Design and testing of a two-monochromator reference spectrofluorimeter for high-accuracy total radiance factor measurements.

A two-monochromator reference spectrofluorimeter has been developed at the National Research Council of Canada in accordance with International Commission on Illumination and American Society for Testing and Materials colorimetry standards to permit high-accuracy total spectral radiance factor measurements of fluorescent materials. This fully automated instrument employs a xenon-arc light source, all-reflective optics, two grating monochromators with order-sorting filters, a cooled photomultiplier tube analyzing detector, and a calibrated silicon photodiode monitor detector. The instrument operating range is 250-1050 nm with a selectable bandpass (optimized for a 5-nm resolution), and the measurement geometry is 45 degrees annular illumination and 0 degrees viewing (45/0). We describe the instrument's design, testing, and verification procedures. Systematic errors that have been determined and corrected for include instrument polarization, beam nonuniformity, wavelength shifts, stray light, and system drift. The wavelength accuracy and reproducibility are estimated to better than +/-0.1 and +/-0.03 nm, respectively. The photometric short-term repeatability and long-term reproducibility are estimated to be better than +/-0.15% and +/-0.5%, respectively. The overall photometric accuracy is better than 1% of the value over a wide range of reflectances, and the reproducibility of the color specification of a fluorescent material is better than 0.25 DeltaE(ab) units.

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%.

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 instrument's 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.

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.

Optical properties of UV transmitting acrylics for use in a heavy water Cerenkov detector.

The absorption, refraction, and scattering properties of several UV transmitting acrylics have been investigated over the wavelength range 300-700 nm using a combination of near-normal incidence regular transmittance and reflectance and diffuse-only reflectance measurements, followed by a Fresnel and a Kubelka-Munk analysis. The samples were evaluated in the as-cast and thermoformed forms, and both before and after an accelerated aging procedure. The results show significant differences in the optical behavior of the various acrylics in the UV region and stress the importance of carefully characterizing acrylic from different sources for each intended use. In our case, acrylic is the proposed material for a heavy water containment vessel for the detection of solar neutrinos. The significance of our findings to the overall performance of this Cerenkov detector, known as the Sudbury neutrino observatory detector, is discussed.

Determination of spectrophotometer polarization and its use in rapid accurate polarized transmission measurements.

The conventional method of measuring polarized transmission components of an anisotropic material requires the insertion of linear polarizers in the beam. For commonly used dichroic polarizers, which have a limited spectral range of use, this represents a time-consuming process, particularly for measurements encompassing a wide spectral range. Since the efficiency of dichroic polarizers is typically 25-40%, there is also a sacrifice of system throughput. This paper presents a more time-efficient and sensitive method for obtaining polarized transmission data by eliminating the need of an external polarizer. The method is based on the very large degree of polarization of our Perkin-Elmer Lambda-9 double-grating spectrophotometer from 250 to 950 nm. This effect has been accurately determined with previously well-characterized dichroic polarizers over the 250-1200-nm range. Examples of its application over a 1-yr period are given. The typical agreement between this method and the conventional method is +/-0.003 transmittance units.