Is there technology or is it theoretically possible to optically
determine AC line frequency with some degree of accuracy ? For
example, could this device observe a functioning 120V incandescnet
light bulb and determine line frequency and it's variations? If so,
what if the observation was recorded on video tape, would the frequency
"information" be lost?

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Clarification of Question by firefox321-ga on 05 Oct 2006 20:20 PDT

Hello and thank you for the comments.  I should clarify my question
again.  I was asking if technology exists for OPTICALLY determine the
AC line frequency powering a light source... so no wires, no physical
contact between the device and the wires/light source......

firefox321-ga

Modern high-speed camera allows you
to select any frame rate from 1,000fps in increments of one frame per second
http://www.visionresearch.com/phantomHD.html

and would handle 60Hz easily.

But do not test it on 
  
   incandescnet light bulb. 

The thermal inertia of the filament does not allow the temperature and so
the light output to follow the current variations.

You would be able to determine that frequency by taking high speed photo
of a fluorescent tube.

"Fluorescent lamps do not give out a steady light, instead they
flicker (fluctuate in intensity) at a rate that depends on the
frequency of the driving voltage. While this is not easily discernable
by the human eye, it can cause a strobe effect posing a safety hazard
in a workshop for example, where something spinning at just the right
speed may appear stationary if illuminated solely by a fluorescent
lamp. It also causes problems for video recording as there can be a
'beat effect' between the periodic reading of a camera's sensor and
the fluctuations in intensity of the fluorescent lamp. Incandescent
lamps, due to the thermal inertia of their element, fluctuate less in
their intensity, although the effect is measurable with instruments.
"
http://en.wikipedia.org/wiki/Fluorescent_lamp


You do not need an expensive lab camera. A photometer can do the job
if it has a fast response - which means 
 it should use a photomultiplier tube rather then calorimeter sensor. 
http://www.dfmengineering.com/photometer.html


Hedgie

One possibility would be to use a digital storage oscilloscope to
capture and display a single cycle of the sine wave, and to measure
the elapsed time from zero-crossing to zero-crossing.  This
instrumentation could be easily automated to repeatedly capture and
record the elapsed times for successive cycles, providing a record of
instantaneous sine wave period (the inverse of frequency) and
cycle-to-cycle variability.

The oscilloscope CRT performs the conversion of 60 Hz electrical
energy to light energy, so I guess this meets your question criteria. 
(The CRT phosphor fluoresces; these phosphors are similar to those
used in the fluorescent light mentioned in your question). Your
question doesn't specify a required accuracy for the measurement of
sine wave period/frequency, nor does it specify whether you would like
to observe short-term or long-term variations.  If you wish to remove
short-term variations, you could use an adaptive filter or phase lock
loop.

Another, more direct measurement method would be to use an
oscilloscope CRT (cathode ray tube) with vertical deflection plates
and horizontal deflection plates.  To visualize how a CRT works, see
the figure in the wikipedia article:

http://en.wikipedia.org/wiki/Cathode_Ray_Tube

Connect the ?AC line voltage? (via a matching transformer) to the
horizontal deflection plates, and a laboratory 60 Hz sine wave
generator source to the vertical deflection plates.  The CRT will
display an ellipse.  If the AC line frequency exactly matches the
frequency of the laboratory sine wave generator, the ellipse will be
perfectly stable.  (Angle of the ellipse axis equals the phase
difference between AC line voltage and laboratory generator sine
wave).

If the AC line frequency deviates from exactly 60 Hz, the ellipse will
slowly rotate ? the rate of ellipse rotation equals the magnitude of
the AC line frequency?s deviation from exactly 60 Hz.  For example, if
the ellipse completes one clockwise rotation every 10 seconds, the AC
line frequency is 60.1 Hz.  If the ellipse completes one
counter-clockwise rotation every 10 seconds, the AC line frequency is
59.9 Hz.

One more note:  If you wish to convert the AC line voltage (sinewave)
directly to an intensity-modulated light source (similar to your
flourescent tube idea), you could use a TV CRT for this purpose. 
Simply connect the AC line voltage, via an appropriate transformer, to
the intensity-modulation grid of the CRT.  The TV CRT's phospher
persistence is probably fast enough for this purpose; if not, a CRT
with custom-specified phosphers can be purchased.  These
short-persistence-phospher CRTs are used in military applications,
where high frame rates (200 frames/second and above) are used to
capture motion.

That's somewhat an odd question, but the answer is yes.  An
incandescent bulb will produce oscillations in a photodiode at twice
the line frequency.  And a video camera with a fast enough frame rate
could be used to record the oscillations.

But if the goal is just to measure line frequency variations, it would
be far simpler to plug a frequency meter directly into the line.  It
might be wise to use a voltage divider to deliver a safe, 5-volts (or
somewhere thereabouts) to the meter.

I think a stroboscope would work.

After reading the answer, I just wanted to reiterate that AC line
variations are indeed visible with an incandescent bulb.  I have seen
it using a simple photodiode.  The light output oscillates at twice
the line frequency about it's average value.

That is correct  redbelly98-ga

 Light output of incadescent does change and can be measurable with proper
 instrumentation.  It is meantioned in one of the links I quote.

    I put simplest facts into quotes and assume that person who
wants details will read the links and use the Search Terms where provided.

To measure fluorescent tube with photomultiplier is just easiest way
for a beginner to detect the AC frequency.

Diode is indeed sufficient. The  frequency is doubled since light output
(power) is proportional to 'current squared'  and current varies with voltage.

 Thanks for you comment.

> "Thanks for your comment."

You're welcome.