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Kevan Shaw • Thomas, I disagree! Both incandescent light and daylight are natural light sources that have continuous spectra within the human visual range. That makes both as near perfect for humans as can be. We evolved with daylight during the day at high lighting levels and with fire in the night at low lighting levels. We have never, until the last 70 or so years, had to contend with discontinuous spectra and are therefore not in any way adapted to deal with these kinds of light source. The fact there is visual similarity between the discontinuous spectra of “modern” light sources is more a defect in the human visual system than it is a suggestion that they are in any way as good as natural light sources with continuous spectra! I would agree that there are a number of practical problems at the incandescent end of the scale. There is no doubt that incandescent light, by its nature, results in the creation of a lot of heat not helpful or necessary to the process of vision but an inevitable byproduct of any light source that uses this natural principal. Daylight as well has similar problems with copious amounts of infra red and ultraviolet energy that exist and also do not contribute to vision. There are limits to what can be realistically achieved. As long as we are working with the Lumen as the method of measurement we are working with the visible spectrum in proportion to our ability to see it. Remember that in pure energy terms that sunlight only achieves 93 lumens per watt of energy that it delivers to earth. This must represent some kind of benchmark as to what is realistically achievable for the delivery of excellent light quality! Another interesting fact I came across is the CRI of daylight once it has passed through heat rejecting glass. This is often as low as 91 for what is supposed to be visually clear glass. Again the glass is seriously distorting the full spectrum of light in the interests of reducing solar gain within a building. We do not have good metrics for quality of light however applying the metrics used to quantify “energy efficiency” we are at risk of gravely damaging the human quality and experience of light.

Thomas Wensma • Yes, incandescent is a natural light source and, just like the sun, it has a continuous spectrum. I also like incandescent because we are ‘wired’ to like it. It resembles a fire like you said. But there is a big difference between daylight and incandescent. Daylight changes in its spectral distribution. An incandescent almost doesn’t. Both keeping a continuous spectra. When I talk about quality I avoid metrics simply because they are too limited. CRI for sure is and only talking about lumens or energy efficiency has a similair problem. I believe which source is better to light an object depends on its colour(s) and what kind of colour rendering it should resemble. So maybe daylight at sunset or more daylight at noon. Because both are different. Red shoes would be better with incandescent but blue jeans with LED.

Kevan Shaw • Still not convinced! Incandescent light in nature changes a lot, at least it does in the fires that I enjoy. The incandescent lamp is bottled light and is somewhat more stable but so is a candle in a proper lantern where it does not get blown about. The incandescent lamp can easily be made to change in appearance and powered by the simplest of dimmers! I would still like to light my denims with a Tungsten Halogen lamp not dimmed at all. This would allow me to determine the subtle differences between different dyes and these days between different treatment processes. Under LED they would all look very blue and the subtleties of other colours would be lost!

Thomas Wensma • Yes we can change the appearance of an incandecent with easy dimming but not close to the dynamics of natural daylight! All those things you want to see in your denims you can with the right LED light (or at least close to) as good as with your tungsten. Also seeing different hues. It won’t just show blue of course. The amazing thing about our visual system (as it turns out after research) is that it can adapt really well to different lighting and light levels. Our love and preference is biological. ‘It’s in our mind that we love it’. Which is good because it is the natural source. I fully agree with you on that. I also believe we have to be careful about LED and its effects. But that’s a different conversation. I truly believe both are great and we should use both. And in the future OLED. Maybe a huge ‘field test’ seeing all the differences would give a definitive answer to this question.

And more reaction... A Letter to the Editor,

The discussion on relighting the National Gallery, London (mondo*arc Aug/Sep, 048-054), touched on an interesting range of topics, but I believe that a couple of issues warrant further comment. Firstly, it was reported that a thermal camera had been used to measure the surface temperatures of two luminaires, one with LEDs and the other with a tungsten halogen lamp, giving readings of 30°C and 300-350°C respectively, and from these data it was concluded that the LED luminaire produced “ten times less heat”. While a halogen lamp obviously runs hotter than a LED, the heat output of an appliance is not proportional to its temperature. Its heating effect on ambient air temperature is determined by its wattage, or rate of energy consumption, and the advantage of LED sources is that they can match the intensity distributions of conventional display lighting with lower wattage. There is, however, another aspect of heat output that is relevant in museums and involves thermal cameras. Some current light sources, and halogen spotlights are notorious, produce significant radiant heating effect on exhibits. To assess this, the thermal camera needs to be directed onto the exhibit, not the light source. Before the display lighting is switched on, it may be confirmed that the exhibit’s surface temperature matches the ambient air temperature, and then, after a few hours of display lighting, the surface temperature may be found to be elevated several degrees above air temperature. The daily cyclic effect of switching can cause damage to an exhibit due to differential expansions and moisture migrations, and here again, LEDs are advantageous as their radiant emission includes no short-wavelength infra-red radiation. While LEDs do have some distinct thermal advantages over conventional display lighting sources, the claim made in this article should be discounted.

The other issue concerns the “blue spike” at around 450 nanometres in the spectral power distribution (SPD) of white LEDs, for which the discussers seemed to be in agreement that this is an undesirable feature that should be at least reduced, or better still eliminated. It is a fact that LEDs emit radiation in narrow wavelength bands, and the SPD of a “white LED” is the sum of the narrow-band, short- wavelength output of a blue LED and the broad-band, mid- and long-wavelength output of the phosphor that it stimulates. However, to eliminate the short-wavelength component from the emitted SPD would not only have the effect of lowering the colour temperature and losing the LED’s admired “cool” appearance, but it would destroy the colour rendering. It is absolutely essential to have radiant power in the “blue” waveband of 400-490nm, but it makes little difference whether that power is concentrated into a peaky narrow-band emission around the middle of the waveband, or is a flatter, more uniform distribution over the waveband. The notion that the colour rendering of a light source is indicated by the smoothness of the SPD curve is a myth. The CIE General Colour Rendering Index goes some way towards taking account of complexity of colour perception, but at the end of the day, the most reliable way of assessing colour rendering properties is to make critical observations of a range of colour samples, such as the Macbeth Colorchecker chart, illuminated by the source. If certain pigments are of particular concern, then samples of them should be included. If no hues appear brightened, dulled, or distorted, this serves the double purpose of not only confirming the acceptability of the colour rendering, but also indicating that none of those pigments are being selectively radiated at wavelengths to which they are particularly susceptible. It is only the absorbed (not reflected) energy that causes pigment degradation, and should a spectral absorptance peak of a pigment coincide with a spectral emission peak of the illuminant, causing an unduly high rate of energy absorption at a particular wavelength, the appearance of that pigment will be affected. Colour rendering should be recognised as an aspect of lighting for which critical observation is both more reliable and more useful than numerical data.

Kit Cuttle Havelock North, New Zealand

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