LED lights: A Few Facts

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Hey Greg,

No offense, but I think you are confusing this thread with insignificant technical detail.

As a general rule with a dive light, 5x lumens = 2x perceived brightness. A 500-lumen light will appear about twice as bright as a 100-lumen light.

As I already said, this is a highly simplified heuristic. In an ideal world, everyone would also consider the beam angle, which makes a huge difference in how those lumens are spread over the target surface.

But the reality is that most manufacturers quote only lumens, not lux@distance and/or beam angles. And these lights are generally used in short-medium range applications where most of the light is reflected back to our eyes. So the 5x output = 2x visible rule is close enough for sport.

As for photopic vs scotopic vision...it is not a major factor here. The current generation of LED dive lights output 100+ lumens, and again, they are typically used at short-medium range. Also, they are frequently used in daytime dives, where you have lots of ambient light but you need to look in a crevice, or restore color to a patch of reef at depth.

Hope this helps. :)
-Jeff

Ok, I'll try to break this down (this is not insignificant detail, and no offense has been taken):

Lumens != brightness, brightness is a luminance measurement, and it is very subjective. Please stop confusing the units (and me in the process). If you changed your equation to equate to cd/m^2 (luminance) it would make sense according to the research you linked to (although it is closer to 6x).

You say that a 50% drop in lumen output does not equate to a noticeable difference in perceived lightness.

I say it does. Again, you should buy a light that has half as many lumens if you can't tell the difference and save yourself some money :wink: .

The scotopic response changes the perceived lightness function to be more sensitive when using cool white LEDs and in dark conditions (as in the cold waters here in the Pacific Northwest). The function you referred to is old and does not account for lumens, nor the scotopic response, nor the cool color (wavelength) of LED light.

Lumens is what this whole discussion has been about (not lux, not cd/m^2). I do not buy the small beam angle and all light is reflected back to the eye. At the end of the day, you have reduced lumens (.: lux) on the illuminating subject and cannot see as much as before, regardless of how bright the center appears. If I went to the USCG, ICAO, IALA, CCG, DOE, USAF or the FAA with a LED light that was quoted in the terms you are saying, I would not be making a sale. That is based solely on safety and divers should be no different.

Here is a quick calculation for you to try:
Light = ? lumens, beam angle 6 degrees

Lux @ center, 1 m away = 1 lx
Lux @ 6 degrees or 52mm on each side of center = 0.5lx

You can't figure out the lumens as you don't know the gradient or change in lumen output as it approaches the divergence angle points. Believe me, more than one engineer I work with has tried to do this with a goniometer and it near impossible unless you measure EVERY point. You can compare lx at a single reference point if you prefer though.

Hopefully this has clarified some misconceptions you may have had. :)

BTW this thread has gotten way of topic, hopefully Gcbyran won't be too upset.
 
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You say that a 50% drop in lumen output does not equate to a noticeable difference in perceived lightness.
No...I never said that. Go directly to jail. Do not collect $200. Please go back and read my earlier posts. :)

If I went to the USCG, ICAO, IALA, CCG, DOE, USAF or the FAA with a LED light that was quoted in the terms you are saying, I would not be making a sale.
Sure...but I'm not a salesman. :)

I'm an engineer with several decades of experience, plus some ability to translate the relevant technical bits for non-specialists who share my interest in diving. :)

Greg, I'm going to guess that you are either not an engineer...or that you are a fairly young engineer (in your early 30s?). FYI, scotopic vs photopic response curves are not something that we discovered "recently". Again...check out the CIE bibliography for more info. If your company cannot afford it, try a good university library.

You can't figure out the lumens as you don't know the gradient or change in lumen output as it approaches the divergence angle points. Believe me, more than one engineer I work with has tried to do this with a goniometer and it near impossible unless you measure EVERY point.
Two words...integrating sphere. Apparently your company doesn't have one?

BTW this thread has gotten way of topic
Agreed. This is a diving forum. Most folks here could care less about the technical minutiae of brightness vs lightness, photopic vs scotopic response, relative spectral power distribution, etc.

Most divers just want to know how to choose their lights.

