The Nature of Light

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Radiative Transfer

Light is what organisms can see. The most common sort of human vision responds to electromagnetic radiation between 360 and 830 nm (nanometers) in wavelength. Electromagnetic radiation pervades space, which physicists understand as a "plenum"—a fullness. All objects constantly both radiate and absorb electromagnetic radiation; this is called "radiative transfer."

In measuring light there is always the problem of whether what is measured is physical radiation or sensible light. Radiation can be reliably and consistently measured by physical means. This is called radiometry. Sensible light, on the other hand, can only be measured with reference to organisms that experience sensation. The rigorous characterization of sensation is a branch of psychology known as psychophysics which seeks to construct predictive models of sensation. The psychophysical quantities most of interest to in the study of lighting are luminance, which is described by photometry, and measured color, which is described by colorimetry.

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Lead Author(s): Randolph Fritz

Measurement of Sensible Light & Visible Radiation

Sensible light is measured in photometric and colorimetric units; radiation is measured in radiometric units and wavelengths. The measurement of spectra is called spectrophotometry. Spectrophotometry is used to characterize the color of light, and also the color of materials.

Photometry, the measurement of luminance, can be done in two ways:

  1. Visual photometry: having an observer or observers evaluate the luminance of an object or scene
  2. Physical photometry: using a radiometer calibrated with anthropometric data about vision generated by visual photometry, to numerically evaluate luminance.

In current practice physical photometry is near-universally used.

Some Photometric and Radiometric Units
Quantity Radiometry Photometry
Intensity Radiant Intensity W/sr (watts per steradian) Candlepower cd (candela, lumen per steradian)
Flux Radiant Flux W (watt) Luminous Flux lm (lumen)
Emitted Energy Radiance W/(sr·m2) Luminance lm/(sr·m2) (nit, nt)
lm/(sr·ft2) (footlambert, fL)
Received Energy Irradiance W/m2 Illuminance lm/m2(lux, lx)
lm/ft2 (footcandle, fc)

There is so far no completely satisfactory way to calibrate a photometer. There is, however, some uniformity in the perception of the luminance of stimuli of different wavelengths. When perceived luminance is plotted against wavelength a plot like the perceived luminance chart emerges.

Perceived Luminance, V(λ), V'(λ) vs. Wavelength, λ, in air

The blue curve, V(λ), describes photopic, that is, daytime color, vision; the red curve, V'(λ), describes scotopic, that is, nighttime monochrome, vision. Colors are approximate. In reading such a chart, always keep in mind that color vision varies from person to person, and the representation of color varies from computer to computer, so what you see will at best approximate what someone else sees.

Photometric values are measured by adapting radiometric sensors to match the appropriate luminous efficiency curves. It can only be stressed that physical photometric measurements are radiometric measurements transformed by partial anthropometric data. Hence, they are guides to the human experience of light and space, rather than certain predictors of that experience.

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Lead Author(s): Randolph Fritz

Luminous Flux, Light Sources, and Geometry

Luminous intensity, luminance, and illuminance are measurements of luminous flux (flow) adapted to the geometry of different light sources. Luminous flux is always measured in lumens.

Luminous Intensity

Candela: The portion of a sphere shown is one steradian.

The amount of light radiated by a point source in particular direction, which is measured in candelas. This unit exists as a result of an attempt to capture the idea of the "brightness" of a candle or incandescent light bulb. One candela is one lumen per steradian.

In practice, since there are no true point sources, the usual approximation is that a light source may be treated as a point source if viewed from a distance of more than five times the "diameter" of the source.

For historical reasons, the candela is the fundamental unit of light, even though the lumen is more simply defined.


The amount of light reflected from an area of a surface back to a point, which is measured in lumens per square meter per steradian, lm/(sr·m2), in the International System of Units (SI) or footlamberts, lm/(sr·ft2), in United States Customary Units (AS.) This attempts to capture the "brightness" of a surface.

Lumens per square meter per steradian are also sometimes called nits from Latin nitere, shining.


The amount of light falling on an area of a surface, which is measured in lux (lx, lm/m2) in SI units and footcandles (fc, lm/ft2) in AS units. This attempts to capture the illumination of a flat surface.

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Lead Author(s): Randolph Fritz

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