DOES Vλ SPD MEASURE UP (2)?

DOES Vλ SPD MEASURE UP? PART 2

Following the first pilot experiment on illumination metrology, David Loe continues the investigation by looking at quantifying colour rendering and colour appearance

The 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. (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

11 PILOT EXPERIMENT 2 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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thought is that if the observer is looking at, say, three adjacent samples to make their judgements over the normal reading distance, this equates to a conical zone of approximately 5 degrees, a zone covered by foveal vision, and an SPD Vλ which the earlier experiment showed to be, in terms of colour vision, limited. This again suggests that the FarnsworthMunsell test is probably inappropriate.

However, by visually studying the SPDs of the two LED sources similar to those used in the experiment (Fig 2), it can be seen that for the cold white source the radiation is predominantly in the blue part of the visual spectrum and reducing rapidly into the red part of the spectrum. For the warm white lamp the reverse is the case, in other words, showing a peak around 620nm in the warm part of the visual SPD. This suggests that neither is very good considering their effectiveness for the whole visual colour range. This is similar to the early fluorescent lamps which also had poor colour performances. It was only when multi-phosphor coatings were introduced, with radiation covering more evenly across the whole visual range, that good colour performance was achieved.

Comparing them both with daylight, using the suggested DVλ as the basis for illumination metrology, although not uniform over the visual range, there is only a difference between the two ends of the visual range of around 30 per cent. This is probably the reason why daylight scores so highly in colour recognition and matching. COLOUR RENDERING AND COLOUR APPEARANCE It would seem that since daylight is recognised as the preferred light source for assessing colours and colour matches, this should form the basis by which to classify human colour assessment of light source illumination. Also, that it should be the DVλ SPD that is used as the standard on which to base a measurement system, at least until a more accurate understanding of the relationship between light and vision is found.

Regarding colour rendering, it would seem that a way forward would be to compare the spectral distribution of a light source in question with the distribution DVλ divided into a number of wavelength bands. Six has been suggested here, each of the same wavelength width and spread equally across the human spectral range 400-700nm (Fig 1). The experience of the Farnsworth-Munsell colour test pilot experiment, described above, indicates that a single number is unlikely to be the solution. However, a system that compares the closeness of the SPD of a light source to the DVλ distribution in terms of the percentage difference for each of the six Summary of the thinking behind the study The human visual system allows us to see fine detail, and gives us the ability to see all colours across the visual spectrum (400-700nm). It also allows us to see volume and texture through patterns of light and shade, and probably much more. Early humans had these abilities, but as the only illumination many thousands of years ago was from daylight, it would be reasonable to expect that human sight would have evolved under a daylight-type spectral distribution. Early in the development of electric lighting the need arose for a standard spectral distribution, to be agreed internationally, by which light measurements would be made. In 1924 the CIE Vλ distribution was agreed and remains the standard by which all photometric measurements are made. However, because the measurements were made by observers using an optical instrument rather like a telescope, the field of view would have been limited, probably to the human visual region known as the fovea, where the retina has the highest density of sensors, enabling sight of very fine discrimination. However, the field only encompasses a cone of around 2-5 degrees in diameter, hence it does not represent all of human sight, which encompasses a cone of around a 40-degree diameter. Nonetheless it has been checked a number of times with similar results. The Vλ distribution is a bell-shaped graph centred approximately on 555nm and encompasses the yellow-orange part of the human spectral range. This suggests that the ends of the human spectral range, the purple and red parts, are seriously reduced or discounted. 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2

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460 500 550 600 650 700 Relative spectral power distribution ≈20˚ Cold White ? Wavelength / nm Warm White ?  Fig 1: suggested DVλ SPD as an approximate for daylight spectral distribution, together with the area covering the human visual colour range (400–700nm) divided into six equal band widths. It also shows how warm and cool lamps might be accommodated

Fig 2: spectral distributions of warm white LED and cool white LED light sources

Incandescent lamp

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Fig 3: spectral distributions of flame and incandescent light sources Wavelength / nm

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wavelength bands seems possible.

For example, if a light source’s radiation for each of the bands was within say 10 per cent (positive or negative) the lamp would be classified to be of very high CRI. Or if each of the differences were within, say, 30 per cent it would have a lesser colour rendering quality. Further work would be necessary to determine the actual percentage values and so on.

Regarding colour appearance identification, since daylight is not described as either warm or cool, the suggestion is to provide different test SPDs as shown by red and blue dashed

Twitter: @sll100 lines (Fig 1). This indicates that reddish light is increased or decreased for the two light appearances, but the whole colour range is still included.

