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DPRG: Re: laser weapons

Subject: DPRG: Re: laser weapons
From: Michael Kennan kennanm at efn.org
Date: Wed Mar 3 12:01:47 CST 1999

>I set up a simple experiment using a common cadmium-sulfide photo-
>cell and an 8 bit A/D converter.  I shined two different laser
>pointers at the cell from a distance of four feet and took
At least you didn't shine them in your eyes... ;)

>I also set the cell in direct sunlight, at noon and
>4 pm, and took readings.  Admittedly this is un-calibrated and
>not very scientific, but the direct sunlight readings were
>consistently higher (brighter) than either laser pointer, or
>even both combined.
>Science, folks, science.

I'm not sure I understand the experiment, here...  Check the response curve
of your CdS cells. There should be a spectral response in the datasheets if
you can find them.  There is going to be a large response in the visible
light range (which is their primary target usage range) and a much smaller
response out on the edges in the near IR.  The overall wattage of direct
sunlight is going to be much larger, of course, than the wattage in the
laser.  The important distinction is that our eyes are by definition tuned
to the visible range, so when we are being confronted with more light than
the organism can safely handle, we are aware of it.  With lasers, however,
there is often a significant spectral power density *outside* of the
visible range...

Some references:

"Class II lasers have a maximum power of 1/1000th of a watt, a power judged
to be eye-safe, except possibly in case of deliberate, long-term direct
staring into the beam."  (Keychain lasers are Class 3, up to 5mW).

The United States Department of Health, Education, and Welfare and Food and
Drug Administration, Bureau of Radiological Health, Rockville, Maryland,

Laser-induced bubble formation in the retina 
Bubble formation in the retinal pigment epithelium by submicrosecond laser
pulses may be a source of laser induced retinal damage. Heat conduction
away from absorbing melanin granules requires timescales on the order of
microseconds. For pulses of shorter duration, all energy absorbed is
effectively absorbed as a delta-function in time, and energy concentration
may be high enough to cause vaporization of the surrounding medium. This
occurs at lower fluences than required for thermal denaturation of a
significant volume of cellular material. The adiabatic nature of the
absorption and subsequent expansion is used to develop expressions for the
calculation of maximum bubble size as a function of laser intensity and
melanosome properties such as radius and absorption coefficients.

B. S. Gerstman, C. R. Thompson, S. L. Jacques, M. E. Rogers, "Laser-induced
bubble formation in the retina," SPIE Proceedings of Laser-Tissue
Interaction VI, edited by S. L. Jacques, 2391, 60-71 (1995). 

Safety Recommendations of Laser Pointers

          The major potential hazard from pointers is limited to the
unprotected eyes of individuals who look at the direct beam emitted from
the laser since no skin hazard usually exists. The natural aversion
response or blink reflex (~0.25 sec.) of the eye from the bright laser
light normally limits exposure to a safe level for those devices. 

          The ANSI Z136.1 Standard bases the "blink reflex" MPE on an
exposure on 0.25 second exposure. This yields an MPE of 2.5mW/cm2. When
this irradiance is spread over a "worst case" 7mm pupil opening (0.4 cm2),
the total power entering the eye can be then computed as follows: Power =
(2.5 mW/cm2) x (0.4 cm2) = 1.0 mW. This suggests that laser pointer type
devices might be limited to an output of 1 mW (Class II). 
          In some darkly lit environments, and at some wavelengths, a 1 mW
pointer power is perhaps an option, but in rooms with a high ambient light
level and if operation is at the longer 670 nm wavelength, 1 mW is just
marginal for visibility and, therefore, 3-5 mW is generally required for
better visibility. Note that if the exposure is raised to a maximum of 5 mW
(Class IIIA), then an eye filter with an optical density of 0.7 would be
required for protection in the event of an intrabeam exposure of 0.25
seconds. This suggests that caution is needed when the pointer emits near
the 5 mW power level! 

R. James Rockwell, Jr. and William J. Ertle, ROCKWELL LASER INDUSTRIES 

Laser Safety Guidelines

Radiation at visible wavelengths, 400 to 780 nm, and near-infrared
wavelengths, 780 to 1400nm is transmitted thorough the ocular media with
little loss of intensity and is focused to a spot on the retina 10 to 20
micrometers in diameter. Such focusing can cause intensities high enough to
damage the retina.

Columbia University 2nd Level Policy

Ability of the laser to cause focal coagulation

The ability of the laser to cause focal coagulation within the eye makes it
suitable for treating conditions such as neovascularisation, retinal
detachments and intraocular tumours. Permanent retinal lesions can be
produced by focusing a pulsed Krypton laser beam (647 nm) of 100mW or less
for 0.1 seconds onto a 100mm spot on the retina.
Likewise, the energy emitted from a 5mW laser pen, of similar wavelength,
when held 10 metres from the eye, will form a retinal spot which the eye
can theoretically focus to a diameter of less than 10mm. Because intensity
is inversely proportional to area, it can be appreciated that the intensity
is actually greater for the lower powered device. The wavelength of light
emitted by the laser pens is such that little energy will be dissipated by
the ocular media before being absorbed by the pigment epithelium. Little is
known about the minimum threshold levels of laser radiation, of this
wavelength, required to cause retinal damage, but extensive work on the
effects of direct exposure, even to non-coherent incandescent or
fluorescent light, suggests that concern in relation to these laser pens is
not misplaced.

Eye News

UMIST safety manual

For laser radiation in the visible and infra-red regions, the tissue at
risk is generally recognised to be the retina of the eye where irreversible
damage can be caused by overexposure.... Class 3 lasers pose a hazard from
direct beam viewing and from specular reflections.

Sign on door of the lab where I work:

	Don't look into laser with remaining eye.


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