# optic or reflector CD/LM factor, what does it mean ?



## videoman (Feb 4, 2013)

I noticed that as the optic or reflector beam angle is wider the cd/lm number increases as in the specs. What does that relate to brightness. Can I assume that a factor of 3 be that it will be 3X the lumens or lux value taken at the same distance ?
I have a XM-L bare emitter at 3 ft. from a white wall and the reflected meter reads it out as 500 lux. By applying a narrow optic with a spec of 3 cd/lm, will that reading become 1500 lux. or there is another method of calculation.?:thinking:


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## AnAppleSnail (Feb 5, 2013)

Lumen and lux are very different. Lumens is total output. A.reflector can only decrease total output.

Lux is concentration. reflectors and optics affect concentration of light. A given reflector may gather many or few lumens, into a large or small spot.

LED emission angle is important. Wide angle seems to help reflectors by putting more lumens out to the sides. The deep reflectors gather more light this way. ONLY light hitting the reflector or optic surface (the outside of the TIR) goes to the spot.

I'm skeptical of a lumen to lux ratio given as a useful number. Either it's for exactly one LED.type perfectly mounted.(and valid), or it's bunk.

In that case, they refer to the fact that lux and lumens of a given optical system scale together. My flashlight on 50% output has half the lumen and lux as that light on 100%. So: if I understand them, and you mount the reflector well, you'll get 1500 lux from 500 lumen. But you would not get 1500lux from 500 lux. That's not how optics behave. You can't go stacking two of these to get double performance.


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## videoman (Feb 5, 2013)

What I really want to do is getting a much brighter illumination on a subject, in my case, a white wall as comparison. Using the same led at the same distance but with a narrow reflector to achieve this. I know that the narrower you make the beam, the brighter that projected beam area is, the question is by approximately how much. I tried with different reflectors and different optics with different beam angles. The results were from slightly brighter to super bright, but there is no rough guide prior to purchasing an optic or reflector to know approximately what brightness increase ratio or percentage that will make. I thought that the cd/lm number given in the specs would be an indication of what to expect. I am really measuring a reflected meter reading from within the resultant projected beam something like a 10 degree spot reading. This gives me an exact comparison of useful info to other optics tested, but this can only be done after the optics or reflectors are purchased and tested. I would like to know if that cd/lm spec can somehow be used in a way that gives a rough idea of what to expect and how can I apply that figure in determining a ratio or percentage to a bare emitter's specs.


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## AnAppleSnail (Feb 5, 2013)

At its simplest, a reflector has two components: spill and spot.Spill never touches the reflector, and behaves like a bare LED.The spot takes all the other light and focuses it into a projected image of a point on the LED that expands at some angle.You can use lux to estimate intensity of that spot, but a direct comparison is tough until you account for LED and drive current.An aspheric lens may be better for your purpose.


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## TEEJ (Feb 5, 2013)

A simple break down:

LUX
Lux is invisible...you can't see it. Its essentially the light reflected BACK to your eye from the target...how bright the TARGET is. (What you said you wanted)

So, the higher the lux, the brighter the target will look.

The lux at one meter is the cd (candela)

(which is NOT candlepower (cp).)




LUMENS
Lumens are the entire load of photons the light is sending out there, whether they hit something or not.



REFLECTOR
The reflector focuses the photons coming from the LED. A narrow reflector does NOT make the BEAM narrower as you mentioned above. Depending on the depth and diameter, and the type of LED, the beam is shaped to have THREE basic parts.

A WIDER reflector is harder to miss, as is a DEEPER reflector, so, it generally focuses more of the light than a narrower or more shallow reflector could. This can mean less spill, and more light thrown at a target.

The spill, mentioned above, is the part that MISSED the reflector, and just "spilled" out of the light's head...its not focused.

The Hot Spot, also mentioned above, is the central brightest part of a beam, the most focused area. (That is the ONLY area cd is measured from) That is the part that will project the furthest.

The Corona, not mentioned yet, is the "donut" of light that surrounds the hot spot. Its a lot brighter than the spill, and a lot dimmer than the hot spot. It is focused, but not as tightly as the hot spot of the beam is. On some lights, the hot spot transitions more gently to the corona, and its less obvious where one starts and the other stops, and on others, is glaringly obvious.

Spill, despite being essentially a "leak" in the system, can be useful. If you have a strong long range search-type light, you can't use it to see where your FEET are/if there's a root on the ground you might trip over, etc...because the burning bright circle of light on the ground is too bright for your eyes to stop down enough to see anything EXCEPT that burning bright circle of light. If the light has some spill, it might be enough to also see where you're going.

