LED-How to tell the real ligthing output of led lights?How to compare lighting power?

hasselblad

hasselblad

Well-known member
I am looking at several led light options. Obviously, one of the considerations is how much light the led panel emits.
Some companies list their products specifying out put via "lux"
http://www.adorama.com/fpvl500c.html
Others list Watt, W.
http://ikancorp.com/productdetail.php?id=998
Others use lumens...

My questions: How to measure/compare the lighting output of led lights? Is there a way to convert these numbers? All I want to know how much light these lights output so I can compare them.
 
The best way is to look at photometrics for similar beam angles. Anything else may not be giving you much to go on since different lights have different efficiencies and beam angles and technologies. Some lights are very difficult to compare because they cannot do the same thing so it's impossible to compare them directly. However LED light arrays are not too hard to compare since most of them have fairly similar beam angles and use similar designs.

Looking at the two units you've linked, one lists 530 lux at 6ft and the other lists 770 lux at 2 meters so most likely you'll find the second one to be brighter even if both draw about 40W. As these are fixed beam angle and one is bi-color but the other is daylight-only, however, the second one could just have a narrower beam angle or the first one may only use half it's power if it maintains intensity through the color temperature range.

Having seen and used lights that look identical to these that were probably made in the same chinese factory but sold under a different name, I would probably trust the second better for color rendering and build but neither would be my choice as they are both 4+ year old designs. The first one you linked is a rebrand of a light that was one of the first inexpensive chinese LEDs to be available 5 years ago or so and I recall them being fairly green and bulky without being very durable. Some of my first fluorescent lights were biax lights using essentially the same housing as many cheap lights reused that housing. I had a friend with that 500LED light under a different brand and it was quite green and quickly superseded by lights with better color, build, size, and weight. One of my first LEDs was the smaller bicolor version of the second light you linked which was a big improvement but still can't really compare to newer lights of the past 4 years. It's still in my kit but I haven't used it in years.
 
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Let me just say how much I hate the fact that we usually rate lights in terms of watts. It's like saying "this car has a very powerful engine, it does just 20 mpg!".
 
Really? I find wattage very useful both for understanding power supply and for getting a reasonable sense of at least luminous flux to expect given a basic knowledge of the efficiency of whatever technology is used.

For example tungsten is around 25 lumens/watt, and just about all modern high-CRI HMI/LED/FLO is around 75-100 lumens/watt, so the emitters are about 3-4x as bright per watt if you forget about the design of the reflector, lens, housing, diffuser, etc. So you can be pretty sure when an LED is only 40W and advertised as 500w or even 650w equivalent you know they are fudging the numbers somewhere (hint: usually it's in the beam angle and quality of light).
 
For tungsten it works OK, but for CFL and specially for LED, where efficiency can change a lot, it promotes inefficiency (bad AC-CD converters and drivers).
My LED fresnel has the same light output as a 500W traditional fresnel. That's useful information. It consumes 75W at the wall. That's useless information: even without going for a less efficient LED, it could very well be 150W if I went for a bad AC-DC converter and an inefficient driver. I see this already happening on the super-cheap ebay lights, and on household fixtures.

(I have a cheap 150W CFL bulb that I bought on ebay, and also a 105W CFL bulb that I bought on amazon and which emits at least twice the amount of light)
 
nyvz said:
Really? I find wattage very useful both for understanding power supply and for getting a reasonable sense of at least luminous flux to expect given a basic knowledge of the efficiency of whatever technology is used.

For example tungsten is around 25 lumens/watt, and just about all modern high-CRI HMI/LED/FLO is around 75-100 lumens/watt, so the emitters are about 3-4x as bright per watt if you forget about the design of the reflector, lens, housing, diffuser, etc. So you can be pretty sure when an LED is only 40W and advertised as 500w or even 650w equivalent you know they are fudging the numbers somewhere (hint: usually it's in the beam angle and quality of light).
Click to expand...

Listing the 'wattage' does nothing to say how much light to expect at a given distance from the lamp head.

Beam angle, and attendant fall off chart will indicate this... and a meter that reads out fc/lx directly will give the necessary information for setting exposure, or indicating more light is needed...

Lumen/watt is even useless, because it doesn't say how much is falling on the subject... only that in the case of a omni directional 'bulb' X number of lumens is passing through the sphere, centered at the bulb.

Unless of course, you have your solid conic trig functions on the tip of your tongue... and know the type of beam produce by the the lamp...
 
Samuel H said:
It's like saying "this car has a very powerful engine, it does just 20 mpg!".
Click to expand...

I appreciate the analogy but the funny thing is that the car industry already does do that when they advertise horsepower. It's meaningless without also knowing the weight of the car.
 
^ In lighting terms, that would be the equivalent of talking about lux without talking about the beam angle, or the quality of the glass (my LED fresnel uses a set of optical-grade lenses; using cheap glass instead would easily cut the light output in half, using coated glass could increase it).

It seems that probably the most useful way to talk is "my LED fresnel is equivalent to a 500W traditional fresnel", meaning that with the same beam angle I get the same exposure.
 
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