Re: Dial face colouration

[Steve Lelievre]

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


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Re: Sun path polar chart generator

[Dan-George Uza]

Something like this.


​

Dan Uza

On Mon, Feb 27, 2017 at 8:40 PM, Helmut Haase 
wrote:

> Hell Dan-George,
> Could you post an example graph how the plot should approximately look
> like?
>
> Regards
> Helmut Haase
>
> --
> Am 27.02.2017 15:49, schrieb Dan-George Uza:
>
> Hello,
>
> I'm looking for a freeware solution to generate customizable polar sun
> path charts in vector format. The software should be able to display sun
> paths for a custom time interval (like daily, weekly) and feature analemma
> curves for standard hours. Any help will be appreciated!
>
> Dan Uza
>
>
> ---https://lists.uni-koeln.de/mailman/listinfo/sundial
>
>
>
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Re: Sun path polar chart generator

[Helmut Haase]

Hell Dan-George,
Could you post an example graph how the plot should approximately look like?

Regards
Helmut Haase

--
Am 27.02.2017 15:49, schrieb Dan-George Uza:

Hello,

I'm looking for a freeware solution to generate customizable polar sun 
path charts in vector format. The software should be able to display 
sun paths for a custom time interval (like daily, weekly) and feature 
analemma curves for standard hours. Any help will be appreciated!


Dan Uza


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Sun path polar chart generator

[Dan-George Uza]

Hello,

I'm looking for a freeware solution to generate customizable polar sun path
charts in vector format. The software should be able to display sun paths
for a custom time interval (like daily, weekly) and feature analemma curves
for standard hours. Any help will be appreciated!

Dan Uza
---
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