SUBCONSCIOUS EFFECTS A subconscious effect of light on humans has long been known as a process within the human brain caused by radiation of around 460nm. However, the actual effect and the process have not yet been properly explained, and whether it is present or not, except that it affects the body clock.

13 The position is shown in Fig 1 by a vertical purple dashed line. This indicates that the radiation is prominent in daylight. Further, it has been suggested by a research team at Rensselaer Polytechnic Institute, Lighting Research Center, New York, led by MG Figueiro et al, that when this radiation is present human performance is enhanced. 2 This would mean that having daylight, or electric light with a similar SPD, in a working environment could provide performance benefits, though the work so far has not indicated the amount or the nature of the benefits, or whether there are any detrimental effects.

Counter to this is that if a light source does not have daylight-type SPD, but that of a flame source or incandescent lamp, which have similar SPDs where there is no, or very little radiation at 460nm, then they might create a more relaxed human condition. This has been suggested in care homes where red light sources have been used at night-time to help poor sleep quality in the elderly. 3

A further possibility is whether exposure to daylight has any effect on health or on the recovery from surgery. Some readers may remember the work of Roger S Ulrich, who compared the recovery rate of hospital patients who had a view out of their window of sky and trees with patients ‘More studies will be needed both to confirm the requirement for a new SPD as a basis of illumination metrology, and to test the suggestion of DVλ’ E

whose window looked on to a brick wall. The result showed benefits for the former. 4 So far there has been little research into the mechanics of this process. It was reported in the 1960s that there was a gland within the human brain, the pineal gland, which acts as a switch suppressing the amount of melatonin in the brain and replacing it with a stimulant – or is it the reverse? If this is the case then it would account for the results reported above. But a further consideration is the route by which it gets to the brain. For example, is it possible that it is via the rod retinal sensors? These have an SPD with a peak of around 500nm, and appear to do little during daylight hours.

WHERE NEXT? The above has been an attempt to indicate the possible or even likely shortcomings of the Vλ SPD as a basis of illumination metrology. Ways in which it might be improved have also been suggested. But more studies will be needed both to confirm the requirement for a new SPD as a basis of illumination metrology, and to test the suggestion of DVλ. This will require further scientific research effort into the operation of human sight and its effects. We should remember that human sight can automatically adjust for extreme luminance variations to avoid glare in an attempt to improve sight, and can automatically adapt

Summary of pilot experiment 1 To test the validity of the Vλ distribution the study involved taking a number of M&S socks of different colours from across the visual range, and photographing them under daylight illumination. They were then photographed again with a filter over the lens, claimed to have a spectral transmission similar to Vλ. It shows a yellowish tone over the whole photograph. Predictably, it also shows colour samples in the low wavelength end of the human spectral range (purplish) and the high wavelength end of human spectral range (reddish) to be downgraded. From the photographic images, luminance measurements were made of the different coloured socks from the two light sources (daylight and daylight + Vλ filter) and adjusted to correct for any differences in illuminance. The ratio between the two different images for each of the coloured socks ranged from 1.42-2.38, a difference that indicates that some coloured items are downgraded. It also means that using the Vλ distribution as the basis of illumination provides a distortion of the effectiveness of some parts of a light source’s radiation depending on the particular colours. The results suggested that the Vλ spectral distribution as a basis of illumination metrology is at the very least questionable. It was concluded that it would be useful for the experiment to be repeated with more exacting conditions. For example, it would be useful to know how accurate is the SPD of the Vλ filter. It would be helpful to use a better range of colour samples. Also, if a camera is used to measure the effects described, to test that the spectral distribution response is at least similar to that of daylight. See SLL Newsletter March/April 2018 for full details of the first pilot study

to a colour wash which at least distorts colours. But these mechanisms have not yet been considered. All of this will require extra funding from both industry and governments. It will also require international collaboration through the CIE for anything to happen. In which case we may continue to make poor quality lamps and inaccurate illumination measurements. But there is no reason why the LED source manufacturers can’t explore ways of developing lamps with improved colour performances as suggested. In effect to produce ‘deluxe’ colour performance lamps to surpass the often poor performance experienced.

Fig 4: Farnsworth-Munsell 100 Hue Colour test showing the colour range with the four boxes

References: 1. Does Vλ SPD still measure up? Society of Light and Lighting Newsletter Vol.11, Issue 2, March/April 2018. https:// issuu.com/matrixprint/docs/ sll_mar_apr_18 2. MG Figueiro et al, Circadianeffectiveness light and its impact on alertness in office workers, LR&T, 2019; 51: 171-183. 3. Mercier K. Maximising health and sleep in the elderly. Lighting Design and Application. October 2012; 42-47. 4. Roger S Ulrich, View through a window may influence recovery from surgery, Science 27, April 1984.