can unfocused laser light cause damage?

[Reaver789]

im building a 200mw red laser 650nm, and i dont have googles yet, but if i dont put lenses on it, can it still cause eye damage even tough it is unfocused?

 

[marcuss]

In your case (assuming exactly 200mw) the minimum beam diameter that is safe is approx. 16 cm. In other words you must work with goggles.

 

[Reaver789]

ok thank you

 

[Swordofsalvation]

hey nice rep marcus

 

[suiraM]

marcuss,

You are assuming he is looking at the beam, yes?

A perhaps more interesting question is whether a red laser diode could be used as a flashlight.

Using a set of meniscus lenses, the beam can be set to exit the flashlight with a 50mmØ diameter and a spread of about 30x15° or so. That is reasonably close to the aspect ratio of our central vision, while the initial beam starts off with less energy per surface area than was used in the Mayo Clinic experiments (although still not technically eye-safe at point blank per se, but given the need for intentional abuse to pose a risk, it seems acceptable) when the laser is set for 200mW.

635-660nm is outside the range that bleaches the rods during photopic conditions, while still providing quite adequate power to see with the red cones. Both 635nm and 660nm at the same time might give a more natural appearance, as the green and red cones would be stimulated slightly differently, although the green cones are still going to be pretty much left in the dark (pardon the pun).

The ability to see clearly enough to work in darkness, while retaining scotopic vision is useful in a variety of conditions, and it eliminates the problem with normal flashlights limiting the effective field of vision to a small angle that is illuminated and having everything else completely dark. Moonlight is enough to spot a car with broken headlights, but not if your vision has been bleached down by a strong flashlight.

I've been thinking about putting a high power 635nm diode in a flashlight for ages, but have always had some reservations about the safety of doing so, although I'm not sure what the issues might be, provided that one takes care to ensure a large initial diameter and significant divergence.

If anyone else has any input on it, that'd be good.

 

[Reaver789]

Wow, thanks for that reply suiraM, thats interesting, what if you put a filter that blocks some of the red light as its coming out of the flashlight, to make it less dangerous, kind of like putting goggles directly on the laser diode opening, it would still be bright enough to light up a room, ut not enough to hurt your vision.

 

[suiraM]

I'm pretty sure turning down the power would do the same thing.

And I was thinking outdoor use.

 

[Linkreincarnate]

In your case (assuming exactly 200mw) the minimum beam diameter that is safe is approx. 16 cm. In other words you must work with goggles.

How do you come to that number? Whats the equation to determine the safe diameter? Does safe mean temporary blindness or all symptoms gone when the light isn't in their face?

 

[Benm]

I've been thinking about putting a high power 635nm diode in a flashlight for ages, but have always had some reservations about the safety of doing so, although I'm not sure what the issues might be, provided that one takes care to ensure a large initial diameter and significant divergence.

If anyone else has any input on it, that'd be good.

What exactly would be the point of using a laser? I'm sure its possible to get the desired output beam pattern using a LED, which is a hole lot cheaper per watt of light produced (especially at 635 nm). Also, the light of a led as a broader wavelength range, which for this application isnt bad... so why not just put a red power led emitter in a led flashlight and be done with it?

Whats the equation to determine the safe diameter?

It is based on assuming what is 'safe'. 1 mW entering the eye is generally accepted as safe, as long as you are able to close your eyes normally. A fully dilated pupil has an area of around 0.5cm2, so the 'safe limit' should then be 2 mW/cm2 - or a 200 mW laser illuminating an area of 100 cm2. That would be a circle with radius 5.6 cm, or 11.2 cm diameter*.

* caclulation assumes equal distribution of light over the circle. Typical lasers output a pattern with more intensity towards the center, and you should build in a safety factor for that.

 

[davidgdg]

What exactly would be the point of using a laser? I'm sure its possible to get the desired output beam pattern using a LED, which is a hole lot cheaper per watt of light produced (especially at 635 nm). Also, the light of a led as a broader wavelength range, which for this application isnt bad... so why not just put a red power led emitter in a led flashlight and be done with it?

I have used my Stingray 2 focusable Homemade (great laser!) as a torch. It outputs 280mw of 650nm, is nicely focusable and produces a crisp circle of light any size I want.

That said, it's a lot  for a torch !

 

[Benm]

Well, 200 mW of 650 is very affordable, but getting that power at 635 would make it a prohibitively expensive flashlight

As for the amount of light produced: a good 3 watt power led puts out over 200 mW of light... the efficiency has gone way up for those over the last couple of years. Getting the 'spotlight circle' might be possible with certain optics as well.

 

[marcuss]

[quote author=marcuss link=1231711340/0#1 date=1231712310]In your case (assuming exactly 200mw) the minimum beam diameter that is safe is approx. 16 cm. In other words you must work with goggles.

How do  you come to that number? Whats the equation to determine the safe diameter? Does safe mean temporary blindness or all symptoms gone when the light isn't in their face?[/quote]

Safe is the laser radiation to which an eye can be exposed without damage. There is a table in ANZI to calculate this value. I assumed the worst case scenario. Usually an exposure of 0.25 sec (blink reflex) is considered but according to various experiments the blink reflex is not always present, expecially if one knows that there is a beam, so I assumed a higher value.

 

[Linkreincarnate]

What would be a safe beam diameter for a 300 mW 532nm laser? At what point does temporary flash blindness occur?

 

[marcuss]

What would be a safe beam diameter for a 300 mW 532nm laser?  At what point does temporary flash blindness occur?

In your case a beam diameter of 19.55 cm is safe (for >10 sec) but, if I didn't do any mistake, 12.3 cm would be safe if there is a blink reflex after 0.25 sec of exposure. To be sure, recalculate the values using the following (for 500-700 nm):

MPE (maximum permissible exposure) for ocular exposure (point source) :
Exposure duration t (s): 10 to 30000 -> MPE = 1mW/cm2
Exposure duration t (s): 18 X (10 [sup]-6[/sup]) to 10 -> MPE = 1.8 X (t [sup]0.75[/sup]) mJ/cm2

I don't know what is the minimun value at which temporary blindness occurs.

 

[Benm]

I don't know what is the minimun value at which temporary blindness occurs.

There is no fixed value. The effect of seeing dark spots is usually caused by temporary depletion of sensitive pigments in the retinal cells. The regulation of these pigments is a normal function of the eye, allowing you to see in both bright and dim conditions. It is much slower than the pupil response, which is why the dark spots can last for several minutes (similar to what happens when walking from a bright to a dim environment).

From a given exposure to light, the severity and longevity of the dark spots depends at least on ambient light, where they last longer in a dim environment.

The spots are caused by light/dark adaptation ocurring locally instead of over the entire retina, which is the normal situation when switching environments.