The relative spectral radiant flux error caused by phosphor fluorescence during integrating sphere measurements is
investigated both theoretically and experimentally. Integrating sphere and goniophotometer measurements are compared
and used for model validation, while a case study provides additional clarification. Criteria for reducing fluorescence
errors to a degree of negligibility as well as a fluorescence error correction method based on simple matrix algebra are
presented. Only remote phosphor type LED light sources are studied because of their large phosphor surfaces and high
application potential in general lighting.
The color rendering index (CRI) has been shown to have deficiencies when applied to white light-emitting-diode-based sources. Furthermore, evidence suggests that the restricted scope of the CRI unnecessarily penalizes some light sources with desirable color qualities. To solve the problems of the CRI and include other dimensions of color quality, the color quality scale (CQS) has been developed. Although the CQS uses many of elements of the CRI, there are a number of fundamental differences. Like the CRI, the CQS is a test-samples method that compares the appearance of a set of reflective samples when illuminated by the test lamp to their appearance under a reference illuminant. The CQS uses a larger set of reflective samples, all of high chroma, and combines the color differences of the samples with a root mean square. Additionally, the CQS does not penalize light sources for causing increases in the chroma of object colors but does penalize sources with smaller rendered color gamut areas. The scale of the CQS is converted to span 0-100, and the uniform object color space and chromatic adaptation transform used in the calculations are updated. Supplementary scales have also been developed for expert users.
Alternating-current (AC) driven high-power light-emitting diodes (LEDs) have become available and introduced into
solid-state lighting (SSL) products. AC LEDs operate directly from a mains supply with no need of drivers, and thus can
simplify the design of SSL product and potentially increase product's reliability and lifetime. Similar to direct-current
(DC) LEDs the optical and electrical properties of AC LEDs are strongly dependent on the LED junction temperature. In
addition, the instantaneous junction temperature of an AC LED changes rapidly within an AC power cycle. Accurate
measurement of AC high-power LEDs is required for quality control and product qualifications such as the US Energy
Star. We have developed a simple, robust method for measurement of high-power AC LEDs at any specified junction
temperature under a normal AC operating condition. An active heat sink is used for setting and controlling the junction
temperature of the test AC LED. By using this measurement technique, the measurement of an AC LED also obtains the
thermal resistance between the LED junction and the LED heat sink.
The performance of Light Emitting Diodes (LEDs), including efficiency, flux level, lifetime, and the variation of color, is advancing at a remarkable pace. LEDs are increasingly used for many applications including automotive, aviation, display, transportation and special lighting applications. White LEDs are expected for general lighting applications (solid state lighting) in the near future. Thus, accurate measurements of LEDs and appropriate standards are increasingly important. This paper reviews photometric, radiometric, and colorimetric quantities used for LEDs, and discusses the current state of optical measurements of LEDs and standardization efforts in International Commission on Illumination (CIE). The paper also touches on the issue of color quality (e.g., Color Rendering Index) of light expected from solid state lighting, and the need for a new metric. The calibration facilities and services for LEDs established at NIST are also discussed.
A spectrally tunable light source using a large number of LEDs and an integrating sphere has been designed and is being constructed at the National Institute of Standards and Technology. The source is designed to have a capability of producing any spectral distribution, mimicking various light sources in the visible region by feedback control of individual LEDs. The output spectral irradiance or radiance of the source will be calibrated by a reference instrument, and the source will be used as a spectroradiometric as well as a photometric and colorimetric standard. A series of simulations have been conducted to predict the performance of the designed tunable source when used for calibration of display colorimeters. The results indicate that the errors can be reduced by an order of magnitude when the tunable source is used to calibrate the colorimeters, compared with measurement errors when the colorimeters are calibrated against Illuminant A. The source can also approximate various CIE daylight illuminants and common lamp spectral distributions for other photometric and colorimetric applications.
White LED spectra for general lighting should be designed for high luminous efficacy as well as good color rendering. Multichip and phosphor-type white LED models were analyzed by simulation of their color characteristics and luminous efficacy of radiation, compared with those of conventional light sources for general lighting. Color rendering characteristics were evaluated based on the CIE Color Rendering Index (CRI), examining not only Ra but also the special color rendering indices Ri, as well as on the CIELAB color difference E for the 14 color samples defined in CIE 13.3. Several models of three-chip and four-chip white LEDs as well as phosphor-type LEDs are optimized for various parameters, and some guidance is given for designing these white LEDs. The simulation analysis also demonstrated several problems with the current CRI, and the need for improvements is discussed.
