Tim wrote:
>
> Dear OMer's,
> a periodic question that appears here is: What is the
> relationship between Flash GN and stored energy in the Flash capacitor? This
> depends on reflector efficiency, area of coverage and flash tube efficiency.
> Here is a comparison between the OM T20 and T32 flashes showing that despite
> the differences in flash tube,reflector and stored energy the overall
> efficiency is almost identical.
>
> T32 Flash energy 70 Joules (or Watt-secs) GN 32 (m) coverage 24mm
> Lens
> T20 Flash energy 25 Joules (or W-s) GN 20 (m) coverage
> 24mm Lens
>
> We expect the flash energy per unit area to decrease as the square of the
> distance. (Area covered increases by square of distance)
> So the T32 should theoretically produce a light intensity identical to a T20
> at a distance of square root (70/25) = 1.67 greater than the T20.
> Thus the T32 GN should theoretically be 20*1.67 = 33.4 which is close to the
> nominal 32.
> The T45 apparently is two T32's sharing a common reflector which would then
> be 140J and have a GN 32*sqr root(2) = 44.8
> pretty close to the nominal 45.
>
> Coclusion the overall conversion efficiency of all of the OM T flashes is
> essentially identical.
>
> Regards,
>
> >> Tim Hughes <<
The formula for getting GN from W*s is, if I remember correctly:
GN = SQRT( 0.005 * ISO * B * E * e )
where
B = the reflector concentration factor (1 for bare bulb, about 7 for a
typical reflector
E = energy storage (in W*s)
e = efficiency of conversion (in lumen*s/W*s), a lumen*s is the light
parallel to W*s. Typically up to 50 or so.
GN is for feet.
With B = 7, ISO = 100, E = 70, e = 50
we get 110 ft.
Paul
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