What Is Photometry?

Photometry is the science of measuring light as perceived by the human visual system, quantifying radiation in terms of its ability to evoke brightness sensations in the human eye. It differs from radiometry, which measures radiant energy in absolute physical terms across all electromagnetic wavelengths, by weighting the measured power by the spectral luminous efficiency function V(λ), a standardized model of the eye's sensitivity that peaks near 555 nm in the photopic (daylight) vision regime. The result is a set of human-response-weighted quantities, including luminous flux, luminous intensity, illuminance, and luminance, that predict how bright a source appears and how well it illuminates a surface. These quantities are expressed in units of the lumen, candela, lux, and candela per square meter, respectively.

Photometry draws on optics, vision science, and metrology. Its foundations were laid in the eighteenth and nineteenth centuries through the work of Pierre Bouguer and Johann Heinrich Lambert, who established the inverse-square law of illuminance and the Lambert cosine law of diffuse emission. Modern photometry is governed by the International Commission on Illumination (CIE), which maintains the standard observer functions and defines the photometric unit system adopted by the International Bureau of Weights and Measures.

Photometric Quantities and Units

The candela, the SI base unit of luminous intensity, is defined in terms of a fixed numerical value of the luminous efficacy of monochromatic radiation at 540 THz, which is 683 lm/W at that frequency. All other photometric units derive from the candela. The NIST photometry program maintains the national realization of the candela using a group of standard photometers traceable to the primary optical watt radiometer, and provides calibration services for lamps, photometers, and reflectance standards used by industry and research laboratories. Luminous flux, measured in lumens, represents the total luminous power emitted by a source integrated over all directions; illuminance, measured in lux, is luminous flux per unit area at a receiving surface; and luminance, measured in candela per square meter, characterizes the brightness of an extended source or surface as seen by an observer.

Radiometry and the Relationship to Photometry

Radiometry provides the physical foundation on which photometry is built. Radiometric quantities such as radiant flux (watts), irradiance (W/m²), and radiance (W/m²·sr) describe electromagnetic energy without reference to human perception. Converting radiometric to photometric quantities requires multiplying the spectral power distribution of the source by the V(λ) function and integrating over wavelength. For the scotopic (low-light) regime, a separate function V'(λ) applies, peaking near 507 nm, reflecting the shift in spectral sensitivity from cone photoreceptors to rod photoreceptors. The NIST realization of the candela describes how detector-based methods anchor the photometric scale to absolute radiometric measurements, achieving luminous intensity uncertainties below 0.5% at the primary level.

Light Sources and Photometric Measurement

Photometric testing of light sources quantifies parameters such as luminous efficacy (lumens per watt), color temperature, color rendering index (CRI), and luminous flux. These parameters are measured using integrating spheres, goniophotometers, or array spectroradiometers depending on the source geometry and the required accuracy. Solid-state light sources, particularly LEDs, present measurement challenges because their output spectrum, flux, and color shift with drive current, temperature, and aging. The NIST solid-state lighting measurement framework describes calibration procedures and uncertainty budgets for LED photometry, including the correction factors required for goniophotometric systems when measuring non-Lambertian emission patterns.

Applications

Photometry has applications in a wide range of disciplines and industries, including:

• Architectural and interior lighting design: ensuring adequate illuminance levels for visual comfort and safety
• Display and monitor calibration: specifying luminance, contrast ratio, and color accuracy
• Automotive lighting: headlamp and signal lamp compliance with regulatory standards
• Astronomy and astrophysics: measuring stellar brightness and the luminosity of celestial objects
• Safety and signage: verification of emergency exit lighting and visibility of road markings