# Candlepower/Lumens



## HIDSGT (Jan 12, 2008)

I realize it is very difficult if not impossible to convert one to the other. I'm am aware of that. 

But if someone had to guess in relative terms what would 4000 lumens (Polarian PF40) be in candlepower if one had to guess? Im just looking for something close. Thanx...


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## Ken J. Good (Jan 13, 2008)

Hmmm…A lot in play here:

Taken from Wikipedia:

The lumen (symbol: lm) is the SI unit of luminous flux (also called luminous power) a measure of the perceived power of light.

Keeping in mind that candela and candlepower are the same term:
If a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian, its total luminous flux emitted into that angle is one lumen.

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

The exact standard and measuring methodologies have changed over time.

Or that could be stated:

If a light source emits one candlepower (this unit was originally based on the light emission from a flame) of luminous intensity (that is, power emitted by a light source in a particular direction, weighted by the luminosity function (the average sensitivity of the human eye to light of different wavelengths), a standardized model of the sensitivity of the human eye to different wavelengths) uniformly across a solid angle of one steradian (think cone instead of a piece of pie), its total luminous flux (the measure of the perceived power of light) emitted into that angle is one lumen.

*Getting down to it - Relationship between luminous intensity and luminous flux*
If a source emits a known intensity (in candelas/candlepower) in a well-defined cone, the total luminous flux in lumens can be calculated by taking the number of candelas, and dividing it by the number in the table below that corresponds to the "radiation angle" of the lamp (the full vertex angle of the emission cone). 

Example: A lamp that emits 590 cd with a radiation angle of 40°: 590/2.64 = approximately 223 lumens.

Radiation angle	Divide by
5° / 167.22
10° /	41.82
15° /	18.50
20° /	10.48
25° / 6.71
30° / 4.67
35° / 3.44
40° /	2.64
45°/ 2.09

If the source emits light uniformly in all directions, the flux can be found by multiplying the intensity by 4π: a uniform 1 candela source emits 12.6 lumens.

It looks like you will have to work backwards:

Lamp Lumious flux of the PF40 is really about 4,150 lumens. Radiation angle is 3 degrees in the collimated light so I will have to use the closest thing on the table (5%), which will make the number come up way short as you can see the radical upward curve on these numbers.

But the math as I see it (no pun intended) is:

cd / 167.22 = 4,150
cd = 693,363 (candlepower with this area of the beam)

The math may be closer to:

cd / 350 = 4,150
cd = 1,1452,500

or 

cd / 600 = 4,150
cd = 2,490,000 

Keep in mind this is a total guess, as I don’t know the accuracy of this concept/table actually comes from, what the 3-degree factor actually is.

What this does show is as the beam is spread out the candela/candlepower is lower when measured at any particular location.

From: http://www.electro-optical.com/whitepapers/candela.htm

The candela value is independent of distance. One can think of it as the emission from the lamp, which then loses interest in what happens to the photons it has ejected. We need a new unit for the light energy moving through space in the direction of our object.

This unit of invisible light in transit is the lumen.

The official definition of the lumen, the unit of luminous flux, is:   The luminous flux dF of a source of luminous intensity I (cd) in an element of solid angle dR is given by dF = IdR

In plain English: The flux from a light source is equal to the intensity in candela multiplied by the solid angle over which the light is emitted, taking account of the varying intensity in different directions. 

This is the statement that is most useful for me in terms of grasping the concept:

The candela is a unit of intensity: a light source can be emitting with an intensity of one candela in all directions, or one candela in just a narrow beam. The intensity is the same but the total energy flux from the lamp, in lumens, is not the same. The output from a lamp is usually quoted in lumens, summed over all directions, together with the distribution diagram in candela, shown above.

What I get out if this I that you can have lights that have high candela, but not necessarily putting out a lot of light (Lumens)

You can have a lot of lumens, but if it not directed, your candela will be low when measured in any given point.

Maybe some of you brainiacs can do a better job of the relationship between lumens and candela as it relates to directed light sources….It is still Greek to me.

Oh by the way, most spotlights have candela ratings that are wildly inflated.


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## HIDSGT (Jan 13, 2008)

That was interesting Ken. Thanx!


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## mtbkndad (Jan 13, 2008)

This is really an apples and oranges question.
A very oversimplified explanation is that lumens would be area filling capacity and candlepower is used to determine how far a focused beam can shine.

Now lux readings can be taken at a set distance and then candlepower can be derived from those.

The Polarion X1 has a lux reading of 2881 on my Lux meter at 37.5 feet.
This can be converted mathamatically to candlepower and the result is 376,388 candlepower. This is actually pretty good and a Helios will likely be very close though very slightly lower.

