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Andrew Ko

Should I think in Watts or Lumens?

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Hi guys,

So I know that a lot of LED sources are more energy efficient than tungsten sources, and so could output more light with similar or even less wattage.
I'm planning on purchasing some lights next year for a short film. I'm used to using Arri 650s from my school, and am trying to get some kind of frame of reference for how much light I'll be getting for my money, and just a general way to "compare" lights and have an idea what I could be working with.

My instinct tells me I should be comparing lights by lumen or lux, but I know that generally on set people refer to lights only by wattage.
So should I be thinking in Watts or Lumens?

Thanks!
Andrew

To give an example from school, I generally worked with a set of 3 Arri 650s (Tungsten bulb) along with a 1.2K HMI (LED source).
How could I calculate how many 650s I would need to equal the output of the HMI when their sources are completely different?
The HMI, although it was only 1.2K seemed to output way more than all 3 650 lights combined!

Edited by Andrew Ko

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OK, so your 1.2K HMI is probably an HMI, not an LED. An HMI is an arc lamp. Different thing to an LED. There are very few LEDs that big.

As you seem to have figured out, it's not a very simple question. For instance, the Arri 650s you have are fresnel lights and they suffer some quite large optical inefficiency because a lot of the light coming from the bulb just smacks into the inside of the light housing and is lost. LED equivalents are often built quite differently, which can really make a big difference.

In general, you can assume a 4:1 advantage for LED and for HMI. A 200W LED is likely to be significantly more powerful than your 650W fresnels, give or take however the light itself is built.

This is why we like it.

P

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I'm a big proponent of comparing fixtures based on illuminance values such as foot-candles and lux, rather than luminance values given in lumens. Lumens represent the total amount of light produced by the source, be it the tungsten bulb, the HMI lamp, or the LED engine. The problem with this is that it does not account for the beam angle, so it does not speak to how wide a source is. It is also my understanding that the lumen measurements are often made independent of the fixture that the light source is installed in. As Phil mentioned, light loss in optical elements is an important factor, and this might not be represented in the lumen measurement. Let's say that five different fixtures are using the same LED engine made by Osram, well all five might list the same lumen value suppled by Osram, but this does not factor in the fixtures optical loss, reflector efficiency, etc.

So, I like foot-candles and lux. These units measure the number of lumens falling over a 1 square foot or 1 square meter area. Instead of measuring just the light source in an integrating sphere like in this photo, foot-candles and lux are measured as the light falls upon a surface at various distances from the fixture. Because of this, we are able to factor in optical efficiencies that might separate two fixtures with the same OEM light engine. Most foot-candle/lux charts will also show the beam angle, illuminance, beam diameter, distance relationship. This has the benefit of telling you how large of a space the fixture can illuminate to X level at Y distance when at whatever variable focal length, if applicable. Here is an example of an illuminance photometric chart.

Comparing lumens reminds me a bit of comparing the expected lifetime of LED engines. There are only a handful of LED manufacturers in the world, and our tiny little industry mostly buys from that same handful. It's the LED OEM that calculates the commonly referenced LED lifetime "L70" metric, which measures how many hours an LED engine can run before efficiency drops to 70%. That's all good and well, and it speaks to the quality of LEDs that the lighting fixture uses, but it is not analogous to the fixture's expected lifetime. When you see a streetlight flickering, it's because of an issue with the LED driver or power supply, not the LEDs themselves. The same is true in our industry, the electronics will likely fail before the L70 time is reached.

 

This is long winded. Like Phil said, you'll probably just get used to thinking of LED, tungsten, and HMI as three different beasts and have a general sense of the different degrees of lumen-to-watt efficiency between them. Just make to sure to double-check the photometric sheets when marketing material claims that X LED fixture is as bright as Y HMI fixture. HMI is still the brightest-per-watt, and I just don't know where some of these marketing comparisons come from...

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Efficacy (Lumen to watt ratio) of LEDs is constantly changing and getting better over time. Every LED manufacturer has different efficacy ratings, meaning a 300watt from one brand could be brighter or less bright from another company boasting the same/similar wattage. This fact alone means it’s better to judge light by ftcandles , lux etc.

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Wow ... Amazing answers guys, this forum is truly a blessing.

Thanks especially to Brian for such a detailed answer, I appreciate you taking the time to write that out, I'll make sure to keep it as reference.

 

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Manufacturers will create photometric charts for their fixtures detailing the output at various distances in f-candles or lux. That allows you to compare different tech fixtures (like Xenon, LED, and HMI) as just lights with an output value. Perhaps this will help you out.

If you start messing with a light meter, it'll have f-candles/lux, and you'll grow used to how much light you'll need for a scenario.

Lumens is more consumer commonplace, probably as a way to outmeasure the competing light bulb.

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For the record:

To achieve proper exposure in light of a given brightness, required exposure value is the binary logarithm of lux multiplied by ISO over (usually) 330. For 1000 lux and 100 ISO, calculate log2(1000×100÷330), giving 8.2. On a calculator:

1000 × 100 ÷ 330 = ln / 2 ln = 8.24...

Furthermore, the exposure value of a camera setup is the binary logarithm of the aperture squared over the shutter time in seconds. For 1/48s at f/2.8, calculate log2(2.82÷(1/48)), giving 8.56. On a calculator:

2.8 x2 ÷ (1 ÷ 48) = ln ÷ 2 ln = 8.56...

Thus, 1000 lux is just under a third of a stop overexposed at f/2.8 and 1/48s at 100 ISO.

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Posted (edited)

Curious as to why companies give lux and footcandles for distances that are not practical. Like one company will give you the fc for 1metre, 3metres then 50metres. Another will give you fc for 5metres then 10metres. It's so frustrating, does anyone know the algorithm to be able to compare two fixtures from two different companies given the photometrics they give you?

Edited by Berry Spinx

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One reason you might hear people talk in watts on set is to make sure the don't trip a breaker when they're on house power.

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I think the HMI is supposed to be about 4 times more energy efficient than tungsten bulb (something like 20% efficient compared to the close to 5% efficiency of a tungsten bulb). If it's a HMI Par vs. tungsten fresnel then it may have even more output depending on the fixture. 

You have to also take into account that you generally need to gel the tungsten or HMI to compare them against each other for same application. If you need cold light and will need to gel the tungsten up you will lose 1.5 or 2 stops of light compared to gelling the HMI down which would take approx. 2/3 of a stop. 

So the HMI can give from 8 to 16 times more output compared to similar wattage tungsten if you are intending to use both at 5400K or more.

For LED vs. HMI the difference is smaller because the gelling is pretty much the same for both. Even when the LED is more energy efficient the difference is something like 20% for HMI and 30 to 60% for LED. The HMI being a more pointy source so it can have advantages when needing bright and hard light. 

But the gelling matters a lot and it is very signifiant variable when comparing either LED or HMI to tungsten in any application

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