Technical question: How many Lumens in a Watt and vice versa?!

Hello all,

I saw a big led light for photographers online that was rated at 100w. Nowhere on the product page could I see how many lumens that is though...

I understand that Lumens per watt are different for different light sources, e.g. for old fashioned household filament bulbs its around 15 lumens per watt if I remember correctly. But what is it for LED's?

Thanks!

It really depends on the make and model of LED. Some LEDs are as high as 200lm/W, while others can be much less.

It also depends on power. A 5W LED could produce 500lm, while a 20W LED could produce only 1000lm.

Generally speaking, your 100W LED (if genuine) will probably produce around 5000lm.

Hope that helps!

I think you already realize you're kinda-sorta asking "how much horsepower per gallon?", although arguably lumens-per-watt has far fewer variables. Each of the myriad LED types, with their different ratings (you've seen R5, S2, etc) has a different answer to "what's the rule of thumb on power-to-lumens?". You're best off finding out which specific type of LEDs -- and how many -- are in that light, and asking about that specifically, IMO

A Watt is a measure of one of two things:
1. Power consumed
2. Heat produced

Some of the current going through the LED is converted to visible light, a little bit to other radiation, and most of it to heat. IIRC, they're now around 70-80% of power converted to heat, but I can't find a source to back that up, so that might be way off.

The first power LEDs were only a bit better than incandescent with wall power (where incandescent could very well run at >=20lm/W constantly), and even with batteries, mainly sold on durability and high CCTs, though they were several times as efficient. Incandescent light sources also vary in efficiency, but typically cap out around 20lm/W, and worse when under-driven. 15W is typical for a fixed light, though (900lm is a typical 60W rating, FI, which comes to 15lm/W).

The lm/W measurement is based on power consumed. LEDs vary greatly in this measurement. Current power LEDs are generally around 100lm/W, but then, for any device, there's loss as it heats up, loss with higher current (both things that are being worked on more than the raw efficiency, now), and light lost to whatever optics the device has. So, out of the lens, anywhere from 120lm/W (at low drive currents) down to 60lm/W would be reasonable, with modern quality LEDs.

So, any Watt rating is totally worthless, unless it's a rating of wall power used, in case you need to calculate that (such as for portable work off of a generator or inverter). Even with lumen ratings, they need to, at the very least, follow relevant ANSI or ISO standards, and have complying measurements. There are different ones for different industries, but even consumer flashlights have one, these days.

Why? Because they will often state the datasheet's lumens at room temperature for each emitter, which are totally bogus, or use the datasheet to extrapolate what the output should be from the emitter, which will always be higher than reality. Reality is never so kind. Just try to count the number of posters that found their XXX lumen torch dimmer than a new Surefire, HDS, Malkoff, Inova, or other lower--but accurately--rated torch. The driver circuitry, heat dissipation efficiency, temperature once warmed up, and optics, all affect real LED efficiency, on top of the potential efficiency of the emitters themselves.

snakeplissken83 said:

Hello all,

I saw a big led light for photographers online that was rated at 100w. Nowhere on the product page could I see how many lumens that is though...

I understand that Lumens per watt are different for different light sources, e.g. for old fashioned household filament bulbs its around 15 lumens per watt if I remember correctly. But what is it for LED's?

Thanks!

Click to expand...

Why don't you provide a link to the product in question, or at least give us the full name / model number so we can look it up ourselves. A "100W" LED light could be anything from a 100W equivalent LED, meaning it's got the light output of a 100W incandescent ~1600lm, but uses 12-15W, to a true 100W LED light which if it was using the latest LEDs, good electronics, and good cooling could push ~10000+ lumen.

I think this is sort of simply taking the watts and lumens for a PARTICULAR source, and dividing to see how many of what per what.

Other than that, its (As mentioned) akin to asking how many horsepower per pound a car has, etc.

If you know which car is involved, you CAN do the math...otherwise, not really.

You can't say if the car has so many HP it will weight a certain weight, or, if it has a certain weight, that it will have a certain HP, etc.

Plug in "LED" for "car" and its the same thing...different LEDs are going to be like different cars.

There's no set mathematical relationship between them in practical use....as there are variables that are not in the question.