Even a very well focused "30 MCP" Costco HID only actually has 996,007 candlepower. A "15 MCP" Thor only has 436,458 candlepower.

While the Helios and X1 have very similar candlepower ratings, the Helios puts more light down field in a larger brighter corona. In other words, it has more down field light filling capacity an therefore puts more lumens down field.

Take a close look at this thread and you will be able to get a good handle of the difference between lights with High CP ratings that don't put much light down field and lights with lower CP ratings that do put lots of light down field.

https://www.candlepowerforums.com/threads/184465

Take Care,
mtbkndad :wave:


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## Flashanator (Jan 13, 2008)

> Costco HID only actually has 996,007 candlepower. A "15 MCP" Thor only has 436,458 candlepower.


 
Thats some nice accurate info.


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## NextLight (Jan 13, 2008)

Ken J. Good said:


> Hmmm…A lot in play here:



Indeed! And you have obviously spent some time researching and thinking about the math & physics. Its late, and I am rather rusty, but here are my thoughts:

The mathematical conversion from lumens to beam candlepower can hardly be approximated for most flashlights.

From your post "*If a source emits a known intensity (in candelas/candlepower) in a well-defined cone..*. " For this conversion from lumens to work, this means no spill; all light must go uniformly into the specified arc of the cone of light. 

We know this neither occurs, nor is it desirable for most flashlight applications. So from the total lumens, we would have to know the main beam angle, the spill beam angle, and the ratio of spill to main (hot spot) beam energy.. Then assuming the both fields/beams are perfectly uniform, we can subtract the spill lumens from the total lumens. Now with that result we could _begin_ calculation of the candlepower in the X degree main beam, using the tables you posted. 

This calculation falls apart because of the calculus of the reflector shape and finish, and to a lesser degree (NPI) the complex reflections from any front lens. While there is a relationship, I don't think we mortals can calculate actual beam candlepower from lumens, with _any _confidence. (Even if we actually knew the actual exact lumen value. That is already a problem, as you have observed.) This is also true for attempts to use "Aperture" calculations.

Fortunately, peak beam candlepower is easy enough to measure, even on a superlight. (But there appears to be no world standard as to how! The Candela (metric) or candlepower (english) is the unit of measure indicating the luminous intensity of a light source in a specific direction; any given light source will have many different intensities, depending upon the direction considered. 

The technique from Maxa-Beam - (Hardly the definitive source, but there is none for this measurement) Shine your light at a target at 100 feet. Take your light meter and measure the light level in footcandles (lumens per sq ft) at the brightest point in the beam. The peak beam candlepower is then the distance squared (100x100=10,000) times the foot candle reading. 

For major superlights, you can increase the distance if the light meter can't go high enough, or do it in your 10 foot kitchen with more modest lights. (10x10=100 so at 10 ft, multiply your peak beam footcandles by 100, to get peak beam candlepower.) These are just hotspot in the hotspot (sic) measurements; They don't really mean much, IMO

FYI, one can also use an optical attenuator (aka Solar Filter) over your light meter's sensor, if your meter won't go high enough on its own, for your chosen working distance. An "ND4" filter attenuates light by 10^4 or 10,000 times. (This filter looks completely black. It is NOT a "4 F-stop Filter") You can even use filters like this on your video or still camera to photograph or video the arc in your HID tube (or filament on your incan) to look at uniformity...just don't get too close, or the filter will get hot.

Good Hunting,


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## Ra (Jan 13, 2008)

If you want to theoretically calculate the cp-output of a light, you need a few things:

The surface brightness (luminance) of the filament or arc, the reflector-diameter, and the % reflectivity of the reflective material on the reflector.
The reflector "magnifies" the surface of the filament or arc, theoretically, the apparent surface brightness stayes the same (minus the reflection loss on the reflector..)

When I point a light at you from 1000 meters away, with a 200mm diameter reflector, and a 2x5mm, 3000 lumens arc or filament (omnidirectional): Theoretics go as follows:

For an omnidirectional source, lumens can be converted to cp!!: lumens divided by 4pi! So 3000 lumens omnidirectional equals 239 cp.. This is called MSCP: Mean Spherical Candle Power
With a surface of 2x5=10 square mm that makes 23.9 cp/mm^2
That is the surface brightness of the source..

The reflector has 100mm^2*pi surface: 31,416 square mm. So the reflector emits 31,416x23.9=750,842 cp. But, due to the central hole and lower reflectivity of the reflector material, The optical efficiency will be about 75%: 75% of 750,842 equals 563,132 cp

As this is not omnidirectional, we call this Beam CandlePower (Bcp)

Note that this is theoretical: With an uniform arc or filament and a perfect parabolic reflector! The surface brightness may not be uniform over the arc's surface, and reflectors can have imperfections in the shape that also can affect the actual outcome.