To that end, the 5x lumens = 2x brightness rule is quite useful. It corrects the common misperception that lumens (which manufacturers love to quote) equate with brightness (which is what most folks call the intensity of their lights).

I was hoping this thread would move on to other practical topics...like beam profile, LED tint bins, LED drive levels, etc. Technical navel gazing is a waste of everyone's time.

J

p.s. fwiw, my current daytime dive lights are two LED lights. Both Cree XR-E, both spec'd at 130 lumens. One has a reflector only, putting about 75% of the light into a 7° spot beam and the remainder in spill. The other has a lens, putting about 95% of the light into a 40° flood beam. Both are cool white (>5000K), tending towards green when the batteries run down. I'll probably upgrade them to neutral white (~4000K) emitters in the next year. Unless I break down and buy one of the nifty variable focus SST-50 lights from mb-sub. :)
 
...

Agreed. This is a diving forum. Most folks here could care less about the technical minutiae of brightness vs lightness, photopic vs scotopic response, relative spectral power distribution, etc.

Most divers just want to know how to choose their lights.

To that end, the 5x lumens = 2x brightness rule is quite useful. It corrects the common misperception that lumens (which manufacturers love to quote) equate with brightness (which is what most folks call the intensity of their lights).

I was hoping this thread would move on to other practical topics...like beam profile, LED tint bins, LED drive levels, etc. Technical navel gazing is a waste of everyone's time.

J

p.s. fwiw, my current daytime dive lights are two LED lights. Both Cree XR-E, both spec'd at 130 lumens. One has a reflector only, putting about 75% of the light into a 7° spot beam and the remainder in spill. The other has a lens, putting about 95% of the light into a 40° flood beam. Both are cool white (>5000K), tending towards green when the batteries run down. I'll probably upgrade them to neutral white (~4000K) emitters in the next year. Unless I break down and buy one of the nifty variable focus SST-50 lights from mb-sub. :)

You are correct regarding the purpose of this thread.:wink:

I think it would be informative and in keeping with the purpose of the thread if any of the subject matters you brought up were discussed as well.

I'm especially interested in TIR optics when paired with XR-E emitters since some are able to reduce the overall spill and increase the "hotspot" where it is more usefull. Some reduce that hard line at the outer edge of the spill as well.

Aspheric lens work well with XR-E's as well due to the XR-E narrower beam angle but not everyone cares for the no spill or some of the other characteristics of aspherics.

Personally, for diving, especially for night diving I think more dive lights should be using TIR optics and well as better aspherics.
 
No...I never said that. Go directly to jail. Do not collect $200. Please go back and read my earlier posts. :)

Here is what you said:

Plus, 50% makes more sense w.r.t. our real-world visual response. Our eyes do not respond linearly to light intensity. You have to drop the intensity by 50% to really notice a difference...and then it looks, subjectively, 10-20% dimmer.

Basically you are saying that less than a 50% drop in lumen output is barely (or not) noticeable. That's what I got from this. If you are saying something different, please clarify as this has been the basis of our discussion.

I'm an engineer with several decades of experience, plus some ability to translate the relevant technical bits for non-specialists who share my interest in diving. :)

Jay, how many of these 40+ so called years of your engineering career have been spent in optics and LED engineering? any? Your experience is irrelevant to this conversation as we are discussing just the facts. I am surprised you played this card.

Two words...integrating sphere. Apparently your company doesn't have one?

Nope. We don't. We send out LED lights that need lumen testing to a lab that is certified to do LM-79 and LM-80 testing. We do all the intensity testing in house with calibrated goniometer equipment, as that is the majority of our business.

To your point about old and new news, again it is irrelevant. I am saying that the formula presented does not account for scotopic, wavelength or colour temperature. That is all. I know people that have ran tests at different LED lumen levels and found there to be visible differences noted with small changes in LED lumen output. Do you have any test data to show otherwise? Remember we are talking about white LED lights here.

My main point is that we should be rating the LED light's runtime to whatever the lumen output that is specified. If it drops below that, then it is not included in the runtime. Allow for a small 10-15% deviation in lumens to allow for some slight variance in the LEDs. Jay you have not convinced me otherwise.
 
