Joanne C. Zwinkels

Photometry, radiometry and 'the candela': evolution in the classical and quantum world

The metrological fields of photometry and radiometry and their associated units are closely linked through the current definition of the base unit of luminous intensity—the candela. These fields are important to a wide range of applications requiring precise and accurate measurements of electromagnetic radiation and, in particular, the amount of radiant energy (light) that is perceived by the human eye. The candela has been one of the base units since the inception of the International System of Units (SI) and is the only base unit that quantifies a fundamental biological process—human vision. This photobiological process spans an enormous dynamic range of light levels from a few-photon interaction involved in triggering the vision mechanism to a level of more than 1015 photons per second that is accommodated by the visual response under bright daylight conditions. This position paper, prepared by members of the Task Group on the SI of the Consultative Committee for Photometry and Radiometry Strategic Planning Working Group (CCPR WG-SP), reviews the evolution of these fields of optical radiation measurements and their consequent impact on definitions and realization of the candela. Over the past several decades, there have been significant developments in sources, detectors, measuring instruments and techniques, that have improved the measurement of photometric and radiometric quantities for classical applications in lighting design, manufacturing and quality control processes involving optical sources, detectors and materials. These improved realizations largely underpin the present (1979) definition of the candela. There is no consensus on whether this radiant-based definition fully satisfies the current and projected needs of the optical radiation community. There is also no consensus on whether a reformulation of the definition of the candela in terms of photon flux will be applicable to the lighting community. However, there have been significant recent advances in radiometry in the development of single-photon sources and single-photon detectors and the growth of associated technologies, such as quantum computing and quantum cryptography. The international acceptance of these new quantum-based technologies requires improved traceability and reliability of measurements at the level of a few photons. This review of the evolution of the candela and the impact of its possible reformulation might lead, in the future, to a reformulation in terms of quantum units (photons). This discussion is timely since redefinitions of four of the other SI base units are being considered now in terms of fundamental constants to provide a more universally realizable quantum-based SI system. This paper also introduces for the first time a fundamental constant for photometry.

Colour-measuring instruments and their calibration

The types of instruments used for colour measurement of selfluminous and surface colours, and the choices available for their optical design, are described. Guidelines are given for selection of measurement conditions appropriate to different measurement applications. The important steps in calibrating and verifying the performance of colour-measuring instruments are described and the standards available for these procedures are discussed in detail.

State-of-the Art Comparability of Corrected Emission Spectra.1. Spectral Correction with Physical Transfer Standards and Spectral Fluorescence Standards by Expert Laboratories

The development of fluorescence applications in the life and material sciences has proceeded largely without sufficient concern for the measurement uncertainties related to the characterization of fluorescence instruments. In this first part of a two-part series on the state-of-the-art comparability of corrected emission spectra, four National Metrology Institutes active in high-precision steady-state fluorometry performed a first comparison of fluorescence measurement capabilities by evaluating physical transfer standard (PTS)-based and reference material (RM)-based calibration methods. To identify achievable comparability and sources of error in instrument calibration, the emission spectra of three test dyes in the wavelength region from 300 to 770 nm were corrected and compared using both calibration methods. The results, obtained for typical spectrofluorometric (0°/90° transmitting) and colorimetric (45°/0° front-face) measurement geometries, demonstrated a comparability of corrected emission spectra within a relative standard uncertainty of 4.2% for PTS- and 2.4% for RM-based spectral correction when measurements and calibrations were performed under identical conditions. Moreover, the emission spectra of RMs F001 to F005, certified by BAM, Federal Institute for Materials Research and Testing, were confirmed. These RMs were subsequently used for the assessment of the comparability of RM-based corrected emission spectra of field laboratories using common commercial spectrofluorometers and routine measurement conditions in part 2 of this series (subsequent paper in this issue).

Optical dispersion relationships in amorphous silicon grown by molecular beam epitaxy

We have produced a novel form of amorphous silicon (a-Si) using ultra-high-vacuum molecular beam epitaxy (MBE). From measurements of the specular reflectance spectrum at near normal incidence and the regular transmittance spectrum at normal incidence we have determined the spectral dependence of the refractive index, the extinction coefficient, the optical absorption coefficient, and the real and complex components of the dielectric function. These optical dispersion relationships are contrasted with those corresponding to other forms of a-Si.

Automated high precision variable aperture for spectrophotometer linearity testing

A new automated linearity tester with a single variable aperture has been designed and built. It uses piezoelectric motors to define precisely the apertures required for application of the double aperture method of light addition. This design avoids many of the inherent shortcomings of two fixed physically separated apertures, such as interference and coherence between two separated beams and the need for an averaging sphere to compensate for beam noncoincidence at the photoreceiver. It also permits the assessment of system nonlinearity for arbitrary flux levels over an approximately 70:1 dynamic range without the use of a supplementary means of optical attenuation. The tester was specifically designed for use with the National Research Council of Canada Reference spectrophotometer, but it can be adapted for use with any instrument with a large stable measurement beam. The paper discusses the correct placement and operation of this device. The performance, as evaluated by nonlinearity measurements of a known highly linear silicon photodiode, shows a reliability of <1-3 parts in 10(4) over a 3400:1 dynamic range at a 97% confidence level. Several applications of this linearity tester to both photomultipliers and photodiodes are described. Transmittance results for several reference materials using these linearity corrected photodetectors are compared and show a typical agreement of better than 0.025% of the value.