So in the Polarion case, you need the surface brightness of the HID arc in cd/mm^2, the reflector diameter, reflectivity and the calculations I did above!

I'm shure that there is someone here who can provide you with this info!!



Best 

Ra.


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## Davekan (Jan 13, 2008)

Ra said:


> If you want to theoretically calculate the cp-output of a light, you need a few things:
> 
> The surface brightness (luminance) of the filament or arc, the reflector-diameter, and the % reflectivity of the reflective material on the reflector.
> The reflector "magnifies" the surface of the filament or arc, theoretically, the apparent surface brightness stayes the same (minus the reflection loss on the reflector..)
> ...


 

Thanks Ra

This clearly explains why small arc in big reflector can give very large Bcp.

Thanks Dave


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## mtbkndad (Jan 13, 2008)

Ra said:


> If you want to theoretically calculate the cp-output of a light, you need a few things:
> 
> 
> ...
> ...



HDSGT,

For practical purposes the measurement I got from my lux meter at 37.5 feet and then using the proper formula to convert that reading to cp and the resulting 376,388 cp is an easier way to get a real world rather then theoretical approximation of the cp. 


Ra,

When you measured a 15 MCP Thor and calculated the true cp what did you get?
Was is close to the 436,458 that That Guy got using my lux readings at 37.5 feet?
If yes, then we can assume the Polarion X1 numbers I posted are a good real world estimate of it's cp and the Helios throws very slightly less then the X1.


After all it is very clear to anybody who has seen a Helios that the parabollic reflector and the position of the bulb in relation to it are not optimized for maximum cp and you mentioned variables like this will throw off theoretical calculations.

Take Care,
mtbkndad :wave:


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## Ra (Jan 13, 2008)

mtbkndad said:


> HDSGT,
> 
> For practical purposes the measurement I got from my lux meter at 37.5 feet and then using the proper formula to convert that reading to cp and the resulting 376,388 cp is an easier way to get a real world rather then theoretical approximation of the cp.
> 
> ...



I totally agree ! 

Like I said with the finishing words in my post, theoretical calculations can have some impuryties. But this was theoretical in the first place.. HIDSGT can easily measure the output himself when he has a lux-meter..

I measured some 15Mcp stock Thor's before modding them and got values of between 380,000 and 410,000 calibrated cp ! The difference can very well be in lux-meter calibration.. And is small enough to not even be noticed, if it's not the calibration..


Best,

Ra.


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## NextLight (Jan 13, 2008)

My apologies to all in this thread. I was not happy with my first post (#6) to this thread, and its lack of technical rigor. I have been trying for hours to edit my post, to no avail.

Thanks Ra for your technical inputs. I have no basis to doubt your formula. Your current understanding of optics and light is probably above mine. Unless addressed directly, my comments below are not directed at you or Ken in particular

I still hold that we in the field, no make that >> I << (in the field) can not calculate beam candle power (with any useful accuracy) from lumens. Knowing or measuring "the (actual) surface area of the source...the uniformity of the reflector...and...the lower reflectivity" to the required accuracy is simply not practical. (Ra, is your math considered part of the "Aperture Method" I mentioned in my post?)

I am an Electrical Engineer (by education) working mostly in the Test & Measurement world, and have demo'd, used, and measured my SureFire 6P for many years, but have absolutely NO clue as to the values of the three parameters mentioned above, on the 6P, my oldest and best known (current production) flashlight.

It is nice to know that specific lumen to beam candlepower conversion formula exist, and I see it could be done in an optical design lab, by some very smart guys who have access to many instruments and CAD design data, to arrive at some value for a given light. The numbers would apply to no other design, of course. This limits the usefulness of the number, and is one of several reasons why at least one leading manufacturer avoids it (and BCP) like the plague.

The Peak Beam Candle Power measurement method I described in post #6 s not universally accepted (among manufacturers) and not without its flaws; It does little to describe the overall usefulness of the beam, and nothing to describe the total light output. (Again, PBCP not having a fixed numerical relationship to lumens) I regard the method I described more as (semi) common practice, than "Science of Light" as SureFire likes to say. But the method is not without merit.

The most obvious merit to the footcandles at distance X method is the required equipment and math are simple; We can all work from the same page, and compare notes. This leads to data validation, and thus a reduction of uncertainty. (though I still fully expect +/- 50%+ variations!) Two, whether we are lighting a wolf, wild man, waterfall, or wall, knowing the intensity with which we can illuminate a target at a given distance, combined with comparative beamshots at a fixed exposure, tells a lot about that light. 

