Wh / Lumens / divetime relationship

[Walde]

Could you give me some basic rules of how Wh, lumens and divetime are related.
There are so big differences between promised lumens and divetime with similar Wh batteries that I don't know who to believe. I know some manufacturers try to hide the real performance by lowering the output when battery runs low but they forget to tell this little detail.
How has LED lumen/watt changed in say last 5-10 years? There are a lot of torches that are kind of old design.

 

[Angelo Farina]

Most manufacturers lie heavily on the lumens produced.
Here some simple facts.
Lumens express the fraction of the power (in watts) which is converted to visible light.
If the conversion was 100% effective, each watt produces 680 lumens.
The best LED bulbs, indeed, produce just 120 lumens/W, the rest is heat. Old halogen lamps were much worst, around 25 lumens/W:
If you want good light quality (high Colour Rating Index CRI>90, warm colour temperature of 3500-4500 K) the max efficiency is around 100 lumen/W.
So, if a lamp in consuming 10 W, it will produce roughly 1000 lumen, whatever the crazy number is declared by the manufacturer.
The duration of the battery is given by its capacity, in Wh (Watt x hour), divided by the power of the lamp.
So, if your battery is rated 100 Wh, and your lamp consumes 10W, the battery should last 10 hours.
In reality, the battery should never be fully discharged, and a protection circuit will switch off the lamp when there is a residual charge of 10-15%. So in reality the duration of your battery will be 8 - 8.5 h.
Often the capacity of the battery is not rated in Wh, but in Ah (or in mAh).
The real capacity in Wh is given by the product of the value on Ah for the voltage of the battery.
So, for a typical Lithium cell at 3.6V and with a rated capacity of 10000 mAh (10 Ah), the total capacity in Wh will be 3.6x10= 36 Wh.
I hope that what provided here is enough information for debunking the crazy claims of lamp manufacturers...

 

[stuartv]

The vast majority of dive lights do not have constant output. That means they dim as the battery runs down.

A couple of examples of lights that do have constant ouput are the canister lights from Underwater Light Dude, and the Dive Rite LX20+ and the Apeks Luna ADV.

If the manufacturer's info about a light does not say "constant output" then it's pretty safe to assume it gradually tapers off in brightness as the battery runs down.

Also, most of the manufacturers *ahem* greatly exaggerate the amount of light their light puts out. For example, the Orcatorch D710 claims to offer 1700 lumens on High and 3000 lumens on Turbo. There is NO WAY that light is putting out that many lumens. Not even when it is fully charged and first turned on.

If you see two lights with the same LED emitter, it is pretty safe to assume they will be about the same brightness, even if one manufacturer says 700 lumens and the other says 1700.

The standard LED emitter for years now has been the Cree XM-L2. They are generally good for around 1000-1100 lumens maximum raw output. If you look at the Cree specs sheet for it, you can find that it takes about (very roughly) 6 Watts of power to run that LED at (very roughly) 1000 lumens output.

With the now-standard 18650 battery at 3500mAh of capacity, some simple arithmetic will tell you that the battery is about (3.5 Amps * 3.7 Volts =) 13 Watt-Hours of capacity.

So, with a 6 Watt burn, a light with the Cree XM-L2 and a single 18650 battery could possibly output close to 1000 lumens continuously for about 2 hours.

In real life, those are approximations and there is some inefficiency, and it will actually be noticeably less.

Nowadays, there are some lights using newer LED tech (e.g. emitters from Luminus, instead of Cree) which are potentially more efficient at the same output level as the older Cree and also can go brighter while maintaining the same power efficiency.

Plus, 5000 mAh 21700 batteries are a lot more common now. So, you can now get lights that will burn brighter for the same amount of time, or burn the same brightness and last longer.


Regardless, the same type of simple math is easy to use to check many of these claims from the manufacturers, where you can determine for yourself that their claims must be false in some way.

For example: I recently got a Big Blue VTL9000P-Max light. They Big Blue rep explicitly told me that it would output 9000 lumens for just shy of 3 hours and then go into low power mode. I used time lapse photography with my camera in manual mode and found that it did not stay at 9000 lumens (or whatever its true max output is) for very long and after 2 hours it is a LOT dimmer.

The light holds 4 x 18650 batteries. Looking at spec sheets and doing some math, I verified that it would need roughly 3 times the battery capacity that it actually has in order to put out a constant 9000 lumens for 3 hours.

