18% gray is used because that was thought to be the average reflectance of photographic subjects.
----- Original Message ----- From: "Patrick Vyvyan" <patrickvyv...@gmail.com> To: "John Lynes" <jly...@iee.org> Cc: "sundial list" <sundial@uni-koeln.de> Sent: Sun, 26 Feb 2017 13:01:10 -0300 Subject: Re: Dial face colouration When taking light meter readings for photography, it is common to use an 18% grey card in place of the object which will actually be photographed. This is generally considered to give a reading which will accurately balance light and dark. See: https://en.wikipedia.org/wiki/Gray_card [1] Might be worth examining as an ideal tone for a sundial? Patrick Vyvyan Presidente Corporación Cultural de Putaendo On 26 February 2017 at 12:08, John Lynes <jly...@iee.org [2]> wrote: There is no single optimum reflectance for a flat dial face. Obviously under dim sunlight the optimum reflectance would be 100 per cent, i.e.perfect white. Under intense sunlight, contrast sensitivity would be optimised for a lower value of reflectance. Thousands of papers have been written on contrast sensitivity. One classical study is "Brightness and contrast in illuminating engineering" by RG Hopkinson, WR Stevens and JM Waldram, Transactions of the Illuminating Engineering Society (London), Vol 6, No 3, pp 37-48 (1941). This indicates that when the sky illuminance on a matt dial face is over about 50 lumens per square foot (about 500 lux) the optimum reflectance would be about 60 per cent (a light grey, about Munsell Value 8). Below this illuminance (which would correspond to a solar altitude close to sunrise or sunset) the optimum reflectance would rise quite sharply. Note however that the maximum sky illuminance considered by the authors was 1000 lumens per square foot (corresponding to a solar altitude of about 20 degrees). Higher illuminances might further reduce the optimum reflectance. John Lynes On 26 February 2017 at 02:25, Michael Ossipoff <email9648...@gmail.com [3]> wrote: It seems to me that Steve's question has been mostly disregarded rather than answered. Not having experience with translucent dial-faces, I didn't know about their lack of accuracy, and I certainly can't disagree with what two people have said about that. It means that the advantage of a translucent dial, for omnidirectional reading, comes with a disadvantage of less precise accuracy. But of course a high-mounted dial intended for relatively distant reading might not be as concerned with fine accuracy as with omnidirectional viewing. And so translucent dials for all-directions viewing certainly aren't ruled-out. Steve's main question was about the choice of dial-face hue, saturation and brilliance, for easy and safe dial-reading. It seems to me that Steve's question has been mostly disregarded and discounted rather than answered. I lied. I said that I can't speak from experience on that matter. But my experience with a few paper-on-cardboard tablet-dials is sufficient to say this: From my experience, I can say that you definitely don't want a white dial-face. As I said, my first dial had a white dial-face. After that, I switched to brown, which was a big improvement in usability. I suggest brown instead of white. Someone implied that, the more contrast (between light and shadow), the better. Not so, when the dial-face is too white to look at in bright sunlight. As for gray: Gray reflects the visible wavelengths in a relatively equal mix, resulting in no perceived hue. If some hues are (at least relatively) to be avoided, then obviously gray isn't what you want. At each end of the visible spectrum, there is, of course, radiation that isn't visible. Infrared (IR) and ultraviolet (UV). One possible disadvantage of that is that, when you don't perceive it or its intensity, then of course you could conceivably get a dangerous amount (accutely or cumulatively) without any perception of it. For example, never look at the sun when, due to a haze, or due to the sun being low in the sky, the sun doesn't look bright. You don't have any perception of how ingtense the UV or IR is. It could burn your eye without any feeling of discomfort. (I don't know which of those is more dangerous, but there have been official warnings to never look at the sun when it seems less bright due to haze or low altitude.) Aside from that, there's been evidence that, when people spend a lot of time outdoors, in bright sunny climate, then many years of exposure to the bright blue light can cause some long-term cumulative damage. So maybe blue isn't the most desirable hue. Yellow, beings the complement of blue, looks yellow because it absorbs blue, removes blue from the light that it reflects. Also, yellow isn't particularly close to either end of the visible spectrum. Brown is defined as: "Any of a group of colors between red and yellow in hue, of medium to low brilliance, and of moderate to low saturation." Then, dark brown would be brown with particularly low brilliance--a desirable attribute for a sundial-face. Might that be the best color for a dial-face? Tan is defined as: "Light yellowish brown." ...suggesting more brilliance than brown (but surely a lot less than white), and enough saturation to be perceived as yellow, which seems a good thing. Brown, especially dark brown, or maybe tan, sound like acceptable colors for a dial-face. By the way, beige is defined as: "A variable color averaging light grayish yellowish brown." Sounds like tan, but with distinct grayness, lower saturation, making it probably less desirable. In my previous post I said that I bought brown construction-paper, but didn't use it, and, instead, just marked the hour-lines on the corrugated cardboard instead of using paper. Actually, I probably did use the brown construction-paper. It looks better of course, and it allowed me to conveniently use a carbon-paper template that I'd prepared for drawing the hour-lines. Maybe the plain cardboard dial-face would have easier construction in one way, and less easy construction in another way. Maybe I tried one all-cardboard dial. It was a long time ago. Michael Ossipoff On Fri, Feb 24, 2017 at 1:04 PM, Steve Lelievre <steve.lelievre.can...@gmail.com [4]> wrote: Fellow sundiallers, I’m planning to make my next sundial from outdoor grade UV resistant plastic sheeting. These come in a range of colours and I want to choose one that works well for a sundial. Assuming I get the material grit-blasted or somehow treated so that it not shiny, and leaving aesthetic considerations aside, what light-related attributes should I be looking for? As anyone who has played with paper sundials knows, a white surface is hard to look at in full sun, even if non-shiny; black would not show any shadow. I need something in between: light enough to catch a shadow, but dark enough to avoid glare in full sun. I assume that latitude has a bearing on this, as the midday sun illuminates more strongly as we approach the equator. In my case, the design latitude is around 45 N. My dial will be about 25cm in diameter. Are there any conventions or empirical guidelines, or even practical experience, to help me choose? Which properties matter? I quick read of Wikipedia suggests colours seem to involve hue, saturation or luminosity (or parallel concepts in other classifications). Cheers, Steve --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial [5] --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial [6] --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial [7] Links: ------ [1] https://en.wikipedia.org/wiki/Gray_card [2] mailto:jly...@iee.org [3] mailto:email9648...@gmail.com [4] mailto:steve.lelievre.can...@gmail.com [5] https://lists.uni-koeln.de/mailman/listinfo/sundial [6] https://lists.uni-koeln.de/mailman/listinfo/sundial [7] https://lists.uni-koeln.de/mailman/listinfo/sundial
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