Photometry is the science of measuring light as the human eye perceives it. Unlike radiometry, which measures all electromagnetic radiation in raw physical power (watts), photometry specifically measures visible light weighted by how sensitive your eyes are to different colors. This distinction matters because your eyes don’t respond equally to all wavelengths. A watt of green light looks far brighter to you than a watt of deep red or violet, and photometry accounts for that.
How Photometry Differs From Radiometry
Radiometry covers the full optical spectrum, from ultraviolet through visible light to infrared, spanning wavelengths from 10 nanometers to 1,000 micrometers. It measures energy output in watts without any regard for whether a human could see that light. Photometry narrows the focus to the visible spectrum (roughly 360 to 830 nanometers) and weights every measurement by how the eye actually responds.
This weighting comes from a standardized model of human vision called the luminous efficiency function, adopted by the International Commission on Illumination in 1924. Researchers tested many observers to map how sensitive the average eye is at each wavelength. The result is a bell-shaped curve that peaks at 555 nanometers, which falls in the yellow-green part of the spectrum. At that peak, one watt of light produces 683 lumens. Move toward red or violet, and the same watt produces progressively fewer lumens because your eye is less sensitive there. A separate curve exists for night vision (scotopic vision), which peaks at 507 nanometers and skews toward blue, reflecting how your eyes shift sensitivity in low light.
The Core Units of Photometry
Photometry uses a handful of interrelated units, each describing a different aspect of how light is produced, travels, and lands on surfaces.
- Candela (cd): The base SI unit of luminous intensity, describing how strong a light source appears in a specific direction. Its formal definition is tied to the luminous efficacy of monochromatic radiation at a frequency of 540 × 10¹² Hz (green light), fixed at exactly 683 lumens per watt.
- Lumen (lm): The total amount of visible light a source emits in all directions. A light source rated at one candela that radiates uniformly in every direction produces a total of about 12.6 lumens (4π candelas). Lumens are what you see on lightbulb packaging.
- Lux (lx): The amount of light landing on a surface, measured in lumens per square meter. If you spread one lumen evenly across one square meter, that surface receives one lux of illumination.
- Nit (cd/m²): A measure of how bright a surface or screen appears to you, formally called luminance. One nit equals one candela per square meter. This is the unit used to rate display brightness on phones, monitors, and TVs.
These units connect through straightforward relationships. Illuminance from a point source drops with distance following the inverse square law: double your distance from a lamp and the light reaching you falls to one quarter. At one meter from a one-candela source, you measure one lux. At two meters, you measure 0.25 lux.
How Photometric Measurements Are Made
One of the most common laboratory tools for photometry is the integrating sphere. This is a hollow sphere coated on the inside with a highly reflective white material. When a light source is placed inside, the sphere bounces and mixes the light in every direction until the illumination on its inner wall is uniform. A detector mounted on the sphere wall then measures this averaged light level. By comparing the reading against a known reference lamp, technicians can calculate the total luminous flux of the test source. Every surface inside the sphere, including baffles and lamp holders, gets coated to minimize light absorption, since even small losses affect accuracy.
For simpler measurements, handheld lux meters use a sensor filtered to mimic the human eye’s sensitivity curve. These are common in workplace assessments, photography, and building design. OSHA guidelines, for instance, recommend office lighting between roughly 200 and 540 lux for paper tasks and computer work, with somewhat higher levels when LCD monitors are in use.
Photometry in Astronomy
Astronomers were among the earliest practitioners of photometry, and they still use a system rooted in ancient Greece. The magnitude scale, inherited from the astronomer Hipparchus, ranks stars so that brighter objects get smaller numbers and fainter objects get larger ones. Each step of one magnitude corresponds to a brightness change of about 2.5 times. Two stars differing by five magnitudes are exactly 100 times different in brightness.
Modern astronomical photometry uses calibrated detectors and standardized color filters to measure starlight precisely. By comparing a star’s brightness through different filters (capturing blue, green, and red portions of the spectrum separately), astronomers can determine a star’s surface temperature and chemical composition. A star that appears brighter through a blue filter than a red one is hotter than one that shows the opposite pattern. This technique, called multiband photometry, is one of the most efficient ways to classify millions of stars in large sky surveys.
Photometry in Chemistry and Medicine
Flame photometry is a laboratory technique that identifies and measures certain elements by the color of light they emit when heated. When a sample containing metals like sodium or potassium is sprayed into a flame, each element emits light at a characteristic wavelength: sodium glows at 589 nanometers (yellow-orange), potassium at 766.5 nanometers (red). A detector measures the intensity of that emitted light, which is proportional to the element’s concentration in the sample. This method is especially sensitive for alkali metals and is still used in clinical labs to measure sodium and potassium levels in blood, where precision at very low concentrations matters.
Everyday Uses of Photometric Data
Photometric units show up in more places than most people realize. When you compare lightbulbs, the lumen rating tells you how much visible light the bulb produces, which is far more useful than wattage for judging brightness across LED, fluorescent, and incandescent options. When you shop for a phone or monitor, the nit rating tells you how well the screen will perform in bright environments. A typical office display runs between 250 and 350 nits. Outdoor digital signage needs 2,000 to 4,000 or more nits to remain readable in direct sunlight.
Lighting designers use lux measurements to ensure spaces meet safety and comfort standards. A warehouse needs far less illumination than a surgical suite, and photometry provides the common language to specify, measure, and verify those levels. Photographers rely on incident and reflected light meters, both photometric instruments, to set correct exposures. Even automotive headlamp regulations are defined in candelas and lux, specifying how intense the beam must be and how much light must reach the road surface at given distances.
At its core, photometry bridges physics and biology. It takes the raw energy of electromagnetic radiation and translates it into numbers that reflect what humans actually see, making it indispensable across science, industry, and daily life.