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Im building a laser target. It consists of a box, with a black plexiglass circle in the center of one of its sides, and a larger white circle around it. The black for the inside shots, and the white for the shots close to being inside.

There will be a camera(and a fresnel lens) to detect shots inside the inner circle, but for the outer circle im relying on light sensors.

The problem is that for a light sensor to work... well, there has to be light, and while a laser has light its way too focused to be measurable when not aiming directly at the light sensor.

To be more precise, aiming directly at the sensor means 50k+ lux(sensor limit), but aiming outside results in less than 4 lux of variation (the smallest unit that the sensor is able to measure when at its fastest timming), which is not enough to measure with confidence.

I tried to use a thin layer of spongy polyethylene, and it does glow and diffuses a good part of the light evenly, but still has a low overall variation. There's still one focused point, and a slight gradient coming from it.

The box is not thick, it should have at most 30cm from the target to the opposite side. Any material which glows even more than polyethylene when illuminated? Mirrors and a polyethylene cylinder around the sensor would also work, but it would be a pain to build, and it can send a laser back to someone's eye. And that's not to mention that it would be possible to reflect the shot to the area which the camera evaluates as a valid shot, unless isolated, which would mean at least three light sensors to cover the area around the valid shot area, and ... enough pain!

Ideas to diffuse that light? ;)

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    $\begingroup$ If you have an old (dead) laptop, you will find there is a diffusing screen behind the LCD (part of the back illumination). But I don't know how to take it out without breaking... $\endgroup$ – Floris Jul 2 '15 at 3:02
  • $\begingroup$ Maybe, the inverse arrangement would work better? You put the light sensor in the middle of your target (maybe with some lens to increase the detection radius). And you put the camera looking at the backside of the target: if the white circle is thin, the camera should be able to see the light spot. $\endgroup$ – gigacyan Jul 2 '15 at 6:54
  • $\begingroup$ I have no dead laptops unfortunately. I'll see if I can get one to try. @gigacyan The inner circle has a 60mm diameter, but it goes from 7.0 points (diameter) to 10.9(middle). Since the camera has to shoot in VGA to have the required 90fps, it would become impresise if i deviate it too much from the center, or simply turn it to the backside. The fresnel is there to help it, but it doesn't if the camera is not centered, or simply not looking at it. This is meant to be for precision shooting, so those tricks probably don't work :( $\endgroup$ – SOMN Jul 2 '15 at 15:11
  • $\begingroup$ Craigslist can be a source of cheap dead laptops, if you have the cash to pay for it. $\endgroup$ – Kyle Kanos Jul 4 '15 at 0:55
  • $\begingroup$ People here don't really use craiglist. I think that's used mostly in the North America. I took a look at the classifieds, but the mentality where I live seems to be "Hmm, I have a broken laptop. What if I tried to sell it like if it was brand new?" I'll try to find some broken laptop screen and pay like 1-2€ for it instead of 100€. After all I just need a bit of the screen to test. $\endgroup$ – SOMN Jul 5 '15 at 2:28
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White paper works well as a diffusor. If you can make the box a couple centimeters thick, then you can place sensors at least 10cm apart and they will easily detect the laser's light. For signal processing purposes you always want to modulate the laser with a couple kHz and use narrowband AC amplifiers to detect the signal of your photodiodes. This will basically eliminate most background signals and give you orders of magnitude more dynamic range on your sensors. If you are detecting with a microcontroller, I would also use a logarithmic amplifier to limit the range of the signal to something that is useful for the controller (i.e. 40dB signal range, at most). The overall electronics circuit won't require more than maybe a quad opamp (one AC preamp x10, a bandpass filter and one opamp for the logarithmic amplifier, which leaves you with one more amp for a second bandpass or more gain, if necessary).

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  • $\begingroup$ Maybe two separated layers of greaseproof paper, and sides filled with regular white paper. Modulation is not possible for this prototype. There are guns which use it but with different signals, and guns which don't at all. I want something universal. If possible I would prefer to use my sensor module (BH1750FVI) instead of having to get photoresistors/diodes, just because of the time I would have to wait for them to arrive. This sensor is not as sensible, nor as fast as I wanted. A few layers of regular paper, or an even denser material would make the sensor unable to detect the laser. $\endgroup$ – SOMN Jul 2 '15 at 2:23
  • $\begingroup$ Give it a try. If your sensor does the trick, even better, and if it doesn't, photodiodes are cheap. I don't know where you live, but in the US you can get delivery in one or two business days... but I get why you like the idea of a prefab sensor that works out of the box. It takes a couple of days to get a well working circuit, even for those who know what you are doing. Good luck! $\endgroup$ – CuriousOne Jul 2 '15 at 4:46
  • $\begingroup$ I live in Portugal, and here the retailers just abuse with the cost. Most of the things they sell come from china, but while buying these things straight from china costs less than 10 cents for a part, from retailers its really hard to get them for less than some euros. I can order some, but I was hoping to have something ready before August. Last night I tested a laser@10 meters with several materials, and greaseproof paper seems like a winner from what I have at home. Doesn't reflects, diffuses evenly @30cm, and its dirt cheap. I'll try it and then regular paper. $\endgroup$ – SOMN Jul 2 '15 at 15:17
  • $\begingroup$ @CláudioPereira: Sorry to hear that. I know that it's harder to get things in Europe than in the US but I wasn't aware it's that bad (my experience is with Germany only). Do you know about professional electronics distributors like Mouser? They are not cheap, but at quantities above 10-50 the prices are OK and these guys are serving professional needs, i.e. shipping is fast. Anyway, I am happy that you have found a suitable material. Paper is amazing, it's cheap and you have so much selection. $\endgroup$ – CuriousOne Jul 2 '15 at 18:17
  • $\begingroup$ Its not about being harder, its about shipping costs, warranties and such things. (Nothing beats the chinese free worldwide shipping, without any kind of warranty and huge lack of quality) Either we have better costumer service, or better prices, can't have both. I know about those retailers, but I don't think it is worth it to pay for a few photoresistors/diodes, only for big-ish orders. I'll try the given tips and mark the answer after finding the one that gives the best result. If nothing works I'll just fill the thing with photodiodes and repeat the experience. $\endgroup$ – SOMN Jul 2 '15 at 23:16
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You might try something like a ring of plexiglass in front of the white layer, which would use internal reflection to carry the light to sensors.

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  • $\begingroup$ It would not diffuse the laser enough. After it would be as focused as after glass... I'll test it, but I highly doubt it will work as pretended. $\endgroup$ – SOMN Jul 2 '15 at 2:14
  • $\begingroup$ No, the idea is that you have a layer of clear plexi in front of a white diffusive reflector. The internal reflection of the diffused light might improve illumination of several sensors. $\endgroup$ – WhatRoughBeast Jul 2 '15 at 2:56
  • $\begingroup$ Trying doesn't hurts I guess. Since I'll have to get plexi either way, I'll test it. $\endgroup$ – SOMN Jul 2 '15 at 15:19
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How about one or more layers of the plastic diffusers used in ceiling fluorescent fixtures? Go to a Big Box Hardware Store and you'll likely find acrylic sheets embossed with various diffusive patterns.

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  • $\begingroup$ Yea, good idea. I'll try to find a "frost effect" one and see if it is enough to diffuse a laser. They seem to have a week diffusion capability for a laser, but since they are translucid I can simply get a few layers of them ;) $\endgroup$ – SOMN Jul 2 '15 at 2:27

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