John,
Will you clarify some things for me?
You mention that 50 lumens per square foot is about 500 lux, and that
the cited article recommends a limit of 60% reflectance for sky
illuminance of up to 1,000 lumens per sq. ft. If I multiple all that
out, it would appear to suggest a a limit of 6,000 lux of reflected
light for comfortable viewing. Is that the case?
Through Google, I found empirical rules for calculating the wattage of
solar radiation reaching the ground, depending on season, altitude,
declination, hour angle, and geographic elevation. I also found a
conversation factor for converting sky illumination in watts per square
metre to lux. Putting it all together, I get a figure of about 72,000 -
91,000 lux for the incident illumination, at noon on the northern
hemisphere summer solstice at sea level, depending on latitude (and
valid for mid-latitudes only). Using the mid figure of 80,000 lux, if I
want to limit the reflected light to 6,000 lux then the reflectivity has
to be less than 22.5%, which corresponds to a lightness of only 3.5 on
the Munsell scale.
Does this conversion make sense, or don't things work like that?
Of course, if we've gone out in midday sun, we should be wearing
sunglasses and, again from the web, sunglasses reduce the visible light
reaching our eyes by two thirds or more. If I factor that in, my Munsell
value rises to 5.3. And, as you pointed out, when the sun isn't so high
in the sky, we can tolerate a more reflectivity on our dial face.
Thanks for any further comments or advice,
Steve
P.S. Based on what I've learned so far, I'm leaning towards using a
material with a Munsell value of 6 or 7, which would correspond to the
mid-grays, tans and browns that people have been suggesting may work in
practice. It would be the number you mentioned but with the lightness
notched down a little. My design latitude of 45N is a little further
south than England (where the article's authors came from) and the
summer sun is a tad brighter. As well, I reckon a suitable colour with a
number of 6 or 7 would look OK against a lawn, flowerbed or other greenery.
On 2017-02-26 7:08 AM, John Lynes 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
---------------------------------------------------
https://lists.uni-koeln.de/mailman/listinfo/sundial
