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 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> 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> > 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> 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 >>> >>> >> >> --------------------------------------------------- >> https://lists.uni-koeln.de/mailman/listinfo/sundial >> >> >> > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > >
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