Fellow dialists, Re:
> >In regards to finishes friendly to shadow definition: matte white is my > >choice. >> >>I don't think anyone could disagree with that. >>Chuck Nafziger >Well, you might consider a slightly darker colour like sand or beige. My >experience is that when you use white the shadow will be less sharp because >the white colour is also reflecting some sunlight. I found this when I >noticed that the shadow on a dark brown surface seemed to be better visible >than the shadow on a white surface >Thibaud (Thibaud Taudin-Chabot) Out of my own observations, I side with Thibaud on this point. Moreover, I'd like to add a bit of speculative interpretation: Of the many factors involved, an important one is the effect of diffuse indirect sunlight that illuminates that dial area which is shadowed from rectilinearly propagated sunlight by the gnomon. Some of this light comes from the sky, some is reflected from structures, foliage, etc. within the large solid angle to which the dial is exposed. This light tends to "dilute" the contrast of the shadow as seen by a logarithmically responding eye. The spectral makeup of this diffuse light is, in general, altered from that of direct (white) sunlight. Most obviously, the sky is blue, the result of differential Rayleigh scattering by tiny particles in the atmosphere; the longer wavelength light being more likely to stay on it's straight course from the sun. Bounce-light from terrestrial objects' surfaces is less predictable in color, but green landscapes are not infrequently associated with sundials. In short, the shadow-revealing light is likely to contain more red and yellow than does the diffuse general illumination, which diverges toward excess blue and green. Thibaud Taudin-Chabot's, "slightly darker colour like sand or beige," or even his "dark brown surface" --- on which the shadow --- "seemed to be better visible than the shadow on a white surface," should by these lights <g> indeed be favorable. Brown and beige pigmented surfaces appear to us as they do by subtraction, for they selectively reflect relatively less of received light in the blue and green parts of the visible spectrum than they reflect in the yellow and red. So they tend to favor the contrast-producing direct sunlight over the contrast-dilluting indirect light. Bill Maddux