I am always willing to learn from those who know more about light. (Especially if the training is worth more than I paid for it but as a consumer, I am unlikely to hold light X in greater regard if the manufacturer claims/calculates 50 million beam candlepower, rather than 5.

Whew!


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## Ken J. Good (Jan 13, 2008)

No offense taken and I clearly am out of my technical league on this issue.

Hence my caveats indicating that the more educated of the group needs to chime in here.

I like to use these things not necessasarily figure out how they work.... 

I do have my handy, dandy lux meter and will get a reading unless somebody beats me to the punch.


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## mtbkndad (Jan 13, 2008)

Ken J. Good said:


> No offense taken and I clearly am out of my technical league on this issue.
> 
> Hence my caveats indicating that the more educated of the group needs to chime in here.
> 
> ...



Ken,

When you do lux readings, be sure they are at a distance beyond the convergence point of the beam. The reason I use 37.5 feet is that is the longest unobstructed distance in my home. Fifty or one hundred feet would even be better. You also want an area that will not have light from other sources that can alter results.

The formula that That Guy used in his calculations in the Amondotech N30 announcement thread is very simple.

One interesting thing in your post is your lux measurements which can be turned into candlepower. Converting the feet to metres and then squaring it gives a number which when multiplied with the lux readings gives the candlepower.

Your measurements in candlepower:

Costco HID (good focus)----------996 007
Costco HID (okay focus)----------741 428
AI Proto 6000K HID---------------446 300
15 MCP Thor Magnum-------------436 458
AI 4200K-------------------------420 449
Polarion X1-----------------------376 388
Polarion P1------------------------335 677
X990 Spot------------------------256 597
N30------------------------------254 972
L35-------------------------------229 908
10 MCP Thor Focused--------------205 631
10 MCP Thor Stock----------------195 002
Dorcy 3 MCP----------------------138 181
X990 Flood------------------------128 208 

I added the X1 results to this list that That Guy put together.
I did not have a X1 back when I did the announcement thread.
The other thing you will need to be careful with when measuring the lux reading of a light like the Helios is to actually measure the brightest spot in the beam. This is more challenging then it may seem as the light meter will pick up differences that the eye will not.
Last of all make sure your distances are accurately measured and repeatable.

Take Care,
mtbkndad :wave:


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## Ra (Jan 14, 2008)

Ken,

Best is to setup the light in a fixed position, and walk to a marked point, 50-100ft away: You can easily find the hotspot by moving the light-sensor in the x-y direction. At those distances, the hotspot should be within armslength!

For the X-1, 37.5 feet should be sufficient: The reflector/arc-size ratio does not need longer measurement-distances IMO..But ofcource, you can do measurements at various distances and compare them: If you are too close, cp calculations will drop significantly compared to measurements at longer distances..

With Maxablaster, I need to step away at least 300ft to obtain usefull measurements!!

Best,

Ra.


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## Flashanator (Jan 14, 2008)

10 MCP Thor Stock----------------195 002




hey Ra, Is there a lux meter of such that you can measure close as in 1m to the light? Ok that might sound stupid, but im newbie so.

later.


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## Ra (Jan 14, 2008)

Flashanator 500mW said:


> hey Ra, Is there a lux meter of such that you can measure close as in 1m to the light? Ok that might sound stupid, but im newbie so.
> 
> later.





Sorry Flash, doesn't exist!

CP-measurement is a point-measurement, the sensor of a lux-meter always has certain surface dimensions. If a light doesn't use the entire reflector-surface to concentrate the light at one point (or total surface of your lux-meter), you cannot measure the actual cp-output. You need to step away from the light, to a distance where the entire reflector is in play! You can easily test this by looking dirctly into the reflector (with the bulb operating..) from a distance, with the use of a dark (welding-) filter. You can see wether or not the entire reflector is lit by the filament (halogen) or arc..

Note that when you cannot find a distance where the reflector is totally lit, bulb-missalignment and/or imperfections in the reflector-shape can and may well be the cause of that!


Best,

Ra.


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## NextLight (Jan 14, 2008)

Ra, your comments about minimum distance explain why I could get repeatable results with my smaller (reflector) lights at 50 feet down to 10, but not so with my larger lights. 

Thanks!


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## Flashanator (Jan 15, 2008)

on the topic of cp, how do manurfactures measure to get ridiculas CP ratings like 15mcp out of a halogen light? Or is it made up bs.


Also how accurate is the cp ratings of seald beam aircraft landing lamps like the par64 with 1.35Mcp center beam?


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