[chbot] LED displays coloured filters

[Charles Manning]

On Sat, Jan 14, 2017 at 2:58 PM, Helmut Walle <helmut.walle at gmail.com>
wrote:

> On 13/01/17 23:06, Volker Kuhlmann wrote:
>
>> On Fri 13 Jan 2017 22:20:50 NZDT +1300, Colin Isenman wrote:
>>
>> You could consider a plain grey polarizing film instead of colour
>>> filters. The polarization should significantly reduce reflections and
>>> hence improve the contrast of the display. To be really slick, an
>>> antireflective coating on top would be nice. But maybe not necessary
>>> unless you want sunlight readable.
>>>
>>
>> In a bright sunlit room I find the white/grey background of the display
>> front and the LED-segments when not lit to be close to indistinguishable
>> from lit segments. Removing the light refelcted from unlit segments
>> would go a very long way.
>>
>> I'm not sure the polarizing would help, because neither the LED light
>> nor the ambient diffuse light is polarized, but it'd be worth a try.
>>
>> [...]
>
> I can't but agree with Volker that a polarising filter is very unlikely to
> improve the situation, mainly because it will remove 50% of the desired LED
> light, too, while making little difference on diffuse reflected light. BTW,
> it doesn't matter that the ambient light is not polarised, as long as the
> reflected light is - but the polarisation of the reflected light if any
> will be across so many planes due to the geometry of diffuse ambient light
> rays being reflected off the 3D structure of the LED module that it will be
> pretty much near impossible to reduce the reflection by using a polarising
> filter. I would expect that the loss of 50% of the desired LED light would
> more than cancel out any minimal gains made by reducing reflected light...
>
> A matching colour filter OTOH will transmit almost all of the desired LED
> light, while blocking everything else.
>

Yup that's the best IMHO.

One of the best things to do use contrasting colour filters in different
places.

For example if you have red LEDs, then fit a red filter in front of the
LEDs and a green filter behind the LEDs. If white light is shone from the
front then the red filter will absorb other colours letting through the red
light only. The green filter will then absorb the remaining red light so
that it does not reflect off the back of the inside of the box.



> 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.
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