Laser centre Finder Safety

I was a little worried about the article on the laser centre finder in the latest ME. Max output at 5mW seems very high for this use.

Using my laser safety data from work

The max permissible exposure at this wavelength is 6.36Joules/sq or 25.4 Watts/sqm. This is assuming a point source and a 0.25 sec aversion reaction time. I don’t know the spot size so I am assuming 1mm giving an area of 3.85 x(ten to the minus 5) sqm. Therefor the irradience is 5mW/the area of the spot which gives 130Watts/sqm which is just over five times the safe value. My conclusion is this laser is not safe to view directly. With the amount of metal around sooner or later you are bound to get a reflection which your normal aversion reaction to bright light would not protect you from. I take on board the comments in the article re safety especially the viewing via USB microscope and I am not suggesting things cannot be arranged in a safe manner but my view is that the laser output needs to be reduced to under 1mw to be completely safe. I would suggest that the use of the polarizer is an extremely good idea as it should cut the output but I would like to see a full data sheet on the laser. Please don’t think I am scare mongering but just be aware what you are doing. I am not a laser safety officer but I my work involves optical safety issues from time to time.

I would welcome further comments.

regards Martin

Judging by photo No 5 in the article I would say the dot is no more than 0.2mm dia, so somewhat less area than you calculate.

Can you also say how you arrived at your 3.85 area as I can't get that from 1mm dia

Suppliers data states 0.1-0.5mm depending on reflectivity of material

J

Edited By JasonB on 16/04/2013 13:20:45

A bit more reading suggests the laser is the old class 3A which places it in 1M or 2M which is safe unless viewed through opical instruments.

Also how is viewing via a USB scope dangerous as you will be looking at the screen which will not be the same wave length as the actual lazer?

My notes say if the beam diameter is less than the limiting aperture (the iris at 7mm dia) then calculate the area using the limiting aperture dia ie Pi x 0.007squared/4 = 0.0000385

Class 2 lasers have an upper limit of 1mW

I agree viewing via USB scope is safe. I think I said that.

Martin

Lasers for scientific and alignment purposes were pretty much a fixture of my working life prior to redundancy in 2004. As were arguments concerning whether or not the thing in question was actually safe! The calculations involved to resolve a specific case are not simple and justifying the assumptions on the optical side can be even harder. By the early 2000s the expanding elf'n safety industry had produced regulations whose local interpretation tended to go well past the sanity event horizon accelerating hard towards total lunacy. Much unofficial internet data is a reaction to this and does assume a certain level of "what's actually going on" understanding so that you can use safe working practices appropriate to your situation.

Fortunately the official side seems to have come back down to earth with some practical guidelines. See **LINK** for chapter and verse. As Martin says for red diodes the basic eye-safe rule is no more than 1 mW into a dark adapted (7 mm pupil diameter) eye. In practice for a device emitting a laser beam this means class 2, or class 2M where the beam from a more powerful source is expanded so only 1 mW can enter the eye which is clearly not relevant here. Eye safe in this context refers to distraction and visual after effects not tissue damage.

Class 3a was devised as a fudge to allow a more useful range of laser powers to be employed in fairly open environments without the rigourous precautions necessary when using something powerful enough to do physical damage. In a device emitting a beam accessible to the untrained user class 3a implies that the 1 mW into a dark adapted eye is met for the intelligent user by suitable beam expansion. So for a laser centre finder with a class 3a emitter would be expected to focus down to a small dot at a fairly short working distance after which it would expand so as to meet the 1 mw criteria at a reasonably short range. I guess a couple or three feet would do to meet the letter of the regulation. In practice class 3a can be considered a low power subset of class 1M assuming that no one would market an unconditionally safe class 2 device as class 3a. I saw plenty of class 3a labels but never saw a true class 3a emitting device. The (valid) assumption was that user safety would be arranged by the experienced designer or researcher by beam expansion, beam blocks or access control as appropriate.

Bottom line is that any laser centre finder sold into the general market should be a class 2. 3a power levels are way over the top, except maybe in very special circumstances. I've used 3a emitters as alignment devices at over 5 miles!

When it comes to laser reflections its unwise to assume you won't do something daft. Optics labs are matt black pretty much everywhere for good reason.

Clive

Thanks Clive for your helpfull contribution especially regarding the divergence of the beam.

The device does appear to encorporate both a polariser and an objective lens which should help the safety situation. The polariser will reduce beam intensity and the objective lens will make the beam divergent so there will be some drop of with distance as Clive indicates. My original calcs assume a non divergent beam of max output 5mW. If I bought one the first thing I would do is take it to work and measure the beam intensity with the polariser fitted at various distances.

It does seem to be a well made and usefull device but I would encourage people to be carefull with it, use the polariser and take the safety instruction seriously.

Having read through Clive Fosters link, the Laser Centre Finder which is manufactured in California has an American IIIa classification which would be UK Class 3R

The American Standard ³ does not have the Class 3A irradiance criteria previously specified in the British Standard. Therefore, a correctly classified American Class IIIA laser pointer would have become Class 3B device when classified against the British Standard. The use of Roman numerals should indicate that the product has been classified in accordance with the American Standard. In practice, examples occur where an inappropriate 3A label has been substituted for the IIIA label. All of the American IIIA laser products will now be Class 3R under the British Standard.

Martin

I am surprised at the intensity of this laser. I have a couple of very small, cheap laser pointers (these usually use reject modules that are not prefect - one of them produces a clearly oval spot, although it has very little divergence). One of them was about £1 at a boot sale and came with dozens of tiny holographic projectiuon end caps, (mostly useless trivia like santa), but one of them is a neat cross hair. It is very fine and if I can ever figure out a way of fixing it to an arbor accurately it would make a neat centre finder, and not hugely bright. It has a safety advantage that it diverges with the cap on as well.

With the product in ME, I wonder if the adjustment is only accurate for one distance?

Neil

I've been usig one of these for some time now, and personally I think its one of the most useful tools for alignment and spot finding.

Mine is rated at LESS THAN 5mw @ 0.1 mm spot size, and quite frankly has far less intensity than a decent laser pointer. (you have to turn the lights off in the shop to take a photo of it)

Absolutely ideal for edge finding and setting up the DRO. not cheap but most definately useful.

For ref.: http://www.lasercenteredgefinder.co.uk/

@Loft76 - I've had on of these for a while and I've not been able to get comfortable using it for anything other than aligning edges, a milling vice for example, for which it is really good. For finding a centre (ie a 2D x,y position) I can never work out the tolerance the thing works to.

I certainly hadn't thought about how safe the laser might be, but as you say with the collimator fitted, I do switch off the machine lights to see the spot. Without the collimator, "splashes" of laser light are noticeable as you move the workpiece around, but how dangerous these are, haven't a clue.

Steve

I've been using one of these for a short while. When I received it I was disappointed to discover the shank of the thing is made from aluminium rather than hardened steel, that may be OK in a collet chuck but it's liable to get damaged if held in a keyed drill chuck. I was also thinking it might be useful for aligning my mill head (or a long workpiece mounted on it) by making a little device something like a 1ft spirit level with a prism at one end and a line at the other. Any deviation from 90deg will be immediately apparent by a shift of the dot above/below the line.