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