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Arri Photometric not the same as actual light?


Sam Bignell
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Hey Everyone

I must just be doing something wrong or my lightmeter is wrong but I was testing my 800w open face lights today between a few brands I have and noticed that my own Arrilite 800w Open faces put out considerably less light than what Arri's photometrics says they should?

Arrilite 800w at 3ft away pointing directly at the lightmeter dome Recessed inside not out, settings (25fps, 800iso, 180d) 240v
Full Flood I got a 340fc , Arri's photometrics say it should be roughly 537fc
Full Spot  I got a 2200fc, Arri's photometrics say it should be roughly 9871fc

Is there something I'm doing wrong? I checked the bulb, Cleaned the dome results are the same

I understand photometric arnt 1:1 but mine seem so far off

Thanks

-Samuel

Edited by Sam Bignell
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That head hasn't been manufactured in 10 years or so, if memory serves. How does the reflector look? A bit of burnt-in dust and dirt over the years can easily decrease intensity by 20%. Though that wouldn't account for the increase you measure in spot. Perhaps the reflector has become misshapen.

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1 hour ago, Brian Doran said:

That head hasn't been manufactured in 10 years or so, if memory serves. How does the reflector look? A bit of burnt-in dust and dirt over the years can easily decrease intensity by 20%. Though that wouldn't account for the increase you measure in spot. Perhaps the reflector has become misshapen.

Urhmm I guess must be the case, The reflector is definietly not warped or dented but maybe the dish has just lost it's reflectiveness however it looks pretty darn clean and reflective and as I just cleaned it it looks.. well clean but it must have just "faded" slightly

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On one hand that's a fairly old light the output of which can be affected by a large set of variables, on the other hand I've noticed that the Photometrics app becomes increasingly "optimistic" about output when you go below the recommended safety distance (which for a lot of lamps seems to be overly cautious) and can be somewhat unreliable for the discontinued products.

My recommendation would be to give the lamp and reflector thorough clean and check for any dents in the reflector. Replace the bulb with a new OEM bulb. If you bought the lamp used you have no real way of knowing the age of the bulb or if it is one of the plethora of knockoffs now available.

In general I've found that in real life tests on new lamps, they tend to measure slightly brighter than lab measurements due to the ambient bounce inherent in not testing is pitch black rooms.

Lastly, I find that testing only at 3' can give somewhat unreliable results as due to the inverse square law the smallest inconsistency in the distance, angle and alignment between the light aperture and the measuring device can give significantly different results. As such, I would recommend doing your tests at the intervals the manufacturers mention in their literature and see if/ where it gets close. I tend to measure in SI at 1m, 2m, 3m and 6m for small units and 6m, 12m, 20m and 50m for big ones.

Try comparing your 800 with the current 750 plus, their flood and spot characteristics are different but at a beam of 60° 13' away the old 800 should be no more than a third of a stop dimmer.

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On 6/8/2020 at 3:04 PM, Andrei Pacuraru said:

On one hand that's a fairly old light the output of which can be affected by a large set of variables, on the other hand I've noticed that the Photometrics app becomes increasingly "optimistic" about output when you go below the recommended safety distance (which for a lot of lamps seems to be overly cautious) and can be somewhat unreliable for the discontinued products.

My recommendation would be to give the lamp and reflector thorough clean and check for any dents in the reflector. Replace the bulb with a new OEM bulb. If you bought the lamp used you have no real way of knowing the age of the bulb or if it is one of the plethora of knockoffs now available.

In general I've found that in real life tests on new lamps, they tend to measure slightly brighter than lab measurements due to the ambient bounce inherent in not testing is pitch black rooms.

Lastly, I find that testing only at 3' can give somewhat unreliable results as due to the inverse square law the smallest inconsistency in the distance, angle and alignment between the light aperture and the measuring device can give significantly different results. As such, I would recommend doing your tests at the intervals the manufacturers mention in their literature and see if/ where it gets close. I tend to measure in SI at 1m, 2m, 3m and 6m for small units and 6m, 12m, 20m and 50m for big ones.