Many manufacturer claims are just straight up baloney.

Ones that I generally find to be reasonably accurate and believable are UWLD, Dive Rite, and Dive Gear Express.

 

[tursiops]

In affection to all the above, two light with identical lumens output will only be the same apparent brightness if the beam patterns are the same. A narrow-beam light concentrates it's lumens; a flood light spreads it's lumens out.

 

[Whitrzac]

The vast majority of dive lights do not have constant output. That means they dim as the battery runs down.

A couple of examples of lights that do have constant ouput are the canister lights from Underwater Light Dude, and the Dive Rite LX20+ and the Apeks Luna ADV.

If the manufacturer's info about a light does not say "constant output" then it's pretty safe to assume it gradually tapers off in brightness as the battery runs down.

Also, most of the manufacturers *ahem* greatly exaggerate the amount of light their light puts out. For example, the Orcatorch D710 claims to offer 1700 lumens on High and 3000 lumens on Turbo. There is NO WAY that light is putting out that many lumens. Not even when it is fully charged and first turned on.

If you see two lights with the same LED emitter, it is pretty safe to assume they will be about the same brightness, even if one manufacturer says 700 lumens and the other says 1700.

The standard LED emitter for years now has been the Cree XM-L2. They are generally good for around 1000-1100 lumens maximum raw output. If you look at the Cree specs sheet for it, you can find that it takes about (very roughly) 6 Watts of power to run that LED at (very roughly) 1000 lumens output.

With the now-standard 18650 battery at 3500mAh of capacity, some simple arithmetic will tell you that the battery is about (3.5 Amps * 3.7 Volts =) 13 Watt-Hours of capacity.

So, with a 6 Watt burn, a light with the Cree XM-L2 and a single 18650 battery could possibly output close to 1000 lumens continuously for about 2 hours.

In real life, those are approximations and there is some inefficiency, and it will actually be noticeably less.

Nowadays, there are some lights using newer LED tech (e.g. emitters from Luminus, instead of Cree) which are potentially more efficient at the same output level as the older Cree and also can go brighter while maintaining the same power efficiency.

Plus, 5000 mAh 21700 batteries are a lot more common now. So, you can now get lights that will burn brighter for the same amount of time, or burn the same brightness and last longer.


Regardless, the same type of simple math is easy to use to check many of these claims from the manufacturers, where you can determine for yourself that their claims must be false in some way.

For example: I recently got a Big Blue VTL9000P-Max light. They Big Blue rep explicitly told me that it would output 9000 lumens for just shy of 3 hours and then go into low power mode. I used time lapse photography with my camera in manual mode and found that it did not stay at 9000 lumens (or whatever its true max output is) for very long and after 2 hours it is a LOT dimmer.

The light holds 4 x 18650 batteries. Looking at spec sheets and doing some math, I verified that it would need roughly 3 times the battery capacity that it actually has in order to put out a constant 9000 lumens for 3 hours.

Many manufacturer claims are just straight up baloney.

Ones that I generally find to be reasonably accurate and believable are UWLD, Dive Rite, and Dive Gear Express.

There are output graphs on some of the Flashlight forums of all they typical emitters. The luminous is slight better than the xml2, but not by much.
Dive lights can be overdriven pretty hard because they're water cooled, some mfgs go as far as saying that they shouldn't be used above water because of this.

The xml3 has been out for 2(?) Years now, but I don't know of any lights coming with it from the factory. It gives a 10-15% bump in output for the same wattage. I've upgraded a few of my older lights with them already

 

[stuartv]

There are output graphs on some of the Flashlight forums of all they typical emitters. The luminous is slight better than the xml2, but not by much.
Dive lights can be overdriven pretty hard because they're water cooled, some mfgs go as far as saying that they shouldn't be used above water because of this.

The xml3 has been out for 2(?) Years now, but I don't know of any lights coming with it from the factory. It gives a 10-15% bump in output for the same wattage. I've upgraded a few of my older lights with them already

Dang! It would be nice if Dive Rite would update the LX20+ to use that!

I can't see buying one and upgrading it myself. I don't mind trashing a warranty on a <$100 light, but not a light as expensive as the LX20+.

 

[inquis]

It gives a 10-15% bump in output for the same wattage.

Since our eyes perceive brightness logarithmically, 15% more lumens would be very difficult to notice.