[chbot] LED displays coloured filters

[Helmut Walle]

On 15/01/2017 11:17 a.m., Charles Manning wrote:
>
>
[...]
> [Helmut Walle's earlier comments:]
>     Anyone interested in judging where a polarising filter makes a
>     difference should use a polariser as they are commonly used in
>     photography and try it in different situations. They generally don't
>     make a difference for reflections from metallic surfaces, as the
>     light reflected by metals is not polarised (unless the incident
>     light already was). They do make an amazing difference for
>     portraits, though - saves you a lot of skin powder...
>
>
> Can't agree with that. Reflection and refraction both cause
> polarisation. Light is polarised significantly by bouncing off a
> flattish surface (eg. water, metal or glass) or even by refraction from
> the sun into the atmosphere. The amount of polarisation is dependent on
> the angle of refraction/ reflection. . Hence fly fishers wearing
> polarised glasses to knock out reflected light so they can see in the
> water and photographers use polarising filters to make the sky darker
> blue (or wash it out) and make the clouds "pop out" (or flatten them)..
>
> See https://en.wikipedia.org/wiki/Polarizing_filter_(photography) for
> some examples.
>
> A polarising filter can reduce "plastic shine", but the results will be
> very dependent on angle of incidence and I doubt it would help much.
>
[...]

Here is a quote from the Wikipedia article you are referencing, Charles:
"Light reflected from a non-metallic surface becomes polarized;" - so 
how exactly do you not agree with my comments above?

And by the way, the polarised light from the Sky is polarised due to 
Rayleigh scattering, not refraction - that is also the reason why it 
mostly only can be exploited effectively in photography if you are 
shooting perpendicular to the rays of light coming from the sun; and you 
can see that this is the case in the first example of the Wikipedia page 
with the clouds: the trees' shadows are falling about perpendicular to 
the viewing direction. You can easily try this: either face the sun, or 
face away from it, and try getting a difference out of a polarising 
filter, and there won't be one (or not a practically useful difference 
anyway).

Kind regards,

Helmut.
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