looking for LED filter plastic

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I have a red LED display that is around 30cm by 20 cm.

I need a sheet of UV stabilised plastic with filter properties so that the
display will work outside on a bright sunny day.

Does anyone know what sort of properties parking and traffic sign filters
have these days? The old red filters seem to have disappeared making way for
what appears to be clear tinted plastic. Obviously it's UV stabilised, but
it looks like a matte finish and slightly tinted. Maybe its one way glass or
something.

Also, I wonder what most LED speed limit sign LEDs are made from. In
Melbourne, I see the use of white and red combinations, with yellow circle
outside. I wonder what MCD the leds are.

I'm looking for a long term supplier of the filters with competitive
pricing. Any ideas?




Re: looking for LED filter plastic

"Phil in Melbourne"


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** If your leds are not bright enough to work in bright sun  - then you are
stuffed.

   Get brighter ones.


.......  Phil



Re: looking for LED filter plastic

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Yes, and no. The problem is with sun glare. If the sun shines on the LEDs
directly, it looks like they are lit.

If a filter is used, it should increase the contrast between leds that are
off and leds that are on.






Re: looking for LED filter plastic

"Phil in Melbourne"
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** That is a result of the sunlight being brighter than the LEDs are.

    A coloured filter will not help.

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** All a red filter will do is help darken the background -  painting it
matt black is the most effective way of achieving that.

BTW:

How about you say  JUST  what it is you have and what you are trying to
achieve with it..

So far it looks like something that is not possible.



.......  Phil







Re: looking for LED filter plastic
On Fri, 21 Mar 2008 13:21:37 +1100, "Phil in Melbourne"

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The only thing that came to mind was this
http://www.optigrafix.com/circular.htm
Circular polarizer films.


D from BC
British Columbia
Canada

Re: looking for LED filter plastic

<D from BC> wrote in message
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Thanks D, that looks promising.



Re: looking for LED filter plastic

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    Why not buy some dark red 3mm sheet glass from the nearest
stained glass hobby supplier?.

Re: looking for LED filter plastic
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If you have a small viewing angle, you can use that stuff that 3M
makes that looks like a bunch of slats.  They usually sell it to keep
people for reading your laptops screen from the side.  If you rotate
it 90 degrees, I will keep sunlight from above out of the display.

Circular polarizers like used on LCDs work well to keep sunlight from
going in and then bouncing back out.

Don't use a filter witht he same color as the LED.  They make matters
worse by coloring and reflecting sunlight.

A lot of improvement can be had by careful design of the placement of
the lens and a shield above it.  You can also tilt the lens down so
that any sunlight is reflected down to the ground and the users line
of sight is reflected to the darker ground by the lens surface.

Ignore the makers claims.  LEDs really come in three brightness grades
(1) Are you sure you turned it on (B) Way too dim and (iii) Just
barely bright enough.


Re: looking for LED filter plastic
On Fri, 21 Mar 2008 06:44:54 -0700 (PDT), the renowned MooseFET

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I must be buying good ones- we had a complaint that the display was
*too* bright, even though the requirement stated "sunlight readable"
as a "nice to have".


Best regards,
Spehro Pefhany
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Re: looking for LED filter plastic

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   Ship a set of those disposable paper sunglasses the eye doctors give
you when they dilate your pupils with the product. :)

--
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Ha! You made my morning! (PST)
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DaveC
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Re: looking for LED filter plastic
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No, you just had a whiny customer and the display for once wasn't dim
enough to whine about it being too dim, so he had to whine it was too
bright.  Did he also complain that the color clashed with his shirt?

You can always dim them by PWMing the drive.


Re: looking for LED filter plastic

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Yeah, I think that's what the "clear" windows use.

Re: looking for LED filter plastic

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Various kinds of filters work, even some that actually attenuate the
light from the LEDs themselves. There are several phenomena that work in
your favor:
1) The ambient (e.g., sunlight) light reflected by the LEDs and other
objects inside the filter has to travel through the filter twice, once
on the way in and once on the way out. A filter that absorbs 50% of
ambient light will transmit only 25% of light reflected from inside the
filter. So, almost any kind of absorbing filter (e.g., 'smoked' Perspex)
improves contrast somewhat.
2) Colored filters selectively absorb ambient light, and the human
visual system discriminates more on the basis of local contrast than on
the basis of absolute brightness. Color contrast filters also do the
double filtering trick described in 1.
3) Louvered bezels and micro-louver materials such as 3M Light Control
Film are strongly absorbing from most directions and highly transmissive
in the desired viewing direction, These materials are used to control
the visibility of traffic control lights, among other applications.
4) Polarizing filters can be oriented to selectively absorb ambient
light.

The right combination of filters will allow you to see relatively dim
LEDs in strong sunlight. However, these days designers often simply
choose high brightness LEDs and crank up the drive current, if ample
power is available.

Paul Mathews

Re: looking for LED filter plastic
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I was looking at some traffic lights today. I don't think the LED
versions use any filters.

Re: looking for LED filter plastic

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Yes they do. The Aldridge ones (most common) have a domed filter of the
corresponding colour in front of the LED arrays, they are however rather
light and a not diffused nor are they designed as a lens as such. In traffic
lights this is not so critical as in a large sign, since the traffic light
arrays have shades over them. I have done work with them in the past and can
tell you they are VERY bright. I can't remember the mcd rating but each
different colour array has a different number of LEDs to compensate for the
differing brightness of each LED / filter combination so the total light
output from each array is the same.

Regards
James



Re: looking for LED filter plastic
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If you get off angle to them, you will see very little light.  They
use directional lensing to shoot the light at the drivers.  The same
lensing keeps sumlight out of the LED so it can't reflect back.

Re: looking for LED filter plastic

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  It also helps keep the drivers on the cross road from seeing the
yellow light and jumping the red before it turns green.


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Re: looking for LED filter plastic
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 Ah, an excellent source. Thanks for letting me know. Free/open
news service are getting hard to find these days.


GB
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Re: looking for LED filter plastic
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for
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or
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I meant no color filters. The unlit LEDs looked milky white.

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