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Open-face vs fresnel: diffusion confusion


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#1 Joshua G.

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Posted 06 March 2016 - 07:07 AM

I was after sourcing a light fixture whose main job was to punch through or bounce against diffusion (and possibly, periodically in a chimera), and so naturally, I had my eyes set on open-faced units. Upon closer look however, I see that the fresnel equivalents output a higher lumens than its open-face counterparts, judging from their photometric data (Arrilite 2000 plus v. Arri T2 v. Ianiro blondes). How is this so -- I thought the sense in preferring open-face units on diffusion is their luminance advantage? Have I misunderstood the rationale and/or misread the charts? I'd very much appreciate some clarification.
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#2 Phil Rhodes

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Posted 06 March 2016 - 07:59 AM

Photometric data provided by manufacturers is often about as reliable as a seven pound note. Can you link the documents you looked at?


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#3 Mihnea Snooker

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Posted 06 March 2016 - 10:34 AM

You have to check the beam angle. At 2K, set at spot, the fresnel has more output than openface but it doesnt fill the same surface of diffusion. And if you change the beam angle to get the same beam diameter as the open face unit, you will get less output.


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#4 Phil Rhodes

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Posted 06 March 2016 - 01:49 PM

Strictly speaking, lumens are supposed to be an expression of total output, so the beam angle shouldn't matter.

 

But as I say, there's a thousand thousand ways to fudge all this.


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#5 Satsuki Murashige

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Posted 06 March 2016 - 01:57 PM

As Mihnea says, check that you are looking at the tables for full flood for the fresnel when comparing to the open-face. It should be lower output in the flood position. The output at full spot is measured in the center of the beam, so it will be brighter. But if you're planning on diffusing or bouncing the source, then you will most likely be closer to full flood than spot.
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#6 Phil Rhodes

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Posted 06 March 2016 - 02:34 PM

That's sort of what I'm talking about. If they're measuring a spot intensity, that should be expressed in lux. If they're taking about total output, that should be in candela.
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#7 Joshua G.

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Posted 06 March 2016 - 03:56 PM

Using Arri's own photometric calculator one comes up with the following

Arri T2 with the CP/41 2000w lamp at full flood:
- 5.0m beam diameter = 4.7m distance = 1609lx illuminance
- 5.0m distance = 5.3m beam diameter = 1423lx

Arrilite 2000 Plus with an FEX 2000w lamp at full flood:
- 5.0m beam diameter = 4.2m distance = 1357lx
- 5.0m distance = 6.0m beam diameter = 940lx

Arri T2 at full spot:
- 5.0m beam diameter = 20m distance = 418lx
- 5.0m distance = 1.1m beam diameter = 6693lx

Arrilite 2000 Plus at full spot:
- 5.0m beam diameter = 11.8m distance = 795lx
- 5.0m distance = 2.1m distance = 4400lx

And,

Ianiro Blonde at 5.0m distance:
- full flood = 948lx
- full spot = 3984lx

---

- http://calc.arri.de/calculator
- http://www.ianirouk....p?ProductID=260
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#8 Mihnea Snooker

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Posted 07 March 2016 - 05:22 PM

Full spot for that fresnel fixture is not the same beam angle (and diameter) as full spot on open face, so to fill the same surface with a fresnel as with an open face you'll have to increase the angle on T2, and so you will get less light.


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#9 Joshua G.

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Posted 07 March 2016 - 07:48 PM

Full spot for that fresnel fixture is not the same beam angle (and diameter) as full spot on open face, so to fill the same surface with a fresnel as with an open face you'll have to increase the angle on T2, and so you will get less light.


Yes, but if the aim is to extract the maximum light output to punch through or bounce against a given area, it still stands that a fresnel would be the unit to give you the better results, does it not? This seems to me to go against conventional wisdom of favouring the open-face on such jobs. I suspect I'm misunderstanding an element to this principle.
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#10 John E Clark

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Posted 07 March 2016 - 08:15 PM

I was after sourcing a light fixture whose main job was to punch through or bounce against diffusion (and possibly, periodically in a chimera), and so naturally, I had my eyes set on open-faced units. Upon closer look however, I see that the fresnel equivalents output a higher lumens than its open-face counterparts, judging from their photometric data (Arrilite 2000 plus v. Arri T2 v. Ianiro blondes). How is this so -- I thought the sense in preferring open-face units on diffusion is their luminance advantage? Have I misunderstood the rationale and/or misread the charts? I'd very much appreciate some clarification.

 

You have misunderstood what a 'fresnel' does... It has changed the 'beam angle' of the resulting cone of light.

 

Without going into various trigonometric calculation along with integral of the lumens passing through a portion of a spherical surface, the rule of thumb would be that the circle of light from a Fresnel lensed light is smaller at a give distance, so more of the lumens from the lamp are passing through that circle, than is the case of the open face, which has a larger circle of light at the same distance.

 

This is why getting the photometric data for the lamp is better, since one doesn't need to do any math calculations, and even then... manufacturer's lie, or don't really know the actual light path through their lamps, such that they can a priori calculate the effective lux/footcandles delivered at a given distance.

 

Of course, one should actually verify with a light meter what the actual output is...

 

This is also why cheap light manufacturers what only list the wattage of the lamp are next to useless and must be measured when one gets the lamp if one is ordering on the cheap.


