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Thread: laser eye safety for a scanning laser rangefinder

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    laser eye safety for a scanning laser rangefinder

    Howdy folks,

    I'm working on a design for a low-cost scanning laser rangefinder, and I need some help with the eye safety calculations. I've never done them before, and it's a little convoluted. The system will be using a 905 nm diode. Has anyone done this before?

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    Re: laser eye safety for a scanning laser rangefinder

    Are you doing this for a commercial application? If so I can privately recommend someone who will guide you through the process. Filing with the FDA is non-trivial, and figuring out your maximum exposure is just the first step.

    The FDA safety requirements for an autonomous consumer robot equipped with a scanning laser are a little bit more involved than a laser pointer. (...speaking from experience here!)

    In any case, I'll assume you're at least familiar with the AEL limits. I can show you to calculate them, but no warranty is implied... don't give yourself eye surgery.

    I'll also assume that you're pulsing your laser. If you're not, you should. As you experiment with these equations, you'll notice that you can increase your peak power by shortening your pulse time.

    Finally, I'm guessing you mean 'Class 1'. That is the only class safe enough to blast a human in the eyes without causing any damage. Since your laser wavelenth is 905nm, people will be unaware they're being blasted. This makes the product much more dangerous than a visible laser, so Class 1 is a good plan.

    You'll need to scale your units appropriately. If your pulse time is in milliseconds and your peak power is in milliwatts, divide by 1000!

    One parameter below deserves explanation: 'Pulses In Eye' means how many pulses can strike the eye as your laser scans past. Figure out the closest a human eye can get to the laser beam. Then measure how many degrees of your scan are covered by that eye. You might assume the average cornea is 12mm and go from there.

    If you're actually going for FDA approval, it is vital that you design in safety systems to shut down the laser when the scanner stops rotating due to an unexpected jam or other mechanical failure. Otherwise, the theoretical 'Pulses In Eye' can be very high...

    Energy (J) = Pulse Time (Sec) * Peak Power (W)
    C4 = 10^(0.002*(Wavelength (nm) - 700))
    C5 = (Rotation (Hz) * 10 (Sec) * Pulses In Eye (Per Rotation)) ^ -0.25
    C7 = 1 (1 for 700 to 1150nm)

    AEL Rule 1 (J)
    Maximum for Class 1 = (7*10^-4) * (Pulse Time^0.75) * C4
    Your Value = Energy (J)

    AEL Rule 2 (W)
    Maximum for Class 1 = 0.00039*C4*C7
    Your Value = Energy (J) * Rotation (Hz) * Pulses In Eye (Per Rotation)

    AEL Rule 3 (J)
    Maximum for Class 1 = AEL Rule 1 (J) * C5
    Limit = Energy (J)

    - Nammo

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    Re: laser eye safety for a scanning laser rangefinder

    Nammo,

    Thanks for the reply! I'l take some time and look over what you've got there and compare it to some of the things I've been reading.

    My goal is an open-source open-hardware project, and to target it for a sub $500 sensor. I think it's feasible, but maybe not eye-safe feasible! I'm basing my design on a whitepaper from Maxim-IC - HFRD-40.0. It's a design for a laser tape-measure. Last I checked you had to request it - it's not on their website. They have a neat little circuit with a lot of flexibility.

    Some other options for improving eye safety I'd thought of were:
    1) Increase the wavelength to 1550 nm - much greater safety, but also reduces the detector sources severely. Maxim was using a standard PIN photodiode for detection. I've spec'ed some InGaAs photodiodes at Hamamatsu, but I'm not sure on the price.
    2) Scan with a red diode inline with the IR - the divergence of the IR will be greater than the red, so I don't know that this is a complete option.


    But, basically if this is going to be open then I want to do everything possible to make sure nobody gets hurt. I can't, in good conscience, put something out that isn't properly protected, as you mentioned.

    I'd love to have both an electrical interlock system - with a redundant IC or circuit, and even a mechanical shutter that closes if the scanning mirror stops. Not sure how I'd do this, but I'm open to help on the project.

    Maxim was using a 905 nm diode with 5W peak power. The circuit has an adjustable rep-rate - it's set at 1/128us for their demo unit - and and adjustable number of pulses per measurement. Their average power was 3.5mW, which is too high. I can also adjust the on time of the laser to some extent. Unfortunately, their current tape-measure unit has a very slow measurement time - 16ms, so I'll need to speed things up in order to get any sort of decent scan rate.

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    Re: laser eye safety for a scanning laser rangefinder

    Quote Originally Posted by gallamine View Post
    My goal is an open-source open-hardware project, and to target it for a sub $500 sensor. I think it's feasible, but maybe not eye-safe feasible!
    Neato vacuum cleaner is $400 for the whole thing, uses IR laser and, since it's a household item -it must be eye-safe. So I think it answers feasibility question (and hopefully puts a little pressure on that $500 price ).
    Last edited by RobotAtlas; 11-16-2010 at 08:03 PM.

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    Re: laser eye safety for a scanning laser rangefinder

    Quote Originally Posted by RobotNV View Post
    Neato vacuum cleaner is $400 for the whole thing, uses IR laser and, since it's a household item -it must be eye-safe. So I think it answers feasibility question (and hopefully puts a little pressure on that $500 price ).
    Well, that depends a bit on the environment he wants to work in. If he's trying to make it work outdoors, it's an entirely different deal, low-cost laser range finders tend to have issues in direct sunlight....

