What's the difference between luminance, illuminance and brightness?

The Problem

A Luminance, illuminance, and brightness are terms that are often used interchangeably. This leads to confusion when defining the light output of LED-based thin  backlight panels and perceived brightness.

 

A few problems can arise if lighting is not quantified properly:

-Over or under-performing luminance

-Unnecessary engineering and labor costs

-Prolonged lead times

 

Brightness

The Brightness is the human perception produced by the luminance of a visual target. Observed brightness is a subjective attribute of our vision of illuminated objects. "Brightness" should only be used for non-quantitative references to perceptions of light.

 

Luminance

Luminance (and Illuminance) are terms used to characterize light emission from flat, diffuse    surfaces. Luminance indicates how much light energy will be detected by an instrument looking at a lit surface from a particular angle of view. Luminance can only be used to quantitatively describe the output of a measurement system. Luminance is not an indicator of how bright the surface will appear. Measuring luminance (or more correctly Luminous Emittance) numerically represents the total luminous flux emitted per unit area from a surface. The standard unit of measurement for luminance is candela per square meter (cd/m2) also known as "nits" or lux (lumens/m2). The conversion to lux is to multiply by "pi" (3.1416). 

Please note: this article is limited to discussing white LED light. Brightness is further complicated by color. Different colors lit with the same luminance are perceived as being different brightness by the human eye. The influence of color, contrast, and 

 

Conservation of energy

Luminance in a light system is subject to the physical law of conservation of energy. In a perfect system, the output luminance equals the input luminance. The measured output luminance can never exceed the input luminance since the LED light panel system cannot "add" light.

 

Basis

Stevens' Power Law is a psychophysical based relationship between measured data and human perception. This law provides a method to explain the relationship between luminance and brightness. Stanley Smith Stevens is credited with the law and published a body of support data in the 1950's. The law equates the Brightness (perceived) with the Luminance (measured) using a set of established "x" exponents, like this B=Lx. The exponent "x" varies from 0.33 (constant light in the dark) to 0.5 (point source of light) to 1.0 (point source flashed) to 3.5 for electric shock perception. The graphs below show the relationship between Brightness and Luminance based on the mathematical relationship outlined by Stevens' Power Law.

 

 

 

 

 

 

 

 

 

 

 

 

 

The graph on the left shows that while increasing luminance steadily the perceived  brightness increases slowly above the lower ranges of visual perception. The graph also shows that increasing luminance in a flashing or moving light increases perceived brightness equally. As a point of reference, electric current shock perception is added to the graph on the right. Minor shocks like static discharge create minimal human response. The response then increases very rapidly as the electric current shock increases.

 

The Conclusion

I Above a minimal level of perception, doubling the perceived brightness in a steady light source requires approximately four times as much luminance. In real terms, a single LED light system that is visible would need to be replaced by 4 LEDs to double the perceived brightness. The measured luminance would increase by a factor of 4.

This relationship between perceived brightness and measured luminance questions the practical benefit of "doubling the nits" in a given light system. It is even reasonable to question the specification of measured luminance in nits as a valid measure of brightness. The variables of color, background, contrast, and ambient light have a significant impact on perceived brightness.

 

For more information on Lumvatech backlighting, please see the following links:

 

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