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#11 John E Clark

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Posted 07 March 2016 - 08:26 PM

Strictly speaking, lumens are supposed to be an expression of total output, so the beam angle shouldn't matter.

 

But as I say, there's a thousand thousand ways to fudge all this.

 

Yes, a given light bulb's output is 'lumens', in order to get lux, one needs to divide by 4 x pi x r^2, the surface area of the sphere which origin at the bulb and the r^2 gives the inverse square law...

 

But lens and reflector modified this equation... So if one wants to ignore the loss from the lens and reflector, a focused beam will have the lumens of the bulb passing through the surface of the base resulting 'cone', using the lamp as the apex of the cone.

 

Ignoring the 'curvature' factor of the intersection of the cone with the surface of the sphere, one can 'easily' calculate the lux for a given diameter of the base of the cone...

 

Or look it up on the Arri site... or just measure it with the lamp at hand...


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#12 Gregg MacPherson

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Posted 07 March 2016 - 08:30 PM

Joshua,
Start by getting clear on two concepts.

 

A bulb emits a certain amount of energy,  so many photons if you like.  That can be measured as lumens.  That light can be focused or completely dispersed.  Our eye easily sees relative brightness  when more or less lumens land on a given surface area.  This light per unit area can be defined as lux.  I think 1 Lux = 1 Lumen/m^2  (one lumen per meter squared).

 

When we measure with our meter,  often in foot candles (fc),  it's light per unit area (and 1fc=10.76 lx).

 

The best way to compare the open face to the fresnel of the same watts is to focus them to fill the same diffusion frame size,  yes?  Visit the rental company and they may let you experiment....?

If using an online calculator,  set the distance and the beam spread the same for both types of light.

 

Don't let John make it sound too hard.


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#13 Joshua G.

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Posted 07 March 2016 - 08:38 PM

If using an online calculator,  set the distance and the beam spread the same for both types of light.


Which is what I did, the results of which I posted. And the fresnel came out on top an which I thought didn't add up.

I am an absolute beginner and have no access to a rental house.
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#14 Stuart Brereton

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Posted 07 March 2016 - 08:55 PM

Completely unscientific, but if you're bouncing, use an open face lamp. If you're diffusing, use a fresnel. If you only have one lamp, then use what you've got.


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#15 Joshua G.

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Posted 07 March 2016 - 09:23 PM

Completely unscientific, but if you're bouncing, use an open face lamp. If you're diffusing, use a fresnel. If you only have one lamp, then use what you've got.


And if a situation arises wherein -- and this is if the photometric data or calculations hold -- you have both fixtures or is tasked to bring only one fixture, the one that gives the higher light output given a set distance or area, be it to bounce against or diffuse through, that fixture would have to be the fresnel, isn't that right?
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#16 Stuart Brereton

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Posted 07 March 2016 - 09:49 PM

If I can only have either a fresnel or an open face lamp, I'll take the fresnel. Not because of output, but because of flexibility


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#17 Gregg MacPherson

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Posted 07 March 2016 - 10:13 PM

Joshua,

Sorry,  but your notes are a bit of a mess.  You can easily compare the lamp heads if you use the same beam diameter,  the diameter of the lit circle..  But you were inconsistent with that.  And Ianiro were hard to figure out,  beam diameter was not given.  I had to calculate that.

 

So I went Arri's calculator,  and found that yes,  they give the fresnel as 6% brighter for a 5m beam diameter.  That is still puzzling.  Maybe the reflector is magic.  If so,  I'm sure they will be singing loudly about it somewhere on the web.

 

If it all turns confusing,  Stuart's advice is the best.


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#18 Satsuki Murashige

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Posted 07 March 2016 - 11:38 PM

The Arri T2 is a newer design, so it's possible that the reflector is more efficient than the old fresnels we are used to. I've never opened one up to see. On the other hand, it's quite a bit more expensive than the equivalent Arrilite. Though it is more versatile.

So that is part of the preference for open-face lights when bouncing - it costs less to rent, and if you have one of each then you reserve the fresnel for use as a set light or a key instead. But if you're buying, then the fresnel would be the better all-rounder.
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#19 Phil Rhodes

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Posted 08 March 2016 - 05:13 AM

Bear in mind that the original post talks about lumens, which are a measure of total output.

 

The posted numbers are then in lux, which is a unit of point flux.

 

The fresnel can have higher absolute output at any one point and still have lower total output.

 

P


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#20 Joshua G.

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Posted 08 March 2016 - 05:55 AM

The Arri T2 is a newer design, so it's possible that the reflector is more efficient than the old fresnels we are used to. [...]

But if you're buying, then the fresnel would be the better all-rounder.


Not meaning to sound difficult here, but the Arrilites are also an improved unit: for example, Arri claims the 750 Plus is comparable to or exceeds their now-defunct 1000w open-face model.

If the aim is purchasing or say picking up a single unit between two 2K fixtures -- one a fresnel, the other open-face -- to deliver the higher output light or meter reading off a bounce or diffusion material, the choice is clear based on the charts, right?

This choice being the fresnel strikes me as contradictory to the prevailing wisdom I've gleaned from the DoP community. The rationale was, as I understood it, if one wants the punching power or higher light output alone with which to utilise a diffusion or bounce material, the open-face was the fixture of choice (over an equivalent tungsten fresnel). The numbers, though, do not or no longer bear this out.
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