Hi, 

I'd like to know which type of laser do you think would be the best
for my project. The goal is that the laser should 'vaporize' some
paint (its black.) Basically I'm hoping to have it pulse in such a way
that it will burn the paint into powder but not harm the
paper/canvas... The best thing would even be if it could be infrared
or UV so that you couldnt even see the ray, but if its a visible laser
that would be fine too, if that would be the best idea... Also, do you
think that pulsing it would make the most sense? And if yes, at what
frequency? Also, what power laser do you think would be best, and what
type (I see many kinds out there)...?

Thanks alot,
Lenny

PS- Even if its way too complicated to get into details, if you could
just point me in the right direction or give some ideas that would be
good, and then I could go look into it myself...

PPS- I assume that this question should be pointed at someone who is
well versed in physics and its applications...

PPPS- I put a low price because I dont know how good of an answer I
can receive on such a technical question... If someone really knows
this stuff and answers then I think Google allows to add money for
payment... (though I'm a student so I cant pay too much but I would
add if its a good answer...)

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Clarification of Question by lenny21-ga on 14 Mar 2005 02:31 PST

I just wanted to add that I'm not even sure if 'Physics' is the right
place to post this question- if you think that it should be posted
somewhere else, please let me know where. You see this question isnt
really about theoretical physics as much as practical design...
(though you have to know basic physics principles to have an idea on
how to design something like this...)

Thanks,
Lenny

You need to be very careful with this experiment, especially if you
intend to use a non visible laser.  Powers high enough to oblate
material can very easily destroy an eye.  With your apparent lack of
knowledge I advise you to find another project.

Thanks, but I'd make sure to wear protective goggles and never let it
face anyones face... Also, its not a problem if its in the visible
light spectrum...

I'd like to add that it doesnt have to be able to do it from a
distance of more than 3 inches or so, and it doesnt have to 'vaporize'
the ink as long as it burns it off for a vaccum to be able to suck it
in...

If you could just give me pointers as to where to begin that would be
ok as well. So far I've been told to look for a ND:YAG laser with
1,065 nm near IR and it could be pulsed as well. Now I have no idea
really what that laser it, but if I dont get feedback here, then I'll
do some big research on it.

I am generally aware of the basic principles involved in a laser, but
its the technical details of actual applications that I have no clue
on... (for example, I am cetain that this project would nedd a pulsed
laser so as to keep the energy focused on one specific area...)

Thanks,
Lenny

Do a google on laser ink removal, or laser tatoo removal. Laser
dermabrasion or laser resurfacing may also be good search hits. You
could also look into getting a CO2 laser, but I think pulsed would be
the way to go for higher peak powers needed for good ablation. I think
there was even a national geographic article on laser based
restoration, so Laser and Art Restoration would be a good way to
start. Find out who is doing that research, then contact them. Go
visit the lab if you can. Its going to be a long and costly project,
no matter how you slice it. And, fairly dangerous. That kind of power
doesnt mess around. Plus with NIR or IR, you cant see the beam, which
aside from saftey, make it a real pain the butt to align and work
with. Besides, if the paint is black, it's absorbing visible
wavelengths, which means that you'd want to visible laser anyway.

A NdYAG is a Neodymium Yttrium Aluminum Garnet, which is the lasing
material in the laser.

You would need a pulsed laser to keep the power exposure short, so as
to avoid bulk heating, which is a littler different than saying to
confine it to spatially. That would be the job of the lenses.

Thank you- I'll look into that... (I would accept that as an answer,
but its set as a comment...)

This is a scary experiment, the main thing I would say is get the right goggles 
for your particular laser, and make sure that they are proper laser goggles.
The problem with lasers is that the light is coherent and its
difficult for people to understand why this is so dangerous, often
even physicists.  Coherent light has weird properties and you must
remember that it has enormous peaks of energy, one of the biggest
dangers with lasers is that light reflects refracts and can get to
places where you don?t expect it. With a lot of powerful lasers you
don?t even need to see the beam to be blinded by it. You must build a
special laser room with no way for light to escape anywhere, curtains
are not enough - paint the windows black or for a high energy laser
cover them with sheets of cardboard covered with aluminium foil -
again painted black. Have an alarm sign on the door telling people
when its safe to enter. Have a safety stop button on the outside, to
shut everything down in an emergency.  (You can get all this in a
standard laser book.)
These safety precautions may seem silly and over the top but there?s
an old saying - ?better to be safe than sorry? which was especially
written for lasers.

A final question, how is a student going to buy one of these lasers? A
minimum price if your using ruby is probably $2000 - $5000 ruby is
probably also the safest but its energy comes in pulses so you will
burn a series of spots rather than a line. Co2 lasers are good to but
the absolute minimum price is at least $10,000. A good laser that is
pretty common is a medical laser but your talking $50,000. If you want
a more exotic cavity or chemical laser the costs will climb further,
so you?ll just have to trade in that Ferrari Enzo. (my prices are from
the early 90?s but I doubt they have changed much).

Thanks for the info. The thing is that I'm pretty confused as to why
it should be expensive and complicated? I dont really have too much
experience in this field, but from what I can tell all I'd need is a
relatively low power laser that could use pulses to 'pulverize' just a
thin layer of ink or paint from a white surface... Now being that the
ink or paint is always much darker than the surface, I assume that it
should be relatively simple to make sure that the pulse heats an
vaporizes only the top layer...

Of course this is coming from my student mind and not from years in
the field, so it may be way off. But nonetheless- I dont see why a
relatively small laser with not too much power shouldn't be able to do
this? (Especially if in pulses- this way the actualy power needed for
it would be less, through using a capacitor-like setup...)

Thank again,
Lenny