Converting Guide Numbers from 4" to 6" reflector to 7" reflector

[Sirius Glass]

I have a 2, 3,5 battery Heiland Research Flashgun with a 7" reflector and a bayonet adapter and the Guide Numbers for the 5, 5B, 25, and 25B bulbs with a 4" to 6" reflector. How do I convert the Guide Numbers from a 4" to 6" reflector to a 7" reflector?

Should I take the area for a 4" to 6" reflector of 19.63 square inches and divide it by the area of a 7" reflector 38.48 and multiply by the Guide Number?

7" Guide Number = 5" Guide Number*(19.63/38.48)

Or should I use the 4" to 6" Guide Numbers?

See my calculations below.

Steve

[E. von Hoegh]

How do you get 19.63 sq.in. for a "4" to 6"" reflector?

Multiply your guide number by the ratio of areas of the reflectors.

[Sirius Glass]

How do you get 19.63 sq.in. for a "4" to 6"" reflector?

Multiply your guide number by the ratio of areas of the reflectors.

Yes, that is what I meant.

[Bob Salomon]

But the GN for any diameter reflector will change with its shape and finish. Are they both shiny surface flashes with the same shape or is one more pancake shape? Is one matte or pebble finish?

[Sirius Glass]

Both shiny and both assumed to be the same shape.

[Bob Salomon]

The Kodak Professional Photoguide shows that with a "Polished Deep Bowl Reflector" and a 25 bulb you get one stop more light then a 25B or 5 B in the same reflector and 2 stops more light then a 5B or 25B in a "polished Shallow Bowl or Polished Fan Type Reflector"

It also states that the guide numbers that they indicate were based on a 4 to 8" diameter shallow bowl polished reflector and a 4 to 5" deep bowl polished reflector.
So if you have a shallow bowl reflector it won't make any difference in the exposure.

[johnielvis]

SIZE in this case matters LESS than the focus of the beam---you can have a teeny tiny reflector with the bulb out of the focus so that it's beam is spread all over for a smaller guid number than a larger reflector focused parallel....this is something best measured by a regular bulb in the reflector and a light meter at the distance you're interested in...use those measured ratios---that's how I'd do it...but that's just me.

[Sirius Glass]

I want the comparison of 5" Graflite reflector and 7" Graflite reflector. The same reflecting surface and assuming the same shape. So assuming the same shape is the mathematical calculation that I proposed correct or is there a better model?

Steve

[E. von Hoegh]

You could screw a 120v lightbulb in and check with a meter, as has been suggested.

Or you can calculate the areas of the reflectors and find that the 7" reflector has almost exactly twice the area of the 5" reflector, thus making it one stop brighter.

There isn't much else you can do except to pop a bulb and measure it with a flashmeter, or expose a sheet of film.

[Bob Salomon]

"find that the 7" reflector has almost exactly twice the area of the 5" reflector, thus making it one stop brighter."

Only if they have the same beam spread. The narrower the beam spread the greater th output within the covered area.

[E. von Hoegh]

Post #8, above; "assuming the same shape".

[johnielvis]

you know...I'm thinking the 7" may actually be BRIGHTER since the beam is better focused for the same shape....the shape MUST be different, geometrically speaking since the focus has changed position for the 2 different reflectors

if the shape is identical (focus still in the same location in relation to the vertex), then the 7" would be brighter since more light would be reflected instead of flying off at an extrreme angle out of the reflector.

you have to look at quantity of light reflected as well as the size of the area....more light is reflected with a 7" if it covers more ofthe bulb solid angle---which would be the case with a reflector of the SAME SHAPE--same focus position--the bigger reflector would be LONGER too...cover more solid angle of bulb=more light reflected.

not as simple as you think, mates..you need to do the axisymmetric 3-d integrations to see.....if you ever too a radiation heat transfer class you'llknow what I"m talking about...meant time...measure ratios is actually easier to do and a lot more accurate since real world and theoretical are two very different things here.

[Sirius Glass]

you know...I'm thinking the 7" may actually be BRIGHTER since the beam is better focused for the same shape....the shape MUST be different, geometrically speaking since the focus has changed position for the 2 different reflectors

if the shape is identical (focus still in the same location in relation to the vertex), then the 7" would be brighter since more light would be reflected instead of flying off at an extrreme angle out of the reflector.

you have to look at quantity of light reflected as well as the size of the area....more light is reflected with a 7" if it covers more ofthe bulb solid angle---which would be the case with a reflector of the SAME SHAPE--same focus position--the bigger reflector would be LONGER too...cover more solid angle of bulb=more light reflected.

not as simple as you think, mates..you need to do the axisymmetric 3-d integrations to see.....if you ever too a radiation heat transfer class you'llknow what I"m talking about...meant time...measure ratios is actually easier to do and a lot more accurate since real world and theoretical are two very different things here.

Especially your last paragraph is why I asked the question in the first place. Simply scaling on the diameter is probably not a valid adjustement. Further, more research has lead me to see the the 6" reflector covers 60 degrees and the 7" reflector covers 120 degrees. A flash meter would be a good idea, but I do not have one.

If I had a 5" reflector, I would just use it rather than screwing around like this.

Steve

[johnielvis]

hey--now I remembered where I saw stuff like this..NOT heat transfer....it was the SOLAR ENGINEERING class..yeah...

check out websites for solar collectors--and books...there are tons of tables from the old days for parabolic solar collectors....THAT's where to go for the REAL info...all measured effectiveness too...particularly today with the green energy solar stuff going on---tons of info on that...THAT's where to look for such info...same thing in reverse, right?

[johnielvis]

HERE's an idea--rather than measuring the reflector size...how about putting in a regular bulb...but rather than measure light output with a light meter, you project it accross the room and measure the diameters of the central "hot spots" of the intended coverage zone...then use THAT ratio of squares of them diameters...(areas)--that ought to be something that works pretty good I'm thinking.....you can easily measure them distances and the geometrical properties of the reflectors will be magnified and taken into account by the projection.