Ocular hazard distances?

[wgaf]

Can anyone please give me some gen on why laser safety distances are less at night than during the day?
This is a serious question so sensible answers only please folks.

 

[Hu Jardon]

Can anyone please give me some gen on why laser safety distances are less at night than during the day?
This is a serious question so sensible answers only please folks.

Errrm

Basically it's cos your pupils get bigger in the dark and all the sensors within your eye and brain are at max sensitivity so are an easier target!!!!!!!!!!!!!!!

That's a sensible answer - honest - it's also the reason lighting levels are supposed to be controlled and of a minumum luminosity in LASER Bays

 

[Ex-Bay]

And there's usually less dust & cr@p in the air so there's less attenuation.
Note: A lot will depend upon the frequency of the laser concerned.

 

[OneTimeSooty]

Erm - think a bit of mis-understanding?

I work with wgaf so know what it is we need to know.

We're trying to explain why the safety distance is less at night than at day. (Quoted figures seem to be about a third less at night.)

So you can be as basic as you can for a heavy to understand why you can be closer to a laser at night than during the day?

Or are our quoted figures completely wrong?

Sorry if this seems a bit nit-picky, but we're in the position where people will ask (and have asked) this question!

Many thanks.

[EDIT]

Ex-Bay, wgaf seems to think that the dust and crap in the air is what focuses the laser beam, so less of it around means the beam is more dispersed. Is this correct?

 

[metimmee]

Erm - think a bit of mis-understanding?

I work with wgaf so know what it is we need to know.

We're trying to explain why the safety distance is less at night than at day. (Quoted figures seem to be about a third less at night.)

So you can be as basic as you can for a heavy to understand why you can be closer to a laser at night than during the day?

Or are our quoted figures completely wrong?

Sorry if this seems a bit nit-picky, but we're in the position where people will ask (and have asked) this question!

Many thanks.

[EDIT]

Ex-Bay, wgaf seems to think that the dust and crap in the air is what focuses the laser beam, so less of it around means the beam is more dispersed. Is this correct?

This www linked JSP has contact details for those that are in the know. Probably better to get the pukka from them direct(para 8). Linkio.. The unit Laser Safety Officer should be able to advise on distances.

 

[Trenchards Ghost]

As has been said the main reason is the fact at night you're eyes are at max performance for collecting light.

(Distances and power for example only)

A laser will yield 100w at 100m whether day or night.
during the daylight 100w may be safe as your eye is not as sensitive.
at night you may be only able to take 75w as your eye is operating at full, therefor you need to be at 150m before the level has dropped to the 75w level.

Thats the simple answer.

 

[Tubby]

I get what your saying T.G but they want to kmow why it is less at night in thier safety notes. You have said why you think it should be more at night. Being a laser I would have thought the distances would stay the same as it is at lot more focused allthough not quite like a james bond type laser it will still be strong enough to cause some harm as I am sure you know. Best bet would be to go through the manufacturer if possible via maybe your Trade Manager or as said earlier your laser safety officer to question the descripencies. Doesn't sound right though.

 

[PTR Hoar]

Let me go through some of my old laser notes and i'll try n work if out for you!!

 

[PTR Hoar]

Can anyone please give me some gen on why laser safety distances are less at night than during the day?
This is a serious question so sensible answers only please folks.

Are you definitally sure that the distance will decrease at night? The reason i ask is because the laser itself will not be really be affected by anything on the ground at night, ie temperature, particles in the air etc. The only differance there really is is the fact that your eye will be that more sensitive at night hence the distance should INCREASE during dark hours. When i used to work with lasers the lighting in the room would be increased to reduce the chance of possible damage to your eye. Either it's a simple misunderstanding in reading the AP or you need to get on to someone about getting it changed to read the correct way!

Hope that helps some.

 

[Ex-Bay]

Erm - think a bit of mis-understanding?


[EDIT]

Ex-Bay, wgaf seems to think that the dust and crap in the air is what focuses the laser beam, so less of it around means the beam is more dispersed. Is this correct?

No it doesn't. Dust & crap will cause diffusion of the beam and so lessen the impact during the day. At night the beam stays concentrated for longer. . . .

What frequency is the laser operating at?. It's not all of them that will penetrate the eye. . . . .

 

[PTR Hoar]

The amount of dust in the air will depend on where you are though. In the UK the level of dust in the atmosphere generally does decrease at night "hence how you can hear things from further away in the night time" but while firing a laser that shouldn't make any differance to the safety levels surely as the amount of dust in the air is relative to the region you are in and i don't think that should be relevant to the safety distances. Or am i making stuff up now?!

 

[Trenchards Ghost]

Sorry totally read your post wrong. (Dug out Janet and John book3 for reading practise)

I can't see why the distances are less.

 

[OneTimeSooty]

OK - we think we have the answer.

The laser we've had the figures quoted for is quite a wide focused beam.

During daylight hours, scintillation occurs, focussing the beam to a narrower diameter than during the night, where scintillation is assumed to fall.

So (ballpark figures) if at night the beam was 5mm wide at 50m, during the day it would be 5mm at 75m.

Hence the safety distance reduces.

Any thoughts on this?

Sorry to labour the point, but we need to understand completely!

 

[metimmee]

OK - we think we have the answer.

The laser we've had the figures quoted for is quite a wide focused beam.

During daylight hours, scintillation occurs, focussing the beam to a narrower diameter than during the night, where scintillation is assumed to fall.

So (ballpark figures) if at night the beam was 5mm wide at 50m, during the day it would be 5mm at 75m.

Hence the safety distance reduces.

Any thoughts on this?

Sorry to labour the point, but we need to understand completely!

Have you asked the Unit Laser Safety Officer? Have you contacted the advice line that was within the JSP that I posted?

Forums like this are great for steers, but if you want the pukka gen you absolutely need official info and guidance.

 

[Shugster]

OK - we think we have the answer.

The laser we've had the figures quoted for is quite a wide focused beam.

During daylight hours, scintillation occurs, focussing the beam to a narrower diameter than during the night, where scintillation is assumed to fall.

So (ballpark figures) if at night the beam was 5mm wide at 50m, during the day it would be 5mm at 75m.

Hence the safety distance reduces.

Any thoughts on this?

Sorry to labour the point, but we need to understand completely!

Having had a quick peek at the scintilation aspects in Wiki...

http://en.wikipedia.org/wiki/Laser_rangefinder

The beam / safety area would have to be treated as a cone. The safest distance to stand in front would be when the power has dissapated to the point where the amount entering they eye was at a safe level.

So the divergence and the power of a particular laser will be on a case by case basis.

This is a guess...

100 Watt spread over an area of 5 sq_mm would be 20 watt per sq_mm.

Edit: So double the area, half the power. (Note I said area, not Diameter (PI*Radius*Radius)). (I don't have a square sign on the keyboardDT_Xtremez_30

Your iris will have a maximum and minimum diameter so the amount allowed in will be directly related to that.

Just my semi educated guess... Ask the boffins or even phone a laser supplier what factors are used to determine safe distances.

 

[Shugster]

Looks like my guess wasn't far off...

http://en.wikipedia.org/wiki/Laser_safety

look under," Maximum permissible exposure".

The maximum permissible exposure (MPE) is the highest power or energy density (in W/cm2 or J/cm2) of a light source that is considered safe, i.e. that has a negligible probability for creating a damage. It is usually about 10% of the dose that has a 50% chance of creating damage[7] under worst-case conditions. The MPE is measured at the cornea of the human eye or at the skin, for a given wavelength and exposure time.