BTW, GCbryan I am done with the technical discussion now, this is obviously going no where.
 
BTW, GCbryan I am done with the technical discussion now, this is obviously going no where.

After scanning the five pages in this thread (months later, admittedly), I have to agree. All that's been discussed is "if this" and "if that", but no real information to speak of has emerged.

The OP's opening remark, "If a light manufacturer is being more realistic they will advertise runtimes down to 1/2 of the max lumen rating not all the way until it actually quits." raised a flag for me. "Being more realistic" is analogous to "being more pregnant". Either he is being realistic, or he's not.

But, at the end of the day, it gets down to this for me: If a light is bright enough to do the job I need done, I use it. If it's not, I use something different. YMMV.
 
I work at a company that designs LED luminaires and LED drivers. We have a full photometric test tunnel here for aiding in the design and quality assurance of our products. I do not see the same commitment from dive light manufacturers OR their suppliers.

I see the word "lumen" being used incorrectly. Folks, lumens is the total sum of light (corrected for the human eye response) emitted from a source (in total) regardless of beam angle. A light CANNOT produce more lumens than the sum of its emitters.

Lux is the lumens per square meter it is an intensity measurement at a point.

There is some headway underway at the DOE to get manufacturers to start labeling LED products similar to the nutritional facts you see on food products: check out LightingFacts.com : The Lighting Facts Label

This should solve the problems with LED lights such as DiveRite's LED Lux which has wayyy overstated its actual light output. See this thread for proof:
What's the deal with the Diverite LED LUX? For real or not? - CandlePowerForums

I have asked Diverite to make a statement about how they tested their light:
http://www.scubaboard.com/forums/dive-rite/308949-led-lux-2.html

It is up to us consumers to demand that LED lights have the correct ratings to allow an informed decision. An integrating sphere lumen test costs only a few hundred dollars and there are many labs that can do it. Demand a photometric plot from the manufacturer that has the information such as that in the lighting facts sheet. Do not buy products from those that will not provide this info.

Ever wonder why you keep hearing how LED lights don't have the same "punch" as a HID? Well it's because they don't! Misstated specs make and accurate comparison impossible. A standard 10W HID will have about 500 lumens of light.

As for battery power ratings, most LED product use what's called a buck converter. this will require the input voltage to the driver (battery voltage) to be around 2V more than the LEDs need. A battery may very well have the capacity to run longer, but if the voltage does not meet a minimum requirement the LED driver will not work. Ask if the run time is at full rated value and what % of deviation from the stated run time would be considered faulty.

Thank you. It is hard to get lumens out of the divers head.

Please read here the truth about lumens
 
-just show me the graph of ANSI lumens vs. time. Oh yeah, and a warranted lifetime...

---------- Post added September 6th, 2013 at 03:04 AM ----------

Simple. Have the dive industry agree on a "screen/spot size" and a distance.

You can bet your collective obfuscating asses that I'll measure it with my Gossen Mavolux 5032C...

---------- Post added September 6th, 2013 at 03:19 AM ----------

This is very probably a diver safety issue. I had a contact at UL (Long Island) years ago when I was tasked with providing support for a dental resin cure lamp. I'll check to see how hard it would be to create a UL standard for dive lights...

---------- Post added September 6th, 2013 at 04:51 AM ----------

I'm thinking a 1 meter circular spot (integrated, no beam uniformity) at 5 meters.

This is how regulations come into being, the manufacturers refuse to police themselves and something just appears out of left field...

---------- Post added September 6th, 2013 at 11:00 PM ----------

-still hard at it, nothing new till Monday.

Question, if you only had one "standard" measurement -what would you want that measurement to be? The idea is to come up with a single basic metric for "first order" comparisons of dive lights. (If the OP sees this as a hijack, please ask a mod to split it out.)

The manufacturers could still tout divergence, range of focus, total light output, spectral purity, color temperature, uniform illumination, overall efficiency, total burn time, etc. etc. etc...
 
https://www.shearwater.com/products/peregrine/

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