    -Fergs

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    Re: laser eye safety for a scanning laser rangefinder

    RobotNV,

    Yes, Neato's product is really cool, but it's 1) not open-source or open-hardware 2) fundamentally limited on accuracy based on the pixel density of the CMOS sensor they're using for triangulation and 3) has a low refresh-rate (10hz) and max distance (6m) and only 1 degree resolution (4" separation at 6m).

    The BOM of materials I have is less than $100, so I think the $500 is very high upper end, in case of unforeseen problems.

    LnxFergy,

    Outdoor use would be optimal. A narrow field-of-view and filters can help some of the interference. At 1550nm H2O is pretty absorptive, so that might lend to using that wavelength: http://www.spectralcalc.com/spectral..._intensity.php

    But, then again it looks like 905 nm isn't too bad either: http://en.wikipedia.org/wiki/File:Solar_Spectrum.png
    It would probably have to be pointed at a downwards angle, but that's where user testing and ingenuity come in!

    Thanks for the input folks! I don't want to make something that isn't useful.

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    Re: laser eye safety for a scanning laser rangefinder

    Quote Originally Posted by gallamine View Post
    Yes, Neato's product is really cool, but it's 1) not open-source or open-hardware 2) fundamentally limited on accuracy based on the pixel density of the CMOS sensor they're using for triangulation and 3) has a low refresh-rate (10hz) and max distance (6m) and only 1 degree resolution (4" separation at 6m).
    If you can improve any of these parameters at a reasonable price, then that would be a great project. We definitely need a better laser range finders (mainly cheaper than $1000).
    Last edited by RobotAtlas; 11-16-2010 at 08:05 PM.

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    Re: laser eye safety for a scanning laser rangefinder

    Quote Originally Posted by gallamine View Post
    Yes, Neato's product is really cool, but it's 1) not open-source or open-hardware 2) fundamentally limited on accuracy based on the pixel density of the CMOS sensor they're using for triangulation and 3) has a low refresh-rate (10hz) and max distance (6m) and only 1 degree resolution (4" separation at 6m).
    I think Neato's product is pretty cool too!

    At 10hz and 1 reading per degree, that's 3600 samples per second or one exposure per 277uS. If you intend to significantly exceed that performance, what sample rates are you thinking of? 10KHz? 20KHz?

    If you want to exceed 6 meters, you get into some really crazy analog design. Although the laser stays coherent on the way out, all you get back is the reflection of the beam... a tiny signal that falls off in amplitude with the usual inverse square law.

    In other words, if you're getting 1 volt at 10cm, you can expect 278 microvolts at 6m. By 12 meters you're at 69 microvolts, by 30 meters you're at 11 microvolts.

    Typical time of flight laser distant meters deal with these tiny signals by integrating the phase difference over time. The problem with this approach is that you need to sample for many milliseconds to get better precision than +/- 10cm. One laser tape measure I own samples for 500ms to get under 1mm precision clear out to 30 meters, and it's cheap!

    Of course, if you are really good at analog design, you might know ways to keep the noise out and get more accurate readings much faster. But then you want a really fast, accurate, high resolution, ADC. That's an exciting challenge of its own.

    "Low cost" laser scanners, by which I mean $1K-$10K, generally can't see past 5 meters. For example, the SICK S100 can only see out to 4.5 meters indoors and that still requires a minimum surface reflectively of 10%. You can get better results with highly-reflective surfaces but that's unrealistic for most homes.

    Even the cheap laser scanners have delicate analog electronics. One $2K laser scanner I took apart had metal shielding over all the analog bits, and was layed out like a cell phone radio.

    Could you briefly describe the approach you'll use? Maybe I can offer some more tips!

    - Nammo
    Last edited by Nammo; 09-20-2010 at 10:52 PM.

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    Re: laser eye safety for a scanning laser rangefinder

    Quote Originally Posted by Nammo View Post
    "Low cost" laser scanners, by which I mean $1K-$10K, generally can't see past 5 meters.
    Well I have quite some experience with the Hokuyo UTM-30LX (about 5K$ atm IIRC). It works pretty well in&outdoors, with a range of about 30 meters (definitely more than 10m in direct sunlight outdoors, just checked a logfile).

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    Re: laser eye safety for a scanning laser rangefinder

    Quote Originally Posted by SK. View Post
    Well I have quite some experience with the Hokuyo UTM-30LX (about 5K$ atm IIRC). It works pretty well in&outdoors, with a range of about 30 meters (definitely more than 10m in direct sunlight outdoors, just checked a logfile).
    Wow, the specs on this are really slick. It kills the SICK LMS100 in distance, and for the same cost... Obviously I haven't been paying enough attention, since it came out last year!

    If only I had $5K burning a hole in my pocket... can you tell me any more about your experience with this unit?

    It's definitely inspiring that a Class 1 product can achieve these kinds of distances and speeds.

    I found a tear-down here:
    http://www.iheartrobotics.com/2010/0...0lx-laser.html

    From the pictures, it looks like the analog front-end might be a custom ASIC.

    I think with the right analog expertise, a hobby laser scanner is quite achievable. Even though the Neato's scanner is "cheap", it's good enough to do very impressive indoor navigation. I'd take something that "limited" for $100 any day!

    - Nammo

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