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Twice the footcandles = twice the wattage?


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#1 Salil Sundresh

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Posted 21 February 2009 - 08:22 AM

I just compared a friend's 650w fresnel to my 650w fresnel. At the same distance mine read F8.0 and his reads F5.6 so therefore mine is twice as bright. Why could this be? Does this mean basically my friend's 650w is the equivalent of a little more than a 300w fresnel?
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#2 Walter Graff

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Posted 21 February 2009 - 09:45 AM

I just compared a friend's 650w fresnel to my 650w fresnel. At the same distance mine read F8.0 and his reads F5.6 so therefore mine is twice as bright. Why could this be? Does this mean basically my friend's 650w is the equivalent of a little more than a 300w fresnel?


The photometric's of a particular fixture are based on the combination of the lamp, lens and reflector. Depending on the design, the spot or flood of the lamp, etc, you will always get different readings. A bare lamp that is 650 watts will always give off the same illumination as another bare lamp of the same wattage. And it will always give off more light than a lesser wattage counterpart. Put it in the chain of a fixture and all the rules change. So while one might have a 300 watt fixture verses a 650 watt fixture, because of the design of the 300 watt fixture, it may be more efficient than the 650 watt counterpart. Doesn't mean it's better or brighter, just that based on how you had the fixture set, and it's design, it might show more efficiency.
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#3 Jim Hyslop

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Posted 21 February 2009 - 03:33 PM

A bare lamp that is 650 watts will always give off the same illumination as another bare lamp of the same wattage. And it will always give off more light than a lesser wattage counterpart.


Sorry to pick on such a small part of an otherwise excellent explanation, but wattage measures how much electricity the lamp uses - nothing more, nothing less. "650 watts" means that the lamp will draw approximately 5.4 amps at 120 volts.

Just as there are many factors that affect the photometrics of a fixture, there are many factors that affect the brightness of a lamp, and wattage - or, more precisely, current draw - is only one of those factors. Higher wattage lamps generally give off more light than lower wattage lamps, but you cannot categorically state that two lamps of the same wattage will emit the same amount of light. For example, this comparison chart shows that BTL and BTM lamps are both rated at 500W, but the BTL puts out 11,000 lumens whereas the BTM puts out 13,000 lumens.

All of this just goes to show that you can't light by numbers alone - it requires experience with the various fixtures to understand what you're actually going to see (and that's what cinematography's all about, isn't it? :-)
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#4 timHealy

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Posted 21 February 2009 - 03:40 PM

I just compared a friend's 650w fresnel to my 650w fresnel. At the same distance mine read F8.0 and his reads F5.6 so therefore mine is twice as bright. Why could this be? Does this mean basically my friend's 650w is the equivalent of a little more than a 300w fresnel?



Are these 650's the same lamp from the same manufacturer and the same bulb, from the same manufacturer as well?

Best

Tim
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#5 Walter Graff

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Posted 21 February 2009 - 03:42 PM

Sorry to pick on such a small part of an otherwise excellent explanation, but wattage measures how much electricity the lamp uses - nothing more, nothing less. "650 watts" means that the lamp will draw approximately 5.4 amps at 120 volts.

Just as there are many factors that affect the photometrics of a fixture, there are many factors that affect the brightness of a lamp, and wattage - or, more precisely, current draw - is only one of those factors. Higher wattage lamps generally give off more light than lower wattage lamps, but you cannot categorically state that two lamps of the same wattage will emit the same amount of light. For example, this comparison chart shows that BTL and BTM lamps are both rated at 500W, but the BTL puts out 11,000 lumens whereas the BTM puts out 13,000 lumens.

All of this just goes to show that you can't light by numbers alone - it requires experience with the various fixtures to understand what you're actually going to see (and that's what cinematography's all about, isn't it? :-)


Jim,

Not nit picking. I was not clear. I was trying to say that the same lamp type of the same wattage should basically read the same on a meter as one next to it if the same wattage so I had a comparison of what happens to that lamp when you put it in a fixture. I should have added that different lamp models are themselves different in term so illumination even though they might both say 500 watts as you stated. Different filament winds make for different light levels so different lamp models of the same wattage may not be the same in terms of output.


Now if only we could re=teach the many myths of electricity such as electricity is made up of electrons.
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#6 Salil Sundresh

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Posted 21 February 2009 - 06:08 PM

Jim,

Not nit picking. I was not clear. I was trying to say that the same lamp type of the same wattage should basically read the same on a meter as one next to it if the same wattage so I had a comparison of what happens to that lamp when you put it in a fixture. I should have added that different lamp models are themselves different in term so illumination even though they might both say 500 watts as you stated. Different filament winds make for different light levels so different lamp models of the same wattage may not be the same in terms of output.


Now if only we could re=teach the many myths of electricity such as electricity is made up of electrons.

I see what you guys are saying. Thanks for all the replies. Now that I look at it, it was a pretty dumb question. It just astonished me that there was such a big difference between the two 650s.
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#7 Walter Graff

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Posted 21 February 2009 - 06:09 PM

I see what you guys are saying. Thanks for all the replies. Now that I look at it, it was a pretty dumb question. It just astonished me that there was such a big difference between the two 650s.


It was a good question, not dumb. Thanks for asking.
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#8 Hal Smith

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Posted 21 February 2009 - 11:07 PM

Another factor is beam spread. Two fresnels of different manufacture will probably have different maximum spread angles in their flood position and different minimum spreads in spot as well. The higher foot candle fixture will be the one with the smaller spread angle since it covers less area at a given distance with the same amount of light power.
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#9 Salil Sundresh

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Posted 22 February 2009 - 03:23 AM

Another factor is beam spread. Two fresnels of different manufacture will probably have different maximum spread angles in their flood position and different minimum spreads in spot as well. The higher foot candle fixture will be the one with the smaller spread angle since it covers less area at a given distance with the same amount of light power.

Yeah good point, we actually had to spot mine slightly b/c his had a narrower beam diameter at full flood.
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