[Laser] Depth of modulation for various lamp types

[TWOSIG at aol.com]

Yves,  F1AVY suggested,  "Try  to mount a few lamps in  serial mode so the 
lamps are just visible  red.   At  this  temperature the modulation depth could 
be better and the most of  
radiant  power in the infrared spectrum. "
 
I have not tried such experiments, but I did read some of the history  
reported by Jim Groth many years ago in Australia.  He amplitude modulated  a 
flashlight with voice successfully.  The tone was poor in the high notes,  as one 
would expect.  As I recall, he had no means to limit the spectrum  received and 
the best results were obtained when the filament was biased to  a very dull 
red.  That would suggest that IR was the dominant  emission. 
 
What it says is that a hot filament will cool well enough to send a signal  
even at voice frequencies.  Probably has limited modulation  percentage.
 
If I may be so bold, I suggest a different way to test lamps for their  
usefulness:  Square waves.  And not just at 50 percent duty  cycle.
 
For an incandescent bulb, there is a rise time and decay time associated  
with the thermal mass of the filaments.  The decay waveform is probably  pretty 
consistant if the pulse is long enough to allow the filament temperature  to 
stabilize.  I would guess that the rising waveform is of a consistant  also if 
you look at it from the peak temperature, then backward in time to  whatever 
point the pulse started.
 
A gas discharge bulb has a trigger voltage so with a sinusoidal drive it  
will not emit until the trigger voltage is reached, then it should emit  
depending on the drive current available ( ballast inductors affect this ),  finally 
it will quench at a lower voltage than the trigger voltage.  The  fast rise 
time of a square wave may reduce the trigger voltage.  The sharp  cutoff of a 
square wave will provide an indication of the time and rate of  ionization decay 
of the gas.
 
If I remember correctly, a flourescent bulb is a gas discharge bulb that  
gives off ultraviolet light.  The use of square waves ( remove the ballast  and 
limit the discharge current with a really hefty resistor ) will give you a  
better indication of the phosphor decay times.  Perhaps an even better way  to 
test a flourescent bulb is to put it next to an antenna and send some CW dit  
strings.  At least that way you avoid some safety problems that I fear  might 
occur with other methods.  Also note that there are several types of  phosphors 
that may have very different rise/decay characteristics.  ( Some  bulbs 
provide "warm" light, some are specialized for plant growth.)


I hope this is useful.
 
Best Wishes to all.
 
 
James
N5GUI