Photometry (optics)

Photometry is the part of optics focused on measuring light as the human eye perceives its brightness. It looks at how much light is given off, bounced off surfaces, passed through materials, or taken in by an object or setup. This field is a subset of radiometry, but what sets it apart is the use of standardized weighting functions—like the photopic sensitivity curve—that mimic how the human visual system responds to different wavelengths. These weightings are set by the CIE and ISO.

The human eye doesn't see all wavelengths of visible light equally well. Photometry accounts for this by adjusting the measured power at each wavelength according to a factor that reflects the eye's sensitivity there. This standardized model is called the luminosity function. The eye's response changes depending on whether it's adapted to bright conditions (photopic vision) or dark ones (scotopic vision). Most photometric measurements rely on the photopic response, so they might not accurately show how bright a source looks in dim light, like under moonlight or starlight, where colors are hard to make out. Photopic vision kicks in at luminance levels above three candela per square meter, while scotopic vision takes over below 2 × 10⁻⁵ cd/m². In between is mesopic vision, whose spectral response isn't well defined.

The study of measuring electromagnetic radiation's effects goes back to the late 18th century. Different effects led to different measurement systems. The heating effect of infrared, measured with thermometers, gave rise to radiometric units based on total energy and power. Using the human eye as a detector led to photometric units, weighted by the eye's response. The chemical effects of ultraviolet radiation led to actinometric units, expressed in photons per second. Photometric measurements use many different units. The word "bright" can mean a light source with high luminous flux (lumens), one that focuses its flux into a narrow beam (candelas), or one seen against a dark background. Because light spreads, concentrates, and reflects off various surfaces, and because it contains many wavelengths, there are many fundamentally different kinds of light measurements, along with their quantities and units. For instance, an office is typically "brightly" lit by many recessed fluorescent lights giving a high combined luminous flux. A laser pointer has very low luminous flux—it can't light up a room—but it's blindingly bright in one direction, meaning high luminous intensity there.

There are two parallel systems: photometric and radiometric quantities. Each photometric quantity has a radiometric counterpart. Examples include luminance (photometric) versus radiance (radiometric), luminous flux versus radiant flux, and luminous intensity versus radiant intensity. In photometric quantities, every wavelength is weighted by how sensitive the human eye is to it, while radiometric quantities use unweighted absolute power. The eye is much more sensitive to green light than red, so a green source will have a higher luminous flux than a red source with the same radiant flux. Radiant energy outside the visible spectrum doesn't contribute to photometric quantities at all. A 1000-watt space heater, for example, puts out a lot of radiant flux (1000 watts), but as a light source it produces very few lumens because most of its energy is in the infrared, leaving only a dim red glow.

Watts measure radiant flux, while lumens measure luminous flux, and comparing them shows the difference between radiometric and photometric units. The watt is a unit of power. People often think of light bulbs in terms of watts, but this indicates energy use, not light output. Because incandescent bulbs for general service have similar spectral power distributions, power consumption gives a rough idea of light output. In radiometric terms, an incandescent bulb is about 80% efficient: 20% of the energy is lost, and the rest is emitted as radiation, mostly infrared. So a 60-watt bulb emits a total radiant flux of about 45 watts. Incandescent bulbs are sometimes used as heat sources, like in chick incubators, but when used for light, they're very inefficient because most of the radiant output is invisible.