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Re: [OM] More Calcs on Flash GN

Subject: Re: [OM] More Calcs on Flash GN
From: "Paul Farrar" <farrar@xxxxxxxxxxxx>
Date: Wed, 8 Mar 2000 08:22:08 -0600 (CST)
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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