Photometry (optics)

Photometry is a specialized branch of optics. It focuses on measuring light based on how it is perceived by the human eye. While science can measure the total energy of light, photometry specifically quantifies light that is emitted, reflected, transmitted, or received by an object. This field is essential because human vision does not respond to all light in the same way. By using specific mathematical models, scientists can translate raw energy into a measurement of perceived brightness.

The mechanism of photometry relies on a weighting system. The human eye is not equally sensitive to every wavelength of visible light. To account for this, photometry uses a luminosity function. This function applies a weight to the measured power at each wavelength. This weight represents how sensitive the eye is to that specific color. Most measurements use the photopic sensitivity function, which models how we see in bright light. However, other functions, like the scotopic function, can be used to model vision in the dark.

Vision changes depending on the lighting conditions of the environment. Photopic vision occurs when luminance levels are above three candela per square metre. This is our standard daytime-adapted vision. Scotopic vision occurs in very dim settings, specifically below 2 × 10⁻⁵ cd/m². This is our darkness-adapted vision. Between these two extremes lies mesopic vision. While mesopic vision exists in the middle range, its specific spectral response is not well characterized. Because photometry is usually based on the photopic response, it might not perfectly match how we see dim moonlight or starlight.

Photometry is closely related to radiometry, but they are distinct systems. Radiometry measures the absolute power of all electromagnetic radiation, including invisible types like infrared. Photometry is a subset of radiometry that only considers the visible spectrum and weights it by human sensitivity. Every photometric quantity has a corresponding radiometric analog. For example, luminous flux is the photometric version of radiant flux. Similarly, luminance is the photometric analog to radiance. Luminous intensity also corresponds to radiant intensity.

The history of measuring radiation began in the late 18th century. Scientists developed different units based on the specific effects they observed. Measuring the heating effect of infrared radiation with thermometers led to radiometric units. Studying the chemical effects of ultraviolet radiation led to actinometric units, measured in photons per second. Using the human eye as a detector led to the creation of photometric units. Today, the CIE and ISO provide the standards for these weighting functions to ensure measurements are consistent globally.

Understanding the difference between watts and lumens is vital. A watt is a unit of radiant flux, representing total power or energy used. A lumen is a unit of luminous flux, representing perceived light output. For instance, a 60 watt incandescent bulb and a 15 watt compact fluorescent lamp can both provide 900 lumens. However, the incandescent bulb is much less efficient. Most of its energy is emitted as invisible infrared radiation. In fact, 700 nm red light is only about 0.4% as efficient as 555 nm green light in the human eye. This means one watt of red light provides only about 2.7 lumens, while one watt of 555 nm green light provides 683 lumens.

Scientists use various tools called photodetectors to perform these measurements. These devices produce an electric signal when exposed to light. Simple photodetectors can automatically switch lights on or off based on ambient brightness. In the lighting industry, more complex tools are required. A spherical photometer uses a large globe to measure light from all sides. To test specific fixtures, engineers use goniophotometers or rotating mirror photometers. These tools allow experts to map how light spreads, helping them design better lighting for homes and offices.