Determining light equivalency

[Zak Ray]

Hi all! I'm trying to get all the variables straight in determining light output equivalency. For example, I'd like to compare the Aputure 300D (a claimed tungsten 2K equivalent) to an actual Arri 2K Fresnel. Here's my math:

 

The Arri Photometric calc tells me a T2 fresnel outputs 354 FC at 3m/10ft with the lamp at full flood.

 

The specs on Aputure's website list the 300D at 1800 FC at 1m/3.3ft with the lamp at full flood (using the fresnel attachment). I use an inverse square calculator to convert that to 272 FC at 3m/10ft.

 

So that would lead me to believe the Aputure has about 75% the output of the Arri, meaning it's equivalent to about a 1.5K. However, this doesn't take into account beam angle. The flood beam angle of the Arri is 56º, and the flood of the Aputure is 42º. That's another 25% loss if we account for beam angle. So that theoretically puts the Aputure at 1.1K.

 

I have a hunch that calculating beam angles this way isn't correct. Can anyone weigh in?

 

Thanks!

[Adrian Sierkowski]

Your best way-- rent both and use a light meter. Photometrics of any lamp, specified or otherwise, will always be a bit off. Also, yes, the LED will not be as bright as a tungsten head once you start moving away from it. It's all marketing, honestly, it's marketing. The question really isn't "how does it compare to X" I think the real question is, will this unit, at this distance give me the output and spread I need for this shot given the realities (power/ rig time/ availability) of production.

 

Beam angle wouldn't really enter into the photo metric number which would be measured center beam; generally. Sometimes some lamps may give you the photometrics of the beam edges, but, in this case, you're really just looking at a point in the center of the beam. The angle will tell you, at a given distance, how much area it will cover, but the real measure of the light won't match the spec sheet 100%

[Zak Ray]

Thanks Adrian. I hear what you're saying re: measuring the center of the beam, but wouldn't the center naturally dim as the beam angle increases?

 

I might indeed end up renting some lamps to test, but at the moment I'm just comparing a lot of them online to gauge what's in the ballpark of what I'm looking for. And yeah, I know marketing deparments can go a little wild! (I remember the Litepanels Sola 6+ being marketed as a 1600W HMI equiv-- ended up being closer to a 650W tungsten)

Edited September 7, 2018 by Zak Ray

[Adrian Sierkowski]

I'm sure the Sola probably is a 1600HMI equiv at like 2' distance. The problem with LED isn't so much the output as it is the falloff of that output.

As for the center of the beam dimming-- that depends on the design of the unit but generally, yes it will dim as the angle increases, but how much is a function of the whole design (reflector and all) which is how, for example, the M40 can compete, reasonably with a former 6K HMI.

[Brian Doran]

While I agree with the overall sentiment that LEDs generally cannot compete with HMIs, I don't think that it makes sense to consider the rate of falloff to be anything but directly proportional the initial output. The rate of falloff is not different from one source to another, the inverse square law always remains the same. I suppose you might mean that the lower light output can be offset by how close you can comfortably stage an LED to the talent, in which case, I would agree. The lack of infrared emissions from most RGB LEDs certainly makes the beam feel colder, and I don't just mean color temperature wise.

[Zak Ray]

Thanks guys. So the consensus is I shouldnt be trying any sort of beam angle compensation, meaning in terms of the Aputure light, itd be fair to say that its a 1.5K equivalent. I just have to factor in that its a smaller beam angle.

 

Right?