LED lights: A Few Facts

[Greg_Vic_Diver]

In an ideal theoretical world, yes...but in the real world, any constant-current regulated light will drop out of regulation because batteries are not an infinite power source.

How long a light will run out-of-regulation depends mostly on the power source. If you use protected li-ion cells, they will just cut off...no warning. Unprotected li-ion cells will fall off a short & steep voltage curve.. Alkaline cells will decline down a very long and shallow curve. Lithium primaries and NiMH cells lie in between these extremes.

The important thing is that, when most lights drop out of regulation, they continue to deliver imperceptibly lower levels of light for significant time. And the amount of light that we see is what matters.

When you suggest "-15% rated lumen output" as the runtime metric cutoff, I'm guessing that you actually mean "-15% perceived light output". They are not the same. The human eye responds non-linearly to light intensity.

Put simply, your eyes/brain cannot see the difference between 100% lumens and 85% lumens in most cases. Lumens must be doubled (200%) or halved (50%) before you will really notice a difference. As a general rule of thumb, the lumens must be 500% higher for a light to look twice as bright. A 1000-lumen light looks about twice as bright as 200 lumens. Non-intuitive, but that's the way our eyes work.

So...assuming you were talking about perceived brightness, 50% of lumen output is right on target.

By the way, this is not my idea. The guys in the flashlight communities like candlepowerforums came to the same conclusion long ago...and they demand this rating from the cutting-edge flashlight manufacturers:

runtime = time to 50% of rated lumens

It makes good sense. Think about it.

J

Hi Jaydil,

I think that if you can't see the difference in lightness in a 49% less bright light, you should buy a light with 49% less luminous flux because you can't tell the difference

Seriously though, the human eye (HVS) responds to different wavelengths of light differently. In Photopic (daylight conditions) and Scotopic (night time conditions) these sensitivies change even furthur (towards blue) as the rods take over to perceive lightness.

Every human will have a slightly different response to lightness at a particular wavelength, so how can you generalize that 49% won't be perceived at all? at what wavelength? In what conditions? How fast is the change? When the light changes slowly it is less perceptive until it is changed quickly.

At the end of the day you will have less lumens illuminating the area you wish to illuminate. That is not what I paid for.

The best idea I have heard yet is to rate the light output (in lumens) for the end of the specified runtime. Then you know what you are at least guaranteed to get throughout the runtime. Although I agree you could get this info by just taking half the "quoted" lumen value to obtain this. Stating the runtime to half the lumen value is misleading to those that are not in the know, and not good customer focused marketing.

 

[jtivat]

While I agree for the most part I do see a few thing please see below in bold.

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.

It a light doesn't have a constant current driver (in which case it will be at max brightness until it just suddenly quits) it will only be at it's max advertised lumen output for a short while. As the battery is used the output will decrease. It may be advertised as 200 lumen for instance but after running for 1/2 hour it may be 170.

After using it for several dives it may now only be 100 lumens. When it gets so the battery is only able to produce 1/2 max lumen this should be considered its effective runtime. Maybe that's 6 hours (depends on the batteries).

If a light is advertised as running for days that's somewhat misleading. Sure it is still burning but it may not be any brighter than a candle. It's also something any nonrechargeable battery will do when powering led's. Led's are very efficient at lower power so it's a long, long time before the light actually doesn't come on at all.

However, if you buy a light that is advertised at 200 lumens and that is claimed to run for days and days you might think that means that it's running at 200 lumen for days and days...it's not.

Another fact is that there are only so many led's out there. Most lights, regardless of the sales prices, are using some of the same led's as other much cheaper lights. The actual retail cost of most led's is in the $15 range so the difference between a $700 light and a $50 light isn't the led.But there are also many BIN's of the same LED and better lights often use higher BIN's. These are brighter and more efficient.

It may be the build quality (higher production/test/development costs) or it may be looks or status or marketing but it's not the parts. You can find an expensive light that isn't well made and you can find a cheap light that is. Of course your chances are probably improved with a more expensive light but there is no direct correlation.

One other fact not always appreciated is that two lights with differing lumen outputs can appear to be the same at first glance. You may think that the higher lumen light must be the one that appears to be brighter. That's not necessarily the case.

The higher lumen model is producing more light but it may not be any brighter at the hotspot than a lesser lumen model with a tighter hotspot and no spill. Lumen is just describing the output. Lux is the measurement at a particular spot.

A 100 w bulb in the ceiling of the living room of your house may produce more lumen than the 60 w bulb that is by your chair that you use as a reading lamp. The light falling on your book from that 60 w bulb is brighter than the light falling on that book from the bulb in your ceiling.

