You raised what is sometimes called a teachable moment.

David W Wood <[log in to unmask]> writes:
> Martin, Phil - all
> 
> Lasers are categorized in classes 1 to 5.
> Off hand, I can't recall the power levels, but 10mW is class 1, and 30mW
> class 3.
> Here in the U only class 1 can be sold on the open market, such as those 
> in
> toys, and lecture theatre pointers.
> Having said that, if they are pulsed, the total energy emitted is reduced,
> and hence class 2 can be sold on this basis.

	That is the concept of duty cycle in a nutshell.
Microwave ovens use that idea all the time since you can't just
turn down the voltage to the plate on the magnet Ron and get less
power. They tend to have an optimum operating voltage and will
just quit oscillating if you try to run them with less voltage.

	The idea for cooking, however, is to raise the
temperature of the food so one can hit them with full power but
make those bursts occur at a certain percentage of the period
between pulses.

	I think the duty cycle must be scheduled to take in to
account how much of a heat sink the food or skin tissue presents
before it heats up during the pulse.

	Something that absorbs enough energy to get red hot in a
fraction of a second like thin metal or maybe the cells in your
retina would probably need a very fast period of duty so as not
to be damaged.

> 3 upwards are dangerous, not only to eyes, but to human tissue and 
> physical
> items.
> Quite often laser is used in stead of a traditional scalpel for medical at
> veterinary use.
> 
> Apart from messing up aircraft, they have also been used to distract race
> horses during races.
> 
> I am not sure which class is used to write and read CD's, but on opening
> these, there is a warning about the danger of the laser.

	I am not sure either without looking it up but I bet it
is at least a Class 2 as the CD writers reportedly heat the
little pin-sized pits up to around 700 degrees. I don't know if
that is Fahrenheit or Celsius but that is more than hot enough to
char anything living.

	It helps a bit that the lenses in optical media drives
have an extremely short focal length so the real cooking happens
within a distance consisting of the small gap between the laser
sender, the thickness of the clear plastic of the disk and the
back side of the disk which is the focal point.

	What's really dangerous about these lasers is that they
are not visible at all so one wouldn't even know if it was on or
off and the iris of the eye wouldn't even try to close the pupil
since the brain isn't seeing any light to adjust to.

	I have actually read that some telecommunications
companies have scavenged lasers out of CD players to use as
transmitters in to fiber-optic cables. I don't know if that is
really true or just urban myth but people who work on fiber are
warned all the time not to look directly in to a piece of fiber
because there might be a laser on the other end and you'll cook
your eyeball. It wouldn't really make anything burn or explode,
but it might be strong enough to damage one's vision like looking
in to an eclipse of the Sun.

73
Martin
WB5AGZ