Principles of Light Measurement

The practice of light measurement delves into some complex areas: optics, electronics, the physics of electromagnetic radiation and the workings of the human eye. You can, however, simplify the situation by asking some general questions. Your approach to measurement depends on whether you're concerned about what the human eye sees or if you want to know about the raw physical phenomena, including infrared and ultraviolet light. Once you've narrowed it down, you can focus on doing the measuring.

Photometry

When you're measuring light the way people see it, it's the realm of optical photometry. It deals with the visible light spectrum and ignores infrared and ultraviolet light. In addition, it follows the sensitivity of the eye to some colors, emphasizing green, for example. Photometry, photography, video and printing are connected; they're all about how we perceive color and light.

Radiometry

Radiometry accounts for all the light radiated or reflected from an object, whether we see it or not. This is useful for astronomy, heat lamps, tanning beds---anywhere the eye's perception is secondary or unimportant. This is important when, for example, you compare the efficiency of an incandescent light and a CFL bulb: the compact fluorescent puts out more visible light from fewer watts of power. The incandescent gives off a lot of infrared, which, for lighting, isn't useful.

Brightness

The brightness of an object can mean more than one thing. It can be the light's intensity, or it can be the area the light covers. A pinpoint of light can be bright but cover a tiny area. A large area can have a lot of light without being intense. Photometric light intensity is measured in candelas; perceived light power is measured in lumens. This is in contrast to radiometric energy and power measured in joules and watts.

Color

The light's spectrum gives its color. Spectrometers and related devices show the strength of the light in a spread of wavelengths. The strength of a particular color or wavelength can also be measured with the use of filters. Putting a filter on a detector to block out undesired colors is one way to measure the brightness of one color. The study of the perception of color is called colorimetry. It breaks color into mixtures of red, green and blue, assigning a relative value to each primary color in the mix.

Devices

Before photography, the human eye was the first light-measuring device. With the invention of photographic plates, people could develop more objective standards of measurement. Also, photography allowed long exposure times, letting people record objects too faint to be seen otherwise. The discovery of the photoelectric effect in 1887 led to electronic light-measuring devices. Light could be measured with greater precision. The photomultiplier tube, an extremely sensitive device still in use, came about in the 1930s and is still used in the sciences. The charge-coupled device, or CCD, invented in the late 1960s, senses light with great precision and at low cost. They can be found in fax machines, video cameras, cell phones---almost anything that takes a picture.