The proper way of calculating lux / cd / candlepower

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DB Custom
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All this talk is nice, put it to pictures for provenance. We have an old saying around here, pictures or it didn’t happen.

Reading the spec sheet on the Thrunite TN42 from it’s light… a mile away

Reading the same spec sheet by the light of my 200wW green laser that was used to aim the camera and TN42, same mile away.

(Yes, I know, my wife missed focus on her camera)

The hot spot from the TN42 was some 40’ in diameter at a mile. The hot spot on the 200mW green laser was about 6’ at a mile.

The TN42 from the lights end…

The 8 1/2 × 11 sheet of paper on a tripod can be seen in the middle of the picture, as can my wife in her pink shirt and black slacks, illuminated from a mile away by the factory Thrunite TN42. Sure, it’s fun to do the math. Much more fun to prove it. Wink

I didn’t have her take a picture of the 3.3W blue laser, she couldn’t see where it was hitting with her eye protection on…

Enderman
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scianiac wrote:
I mean it’s not like there is some magic point where etendue compensation is suddenly needed, in effect you are both right the question is what magnitude are we talking here. Lexel is probably right up to surprisingly high kcd/lens size/collimation. Surely well in the range of even the best off the shelf “throwers”, or least that’s how it seems to me, seems like the error with them would be well within the numerous other factors involved. But after 1Mcd things start to get different, we aren’t dealing with the 3mm aperture Lexel used in his example, we are dealing 100mm+ lenses. The divergence is surprisingly small. I of course don’t have measurements yet but things sure look awfully small. If I recall that tree line is 1300m away. So I guess my seat of the pants opinion is that there is some compensation needed at these ranges and perhaps it’s enough to be worth taking into account.

Thank you! Someone that actually read the whole post Smile

Lexel wrote:
Normal flashlights have a hotspot of 10-20 degree with 1” diameter

The virtual sorce point is often not even behind the body of the light, if its a short reflector
We are talking here about 10-30cm, if you measure at 3-5m the difference is very small

A extreeme thrower has less divergence and big head
Both things push it towards bad readings at low distances
Lets say you get 2 degree with 10cm head the virtual point is less than 3 meters behind the flashlight

The fact that your luxmeter has a big aperatur catching light compensates the virtual source behind the front of the flashlight, ideally the luxmeters aperature should be 0mm2
So often the measurement at too low distance is still correct as both values negate each other

The whole calculating cd to 1m is not really useful to get the real brightness if there is not a defined distance for measurement set for beam angle/head diameter, as well how big the aperature of the luxmeter has to be

From my last measurement, my beam has less than 0.25 degrees divergence.
I need to take longer distance measurements though to get the exact divergence.
As I said, for regular flashlights this isn’t really important, but for highly collimated lights it is.

hIKARInoob
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Enderman wrote:
As I said, for regular flashlights this isn’t really important, but for highly collimated lights it is.

Although this is true, I do find it important to realise what REALLY is going on with any flashlight, and what kind of simplifications are used and for what reasons. Your thoughts and this thread are much appreciated. Bravo!

EasyB
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I will say again that the phenomenon that Enderman is explaining makes sense, but actual measurements disagree with the proposed model.

EasyB wrote:
I have to say that this possible effect has caused me some confusion regarding my understanding of throw and how to predict it, so I did some measurements to see what is actually happening.

My test light was a UF-1504, with 62mm diameter lens and XPL HI V2 1A. I put in a 8×7135 driver to keep the output more constant with time. I measured the lux at three different distances measured from the lens.

__d_______lux_____cd
6.05m____7230___264.6Kcd
8.34m____3840___267.1Kcd
10.29m___2530___267.9Kcd

These measurements are consistent with measuring the distance right from the lens and not at some point behind the lens.

Plugging the 10.29m measurement (the beam size at this distance was 36cm) into the calculator you linked (after converting units), results in:
divergence distance behind aperture: 2.14m
candlepower: 390.9Kcd

The calculator results are not consistent with my measurements and how they vary with distance. There are certainly some things in your explanation of the effect that make sense, but actual measurements tell a different story.

A different effect could have caused your conflicting results when you measured the lux at 1m and 2m. At small distances like 1m, the lens might not be filled with the image of the LED, from the point of view of the lux meter. Moving farther back to where the lens is filled would then result in a larger throw number.

Below is a graphical representation of the data and the two models: measuring the distance from the lens and measuring the distance from 2.14m behind the lens, as the calculator predicted.


This plot shows the measured lux vs distance. The blue diamonds are the data points in the quote above that I measured. I estimate the uncertainty at about 100lux. The red line is the lux according to the inverse square law, measured from the lens. The equation is lux=265000cd/(d^2). The blue line is the predicted lux when the distance is measured from 2.14m behind the lens, as the calculator predicts for the measurement. The equation is lux=390900cd/((d+2.14m)^2). The two lines cross at 10.29m because that is the measurement that was input to the calculator.

The two models predict different lux numbers and the measurements are consistent with measuring distance from the lens and not from a point behind the lens.

hIKARInoob
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EasyB wrote:
I will say again that the phenomenon that Enderman is explaining makes sense, but actual measurements disagree with the proposed model.

Yep. I’ve read your post, so I’m also interested what on earth is going on. This makes it all so interesting.

Enderman
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Interesting. I am in the process of reassembling my “lightcanon” and will do more tests, hopefully a lot more accurate than that quick and dirty test I did at the start.
I will do longer distance, hopefully 5, 10, 15, 20, 25m or even 50m.

EasyB
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This recent thread brought me back to thinking a bit about this method. As I talked about above, this method seems to make sense, but my measurements disagree with this method of calculating the candela.

I just thought of a thought experiment which makes me further question this method. Say you have a aspheric lens light with an XPL HI. You take aperture and beam size measurements and the calculator linked in the OP tells you the distance behind the lens the light appears to emanate from and you use this to calculate the candela. Now you put a mask right over the LED which decreases its size to 1mmx1mm instead of the original 2mmx2mm. The lux measurement and aperture size stay the same, but the beam diameter decreases by a factor of 2. Now the calculator says the light is emanating from further behind the lens and the calculated candela goes up. But we know the beam candela would not actually change; the center of the LED is left unchanged and so the properties of the center of the beam should also remain unchanged.

Enderman
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Yes I agree, see this post specifically: http://budgetlightforum.com/comment/1053206#comment-1053206
I made this thread to explain the concept that I read about on CPF, but after some further research it seems to be incorrect.

The light coming out of a lens is not a single cone of light that diverges from one point, it is an infinite amount of light cones coming from the surface of the lens.

Based on this, the total lux of the flashlight is simply the sum of all these cones of light.
Since each cone behaves according to 1/d^2, then the sum of all cones also behaves proportional to 1/d^2

So in fact the light converges at the lens, not behind it.


What is still important though is that the flashlight needs to be measured at a far distance to allow all the cones of light to overlap at your luxmeter, in order to measure the maximum possible intensity.
More info here: http://budgetlightforum.com/node/55428

EasyB
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Ah, that makes sense. Glad that is cleared up.

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I’ll edit the OP later today so people don’t get misinformed.

Should have done it a long time ago when I discovered I was wrong.

Completely forgot I had made this thread Silly

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