Yoshi Ohno

Simple Spectral Stray Light Correction Method for Array Spectroradiometers

A simple, practical method has been developed to correct a spectroradiometers response for measurement errors arising from the instruments spectral stray light. By characterizing the instruments response to a set of monochromatic laser sources that cover the instruments spectral range, one obtains a spectral stray light signal distribution matrix that quantifies the magnitude of the spectral stray light signal within the instrument. By use of these data, a spectral stray light correction matrix is derived and the instruments response can be corrected with a simple matrix multiplication. The method has been implemented and validated with a commercial CCD-array spectrograph. Spectral stray light errors after the correction was applied were reduced by 1-2 orders of magnitude to a level of approximately 10(-5) for a broadband source measurement, equivalent to less than one count of the 15-bit-resolution instrument. This method is fast enough to be integrated into an instruments software to perform real-time corrections with minimal effect on acquisition speed. Using instruments that have been corrected for spectral stray light, we expect significant reductions in overall measurement uncertainties in many applications in which spectrometers are commonly used, including radiometry, colorimetry, photometry, and biotechnology.

Approaches to color rendering measurement

Color rendering refers to a light sources ability to make the colors of illuminated objects appear natural or accurate. The color rendering index (CRI) is currently the only internationally-standardized way to assess a light sources color rendering abilities. The CRI has shortcomings in application, however, and its problems are pronounced when applied to newer lighting technologies, such as light-emitting diodes (LEDs). Since the introduction of the CRI to the present day, alternative methods have been proposed and studied. Some methods are based on the shape of the spectral output of the source, considering broadband sources to have better color rendering than sources with spectral peaks or valleys. Several proposals share the basic method of CRI, with modifications to improve performance. Still other ideas are based on measures of the gamut area of rendered object colors. The International Commission on Illumination (CIE) is in the process of developing and recommending a new metric of color rendition.

Limits on the maximum attainable efficiency for solid-state lighting

Artificial lighting for general illumination purposes accounts for over 8% of global primary energy consumption. However, the traditional lighting technologies in use today, i.e., incandescent, fluorescent, and high-intensity discharge lamps, are not very efficient, with less than about 25% of the input power being converted to useful light. Solid-state lighting is a rapidly evolving, emerging technology whose efficiency of conversion of electricity to visible white light is likely to approach 50% within the next years. This efficiency is significantly higher than that of traditional lighting technologies, with the potential to enable a marked reduction in the rate of world energy consumption. There is no fundamental physical reason why efficiencies well beyond 50% could not be achieved, which could enable even greater world energy savings. The maximum achievable luminous efficacy for a solid-state lighting source depends on many different physical parameters, for example the color rendering quality that is required, the architecture employed to produce the component light colors that are mixed to produce white, and the efficiency of light sources producing each color component. In this article, we discuss in some detail several approaches to solid-state lighting and the maximum luminous efficacy that could be attained, given various constraints such as those listed above.

Spectral matching with an LED-based spectrally tunable light source

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.

Color Quality of White LEDs

This chapter provides an overview of the fundamentals of chromaticity and color rendering, the two important aspects of color quality of light sources for general illumination. There is a special focus on the use of solid state light sources. The section on chromaticity discusses chromaticity coordinates and diagrams, correlated color temperature (CCT), Duv, and specifications for color differences. The section on color rendering discusses object color evaluation, the color rendering index (CRI) and the shortcomings thereof, and color quality beyond CRI, introducing the color quality scale (CQS), a proposed alternative metric. The chapter also discusses luminous efficacy of radiation and the color characteristics used for single-color LEDs. Finally, future considerations on color quality for white LED developments are given.

