[Laser] Sky illumination experiment

[wa4qal at ix.netcom.com]

> Date: Tue, 5 Jul 2005 03:21:24 -0700 (PDT)
> From: Tim Toast <toasty256 at yahoo.com>
> Subject: RE: [Laser] Sky illumination experiment
> To: laser mailinglist <laser at mailman.qth.net>
> Message-ID: <20050705102124.8394.qmail at web30212.mail.mud.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
> 
> Kerry, 
> Angry home owners associations, black helicopters and
> astronomers aside, 80 watts is plenty of light. Much
> more and you'd be attracting the men-in-black :D 

I'm wondering if it might not be better to use a subcarrier 
on the light and modulate that, rather than doing base band
audio modulation.  Base band audio modulation will make the
light flicker, which will attract attention.  Using a modulation
scheme which avoids flicker will make the light look like just
any other ordinary light.

> But yeah, compared to typical football stadiums, light
> pollution is miniscule, especially if you direct most
> of the light straight up and away from the neighbors. 

Every little bit helps (or hurts).

> It might be hard to find a large IR filter for
> something that size but i dont think those tubes put
> out very much light at IR wavelengths anyway, so its
> probably a lost cause trying to stealth them that way.

You can obtain rather large sheets of gel from theater
supply companies, and that will filter the light.  Of course,
be wary when someone tells you to go wash the gel.  ;-)

But, fluorescent lamps typically don't produce much 
infrared radiation.  However, note that it is possible to
obtain fluorescent lamps with special phosphor coatings
which can be tinted (e.g., red, blue, green, etc.).  I'm not
sure if they have an infrared phosphor, but I'm thinking that
it's probably possible.  Then, again, I'm not sure how difficult
it might be to obtain samples of just a few tubes.  But, it
wouldn't hurt to check, or to make a couple of e-mail inquiries.

> A regular filament type light bulb does however put
> out a lot of IR light and would be easier to find IR
> filters big enough cheaply and also to focus a
> narrower beam if you wanted. 

The problem with incandescent lamps is that the thermal
mass of the filament limits the modulation speed (although
I'm told that you can get some audio frequencies through).

> I was thinking since these frequencies are all in the
> audio range, and if one was lacking an exotic power
> supply, you could use a regular off-the-shelf audio
> power amp in the 100 watt range. Then match its 8 or 4
> ohm output to the impedance of the tube(s). Matching
> it to such an oddball capacitive load might be a
> problem with the florescent tubes though. A regular
> light bulb would probably be easier to match. 

Note that the resistance of an incandescent lamp changes
with the amount of power flowing into it.  Thus, as the lamp
brightens, the filament changes resistance.  This might cause
a bit of distortion, unless you compensated for it with feedback
through the amplifier.  Then, again, you might just live with
the distortion.

The resistance of a fluorescent (or, for that matter, any glow
discharge) lamp is negative.  Thus, you need to include a 
suitable ballast resistor (or impedance, if you're driving it from
an AC source).  Also, you have to make sure that the modulation
peaks don't extinguish the discharge, since that will surely result
in severe distortion (as well as causing problems with restarting
the tube).

> At any rate, PSK31 modulation springs to mind since
> this is all nice and sinusoidal. 

Digital might be better.

> good luck
> -_-
> 
> 
> Tim Toast
> http://www.aladal.net/toast/exp.html

Dave
WA4QAL