Several aspects of the Color Rendering Index (CRI) are flawed, limiting its usefulness in assessing the color rendering capabilities of LEDs for general illumination. At NIST, we are developing recommendations to modify the CRI that would overcome these problems. The current CRI is based on only eight reflective samples, all of which are low to medium chromatic saturation. These colors do not adequately span the range of normal object colors. Some lights that are able to accurately render colors of low saturation perform poorly with highly saturated colors. This is particularly prominent with light sources with peaked spectral distributions as realized by solid-state lighting. We have assembled 15 Munsell samples that overcome these problems and have performed analysis to show the improvement. Additionally, the CRI penalizes lamps for showing increases in object chromatic saturation compared to reference lights, which is actually desirable for most applications. We suggest a new computation scheme for determining the color rendering score that differentiates between hue and saturation shifts and takes their directions into account. The uniform color space used in the CRI is outdated and a replacement will be recommended. The CRI matches the CCT of the reference to that of the test light. This can be problematic when lights are substantially bluish or reddish. Lights of extreme CCTs are frequently poor color renderers, though they can score very high on the current CRI. An improved chromatic adaptation correction calculation would eliminate the need to match CCT and an updated correction is being considered.
A spectrally tunable light source using an integrating sphere with a large number of LEDs has been designed and constructed at the National Institute of Standards and Technology (NIST). The source is designed to have a capability of producing any visible spectral distribution, mimicking various light sources in the visible region by feedback control of the radiant power emitted by individual LEDs. The spectral irradiance or radiance of the source is measured by a standard reference instrument; the source will be used as a transfer standard for colorimetric, photometric and radiometric applications. A series of simulations have been conducted to predict the performance of the designed tunable source and source distributions have been realized for a number of target distributions.
Spectrographs are used in a variety of applications in the field of remote sensing for radiometric measurements due to the benefits of measurement speed, sensitivity, and portability. However, spectrographs are single grating instruments that are susceptible to systematic errors arising from stray radiation within the instrument. In the application of measurements of ocean color, stray light of the spectrographs has led to significant measurement errors. In this work, a simple method to correct stray-light errors in a spectrograph is described. By measuring a set of monochromatic laser sources that cover the instrument's spectral range, the instrument's stray-light property is characterized and a stray-light correction matrix is derived. The matrix is then used to correct the stray-light error in measured raw signals by a simple matrix multiplication, which is fast enough to be implemented in the spectrograph's firmware or software to perform real-time corrections: an important feature for remote sensing applications. The results of corrections on real instruments demonstrated that the stray-light errors were reduced by one to two orders of magnitude, to a level of approximately 10-5 for a broadband source measurement, which is a level less than one count of a 15-bit resolution instrument. As a stray-light correction example, the errors in measurement of solar spectral irradiance using a highquality spectrograph optimized for UV measurements are analyzed; the stray-light correction leads to reduction of errors from a 10 % level to a 1 % level in the UV region. This method is expected to contribute to achieving a 0.1 % level of uncertainty required for future remote-sensing applications.
A spectrally tunable light source using a large number of LEDs and an integrating sphere has been designed and being developed at NIST. The source is designed to have a capability of producing any spectral distributions mimicking various light sources in the visible region by feedback control of individual LEDs. The output spectral irradiance or radiance of the source will be calibrated by a reference instrument, and the source will be used as a spectroradiometric as well as photometric and colorimetric standard. The use of the tunable source mimicking spectra of display colors, for example, rather than a traditional incandescent standard lamp for calibration of colorimeters, can reduce the spectral mismatch errors of the colorimeter measuring displays significantly. A series of simulations have been conducted to predict the performance of the designed tunable source when used for calibration of colorimeters. The results indicate that the errors can be reduced by an order of magnitude compared with those when the colorimeters are calibrated against Illuminant A. Stray light errors of a spectroradiometer can also be effectively reduced by using the tunable source producing a blackbody spectrum at higher temperature (e.g., 9000 K). The source can also approximate various CIE daylight illuminants and common lamp spectral distributions for other photometric and colorimetric applications.
Various new light-emitting diodes (LEDs) including white LEDs are being actively developed for solid-state lighting and many other applications, and there are great needs for accurate measurement of various optical quantities of LEDs. Traditional lamp standards do not suffice for specific measurement needs for LEDs. The National Institute of Standards and Technology (NIST) has recently established calibration services for photometric quantities (luminous intensity and luminous flux) of LEDs, but the measurement needs are expanding. This paper covers the current capabilities and services NIST provides for calibration of LEDs and discusses the future needs for optical metrology of LEDs. Work is just completed at NIST to provide official color calibrations of LEDs (chromaticity coordinates, peak wavelength, correlated color temperature, etc.). Another urgent need addressed is radiometric calibration of LEDs, particularly the total radiant flux (watt) of ultraviolet (UV) LEDs used to excite phosphors for white LEDs. Also, as spectroradiometers coupled with an integrating sphere are increasingly used total spectral radiant flux standards from NIST are in urgent demand. Presented is the scope of NIST plans to realize these new radiometric calibration capabilities for LEDs in the near future.