Lights that need to be twisted to work underwater may be preferred as they are simple with no complicated parts involved or they may not be preferred because they can come on by themselves at increasing pressure. They also rely on o-rings to handle the dynamic motion of being twisted at pressure.Not a well designed and built light like the Rat or Photon Torpedo I have never had or heard of them coming on at depth.

Magnetic switches have the advantage of not penetrating the body at all (and can't be a cause of leakage) and don't put any dynamic stress on sealing o-rings. They are more complicated and need to be well designed to keep magnets from falling out or if not sealed by degrading in saltwater. The circuitry internal to the light is more complicated and expensive although once properly designed is not so much of a problem unless the light leaks.

Non-rechargeable batteries have the advantage of lasting longer and rechargeable batteries have the advantage of saving money, being able to produce higher power for shorter periods of time with the right drivers and therefore allow for matching power to anticipated dive times.Not always try a set of Alkaline's and a set of NiMh in your camera with the flash on the NiMh will last longer.

Also, there is a limit to the use of non-rechargeables for higher powered lights. Or rather there is a useful limit. If a 1500 lumen light can only burn for 30 minutes that's not a useful dive light. If it can produce 1500 lumen for only a minute or two but burn for 2 hours at a rapidly declining level of output then that isn't really a 1500 lumen light either.

Led's are more efficient as power is reduced so with multiple power settings on a dive light battery power can be extended by reducing power on those dives when it's not needed. Since led's are more efficient as power is reduced this also means that a 50% reduction in power may only reduce the brightness by 35% so this can be a real benefit.

I see some misconceptions sometimes regarding led lights and that's the reason for this post. It's long and maybe isn't effective because of that but maybe it will help someone.

Anyone knowledgeable regarding led lighting feel free to clear up other misconceptions. Too much of a buyers knowledge regarding led lights frequently comes only from manufacturers' ads. This can be misleading.

 

[gcbryan]

Thanks Jtivat, I would add some of those comments to the original post but it's beyond the time when I can edit my comments now.

Regarding the bin info, that's true that there are greater and lesser bins and that does make a difference but the difference in price between a P4 and a R2 for instance is still not much. Availability may be an issue but price is not generally that much of a factor when you are looking at $50 lights verses a $700 light.

Regarding the twisty light not coming on with better lights, you may be right. I don't have enough data points to say on that one.

Regarding the Nimh comment, I didn't know that. Another factor of alkaline thought is in many cases it has a longer shelf life although lithium is good in that regard as well I believe. This may matter with backup lights.

Thanks for the comments.

 

[jaydil]

I think that if you can't see the difference in lightness in a 49% less bright light, you should buy a light with 49% less luminous flux because you can't tell the difference

Greg, I think you misread my posts above.

What I actually said was that a 50% drop in measurable brightness (lumens) corresponds to a much smaller drop (like 15%) in perceived brightness. That's why a 50% output benchmark is quite useful. In most cases, you will just start to notice the difference in brightness around that point.

On the other hand, A 15% drop in measurable brightness is practically imperceptible. If I show you a 200 lumen light, then turn it off and show you a 170 lumen light, you almost certainly will not see the difference.

Every human will have a slightly different response to lightness at a particular wavelength, so how can you generalize that 49% won't be perceived at all? at what wavelength? In what conditions? How fast is the change? When the light changes slowly it is less perceptive until it is changed quickly.

It's true...visual perception is complicated by many factors, and varies by individual. But there is a huge body of empirical data that allows us to state some general rules of thumb across a population. Much of the foundational research in this area was done in the 1950s-1970s. Google "perception of brightness" and you will find some of it.

The best idea I have heard yet is to rate the light output (in lumens) for the end of the specified runtime.

That would be a nice number to have for any light...however, it does not let you compare different lights. Let's say you have one light that runs for 1 hour, from 200 down to 100 lumens. Another light runs for 3 hours, from 200 down to 50 lumens. Which one is better?

Stating the runtime to half the lumen value is misleading to those that are not in the know, and not good customer focused marketing.

On the contrary, runtime to half lumen value is the BEST customer focused number. It basically says "this is how long the light will run before you notice a difference in the overall brightness we promised". That is what matters most. A noticeable difference in brightness.

J

 

[jaydil]

Regarding the Nimh comment, I didn't know that. Another factor of alkaline thought is in many cases it has a longer shelf life although lithium is good in that regard as well I believe. This may matter with backup lights.

A bit more info on shelf life:

-- Normal NiMH cells will lose their charge in a matter of weeks or months, depending on their construction. Higher capacity cells lose their charge faster.

