Ra
Flashlight Enthusiast
Hi guyzzz,
I think it is about time for me to step in..
Reading this and other threads where the throw discussion starts (or is the main issue..) I cannot help getting the thought that, propably unintentionally, these discussions end up in very long threads that incorporate some confusing remarks, definitely when they are not true! Don't get me wrong here, but this is not very helpfull for the ones that are not quite into this, but still want to know how things work..
Question is: Do you want to know the facts about throw, and live with them without understanding the detailed law's behind them, or not..? (most do propably not..)
We have to face that some members do not have the abillity to understand it all, and when those members are not satisfied with this, long, uninformative threads start to grow..
I sinceraly hope you're not offended by the above, it's absolutely not my intention to lose CPF-friends, but to some, these are maters which are quite demanding to the brain, and so don't allow space for complicated discussions..
Maybe it's a good idea for the OP to edit the first post with a section in which the short conclusions we all agree about are summed up, so members can find a short summary with all the facts about this matter, and can decide for themselves if they want to read how we came up with those facts..
That said,
It's time for a little input from myself:
Definition of throw: The abillity to enlighten distant objects.. It's as simple as that!
But: Different objects have different levels of reflection when enlightened, so we have a problem.
Ok, then lets agree on specify the throw of a torch by stating the distance from the torch at which 1 lux is measured (like most do..)
Don't forget to post the simple square law formula for people who want to recalculate the throw-distance when they need for example 3 lux.
-Throw is not lumens related: A laser pointer throws far but has very poor lumens output!
Years ago, I experimented with lenses and led's on a testbench, and came to the following hard conclusions: (all theoretical, based on actual tests)
-Within the range of aviable lenses: For all lenses with the same diameter: The focus length does absolutely not affect throw!-
If it does, something else is wrong, like difference in quality, or transmittance of the glass,
or the entre surface of the lens doesn't play along. Those are only a few things that can be wrong.
-For all lenses with the same diameter: Focus length does affect the amount of lumens, collimated into the beam, affecting the wideness of the beam.
So what is important about focal length: Angle of emittance that is grabbed:
Most sources emit their lumens in a wide area (for most led's about 140 degrees), so the more you cover that area with a lens or reflector, the more lumens you collimate into the main beam. But we're not talking about lumens on this thread, we're talking about throw.
-Lenses more easily give high throw: With led's, which are front-emitting ofcource, lenses are best suited as they grab the light in front of the source. Conventional reflectors are designed mostly for use with side-emitting sources and are less efficient with led's (but still work to certain extend, when you accept the lower efficiency..)
Theoretical:
If a omnidirectional source emits 250 lumens, the lux measurement at 1 meter should give 20 lux (250 divided by 4 times pi) This is called MSCP (Mean Spherical Candle Power)
That also means, that when you know the size of the source, for example led-die 1x1mm, you can calculate the surface brightness: 1x1x20 equals 20 lux/mm2
Then you simply need to know the surface of the collimator in use: For example, 30mm diameter aspherical lens.. 15x15xpi=706.85 mm2
A high quality lens copies the surface brightness of the source, minus the losses caused by surface reflections, absorbtions and stuff like that. So we need to know the effective transmission of the lens: Uncoated, that will be about 90%..
So here we are:
-Source: 20 lux at one meter comming from a 1x1mm source size
-Effective lens surface (always 2-D, seen from a distance..): 706mm2
-Lens efficiency 90% (note that this is the efficiancy for surface brightness, not for lumens output!!)
Source has 1mm2 surface, measures 20 lux at 1 meter: Source + lens will give:
(Lenssurface divided by sourcesurface) x lux @ 1 meter x lensefficiency:
(706/1)x20x0.9= 12708 lux at one meter ! So there is your formula...well not quite..
with the inverse square law, you now can calculate the throw:
Taking the square root from 12708 (which is the actual CP-output, as this already is at 1 meter) gives 113 meters as the distance at which 1 lux should be the measurement result.
Another formula for throw: Take a calibrated measurement at any distance from the source, but far enough to be sure that the entire lens- or reflector-surface plays along,
and multiply that measurement with the quadratic of the distance:
Example:
Torch-luxmeter distance is 100 meters reading 24 lux: Beam-CP output is 100x100x24= 240,000 B-cp
Now there is a catch: Omnidirectional means emitting in all directions.. (like the sun does)
Led's are not omnidirectional. So IMO, the only way to do this without much uncertainty, is to do a lux-measurement on the bare source at one meter, with a calibrated lux-meter,
and determain the source size
The remaining of the theoretics do have less uncertainties.
Notes: When a torch does not give the theoretically calculated lux reading at one meter, chances are that, among other things, the entire surface of the lens does not play along: You are too close to the torch, or you need to focus the torch.
When does one know, he (or she) has enough distance for an accurate measurement:
Double the distance: according to the inverse square law, you should measure 1/4 of the measurement at half the distance. If this doesn't apply to your results, something is wrong in the way you measure..Propably too close with the first measurement.
Like I said, this is matter that is not easy to understand for some. This is my effort to be clear about the facts in a post that hopefully isn't too long..