The corona is useful, in that it helps to see a larger area at a time than the hot spot alone would allow...at least at short/intermediate ranges for a given light, where the corona's cd is still sufficient to create lux on a target too.

Some lights with the same "beam angle" can have different amounts of spill, corona and hot spot...in different proportions, so - what you SEE will vary greatly.

For example, the smaller LED's generally throw further than the larger ones, because the smaller ones can have a "proportionally) higher surface brightness, which helps the reflector to concentrate the beam into a central hot spot. So a 300 lumen wee LED might throw FURTHER than a 1000 lumen larger LED...

...but the 300 L spot that is thrown further might be a very small circle of light, and the 1000 L circle of light might be a flood of light, but, a flood that doesn't extend as far OUT as the 300 L light did, and so forth.


As mentioned already, there are too many variables to be able to reliably predict EXACTLY what to expect if you have just lux/cd/lumen/beam angle specs.


If you want a general idea of how to interpret the specs, to get a clue at least as to what kind of illumination you might see...first, consider the range YOU want to see AT.

Light falls off according to the inverse square law....so at double the distance, you have 1/4 the lux on target, and so forth.

An easy short cut is to take the square root of the cd, and that's the range the light will give you ONE LUX at. (In meters)

So, if the cd = 100k, you take the square root, and get one lux at 316 meters.

If the cd = 200k, you get 1 lux at 447 meters. (Notice that 447 m (for cd = 200,000) is NOT twice as far as 316 m (for cd = 100,000)


The ANSI specs use 0.25 lux, but, in practice, that's too dim to be useful, especially at longer ranges. Most people want ~ 2 lux to be able to aim a gun at something, etc.

So the 1 lux range is a decent yardstick for visualizing the light's reach.


Then, think about how much of an area you want lit up at that range. 

The higher the lumens, generally, the larger the area that CAN be lit up at one time.


A laser beam has a very high cd...its ALL hot spot, but, the area lit up is miniscule. Many aspheric lensed lights can throw very far/have a higher cd, but, the area lit up is still pretty small, even if huge compared to a laser beam. (Looks like a fish eye lens at the front of the flashlight)


So, if the flashlight has the range you want, you can then think about if it lights up enough stuff AT that range to be more than a big laser pointer.


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## videoman (Feb 5, 2013)

Great info, thanks. Like Columbo used to say " there is just one more question.“
Let us assume that I wish to use the XM-L led as a single led video light. I would not need the bare led super wide coverage of the 120 degrees as it would distract people that are not in the frame of shooting and moreover it is wider than the camera lens would see anyway. I would instead prefer to have all them lumens
channeled or funneled down via an optic or reflector, to about a beam angle of 60 degrees. Would I be correct to assume that that resultant beam would be brighter to the eye on a given subject than without the optic/reflector.? In a theoretical and perfect world where there is no loss in spill or internal light robbing surface etc, can I also assume that the light increase on the subject would be 2X or a full F stop than just the bare led.? That 2X the brightness would mean 4X the lumens ( inverse square law). So I figure that halving that 120 degree bare led angle to 60 degrees I would see an apparent brightness of a full F stop. If I measured the subject with a bare led as to be F4 with a reflected light meter at 3 feet, can that same subject go to 6 feet using the 60 degree optic/reflector at the same F4 and exhibiting the same brightness of 3 feet.? Would that 60 degree optic/reflector have a cd/lm of 2.? Again in a perfect theoretical world.


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## uk_caver (Feb 5, 2013)

TEEJ said:


> A WIDER reflector is harder to miss, as is a DEEPER reflector, so, it generally focuses more of the light than a narrower or more shallow reflector could. This can mean less spill, and more light thrown at a target.


Surely a wider reflector is generally easier to miss, and is only harder to miss if its depth is increased to a greater extent than its width?

The critical thing for a reflector intercepting light would seem to be the angle (viewed from the LED) between the LED axis and the front edge of the reflector.
Excluding very small reflectors, a reflector which is uniformly scaled up or down in size will have similar light-gathering abilities, even if a larger reflector may be easier to make a more-parallel beam from for other reasons.


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## AnAppleSnail (Feb 5, 2013)

You'll want an aspheric. And the short answer is, no in practice.An aspheric lens projects a circle of uniformly bright light. Good, right? And moving the lens and LED between 0mm apart and the focal length of the aspheric ”zooms” this lit circle from wide (low lux, large area) to narrow (high lux, small area). Great!But most are optically inefficient. All the light that doesn't hit this lens is no good. So it's adaptive, versatile for video lights up close... And may ****can 50% of your output.


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