Try comparing your 800 with the current 750 plus, their flood and spot characteristics are different but at a beam of 60° 13' away the old 800 should be no more than a third of a stop dimmer.

This is a great point and did cross my mind reguarding measuring at 3ft an unsafe distance

I'm going to give it another clean sometime soon and will test at a greater distance, The reflectors are dent free and are actually pretty clean looking however I have two of these 800w heads ones an original ARRI arnold & richter really old the others a standard ARRI both have seen a lot of use and the arnold and ritcher I guess is not quite as shiny looking in terms as the dish and does measure les light in the SPOT

I tried a brand new bulb however not an Osram OEM 800w and there was no difference

I will post back later when I get results!

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  • 1 month later...
  • Sustaining Member

I just did the same type of test, and the results were about half of the manufactures published numbers. 

I agree that the photometric apps are optimistic. 

I would love to be able to rely on that data, but at this point, I can't. 

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  • 3 weeks later...

Good for you to test photometrics.  In my experience, most gaffers and DPs will call a lamp based on years of experience rather than actually metering lights, and thats all well and fine, but its great to have some back-pocket stats.

In general, it's probably best to do your own testing obviously.  Just make notes of the lamps you like to use, and that way you'll have it for future reference.  If you have a handle on the smaller lamps you like to use often, then it gets easier to "scale up" when the shows and shots get larger.  In general, for instance, the Mole and Arri fresnel lamps scale up as you would expect photometrically, same with Arri M Series HMI.  But, always test!  DP's (and other gaffers) never believe me when I tell them how much an M18 puts out in Full Spot.  If I had a nickel...

Like others have noted, inspect the lamps optics, and make sure everything is clean.  And the bulb, of course.  With tungsten, remember that the lamp needs it's rated voltage to perform optimally both in terms of output, and color temperature.  Meter the line before you test, and if possible, while under load and see how much the voltage takes a hit.  If the voltage running through the filament is less than the rated voltage, you won't see optimum results.  LED and HMI are obviously not the same, as they have power regulators.   

Edited by Craig Ryan
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On 8/24/2020 at 9:04 PM, Craig Ryan said:

Good for you to test photometrics.  In my experience, most gaffers and DPs will call a lamp based on years of experience rather than actually metering lights, and thats all well and fine, but its great to have some back-pocket stats.

In general, it's probably best to do your own testing obviously.  Just make notes of the lamps you like to use, and that way you'll have it for future reference.  If you have a handle on the smaller lamps you like to use often, then it gets easier to "scale up" when the shows and shots get larger.  In general, for instance, the Mole and Arri fresnel lamps scale up as you would expect photometrically, same with Arri M Series HMI.  But, always test!  DP's (and other gaffers) never believe me when I tell them how much an M18 puts out in Full Spot.  If I had a nickel...

Like others have noted, inspect the lamps optics, and make sure everything is clean.  And the bulb, of course.  With tungsten, remember that the lamp needs it's rated voltage to perform optimally both in terms of output, and color temperature.  Meter the line before you test, and if possible, while under load and see how much the voltage takes a hit.  If the voltage running through the filament is less than the rated voltage, you won't see optimum results.  LED and HMI are obviously not the same, as they have power regulators.   

Absolutely agree in regards to testing and being aware of line-loss for tungsten loads. I just want to add that although electronic ballasts can regulate the mains voltage and their output voltage, you can still incur line-loss across your run of header cable. Trying to use a single 100' run instead of two 50's, metering out your header to find any high resistance wires, and keeping clean contacts can all help keep resistance and thus line-loss down. The newer ARRI ballasts now use CCL (compensation for cable loss) circuitry, wherein a feedback circuit in the head is telling the ballast how much extra voltage is needed to overcome the line-loss. helps keep things predictable.

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