Influence of growth temperature on order within silicon films grown by ultrahigh-vacuum evaporation on silica

We study the role that the growth temperature plays in determining the amount of order present within silicon films deposited on fused silica substrates through ultrahigh-vacuum evaporation at growth temperatures ranging from 98 to 572°C. Through measurements of the Raman and optical absorption spectra, we quantitatively determine how the growth temperature influences the order present within 11 such films. We employ three disparate measures of order for the purposes of this study: the breadth of the transverse-optic phonon Raman peak, this being related to the amount of short-range order present; the area under the transverse-acoustic Raman peak divided by the area under the corresponding transverse-optic peak, this being related to the amount of intermediate-range order present; and the breadth of the optical absorption tail, which is a general measure of the overall amount of order present. All three measures of order indicate a dramatic increase in the amount of order present for growth temperatures a...

Optical absorption in an amorphous silicon superlattice grown by molecular beam epitaxy

Abstract We have fabricated amorphous silicon (a-Si) superlattices, comprised of thin layers of a-Si separated by even thinner layers of SiO2 through ultra-high-vacuum molecular beam epitaxy and an ultraviolet ozone process. From measurements of the specular reflection spectrum at near normal incidence, and the regular transmittance spectrum at normal incidence, we have determined the spectral dependence of the optical absorption coefficient corresponding to the a-Si layers within such a superlattice deposited on sapphire. We contrast these results with those corresponding to thin films of a-Si deposited through ultra-high-vacuum molecular beam epitaxy and find that the optical absorption edge of the a-Si layers within the a-Si/SiO2 superlattices is sharper and occurs at higher energies as compared with the thin films of a-Si. We conjecture that both quantum confinement and impurities may be responsible for this effect.

PROCEDURES AND STANDARDS FOR ACCURATE SPECTROPHOTOMETRIC MEASUREMENTS OF SPECULAR REFLECTANCE

Procedures and standards that have been developed at the National Research Council of Canada for the accurate measurement of specular reflectance are discussed for both absolute and relative methods over the spectral range 250 to 2500 nm. There has been an increasing demand for these types of measurements, particularly for coated samples approaching the extremes of 0% reflectance and 100% reflectance. In some applications of these coatings, such as energy conservation and control, conventional methods of measuring specular reflectance give insufficient accuracies for the prediction of optical performance. Details of alignment procedures for both absolute and relative reflectance methods, the preparation and application of several candidate reflectance standards, and the compensation, attenuation, and verification procedures that have been developed to improve the precision and accuracy of specular reflectance measurements are described. Using these various techniques, one can routinely achieve accuracies of 0.3% reflectance at reflectance values as high as 97% and as low as 4%.

Intrinsic Wavelength Standard Absorption Bands in Holmium Oxide Solution for UV/visible Molecular Absorption Spectrophotometry

The transmittance minima of 18 absorption bands of a solution of 40 g/L holmium oxide in 10% (volume fraction) perchloric acid are certified as intrinsic traceable wavelength standards, by means of a multicenter measurement on material from a single source coupled with comparisons of a variety of preparations of the material evaluated on a single instrument. Fit-for-purpose artifact standards for the experimental calibration or validation of wavelength scales of chemical spectrophotometers can be carefully produced by end users themselves or by commercial standards producers. The intrinsic (data) standard confers traceability to the SI unit of length in place of costly transfer artifacts and repetitive calibration procedures. Certified values are provided for instrumental spectral bandwidths of 0.1–3.0 nm in 0.1 nm intervals, and information values are provided to a spectral bandwidth of 10 nm at wider intervals. Expanded uncertainties are typically less than ±0.1 nm for certified band positions.

Design and testing of a new high-accuracy ultraviolet–visible–near-infrared spectrophotometer

A new high-accuracy spectrophotometer has been developed at the National Research Council of Canada to measure regular transmittance factors over the spectral range from 200 to 2500 nm. The most significant feature of this automated single-beam instrument is a highly collimated normal-incidence beam geometry, which eliminates the need for polarization corrections or for an averaging sphere for the calibration of regular-transmittance reference materials. The instrument also possesses a large uniformmeasurement beam that minimizes errors caused by sample nonuniformity. We describe the instruments design and the testing, optimization, and verification procedures that have been carried out for measurements in the visible and near-infrared regions. Systematic errors that have been determined and corrected for include wavelength shifts, stray light, system drift, and nonlinearity. In the visible and near-infrared regions, the overall photometric accuracy is estimated to be 2.5 and 4.0 parts in 10(4), respectively. The wavelength scale is accurate to within +/-0.1 nm with a reproducibility of +/-0.03 nm over its entire design range from 200 to 2500 nm.