat discernible. At the bright extreme, it is possible to read large text at 100 million lux, which is about one thousand times the illuminance of direct sunlight, though such levels are very uncomfortable and can cause long-lasting afterimages. In astronomy, the illuminance that stars cast on Earth’s atmosphere is used to measure their brightness, typically expressed in apparent magnitudes in the visible band; V-magnitudes can be converted to lux using a specific formula. For a point source, illuminance on a surface relates to luminous intensity through the cosine law and the inverse-square law. For reflecting surfaces, luminance is related to illuminance via the surface’s reflectance and luminous exitance; for a perfectly diffuse (Lambertian) reflector, the relationship simplifies to a direct proportionality.
- Highest readable
- 1.2×10⁵ lux (120 klx) for typical outdoor illuminance
Lore & Background
Illuminance, in photometry, is the total luminous flux incident on a surface per unit area, weighted by the luminosity function to match human brightness perception. It is measured in lux (lx), equivalent to lumens per square metre, while the related quantity luminous exitance (or emittance) is measured only in lm·m⁻². In the CGS system, the unit is the phot, and the foot-candle is a non-metric unit used in photography. Historically, illuminance was often called brightness, but this term is now reserved for nonquantitative references to physiological sensation, as it causes confusion with luminance. The human eye can perceive a range of over 2 trillion-fold: under starlight, at roughly 50 μlx, white objects become somewhat discernible, while at the upper extreme, reading large text is possible at 108 lux (100 Mlx)—about 1000 times direct sunlight—though this is uncomfortable and can cause lasting afterimages. In astronomy, the illuminance stars cast on Earth’s atmosphere is used to measure brightness, with apparent V-magnitudes convertible to lux via the formula Ev = 10^(-0.4 * mv - 14.18). For a point source, illuminance follows the inverse-square law combined with the cosine law, relating luminous intensity, distance, and angle of incidence. For reflecting surfaces, luminance relates to illuminance through the surface’s reflectance and luminous exitance; a perfectly diffuse (Lambertian) reflector yields isotropic luminance.
Reader's Guide
Illuminance is fundamental in photometry for describing light incident on surfaces, distinct from luminous exitance (emitted light). Its SI unit is the lux, while the CGS phot and non-metric foot-candle are also used. The human visual system's vast dynamic range—from starlight to extreme brightness—is expressed in lux values. In astronomy, illuminance links apparent magnitude to physical light levels. The relationship to luminous intensity follows the inverse-square and cosine laws for point sources, and to luminance via reflectance, with a simple formula for Lambertian reflectors. Understanding illuminance is critical in lighting design, photography, and vision science, as it directly affects visibility and perception.
Did You Know?
- Illuminance is measured in lux (lx) or lumens per square metre (lm·m−2) in SI units.
- The human eye can see over a 2 trillion-fold range of illuminance.
- Under starlight, white objects are discernible at around 1×10⁻⁴ to 5×10⁻⁴ lux.
- In astronomy, V-magnitudes can be converted to lux using the formula Ev = 10^(-0.4 * mv - 14.18) or equivalently Ev = 10^(-14.18) * 10^(-0.4 * mv).
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Frequently Asked Questions
What is Illuminance in optics?
Illuminance quantifies how much luminous flux strikes a surface per unit area, weighted by the eye's sensitivity curve so the value tracks perceived brightness. In plain terms, it is the 'incoming light per square metre' as the human visual system would register it.
How does Illuminance differ from luminous exitance?
Illuminance describes light arriving at a surface, while luminous exitance describes light leaving one. Although both carry the lm·m⁻² dimension, illuminance can additionally be expressed in lux, which keeps the two terms distinct in everyday usage.
What is the full range of illuminance the human eye can handle?
The span runs from roughly 5×10⁻⁵ lux under starlight up to about 10⁸ lux for reading large print, a dynamic range exceeding two trillion-to-one. This extraordinary adaptability lets the same visual system function from a dark night sky to a bright reading lamp.
Why is the old term 'brightness' discouraged for Illuminance?
The word 'brightness' was once used as a synonym but is now avoided because it collides with the separate photometric quantity called luminance. Keeping the names distinct prevents confusion between light falling on a surface and the perceived glow of that surface itself.
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