Final report on the key comparison CCPR-K5: Spectral diffuse reflectance

The CCPR K5 key comparison on spectral diffuse reflectance was carried out in the framework of the CIPM Mutual Recognition Arrangement, by 13 national metrology institutes (MMIs) as participants. The participants were CSIR-NML (South Africa), HUT (Finland), IFA-CSIC (Spain), KRISS (Republic of Korea), MSL (New Zealand), NIM (China), NIST (United States of America), NMIJ (Japan), NPL (United Kingdom), NRC (Canada), OMH (Hungary), PTB (Germany) and VNIIOFI (Russia Federation). NIST (USA) piloted the comparison. The aim of this comparison was to check the agreement of measurement of the spectral diffuse reflectance among participants, using the measurement geometry of d/0 or 0/d in the wavelength range of 360 nm to 820 nm at 20 nm increment. The comparison was a star type comparison with the samples provided by the pilot laboratory and with the measurement sequence: Pilot–Participant–Pilot. Spectralon and matte white ceramic tiles were used as the transfer standards. Each participant received three of each type of sample and at least one sample of each type was measured three times on three separate days, and the other two samples were measured once. The report presents the description of the measurement facilities, procedures and uncertainties of all the participants as well as the results of the comparison. Measurement results from the participants and their associated uncertainties were analyzed in accordance with the Guidelines for CCPR Key Comparison Report Preparation, using weighted mean with cut-off. For the calculation of the Key Comparison Reference Value (KCRV), as agreed by the participants, the data of both samples were used for the 460 nm to 820 nm region and only the data of the Spectralon samples were used in the spectral region of 360 nm to 440 nm. The unilateral degrees of equivalence (DoE) calculated for each participant are mostly consistent within the uncertainty (k = 2) of the DoE. This international comparison of spectral diffuse reflectance shows overall good agreement among all participants. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCPR, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Final report on the key comparison CCPR-K2.a-2003: Spectral responsivity in the range of 900 nm to 1600 nm

An international comparison of spectral responsivity in the near infrared region, 900 nm to 1600 nm, designated CCPR-K2.a, has been conducted under the Consultative Committee for Photometry and Radiometry (CCPR) as one of the key comparisons to support the Mutual Recognition Arrangement (MRA). This comparison was participated in by 15 laboratories and was piloted by National Institute of Standards and Technology (NIST). The comparison was carried out through calibration of a group of transfer standard detectors, which were indium gallium arsenide (InGaAs) photodiodes with sapphire windows, mounted with a thermistor. The comparison was organized in a star pattern, and conducted in four groups of participants. The report describes in detail the measurements made at NIST and summarizes the reports submitted by the participants. Key comparison reference values and degrees of equivalence have been determined from the comparison results. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCPR, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Illuminants as visualization tool for clinical diagnostics and surgery

The requirements for diagnostic and surgical lighting have remained largely unchanged over the past several years-illumination level, glare, shadow and tissue heating reduction are the dominant factors in choosing a lighting system. Since human visual perception remains the key tool in clinical diagnostics and surgery, it is worth exploring ways to heighten visual contrast between areas of interest with respect to surrounding tissues. A simulation program for predicting test illuminant spectral distribution that would enhance contrast between standard color patches typical of tissue color is used. Data images of the color patches under the predicted test illuminant as realized using a spectrally tunable source are collected. Details of the simulation program, the equipment used for this test and results of the test will be discussed.

International comparison of the illuminance responsivity scales and units of luminous flux maintained at the HUT (Finland) and the NIST (USA)

An international comparison has been conducted to compare the illuminance responsivity scales (A/lx) and the units of luminous flux (lm) maintained at the National Institute of Standards and Technology (NIST, USA) and the Helsinki University of Technology (HUT, Finland). Both laboratories realize the illuminance unit by absolutely calibrated photometers and the luminous flux unit by the absolute integrating-sphere method. Standard photometers were used as transfer standards for the illuminance responsivity comparison, and standard lamps in the luminous flux comparison. The ratio of the measured illuminance responsivity values (HUT/NIST) was 0.9992 with an expanded uncertainty (k = 2) of 0.0013, and the ratio of the measured luminous flux values was 1.0006 with an expanded uncertainty (k = 2) of 0.0018. The relative expanded uncertainties of the agreement of the units, including the uncertainties of the realizations of the units as well as the uncertainty of the comparison, were 0.0047 and 0.0101, respectively.

Correction of stray light in spectrographs: implications for remote sensing

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 instruments spectral range, the instruments 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 spectrographs 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.