-- Low Self-Discharge (LSD) NiMH cells like sanyo eneloops will self-discharge in about a year or so. Cells that have been through more cycles lose their charge faster.

-- Alkaline cells have a shelf life of about 6 years. BUT you don't want to store them in any device for more than a month or so, since they are prone to leaking and destroying the electronics.

-- Lithium cells have a shelf life of about 15 years, and you can store them in the device. Plus they are lighter, have lower internal resistane, and work over a wider temperature range. Overall they are the best choice if device compatibility, cost, and availability are not factors.

-- Rechargeable lithium ion cells will lose their charge over a couple-few years, depending on the chemistry and storage conditions. They handle moderately high loads well, so you can use a smaller battery pack for a short-run, high-draw device. The downside is that they are easy to damage with over-charging, over-discharging, or excessive current, unless you add cell management electronics to the pack or device.

Hope this helps.

J

 

[Greg_Vic_Diver]

Greg, I think you misread my posts above.

What I actually said was that a 50% drop in measurable brightness (lumens) corresponds to a much smaller drop (like 15%) in perceived brightness. That's why a 50% output benchmark is quite useful. In most cases, you will just start to notice the difference in brightness around that point.

On the other hand, A 15% drop in measurable brightness is practically imperceptible. If I show you a 200 lumen light, then turn it off and show you a 170 lumen light, you almost certainly will not see the difference.


It's true...visual perception is complicated by many factors, and varies by individual. But there is a huge body of empirical data that allows us to state some general rules of thumb across a population. Much of the foundational research in this area was done in the 1950s-1970s. Google "perception of brightness" and you will find some of it.

Care to share some of this? The correct term you are looking for is called "lightness". I did a lot of work in Biopsychology and the HVS so don't worry about it flying over my head.

That would be a nice number to have for any light...however, it does not let you compare different lights. Let's say you have one light that runs for 1 hour, from 200 down to 100 lumens. Another light runs for 3 hours, from 200 down to 50 lumens. Which one is better?

Well if you are looking at total lumen output you would have to monitor and measure the lumens continually over time to integrate to find the one with the most light. Otherwise you could easily confound the results. If you knew what the lumen output was at the end of the runtime you could at least guarantee you have the minimum required lumens for the minimum amount of time for which you paid for.

On the contrary, runtime to half lumen value is the BEST customer focused number. It basically says "this is how long the light will run before you notice a difference in the overall brightness we promised". That is what matters most. A noticeable difference in brightness.

J

Don't agree at all with that, let's just leave it at that.

 

[jaydil]

Care to share some of this? The correct term you are looking for is called "lightness". I did a lot of work in Biopsychology and the HVS so don't worry about it flying over my head.

Start with any intro-level psychology textbook that devotes a chapter to human visual perception. It will certainly cover our non-linear perception of brightness.

If you want to dive deep into the research, check out the CIE 78-1988 bibliography.

This web page nicely summarizes the principle and provides the standard CIE formula for lightness/brightness:
http://www.poynton.com/notes/colour_and_gamma/GammaFAQ.html#lightness

Quoting from that page: "A source having a luminance only 18% of a reference luminance appears about half as bright".

In a simplified nutshell...you need 5x the lumens for a light to appear 2x as bright.

J

 

[Greg_Vic_Diver]

Hi Jaydil,

Recent studies have shown that the human eye response changes when in dimly light condition (pupils dilate).

This means that eyes become more senstive to blue/white spectrum than initially thought when in dark scotopic conditions.

Some research papers have even started ranking lumen efficiency to pupil lumens per watt to reflect the visible lightness one would perceive from the same light.

I think you might want to look at these new revelations, especially when considering LED lights as they have a more blue tint to them vs traditional lighting sources.

Check out: RUUD Led :: Photopic vs Scotopic

for some good, easy to read definitions.

 

[Greg_Vic_Diver]

Start with any intro-level psychology textbook that devotes a chapter to human visual perception. It will certainly cover our non-linear perception of brightness.

If you want to dive deep into the research, check out the CIE 78-1988 bibliography.

This web page nicely summarizes the principle and provides the standard CIE formula for lightness/brightness:
Gamma FAQ - Frequently Asked Questions about Gamma

Quoting from that page: "A source having a luminance only 18% of a reference luminance appears about half as bright".

In a simplified nutshell...you need 5x the lumens for a light to appear 2x as bright.

J

Actually by that formula the light would have to have 6.4565 times more Luminance to appear 2x as bright.

However, we are talking about Lumens here. And yes. They are very different. One is an intensity measurement at a point and the other is the integral sum of all light output. It is easy to get the two confused.

 

[jaydil]

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