> OK. But assuming both flashes have the same shape, say for a 24mm lens,
> what w sec corresponds roughly to the T 32?
To repeat my post from a day or two ago on this thread the T32 is a *** 70
Joule ** Flash. For comparison the Handlemount Sunpak 544 is also a 70 Joule
unit.
However the T32 is more efficient at getting light from the flash tube
to the scene than is the 544. This is illustrated by converting the GN's for
both flashes if the light were spread to cover the same area. If this done
the 544 has a lower GN than the T32. (See previous post)
The overall efficiency of light conversion from (70J) energy stored in
the capacitor to light on the scene depends on about 3 major factors. 1) The
flash tube light conversion efficiency 2) resistive series losses in the
electric circuit from the flash capacitor to the flash tube and the 3) the
reflector's focusing and reflecting efficiency. (The overall battery energy
to flash output conversion efficiency is even lower as the battery efficiency
gets very low at high current and invertor losses are significant.)
The light conversion efficiency of the tube depends on the bore
and length of the tube and the gas pressure in the tube. When the capacitor
is discharged through the tube it produces a plasma arc with a temperature of
around 5800 degrees Kelvin (Hence the color temperature of the light) .
Running higher current and hence higher color temperature improves the
efficiency but shifts the color temperature, thus most photo-flash units run
under similar conditions of arc temperature. Running a higher pressure might
require a higher voltage so since almost all common new small photoflashes
run 330V the tubes tend to operate under similar conditions and hence similar
efficiencies. Some high power studio flashes evidently are more blue (higher
temp). The tubes convert more than half the energy input into light with a
significant amount of light outside the visible spectrum spanning the UV
through infra-red; but what ccounts is how much is in the visible range.
(See the EG&G website for much more information.Site may be renamed Perkin
Elmer who bought EG&G).
The current through the flash tubes is very large about 200Amps+ for a flash
like the T32. Thus even small wiring resistances including the capacitances
own small resistance and the series SCR and series inductor used for TTL
flash control add series resistive losses. At the end of the discharge about
50V is left on the capacitor so not all the energy is discharged although
this is a small amout left as the energy varies as volts squared.
The reflector losses are probably only 50r so (a guess) but the
reflector design may not spread the light optimally. This seems to be
particularly true for some zoom head flashes and the OM zoom adapter which is
probably only optimized at one setting.
There was debate over whether the flash looks like a point source or not
but in a sense this is not the right question since it is an optical
(reflector) design and, for example could be deisgned to give a collimated
beam which certainly would not fall off as the inverse square. However,
clearly it must fall off as a first approximation as the inverse square if
the illumination is uniform since the projected rectangular area covered
increases as the square of distance. A subtle point if you measure at an arc
the illumination should be uniform but at a flat surface it drops off more
rapidly off axis. It is more like Cos^3 (I seem to remember) for a more
diffuse source like a white studio flash reflector. For small angles these
are the same but not for larger angles hence the apparent sudden drop off
with wide angle lenses in rooms with flat walls!
Hope I did not put everbody to sleep!
Regards,
>>> Tim Hughes <<<
_Hi100@xxxxxxx
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