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.
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
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.
Research and Development
The Photometry Project is actively engaged in the research of many pertinent areas in photometry. Current projects include:
The Photometry Project provides various photometric calibration services, the Measurement Assurance Program, and the 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: