Accomplishments in optical radiation standards
Solid-state lighting is becoming a commercial reality. Light emitting diodes (LEDs) are being introduced into more and more products for general purpose and architectural lighting. This is significant because about 22 % of electricity consumption in the U.S. is used for lighting, and white LED lamps are expected to attain at least twice the energy efficiency of fluorescent lamps.
In support of the Department of Energy (DOE), which has set a goal of reducing by half the energy spent on lighting, NIST is developing national and international measurement standards for this emerging technology. We have developed new measurement services for LEDs, a laboratory accreditation program, and are furthering research on photometric and colorimetric aspects of solid-state lighting sources.
In particular, NIST played leadership roles in develop- ing the ANSI (American National Standards Institute) C78.377 chromaticity specifications, and the IESNA (Illuminating Engineering Society of North America) LM-79-08 standard method of photometric measurement for LED products, which have become the key standards used in DOE’s Energy Star program.
NIST scientists continue to work towards further, needed LED measurement standards in committees of ANSI, IESNA, and the CIE (International Commission on Illumination). For example, measurements of high-power LEDs have been difficult due to their strong dependence on temperature and due to differences in measurement practice in the semiconductor and lighting industries. We have developed a novel technique to measure high-power LEDs at a set junction temperature, bridging the gap between the two practices. We have also developed a new metric for color rendering of light sources, which solves the problem of applying traditional metrics to white LEDs. It is being proposed as a new international standard.
For more information, contact Yoshi Ohno.
The spectra of LED sources are dissimilar to those of traditional incandescent and discharge lamps, and some traditional lighting standards are insufficient or deficient when applied to LEDs. Thus, it is important to conduct modern vision experiments to better understand the effects of chromaticity, color rendering, and other aspects of spectra on lighting quality.
NIST’s new Spectrally Tunable Lighting Facility has been developed to enable this research. The facility consists of two room-size, furnished cubicles in which observers can be completely immersed in a real-life setting. Each cubicle is lit by 1,800 variable-power LEDs under computer control. Organized into 22 wavelength channels, the LEDs span the visible spectral range of 440 nm to 640 nm and can be set to simulate the spectra of various types of light sources. This allows testing and evaluation of prospective lighting systems as people would experience them.
NIST researchers are using the facility to develop a new color-quality scale, and to investigate optimum spectral compositions of white light to achieve both superior color quality and high energy efficiency. The results of these experiments will lead to new international standards and provide manufacturers with knowledge they need to develop higher quality products.
For more information, contact Yoshi Ohno.
In partnership with NASA and NOAA, a new radiometric source has been developed for the calibration of satellite optical sensors during the thermal-vacuum stage of testing. During this stage, the conditions on orbit are simulated. Conventional calibration approaches use large lamp-illuminated integrating spheres. However, they would not only require excessively large vacuum chambers for testing, their flux levels and spatial profiles would replicate poorly the radiation field that a satellite sensor would see on orbit.
The interagency team developed a novel, low-profile, vacuum-compatible calibration tool to fit the need. It consists of a flat-plate illuminator fed by xenon arc lamps using optical fibers. Four filter radiometers were used to monitor the spectral radiance of the four quadrants of the flat plate. A diffuser is installed to provide an improved representation of reflected solar radiation, from Earth as seen by the satellite.
The source was demonstrated to have an absolute radiometric uncertainty of 1.9 % (k = 1), sufficient to meet the 2 % requirement of the Visible Infrared Imager Radiometer Suite (VIIRS) sensor being developed for the National Polar-orbiting Operational Environmental Satellite System (NPOESS). Additionally, the team showed that the source could be used at wavelengths as long as 2.4 μm by replacing the xenon lamps with a supercontinuum source. This allows the calibrator to be used throughout the full reflective solar band, from 350 nm to 2.4 μm.
Other applications are being explored, including the calibration of astronomical telescopes, where the space between the entrance aperture of the telescope and the roof of the dome is insufficient for integrating sphere sources. Additionally, the flat-plate system is scalable to large aperture telescopes, whereas integrating spheres would become prohibitively large and expensive.
For more information, contact Steven Brown.