Subject: Re: Sundial finish
I would say just take a walk around ontdoors and look at the shadows cast by
all sorts of objects, and on all types of surfaces. This would give good idea
of suitable surfaces for sundials.
David
Message Received: Jun 07 2013, 09:19 AM
From: Frank King
To: Art Krenzel
Cc: sundial@uni-koeln.de
Subject: Re: Sundial finish
Dear Art,
You ask a challenging question...
what surface (color, texture, finish,
material, etc) is the best choice to
show a gnomon shadow?
This is a big subject and to understand it
properly you will need to do some serious
study! You can make a start by discovering
what a Lambertian Surface is. Essentially,
it is a surface which reflects equally in
all directions. It is absolutely NOT like
a mirror.
In general, a Lambertian Surface is better
than a non-Lambertian Surface for sundials.
I have seen disastrous dials made of polished
stainless steel! The user can get blinded by
reflection from the sun.
Stainless steel CAN work but it must have
a satin finish.
I would add that light-coloured surfaces,
for example limestone, are better than
dark surfaces though, paradoxically, slate
is quite good.
These surfaces must not be polished of
course. Polished marble is almost as
bad as polished stainless steel!
The surface should not break up the shadow
and possibly the worst receiving surfaces
are long grass and round cobbled pavements.
This is enough to start a discussion.
Let's see what others say!
Frank King
Cambridge, U.K.
PS
Try reading these references:
http://en.wikipedia.org/wiki/Lambertian_reflectance
http://en.wikipedia.org/wiki/Lambert%27s_cosine_law
http://en.wikipedia.org/wiki/Oren%E2%80%93Nayar_reflectance_model
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