March/April 2020
Lighting metrics
he second part of the pilot experiment (see p14 for summary of the first) considered whether a daylighttype spectral distribution would make a better basis for illumination metrology. For example, daylight is always seen as the best illumination for making critical colour assessments. With this in mind the following distribution is proposed (see Fig 1). The area proposed ranges from 400-700nm, described as the human spectral range and approximately following the daylight spectral distribution. It is a regular mathematical shape, a polygon; this means that any point within the shape can be described mathematically. This could be useful when comparing it with the spectral distribution of an electric light source. The test distribution SPD is labelled DVλ DV λ . (D for daylight and V for human vision). Also, it has been divided into six equal bands to aid mathematical ratings across the human spectral range.1
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PILOT EXPERIMENT 2
PART 2
DOES Vλ SPD MEASURE UP? Following the first pilot experiment on illumination metrology, David Loe continues the investigation by looking at quantifying colour rendering and colour appearance Twitter: @sll100
Following the first experiment, there remained the question of how the above could be used to quantify colour rendering quality and colour appearance rating. It was decided to test the accuracy of human colour vision under three different light sources: natural daylight, warm white and cold white LEDs (Fig 2). The Farnsworth-Munsell colour test was used, involving 100 colour samples covering a wide range of shades, all of a similar reflectance value. The colour range was divided into four sets. Initially the individual samples in each set were mixed up, with the subject being asked to place the samples in colour order between the two fixed ends (Fig 4). An experiment was carried out by 16 UCL MSc Light and Lighting students with the individual results for all four boxes combined, and for each of the three light sources. The individual results were combined into the average for the group and adjusted where necessary for any illuminance level differences. The averages for the group results for the three sources were compared and indicated similar results. This was surprising since the LED light sources had very different SPDs but both have erratic distributions suggesting that they may be similarly poor. This suggests that the Farnsworth-Munsell 100 hue test is inappropriate in this situation. A further
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