1

someone else was looking for ‘most lumens per watt’

where is that, actually?

i was guessing that there is a curve “efficiency vs lumens”, which goes up from say 1 lumen, then hits a peak at about 100, then goes down as the I^2*R losses go up

but i don;t know

i also don;t know (discuss among yourselves) how or if heat sinking influences the curve of ‘efficiency vs lumens’
my guess is that it does not

wle

2

It depends entirely on the LED.

3

ok - a typical one - or a specific example - say a common one - XP-L (that’s common isn’t it?)

or - how would you look at a data sheet and predict that?

wle

4

You’d look at these handy chart BLFers have made :heart_eyes: :

5

You can use Cree PCT

6

oh
there it is
i was wrong
looks like it just keeps going down, more watts == less lumens/watt

i was thinking it would be shaped like a bell, somewhere in the middle would be a sweet spot
but no

ok thanks

so this dude that wants to get the flashlight with the max lumens per watt, how would anyone advise him?

wle

7

Is it efficient to flood an entire room with light when I’m trying to focus on one object in the room?

8

good question
is it?
maybe the flashlight can do eyeball tracking and only put the light where you are trying to look

it could also sense (brain scan) when you are actually using peripheral vision, then make the beam all floody :slight_smile:

wle

9

You can estimate your design using OEM tools. Available to download from manufacturer’s website. But almost all of those official simulation tools only rated to OEM’s minimum and maximum range. You have to test or extrapolate yourself for current below and above OEM rated.

Theoretically, you will get the most efficiency when you drive any LED with as low as possible current (put consistent binned CCT/CRI/tint etc… aside). You can achieve this by using as much as possible LEDs at as low as possible current per emitter.
Unfortunately that’s NOT the case. There are few limiters:
A. Cost.
B. Design.
C. Space. related to above point
D. Optic compatibility for directional applications.
E. Driver operating range and efficiency limit.

[Clemence]

10

As you can see from the chart above, lm/W varies depending on how hard the LEDs are driven. Unless a flashlight only has a single mode, the dimmer mode will usually be more efficient, with higher lm/W.

“Photopic luminous efficacy of radiation has a maximum possible value of 683 lm/W, for the case of monochromatic light at a wavelength of approximately 555 nm (green).”
https://en.wikipedia.org/wiki/Luminous_efficacy

Lumens are defined as visible light, and the human eye is most sensitive to green light. In theory, a green-only LED will be much more efficient than a high CRI white light.

Useful advice cannot be given until we know more. What is he trying to achieve? How much light does he need? I assume 1 lumen of green light at a very high lm/W isn’t the goal.

11

The driver has a high effect on efficiency. If you run one or multiple LEDs with a Vf of 3.0V at the chosen current with a fully charged 4.2V cell, you are wasting 30% of the power as heat before even reaching the LEDs if you use a linear driver without PWM. The overall efficiency wil be even worse if you are using PWM as the LEDs will be run at higher current.

Still, linear drivers are the most popular because of price, compact size and firmware availability, despite their lower efficiency.

12

Unclosed quotation mark error!

13

yeah i got the impression he isn;t all that technical - he just thought that would have a simple answer, not a science project :slight_smile:

it;s here most efencent small led flashlight

wle

14

I think there is a point at very low power levels where efficiency is poor, so if the graph went down to the 0.1 or 1 lumen level, you could see a bit of a bell shape.

15

I would ask him why he cares about that metric when most lights are SO close to one another in most output ranges

16

he is only asking about lumens, which tells me he is a beginner

most lumens per watt comes from the greenest, coolest color temperature, lowest CRI LEDs

I suggest he expand his horizons to consider the Quality of the light, not just the Quantity

things like, how close is the color of the LED to Sunlight, including is the light High CRI, and is it free of Green Tint!

Cause myself, I have no interest in more lumens of green tint in cool white color temperature, and that makes red look brown and skin look dead. Even though that is what has the most lumens per watt.

sometimes beginners ask questions that show they are not aware of more subtle, and more important variables, than just how bright a light is.

so answering his question, may be a bad idea, because the question is not based on a broad enough understanding of the variables involved.

My suggestion is to get him to start by deciding what battery size he wants to use
then how heavy and large a light he wants
then how much he cares about the color of the light

it helps if he defines the application for the light

what I would use on a rifle to shoot coyotes is totally different than what I would use in my house, to find something under the couch

courses for horses

17

Jon, even a tint snobs would care about max efficiency depending on the applications. For example, in my survival kit, I highly prefer a Zuper efficient flashlight with whatever color it comes with for the longest run time possible.
We don’t have enough data of what he’s trying to achieve. So, it’s better not to assume anything yet.

[Clemence]

18

The simplest answer is usually the best. But, as others have said, there’s a bit of info missing. To get the “most lumens per watt” you first have to know at what light level it will be used. You want the driver to be optimized at exactly the level you wish to use, so there is no need to throttle it with PWM. And then, you want to use the most efficient LED that is capable of that level of output. Some higher output capable emitters are more efficient at low levels than the lower output emitters, but not in all cases. Also, as others have said, all emitters start at their most efficient and get worse with more power, so using multiple emitters will always be more efficient than using just one of the same emitter for the same light output.

19

I think his point is that lumens/watt by itself is a very “odd” request. “Most lumens/watt at around 100 lumens for a whole battery drain cycle” or something similar makes more sense.

20

http://hyperphysics.phy-astr.gsu.edu/hbase/vision/lumpow.html
For 100% luminous efficiency, you need a pure 555nm green light source, like a laser, due to the sensitivity of the human eye which is how lumens are measured.
The efficacy is 683lm/W which is where all the energy is being converted into radiant flux.
Since it is a pure 555nm light, the radiant flux is equal to the luminous flux.