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Daylight visible indicator LEDs - how bright is enough?
Posted: Mon Jun 30, 2008 2:52 pm
I'm working on a project that will be displaying indicator LEDs to the user in broad daylight. They need to be clearly, unmistakably identified as to whether they are lit or not even in direct sunlight. What would be the minimum brightness levels to achieve favorable results for blue, ~520 nm green, red, and yellow (i.e. in millicandelas)? Does anyone have first-hand experience with this who can shed some light on the situation? Any specific suggestions as to what would work/doesn't work would be appreciated, as well as part numbers.
Power consumption is not really a concern here. I'm hoping for relatively cheap, discrete LEDs (e.g. 5 mm as the panel mountings etc. for this is common) that can be driven by a linear multi-channel LED driver (i.e. 16-output shift register that also lets you adjust the current for each channel - this would let me dim the LEDs somewhat at night). At full brightness I'd expect a 100% duty cycle on the LEDs. Since the brightness would be adjustable, I'd prefer LEDs that can be safely driven at values higher than absolutely necessary to achieve daylight visibility, to allow room for brighter values if necessary.
Posted: Mon Jun 30, 2008 3:35 pm
I used to work on military human factors, and don't remember any relevant data in the military standards documents.
You will probably have to experiment and find out for yourself.
Re: Daylight visible indicator LEDs - how bright is enough?
Posted: Mon Jun 30, 2008 4:55 pm
Greetings (No First name Supplied),
PredatorCoder wrote:I'm working on a project that will be displaying indicator LEDs to the user in broad daylight. They need to be clearly, unmistakably identified as to whether they are lit or not even in direct sunlight.
The issue is not absolute brightness but contrast. There
are several good examples of outdoor human readable
signs and signals, starting with vehicle dashboards and
traffic signals, electric and electronic advertizing signage.
You can improve the readability of your display by
increasing the contrast ratio, adding lightshields to block
direct sunlight, and adding optical filters that pass the
indicator light while attenuating incident light.
Posted: Mon Jun 30, 2008 5:07 pm
A blinking LED is much easier to see.
Tiny surface mount LED is also good if direct viewing is possible.
Many kinds of lenses often scatter or reflect other lights and make it difficult to to tell if the LED is on or not.
Posted: Mon Jun 30, 2008 5:08 pm
One of the most effective things to do is to surround the LED with flat black ("increasing the contrast ratio" as mentioned above). In fact, if you can have a piece of flat black plastic with only holes for the LEDs, that would improve daylight readability a lot. What you want to do is eliminate anything near the LEDs that would reflect a lot of ambient light.
If a narrow viewing angle is OK, the next step is to surround each LED with a black plastic tube, or set the LEDs into holes with the insides painted black.
Posted: Mon Jun 30, 2008 11:12 pm
Hmmm.... I shall give some more information on what I had in mind for the panel. This is still very preliminary, I haven't started actual design on the panel yet. The panel needs to be waterproof (it's being used outdoors, somebody might dump a bucket of water on it, the panel gets dirty and gets washed, etc. etc.). I was thinking about using one of the Pelican OEM cases, as it provides a waterproof solution - all you have to do is provide your own panel. http://pelican.com/oem/planning/electronic.php
Now what do I need on the panel? Several indicator LEDs (as previously described), a touch-sensitive transflective graphic LCD, a couple buttons. I was thinking that I could build the panel out of e.g. clear Lexan with cutouts for any LED lenses, the buttons, and the LCD. Then make the surface of the panel out of e.g. 10 mil thick Lexan with the label screen-printed on the bottom. This would then be attached to the main, thicker Lexan panel with an adhesive. So it would be mostly black background (behind the 10 mil Lexan) contrasting with the 5 mm LED. That way I can have descriptive text on the panel, with a backlight for use at night and a reasonably durable surface.
I assume I'm on the right track here? In which case I suppose I need to figure out what LED lenses to use, so that it's visible from many different angles, contrasting against the black background. I was thinking something like http://www.vcclite.com/CLB300
would be in order. Put that underneath a transparent section of the 10 mil Lexan label as described above. Then my remaining question is, how bright do I need my LED to be...?
Posted: Tue Jul 01, 2008 2:00 am
As I said previously, you will have to experiment.
Posted: Tue Jul 01, 2008 2:35 am
He will, but it seems bad that concepts like these have to reinvented each time someone needs to do it.
Posted: Tue Jul 01, 2008 2:43 am
Yeah, I know I'll be experimenting - but it'd sure be nice to be able to get a good start on it and have a better idea of what I'm doing before I go run out and buy some random parts!
I.e. if something's likely not to work, nice to know before I waste time and money on it... See macegr's last comment, it states my feelings exactly - I'm hoping to save some time by learning from others... Thanks for help so far & in advance for any further pointers!
Posted: Tue Jul 01, 2008 4:28 am
You could calculate it, knowing the maximum illuminance of the sun on the unit, and the properties of the visual system.
Posted: Tue Jul 01, 2008 8:47 am
A few weeks ago, I was looking for something similar (an indicator LED for outdoor use), so I asked a rep from Lumex to stop by our office. He brought a demo box with all kinds of LEDs in it, most of which could be seen in direct sunlight (ie. the sun at high-noon shining directly into the LED). We ended up using a blue LED with a diffusing lens. I don't remember any part numbers, and they weren't 5mm LEDs, but check out Lumex for high-brightness LEDs.
Also, Pelican cases rock. We make equipment for law enforcement, and the majority of it goes in a Pelican case. Recently we had an agency drive over a device with an armored truck. The antenna was ripped off the device, but the only damage to the box was a tire impression, and the electronics inside were all undamaged.
Posted: Tue Jul 01, 2008 11:19 pm
On Lumex: do you have any idea about some of that blue LED's specifications? I.e. how much current was used, ballpark figure of how big the LED, what type of package was it (surface mount or not, that sort of thing).
E.g. Lumex has blue LEDs capable of 15 lumen output at 350 mA current, but I'm guessing they use those in police cars and not indicator lights. Point is, there's a lot of possibilities from that company....
Posted: Wed Jul 02, 2008 6:05 am
You could also try with LEDs that are water clear. The contrast on/off is probably better as bright-red/clear rather than bright-red/dark-red
Posted: Wed Jul 02, 2008 10:58 am
I've had success with cheap (30 cent) "high intensity" 0805 SMT LEDs driven at around 10mA in pretty much direct sunlight at a distance of a few feet. As stated, flashing makes it easier to see.
Posted: Thu Jul 03, 2008 11:16 am
Try these: http://www.sparkfun.com/commerce/produc ... cts_id=528
I think all leds that:
1) are waterclear (for maximum contrast ratio)
2) are marked as "high brightness"
When designing something "sunlight viewable" I use a 10 or 15mA driving current.
If you can choose the colors, use red and blue, since are usually the brightest.