Any question's?
Regards,
Ra.
I think it is about time for me to step in..
Reading this and other threads where the throw discussion starts (or is the main issue..) I cannot help getting the thought that, propably unintentionally, these discussions end up in very long threads that incorporate some confusing remarks, definitely when they are not true! Don't get me wrong here, but this is not very helpfull for the ones that are not quite into this, but still want to know how things work..
Question is: Do you want to know the facts about throw, and live with them without understanding the detailed law's behind them, or not..? (most do propably not..)
We have to face that some members do not have the abillity to understand it all, and when those members are not satisfied with this, long, uninformative threads start to grow..
I sinceraly hope you're not offended by the above, it's absolutely not my intention to lose CPF-friends, but to some, these are maters which are quite demanding to the brain, and so don't allow space for complicated discussions..
Maybe it's a good idea for the OP to edit the first post with a section in which the short conclusions we all agree about are summed up, so members can find a short summary with all the facts about this matter, and can decide for themselves if they want to read how we came up with those facts..
That said,
It's time for a little input from myself:
Definition of throw: The abillity to enlighten distant objects.. It's as simple as that!
But: Different objects have different levels of reflection when enlightened, so we have a problem.
Ok, then lets agree on specify the throw of a torch by stating the distance from the torch at which 1 lux is measured (like most do..)
Don't forget to post the simple square law formula for people who want to recalculate the throw-distance when they need for example 3 lux.
-Throw is not lumens related: A laser pointer throws far but has very poor lumens output!
Years ago, I experimented with lenses and led's on a testbench, and came to the following hard conclusions: (all theoretical, based on actual tests)
-Within the range of aviable lenses: For all lenses with the same diameter: The focus length does absolutely not affect throw!-
If it does, something else is wrong, like difference in quality, or transmittance of the glass,
or the entre surface of the lens doesn't play along. Those are only a few things that can be wrong.
-For all lenses with the same diameter: Focus length does affect the amount of lumens, collimated into the beam, affecting the wideness of the beam.
So what is important about focal length: Angle of emittance that is grabbed:
Most sources emit their lumens in a wide area (for most led's about 140 degrees), so the more you cover that area with a lens or reflector, the more lumens you collimate into the main beam. But we're not talking about lumens on this thread, we're talking about throw.
-Lenses more easily give high throw: With led's, which are front-emitting ofcource, lenses are best suited as they grab the light in front of the source. Conventional reflectors are designed mostly for use with side-emitting sources and are less efficient with led's (but still work to certain extend, when you accept the lower efficiency..)
Theoretical:
If a omnidirectional source emits 250 lumens, the lux measurement at 1 meter should give 20 lux (250 divided by 4 times pi) This is called MSCP (Mean Spherical Candle Power)
That also means, that when you know the size of the source, for example led-die 1x1mm, you can calculate the surface brightness: 1x1x20 equals 20 lux/mm2
Then you simply need to know the surface of the collimator in use: For example, 30mm diameter aspherical lens.. 15x15xpi=706.85 mm2
A high quality lens copies the surface brightness of the source, minus the losses caused by surface reflections, absorbtions and stuff like that. So we need to know the effective transmission of the lens: Uncoated, that will be about 90%..
So here we are:
-Source: 20 lux at one meter comming from a 1x1mm source size
-Effective lens surface (always 2-D, seen from a distance..): 706mm2
-Lens efficiency 90% (note that this is the efficiancy for surface brightness, not for lumens output!!)
Source has 1mm2 surface, measures 20 lux at 1 meter: Source + lens will give:
(Lenssurface divided by sourcesurface) x lux @ 1 meter x lensefficiency:
(706/1)x20x0.9= 12708 lux at one meter ! So there is your formula...well not quite..
with the inverse square law, you now can calculate the throw:
Taking the square root from 12708 (which is the actual CP-output, as this already is at 1 meter) gives 113 meters as the distance at which 1 lux should be the measurement result.
Another formula for throw: Take a calibrated measurement at any distance from the source, but far enough to be sure that the entire lens- or reflector-surface plays along,
and multiply that measurement with the quadratic of the distance:
Example:
Torch-luxmeter distance is 100 meters reading 24 lux: Beam-CP output is 100x100x24= 240,000 B-cp
Now there is a catch: Omnidirectional means emitting in all directions.. (like the sun does)
Led's are not omnidirectional. So IMO, the only way to do this without much uncertainty, is to do a lux-measurement on the bare source at one meter, with a calibrated lux-meter,
and determain the source size
The remaining of the theoretics do have less uncertainties.
Notes: When a torch does not give the theoretically calculated lux reading at one meter, chances are that, among other things, the entire surface of the lens does not play along: You are too close to the torch, or you need to focus the torch.
When does one know, he (or she) has enough distance for an accurate measurement:
Double the distance: according to the inverse square law, you should measure 1/4 of the measurement at half the distance. If this doesn't apply to your results, something is wrong in the way you measure..Propably too close with the first measurement.
Like I said, this is matter that is not easy to understand for some. This is my effort to be clear about the facts in a post that hopefully isn't too long..
Any question's?
Regards,
Ra.
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