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[Phys-L] Re: light bulb transient



The section of Levi to which I have already referred begins w/: "The
heating and cooling times of tungsten filament lamps have been
investigated and seem to be governed primarily by the current rating of
the lamp."

Continuing summary:

times (seconds) are 0 => 90% heating, vice versa 100 => 10

gas filled: Time(heating) = 0.15 (I0)^0.7 I(0) is rated current in
amperes
time (cooling) = 0.07 ditto

vacuum lamps:

= 0.06 I(0)^ 0.54 and
= 0.022 I(0)^0.7 [N.B. difference]


When operated on AC freq. intensity freq. is 2X supplied (duh). Total
variation as a fraction of the mean value is

~ (gas filled)
0.0069 D^-1.06 (60 Hz supply and
0.248 D^-0.89 (25 Hz)

Vacuum: 0.0056 D^-1.37 (60 Hz)
0.0189 D^-0.91 (25 Hz)

Where D is the diameter of the wire (mm)

From all the above I suspect the overshoot, previously mentioned, IS
the intensity. Remember Levi refers to high power 6 V lamps, e.g.
ribbon filament 30 A. * (& 6 V, 180W) these are massive filaments.



* 30A/T20/4 may be operated at 14.5 V (44 A; 640 W) [700 cd; 3K deg.
K color temp]

If some one wants to check this out and doesn't have one (The ones * I
have may be only 18 A.) ... It'll be easier for me to ship one than do
it myself.


* inherited from UCB Physics.

bc, who finds it easier to use a book than find a photodiode, etc., but
much less fun.

p.s. incidentally table 31 gives for 40W (115V) 0.065 s. 0.026 (heating
/cooling respec.) and 0.27 fractional intensity variation (60 Hz)

for a 60W, correspondingly:

0.102 and 0.042 (remember 0 => 90% etc.) w/ 0.18 total fractional
variation (P. to P.).

p.p.s. If one uses a strobe (chopper) one might see the intensity
variation. This bring up something I wanted to post regarding the
perception of "brightness". Levi states that this perception is of
about four + orders while the stimulus is fourteen! I think an EE will
appreciate this. The stimulus is measured in Trolands. i.e. the
intensity incident the retina. [One nit w/ a pupil A of one mm^2 = 1 Td.]

Threshold = 40 micro Td and 1E8 Td, where rodopsin bleached after one
sec. [At 30 X retina is damaged]

The response is measured in bril(s).

Transfer function:

at low levels (of stimulation) response is ~ linear B = 5k L bril (L
= nits (cd/ m^2))

at higher levels, B = 0.68 (L)^-1/3

measured response (I suppose done similarly as loudness found.) depends
on may factors in addition to the Td magnitude.




Michael Edmiston wrote:

John Sohl made some measurements and posted them on a web page. I have
done the same. My results can be found at

www.bluffton.edu/~edmistonm/light.bulb.pdf

My results are similar to John's but different.

(1) I don't understand the oscillations in John's plots. If the time
scales are correct, the oscillation period is about 200 ms giving a
frequency of about 5 Hz. This might seem to be a vibrating filament
except John's fluorescent bulb also showed the 5-Hz behavior. That
makes me think his time scale is incorrect.

My filament clealry showed 120-Hz oscillations in light output caused by
the 60-Hz AC power. The magnitude of these oscillations is considerably
more than I would have guessed. I wasn't sure they would be visible at
all, but they amounted to about 18% of the average light output.

(2) John saw his bulbs heat faster than they cooled during turn-on
versus turn-off. I saw the opposite in terms of overall behavior.
However, in the 120-Hz oscillations I observed the cool-down was
noticeably slower than the warm-up.


Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu

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