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Pop Quiz: April 2006

Test your knowledge of light measurement with these true or false questions.

  • The perceived brightness of a light source is related to the number of photons per second the light source emits.


  • Light can be characterized only by its wavelength.


  • One lumen per meter squared per steradian is another way of describing a “nit.”


  • A projector's light output can be specified by either lumens or nits.



1. TRUE. However, because the number of photons at visible frequencies is an extremely large and unwieldy number, we don't quantify brightness in terms of photons. Brightness is also the human perception of light, so it can't be directly measured. Brightness is really two things: illuminance (light radiating from a source) and luminance (light reflecting off a surface). Both could be considered “brightness” and both are measurable. Technically speaking, “illuminance” and “luminance” are the most accurate terms to use.

2. FALSE. Wavelength describes the perceived “color” of the light, but we also need to know how much energy is present to understand its relative brightness. Because light is a stream of photon particles traveling at a constant rate of speed (186,000 miles per second), we can also determine the rate at which it's being transmitted, using this formula: Frequency = Speed of Light/Wavelength.

3. TRUE. But most people use the term cd/m2, which is one candela per meter squared — also equal to one nit.

4. FALSE. Because projectors are light sources, we use the term “lumens” to quantify their light output. The lumen is the fundamental unit for measurement of luminous flux, and is related to the power (watts) of a given light source. On the other hand, nits — or the number of candelas per square meter — is a unit of measurement for the luminous flux density falling on a surface of some specific area (1 square meter), such as a flat-panel display. We use cd/m2 to quantify the “brightness” of a flat-panel display.


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