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The purpose of photometry is to measure light in a way that takes the sensitivity of human visual system into account. While radiometry measures light in all spectral regions, including ultraviolet and infrared, photometry only measures in the visible spectral region from 360 nm to 830 nm, where human eyes are sensitive. Thus, photometry is essential for evaluation of light sources and objects used for lighting, signaling, displays, and other applications where light is intended to be seen by humans.

NIST's program in photometry involves maintaining the SI base unit of luminous intensity, the candela, as well as other photometric quantities; establishing national calibration standards and services for retroreflectivity; and educating the photometric community about the fundamentals and practical aspects of photometry and photometric measurements.


NIST standard photometer and NIST issued luminous intensity standard lamp.

SSL products are rapidly penetrating the huge general lighting market.  Both high and low quality SSL products exist in the market.  The United States DOE Lighting Facts program and the Commercially Available LED Product Evaluation and Reporting (CALiPER) program, United States EPA Energy Star program, and many industrial documentary standards are designed to support manufacturers and consumers to avoid market failure.  All these programs and standards rely on accurate optical metrology since performance of SSL products strongly depends on the thermal and electrical operating conditions.
NIST makes significant contributions to SSL industry for R & D, components, modules, and final products.  Activities cover

  • research for optical metrology and vision science,
  • development of calibration standard artifacts,
  • providing measurement services to industry, and
  • development of national and international industrial documentary measurement standards.

Realization of photometric quantities

The candela, SI base unit of luminous intensity, is realized with a detector-based method, dependent on the absolute responsivity of detectors. It is maintained on a group of eight standard photometers, which are calibrated annually and traceable to Division's reference cryogenic radiometer (POWR). The lumen, the unit of total luminous flux, is realized from the candela using the Absolute Integrating Sphere Method implemented in the NIST 2.5-m integrating sphere.

For more details about the realization of these quantities, see Realization of the candela, Realization of the lumen, and Realization of related photometric units.

Research and Development

The Photometry Project is actively engaged in the research of many pertinent areas in photometry.  Current projects include:

Optical Metrology

  • Calibration standards
  • LED Lifetime
  • Measurement method for HP-LEDs
  • Stray light correction method for compact array spectroradiometer

Vision science

  • Color quality
  • Flicker perception
  • Mesopic vision


The Photometry Project provides various photometric calibration services, the Measurement Assurance Program, and the NIST Photometry Short Course.

  1. The photometry program includes calibration services which provide access to the photometric scales realized and maintained at NIST. These services include lamp standards of luminous intensity, luminous flux, and color temperature; calibration services for retroreflective materials; and reference photometers and materials.  A detailed list of services is available through NIST Calibration Services at Photometric measurements.  Please contact Yuqin Zong to discuss the details of the services and obtain a nist-quote for the requested calibration.
  2. The NIST Solid State Lighting Measurement Assurance Program offers proficiency of solid state lighting products.  Now in the second phase, this MAP has provided a consistent proficiency testing for over 200 customers from all over the world in the last  9 years.  To learn more, see NIST solid State Lighting Measurement Assurance Program.
  3. The NIST Photometry Short Course covers fundamentals in photometry, radiometry, and colorimetry as well as practical aspects of measurements of luminous flux, luminous intensity, illuminance, luminance, color temperature, and chromaticity of light sources. It is offered every two years.  For more information, see NIST Photometry short course.

Stakeholders and Outreach

Many organizations and agencies have interest in the research and services performed in the Photometry Project, and concurrently we have reached out to other organizations in helping to provide standards and cutting edge research.  These organizations include:

  • Lighting industry
  • DOE - Lighting Facts
  • EPA - Energy Star
  • Documentary standards committees - CIE, ISO, ANSI, IES, CCPR, and IEA
Created November 18, 2009, Updated July 18, 2019