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    Simple Laser detector

    Hi optics gurus,
    I'm looking for a way to detect presence of a red or green laser line. If the laser line is within 50mm at least it should be detected. The sensor can be slow.
    I thought about a construction laser line detector. How is the line detected there? Does anyone know what kind of sensor they use? Here's an example: https://www.ebay.com/p/Adirpro-LD-8-...d=251980297151

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    Re: Simple Laser detector

    Most use a simple array of phototransistors. The line is detected in the background light by it's pulsating nature rather than color or intensity.

    You could use a line CCD photodetector as used in a document scanner as well. They are clocked and sequentially scan a line of CMOS detectors, the line position along the detector is found by counting the number of shift pulses before it is found.

    Some ideas here (in German): https://www.heise.de/make/projekte/E...n-1745504.html

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    Re: Simple Laser detector

    Thanks,
    thats really helpful.
    Do you know if these arrays of phototransistors also use some lenses or diffuser or similar? Or would you have to make a very tight packed array to not get any "dead" areas between the components?



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    Re: Simple Laser detector

    Hi,

    It's rather straightforward:
    If the laserbeam is more narrow than the gap between your detectors you get in trouble.

    Klaus
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    Re: Simple Laser detector

    You'll want to setup a specification first, e.g. intended resolution, operation distance.



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    Re: Simple Laser detector

    Well I dont really need any sort of resolution, I only want to detect presence. I looked at some phototransistors but it seems that the chip area of them are small, such as this
    https://eu.mouser.com/ProductDetail/OSRAM-Opto-Semiconductors/SFH-3716
    It has an area of 0.29mm2 which roghly givs 0.5mm length. I dont know the laser line width, I have just looked at it on a wall and it was kind of diffuse. If I take 1mm width the sensors would need to be placed on a row with max 1,5mm pitch. That gives 33 sensors to cover 50mm. Does this makes sense?



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    Re: Simple Laser detector

    If using discrete sensors that does seem correct. Bear in mind that they do not have to be lined up vertically, it's the horizontal spacing that matters.
    An alternative is to use one sensor and a moving reflector, like the line scanner but in reverse. If you use that method, the electronics is simpler but the mechanical part is more difficult. You could also consider another method which is less accurate but may suffice: diffusing the incoming line so it spreads across fewer sensors, in theory you can look for pairs of sensors and decide from their relative outputs where the beam center lies.

    Brian.
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    Re: Simple Laser detector

    Hi,

    theoretically you just need a diffusor and a sensor behind the diffusor.

    You need to consider how to find out if the light comes from the laser or not.
    Pulsing the laser with a known frequency is a good way.

    A filter, that attenuates most other light frequencies could be a benefit.

    Klaus
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    Re: Simple Laser detector

    Thanks, you're quick!
    No moving parts, mechanics not my thing
    I'm not into optics that much either so I dont know how to design a diffuser from 50mm to 30mm2 sensor chip area but the idea intrigues me. Anyone got an idea?



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    Re: Simple Laser detector

    Hi,

    I dont know how to design a diffuser from 50mm to 30mm2 sensor chip area
    in simplest case: diffusor = opaque (translucent, but diffusing) plastics plate. Dimension like you want (50 x 10 mm maybe). place the sensor behind this opaque plate.
    I recommend to keep min distance (plate to sensor) of at least the max dimension of the plate (50mm)

    better:
    use a box with transparent cover.
    Maybe something like this:
    Click image for larger version. 

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    use a dusty foil as diffusor (dimension and place as you like). Use black adhesive foil or adhesive metal foil to protect all other areas from light.
    Place the sensor at the bottom of the box (opposite to the dusty foil)

    Klaus
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    Re: Simple Laser detector

    The 'line' is already a pulsating flash of laser light. Typically, the emitter is a single laser diode pointing at a rotating mirror so the reflection 'sweeps' the surrounding and the speed is fast enough that it looks like a continuous line. A low-pass filter with cut-off just above rotor speed (~ 200Hz) is generally enough but driving the laser at higher frequency (> 10KHz) and using a second filter might help to avoid other light sources.

    Am I right in thinking you are looking for something to detect the laser line height above ground in the same way surveyors do it with a theodolite or do you just want an indication of whether a laser line is present or not?

    Brian.
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    Re: Simple Laser detector

    Quote Originally Posted by betwixt View Post
    The 'line' is already a pulsating flash of laser light.
    You can use cylindrical lens (or other optics or even diffraction pattern) to make a line-like laser dot from ordinary CW laser beam.



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    Re: Simple Laser detector

    Great answers guys! The line is most certainly created with a cylindrical lens in my case. Just out of curiosity, how is a line made with a diffraction pattern?

    I gonna order some photo transistor components and a box to try the idea. I have doubts whether the laser line can be detected with the diffuser method and a laser line at a some meters distance and at some angle but it will be funny to test.



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    Re: Simple Laser detector

    Quote Originally Posted by floke View Post
    If the laser line is within 50mm at least it should be detected.
    The diffuser idea looks practical. How about a clear jar containing liquid with tiny reflective particles suspended in it?



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    Re: Simple Laser detector

    There are four quadrant photodiodes made specifically for this purpose. You can use one of them.

    If the laser beam is frequency modulated, you can easily detect the laser signal even if it is not visually seen. After a couple of meters, the beam will be visible to the photodiode even at a distance of 1 inch or so from the main beamline.

    With a four quadrant detector you can also determine whether the beamline is on the left or right, top or bottom.

    If you are having lots of ambient light, you will need a narrow band filter, perhaps interference filter corresponding to the laser wavelength.



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