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Light Measurement

Light provides humans with stereoscopic images of the world at all distances. This entry considers measuring light as it affects human vision. Humans are most sensitive to electromagnetic radiation in a small window between 400 and 700 nanometer (nm) wavelengths (λ). This response begins with four types of retinal receptors. After sitting in a light-free room for an hour, humans report seeing light with only four to six photons. Snow on a mountaintop sends to the eye 100 million times more photons. Although photographic film has a fixed, unique response to the number of photons/area, human visual appearance has a complex spatial relationship to the light on the retina. Nevertheless, light measurement in psychophysics is important: first, to accurately describe the display presented to observers, so others can reproduce the experiment; and second, to describe the light array, pixel (picture element) by pixel, of the entire field of view, as input for computer appearance models.

Radiometry

Radiometry, a part of physics, describes standards for measuring electromagnetic radiation. Photons with different wavelengths have different energies. From Planck's law, we can calculate that a single 555-nm photon's energy equals 3.6 × 10-19 joules (or, watt × seconds).

Irradiance is the measure of the energy from the number of photons continuously falling on an area. Irradiance meters measure (watts/centimeter [cm]2) using photosensitive electronic elements behind a diffuser integrating the light falling on the meter. Photographers use similar incident light meters to measure the light falling on the scene by standing in front of the subject and aiming the meter at the light source.

Radiance is the measurement of light continuously coming from a particular object to a particular point in space. Ansel Adams' zone system described techniques to capture and print the entire range of all scenes. He measured the light from clouds, sky, rocks, trees, and shadows using a spot photometer, a camera-like instrument that forms an image of a small scene segment on the light-sensitive element. Such telephotometers measure photons per area per angle (watts/[cm2 × steradian]). By replacing the average of the entire scene with individual values for maximum light, minimum light, and objects of interest, the photographer can control the image-making processes to render his or her visualization of the scene.

Photometry

Photometry, a part of psychophysics, is radiometry adjusted for the wavelength sensitivity standards for human vision. The photopic luminosity standard (Vλ) is the nominal sensitivity of the eye to different wavelengths. The peak sensitivity is at 555 nm (100%); half-height (50%) at 510 and 610 nm; (3%) at 446 and 672 nm. The candela/meter2 (cd/m2), the standard of luminous radiance, is 1/683 watts per area per angle at 555 nm. This standard is the basis of lumens per watt used in evaluating the electrical efficiency of lamps (Lumen = [candela × angle]).

To convert the radiometric units (physics) irradiance and radiance to photometric units (psychophysics) of illuminance and luminance, we multiply the radiometric values at each wavelength by the Commission Internationale de l'Eclairage (CIE) standard Vλ curve value of those wavelengths. Illuminance is the integral of (irradianceλ × Vλ); luminance is the integral (radianceλ × Vλ). Before 1930, the term brightness was defined as both the measurement of light and the measurement of appearance. The term luminance was introduced to measure light and to reduce the confusion caused by the two usages of brightness with different meanings.

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