4 ms·

When you assumed the illuminated area to be 0.1m^2, you assumed the light source to not be a laser, spread evenly over a 30×30cm square (to get 1000 lux). Reali

When you assumed the illuminated area to be 0.1m^2, you assumed the light source to not be a laser, spread evenly over a 30×30cm square (to get 1000 lux). Realistically, a several-watt fluorescent bulb would throw 100 lumens. And would of course not be harmful to look at. The point of a laser is that the light is /not/ scattered over a square, its a very tight circle. If it was instead over a 3mm diameter circle, that 100 lumens would be over 20 million lux. You would, of course, not want to stare into that! Its unclear from the picture exactly how long the "line" being drawn is (3mm diameter by even 7m long is only 0.02m^2), but a hypothetical kid staring into the beam would only get hit by part of the figure; the laser beam is moving to project the entire figure. Only the part that actually goes into the kid's eye matters, and the impact is lessened by the fact that its moving and also by the blink response. BTW: Wikipedia tells me direct sunlight is up to 130,000 lux, so that's probably around the illuminance you need, not 1,000 lux, btw.

gjm11 15y ago
I didn't assume that anything is spread evenly over a 30x30cm square, nor did I assume that the light source isn't a laser. I assumed that the optics were arranged to get the light from the laser distributed in the appropriate way. (There are lots of ways to do this. You could do it with a single DOE, which would probably be simplest if you didn't mind poor image quality; or with a few lenses to give you a nicely diverging beam, and then something to absorb light where you don't want it -- but that would waste a lot of power.) It doesn't stop being laser light, and it doesn't stop being subject to laser safety regulations, just because it isn't coming out as a nice collimated beam. The point of a laser is not only that the beam is very small. It's also that it's possible for the light to be focused on a very small region of the retina, even if the beam is diverging when it enters the eye. The illuminance needed for the figure to be clearly visible is of course an average over time (roughly, the integrating time of the eye). If you use a scanning system (which seems a bit of a funny choice for something projecting from a moving object), then indeed the beam may only be in your eye for a small fraction of the time -- but it will also be much brighter (while it is in your eye) than I assumed. The way in which laser safety changes when you consider a pulsed system rather than a continuous one (which is what you effectively get in this scenario) is complicated, but generally for a given average power it's worse to have it delivered in pulses. In any case, though, the critical scenario for a laser safety assessment is when someone manages to get close enough to the laser to get rather a lot of the beam into their eye. (You can have safety mechanisms that aim to make that impossible, of course.) For laser classes higher than class 1, the blink response is already taken into account in the permitted exposure levels. (Class 1 safety, which this device would be miles away from having any chance of achieving, is extremely strict and basically amounts to "if you are really determined to damage your eyes using this device, the best way would be to hit yourself in the face with it". To be class-1 safe, you need to be safe even without the blink response. But that's not the issue here.) I chose the figure of 1000 lux precisely because I wanted a low estimate, to make the point that even being very generous to this thing it would be difficult to do it without a laser powerful enough to raise safety issues. Obviously if you want the image to be visible even in really bright direct sunlight then the numbers need to be considerably higher.
- derobert 15y ago
  I may be missing something, but when you take 1000 lux and 0.1m², and come up with 100 lumen, the obvious way to put those numbers together is 1000lux × 0.1m² = 100 lumen. That's where I got evenly spread over a 30cm square (30 being the square root of 100, to one significant digit). I'm not an expert in the field by any stretch of the imagination, but something seems very wrong with these calculations in that throwing a several-thousand lux image of that size would be easily done with a $500 business projector.
  - gjm11 15y ago
    Yes, I assumed that the light is spread over that area. I didn't assume it's spread evenly over a square of that area. Maybe the shape of the 0.1m² region wasn't important to whatever you had in mind, but then I'm really unsure what your point was since I'd already said I was assuming an area of 0.1m². You can get very bright $500 business projectors. They aren't emitting laser light. If they were, they would have serious problems getting non-scary laser safety classifications. Part of that is because the rules governing laser safety classification are kinda paranoid. Part of it is because coherent light really is specially dangerous. (A 100W light bulb has a luminous efficiency of about 2.5%. A 2.5W laser is really, really, really dangerous.) Anyway, you could do your eyes some serious damage with that $500 business projector. It just happens that there aren't such stringent regulations governing incoherent light sources, because the worst case isn't quite so scary as it is with lasers.

gjm11 15y ago

I didn't assume that anything is spread evenly over a 30x30cm square, nor did I assume that the light source isn't a laser. I assumed that the optics were arranged to get the light from the laser distributed in the appropriate way. (There are lots of ways to do this. You could do it with a single DOE, which would probably be simplest if you didn't mind poor image quality; or with a few lenses to give you a nicely diverging beam, and then something to absorb light where you don't want it -- but that would waste a lot of power.) It doesn't stop being laser light, and it doesn't stop being subject to laser safety regulations, just because it isn't coming out as a nice collimated beam. The point of a laser is not only that the beam is very small. It's also that it's possible for the light to be focused on a very small region of the retina, even if the beam is diverging when it enters the eye. The illuminance needed for the figure to be clearly visible is of course an average over time (roughly, the integrating time of the eye). If you use a scanning system (which seems a bit of a funny choice for something projecting from a moving object), then indeed the beam may only be in your eye for a small fraction of the time -- but it will also be much brighter (while it is in your eye) than I assumed. The way in which laser safety changes when you consider a pulsed system rather than a continuous one (which is what you effectively get in this scenario) is complicated, but generally for a given average power it's worse to have it delivered in pulses. In any case, though, the critical scenario for a laser safety assessment is when someone manages to get close enough to the laser to get rather a lot of the beam into their eye. (You can have safety mechanisms that aim to make that impossible, of course.) For laser classes higher than class 1, the blink response is already taken into account in the permitted exposure levels. (Class 1 safety, which this device would be miles away from having any chance of achieving, is extremely strict and basically amounts to "if you are really determined to damage your eyes using this device, the best way would be to hit yourself in the face with it". To be class-1 safe, you need to be safe even without the blink response. But that's not the issue here.) I chose the figure of 1000 lux precisely because I wanted a low estimate, to make the point that even being very generous to this thing it would be difficult to do it without a laser powerful enough to raise safety issues. Obviously if you want the image to be visible even in really bright direct sunlight then the numbers need to be considerably higher.

derobert 15y ago
I may be missing something, but when you take 1000 lux and 0.1m², and come up with 100 lumen, the obvious way to put those numbers together is 1000lux × 0.1m² = 100 lumen. That's where I got evenly spread over a 30cm square (30 being the square root of 100, to one significant digit). I'm not an expert in the field by any stretch of the imagination, but something seems very wrong with these calculations in that throwing a several-thousand lux image of that size would be easily done with a $500 business projector.
- gjm11 15y ago
  Yes, I assumed that the light is spread over that area. I didn't assume it's spread evenly over a square of that area. Maybe the shape of the 0.1m² region wasn't important to whatever you had in mind, but then I'm really unsure what your point was since I'd already said I was assuming an area of 0.1m². You can get very bright $500 business projectors. They aren't emitting laser light. If they were, they would have serious problems getting non-scary laser safety classifications. Part of that is because the rules governing laser safety classification are kinda paranoid. Part of it is because coherent light really is specially dangerous. (A 100W light bulb has a luminous efficiency of about 2.5%. A 2.5W laser is really, really, really dangerous.) Anyway, you could do your eyes some serious damage with that $500 business projector. It just happens that there aren't such stringent regulations governing incoherent light sources, because the worst case isn't quite so scary as it is with lasers.