In this article, we describe a spectral sensitivity measurement procedure at the National Physical Laboratory, London, with the aim of obtaining ground truth spectral sensitivity functions for Nikon D5100 and Sigma SD1 Merill cameras. The novelty of our data is that the potential measurement errors are estimated at each wavelength. We determine how well the measured spectral sensitivity functions represent the actual camera sensitivity functions (as a function of wavelength). The second contribution of this paper is to test the performance of various leading sensor estimation techniques implemented from the literature using measured and synthetic data and also evaluate them based on ground truth data for the two cameras. We conclude that the estimation techniques tested are not sufficiently accurate when compared with our measured ground truth data and that there remains significant scope to improve estimation algorithms for spectral estimation. To help in this endeavor, we will make all our data available online for the community.
A radiometric realization of the S.I. base unit of photometry, the candela, as performed at the National Physical Laboratory, is described and the results compared with the scale established at NPL in 1937 and disseminated since 1948. The realization has been carried out in accordance with the radiometrically based redefinition given by the Conférence Générale des Poids et Mesures (CGPM) in 1979 which quantifies the spectral luminous efficacy of radiation at a specified frequency (540 × 1012 Hz) as 683 lumens per watt. The excellent performance of the siliconphotodiode photometers developed for this project is demonstrated, together with characteristics of the two types of lamp employed for preserving the results of the realization and methods for improving their alignment and reproducibility.
Achromatic thresholds are measured at three sites to investigate the effects of light spectrum on achromatic detection thresholds in the mesopic region. The results of measurements using quasi-monochromatic targets reveal spectral sensitivity functions with two or three peaks, a so-called 'chromatic effect' which is assumed to be due to the influence of the colour-opponent channels. This chromatic contribution seem to be less significant at lower luminances but it is accentuated for peripheral observation. Results obtained with broadband stimuli show that the contrast threshold, defined in terms of V(), is markedly higher for red targets than for other colours, particularly at lower light levels. For these broadband targets, contrast threshold values calculated using the new spectral sensitivity curves instead of V() are in better agreement with the experimental results, particularly for the peripheral detection task.
We present a description of a new goniospectroradiometric measurement system developed at the National Physical Laboratory (NPL). The instrument incorporates a modified array spectrometer and a series of rotary stages to allow measurement of the spectral radiant intensity distribution of a variety of different types of light source from 350 to 830 nm. Associated source properties such as chromaticity and correlated color temperature distributions and total spectral flux are then calculated from the radiant intensity data. A preliminary comparison with NPL's integrating sphere-based luminous flux scale shows agreement to within 0.4%, well within the combined measurement uncertainty. Measurements of the luminous intensity and color temperature distributions and the spectral total flux of a tungsten filament flux standard, a white LED cluster and a compact fluorescent source made using the goniospectroradiometer, are also presented.
The authors represent a research consortium which has adopted a task performance based approach for nighttime driving to obtain mesopic spectral sensitivity functions. This study describes investigations based on discrimination threshold techniques and compares the results with those obtained from detection threshold measurements. It forms a continuation of `Mesopic models — from brightness matching to visual performance in night-time driving: a review'. 1 The achromatic increment contrast threshold for discrimination of a quasi-static target represents the ability to correctly identify a target which has low contrast with the background and is therefore a highly relevant visual task in the context of nighttime driving. The psychophysical experiments reported here are carried out in the laboratory and in dynamic nighttime driving tests. The spectral sensitivity functions resulting from the discrimination threshold contrast investigations show a significant contribution from the colour-opponent (or chromatic) channels of the visual system. Similar features have been observed in results from investigations using detection threshold techniques.
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