White LED spectra for general lighting should be designed for high luminous efficacy as well as good color rendering. Multi-chip and phosphor-type white LED models were analyzed by simulation on their color characteristics and luminous efficacy of radiation, compared with those of conventional light sources for general lighting. Color rendering characteristics were evaluated based on the CIE Color Rendering Index (CRI), using not only Ra but also the special color rendering indices Ri as well as the CIELAB color difference ΔE*ab for the 14 color samples defined in CIE 13.3. Several models of 3-chip and 4-chip white LEDs as well as phosphor-type LEDs are optimized for various parameters, and some guidance is given for designing these white LEDs. The simulation analysis also demonstrated several problems with the current CIE Color Rendering Index (CRI), and the need for improvements is discussed.
A facility has been developed at the National Institute of Standards and Technology (NIST) to provide calibration services for color-measuring instruments to address the need for improving and certifying the measurement uncertainties of this type of instrument. While NIST has active programs in photometry, flat panel display metrology, and color and appearance measurements, these are the first services offered by NIST tailored to color-measuring instruments for displays. An overview of the facility, the calibration approach, and associated uncertainties are presented. Details of a new tunable colorimetric source and the development of new transfer standard instruments are discussed.
We have developed a tunable laser-based facility for the absolute radiometric calibration of digital imaging system such as CCD cameras, spectrographs, and microscopes. Several types of silicon-based digital imaging systems have been calibrated in this new facility, including a commercially available camera equipped with a removable photopic filter, a custom-designed digital microscope, and a CCD spectrography. We present result of the CCD camera calibration in detail and discuss relevant aspects of the microscope and spectrograph calibrations. During the radiometric calibration, the pixel-to-pixel uniformity, linearity, and absolute spectral responsivity of each system were determined over the visible spectral range. Each of these aspects of the CCD camera calibration will be presented, along with a discussion of the measurement uncertainties.
A prototype display measurement assessment transfer standard (DMATS) is being developed by the NIST to assist the display industry in standardizing measurement methods used to quantify and specify the performance of electronic display. Designed as an idealized electronic display, the DMATS illumination source emulates photometric and colorimetric measurement problems commonly encountered in measuring electronic displays. NIST will calibrate DMATS units and distribute them to participating laboratories for measurement. Analysis of initial interlaboratory comparison results will provide a baseline assessment of display measurement uncertainties. Also, diagnostic indicators expected to emerge from the data will be used to assist laboratories in correcting deficiencies or in identifying metrology problem areas for further research, such as measurement techniques tailored to new display technologies. This paper describes the design and construction of a prototype DMATS source and preliminary photometric and colorimetric characterization. Also, this paper compares measurements obtained by several instruments under constant environmental conditions and examines the effects of veiling glare on chromaticity measurements.
A calibration facility has been developed at the National Institute of Standards and Technology (NIST) to address the need for high accuracy color measurements of displays. Calibration services are planned for colorimeters and spectroradiometers, tailored to display measurements. A key component of the facility, a reference spectroradiometer, has been developed and its uncertainty for display measurements estimated using a series of computer simulations. The simulations predict that the reference spectroradiometer - corrected for wavelength error and variable bandpass - can measure any color of a cathode ray tube or liquid crystal display with a combined standard uncertainty of approximately 0.001 in chromaticity (x,y) and 1 percent in luminance (Y). In addition a new matrix correction technique has been developed as a means to transfer the calibration from the reference instrument to a test instrument. Using the four-color method, the residual errors with the calibrated instrument for one type of display are reduced to within 0.001 in x,y with respect to the reference instrument. To evaluate the overall performance of the system, commercial instruments - spectroradiometers and tristimulus colorimeter - were calibrated against the reference instrument, measuring both a CRT and an LCD display. The results show that calibrated target instruments can measure various colors of a particular display with a combined standard uncertainty of approximately 0.002 in x,y and 2 percent in Y.
There is a need for accurate measurement of flashing lights for the proper maintenance of aircraft anticollision lights. A large variation in the measured intensities of anticollision lights has been a problem, and thus, NIST has undertaken the task to establish flashing-light photometric standards to provide calibration services in this area. A flashing-light photometric unit [lux second, (lx (DOT) s)] has been realized based on the NIST detector-based candela, using four standard photometers equipped with current integrators. Two different approaches have been taken to calibrate these standard photometers: one based on electrical calibration of the current integrator, and the other based on electronic pulsing of a steady-state photometric standard. The units realized using these two independent methods agreed to within 0.2%. The relative expanded uncertainty (k equals 2) of the standard photometers, in the measurement of the white xenon flash, is estimated to be 0.6%. The standard photometers are characterized for temporal response, linearity, and spectral responsivity, to be used for measurement of xenon flash sources of various waveforms and colors. Calibration services have been established at NIST for flashing-light photometers with white and red anticollision lights.