John,
At 9:35 AM +0000 12/19/01, olympus-digest wrote:
>Date: Tue, 18 Dec 2001 21:34:26 -0800
>From: John Hudson <jahudson@xxxxxxx>
>Subject: [OM] rating of studio strobes
>
>I was told today that any two different brands of studio strobes each
>rated at 400 watts / second [or any other rating for that matter] might
>not deliver the same intensity of light output. By way of analogy my
>source said that in and of itself any particular strobe rating was like
>saying that an automobile's engine had four, or five, or six cylinders
>and so many valves but did not imply or specify the actual power output
>other than suggest, for example, that a 800 w/s rating was likely to be
>twice as intense as a 400 w/s rating.
>
>Can any one out there explain just how you measure the intensity of
>light output by reference to the watt per second rating and how this can
>be converted into a guide number for any particular ISO film speed?
Watt-seconds are units of energy, equivalent to Joules. The watt-second rating
of a flash is how much electrical energy is stored in the flash capacitor.
The equation is: energy = (1/2)*(capacitance)*(voltage)^2.
So, for a reasonable example, voltage=300 volts, capacitance=4500 microfarads,
so energy=(1/2)*(4500e-6)*(300)^2= 202.5 Joules= about 200 watt-seconds. The
example is the "White Lightning 10000" studio flash from Paul C Buff.
The problem is that this is the electrical energy, but we want optical energy.
When the flash capacitor discharges through the xenon flashtube, some fraction
of the electrical energy is converted to optical energy (light), and flash unit
designs vary greatly in how efficiently this conversion is done. (Paul Buff's
claim to fame is that their units are particularly efficient, and it appears to
be true.)
Another issue is that flash units vary in how tightly focused the light is.
Rating flash units by flash guide number (which is proportional to brightness
on axis) tempts the flash manufacturer to use sharp focus for max brigtness on
axis, at the expense of non-uniform illumination, and making comparisons quite
difficult.
The simplest way to tell the total optical output of a flash is to fire it into
a white box and use a flashmeter to measure the flash coming out of the box.
This box is a primitive integrating sphere, but sufficient to the purpose.
I use a standard "china barrel" mover's box that I painted white on the inside
using two or three coats of Krylon spray paint. China barrels are about 3x3x4
feet. The details don't much matter, except that it's essential to put a piece
of painted cardboard between flash and flashmeter, so there is no direct path
from flash to meter.
Anyway, using this setup, one can compare flashes quite well for their total
optical output. To calibrate the setup to get the optical output in
lumen-seconds (rather than arbitrary units of optical power) would be a bit
more work.
One issue I've ignored is the fact that the lumen is a measure of visual
intensity, not optical power per se. The extreme case would be infra red
light, which carries significant optical power, but being invisible has a
visual intensity of zero lumens. Given that xenon flashtubes all give pretty
much the same spectrum, it's not much of a problem.
Joe Gwinn
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