Formula to convert Lumens to Candle Power?

Formula to convert Lumens to Candle Power?

This is a discussion on Formula to convert Lumens to Candle Power? within the Related Gear & Equipment forums, part of the Defensive Carry Discussions category; Is there a formula to help compair flashlights where some have their power listed in Candle Power and others listed in units of Lumens?...

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Thread: Formula to convert Lumens to Candle Power?

  1. #1
    Distinguished Member Array ArmyCop's Avatar
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    Formula to convert Lumens to Candle Power?

    Is there a formula to help compair flashlights where some have their power listed in Candle Power and others listed in units of Lumens?
    For God, Family and Country!


  2. #2
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    IIRC the units are based on different criteria - one is Illuminance the other Luminance - something like that so not sure if a direct conversion is absolute.

    Anyways try here

    http://www.onlineconversion.com/light.htm
    Chris - P95
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    VIP Member Array Redneck Repairs's Avatar
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    I dont know of the forumla but if i rember right master converter is a freeware/shareware progy that should do it , i dont have the link handy sorry and am running on a deal ... post back if you cant find it or it is non suitable .
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    Senior Member Array David III's Avatar
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    I looked a while back and never could find a direct way to compare them - finally decided that there are two units just so I can't compare Surefire with Streamlight!

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    Unhappy Just Quick Searching The Web

    I have a funny feeling that you're going to feel sorry that you asked if (below) is any indication.

    Date: Tue Apr 10 15:43:32 2001
    Posted By: Adrian Popa, Director Emeritus, Hughes Research Laboratories
    Area of science: Physics
    ID: 986426234.Ph
    Message:
    Conversion Of Lumens To Candlepower
    Greetings:

    Reference: F. A. Jenkins and H. E. White,
    Fundamentals of Optics,
    McGraw-Hill, 1950

    Your question goes to the heart of why we need international standards.
    Back in the middle ages the size of the kings foot was how long a foot was resulting in every kingdom having a different unit of length.
    If something valuable was measured by length, such as cloth, then it was best to have a king with a small foot.

    The same is true for other archaic units of measure such as candlepower and candles.

    The wax, wick, size parameters can all be different and each candle would give a different answer.
    Also the temperature of the candle flame and color wavelength distribution) are related key parameters that determine how bright candles are.

    When electric light bulbs were invented in the late 1800s they rated
    the brightness relative to candles just as steam engine manufacturers rated there engines in horse power.
    As you well know there also are many different sized horses.
    So what is a horse power?
    Thus an international system of standard units came into being to have everyone measure things using the same standard units.

    The standards that you are seeking are part of the science of photometry.
    These standards have metric and English units making it difficult for students to understand all of the different names.
    Just as the here in the U.S.A. we are still using yards instead of meters.
    The bottom line is that the world (at least the world of science and engineering) has given up on using candle power and candles and that currently the candela replaces the old candle.

    The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 hertz (a green laser) and that has a radiant intensity in that direction of 1/683 watt per steradian.

    References about the history and how we currently measure one candle power
    (luminous intensity) today are based on the work of the National Institute for Standards and Technology (NIST)
    http://www.physics.nist.gov/cuu/Units/index.html http://
    www.physics.nist.gov/cuu/Units/index.html

    Your question asks about white light sources and in that case the candle power, the candle and the candela are all equal to one lumen per steradian.
    There are 4 times Pi steradians in a sphere.
    If you are not familiar with working with steradians as solid angles we can convert the standard to square degrees.
    There are 41,252.96 square degrees in a sphere. Divided by 4 times Pi that gives 3,282.81 square degrees per steradian.
    Thus a candle power (CP) = one lumen per
    3,282.81 square degrees or inversely , 304.6 x 10^-6 (0.0003046) lumen per square degree per candlepower.
    As you can see a one CP is a very weak light source.

    Because a flashlight or spotlight is an extended light source (the diameter of the reflector or lens) not a point source, it is possible, but difficult to calculate your answer.
    However, experimental measurements will probably give a better answer.
    All that you need to know are the number of square degrees that the flashlights illuminate at a distance of 12 feet and 60 feet from the light source.
    If the beam spots are round (a cone of light) then you can measure the diameter of the beam in degrees from the light source and calculate the number of square degrees from Pi times the diameter (in degrees) squared divided by 4.

    To calculate the CP at each location first calculate the number of lumens from the flashlight divided by the number of square degrees in the spot.
    This gives the number of lumens per square degree.
    Then divide by divide by 304.6 x 10^-6 (0.0003046) lumens per
    square degree per candle power to give the total candle power.

    The problem with this measurement or the aperture calculation is that the light flux in the spot or in the aperture of the flashlight is not uniform across the beam diameter.
    So which beam diameter do you use? My flashlight has a 30 inch (12 degrees) diameter beam at 12 feet with a 10 inch diameter
    (4 degrees) bright spot (the filament)in the center.

    Using your 125 lumens and the 4 degree hot spot cone angle (113 square degrees) from my flashlight optics gives 32,567 CP.
    The 12 degree cone angle (12.6 square degrees)results in 3691 CP.

    However, not all of the lumens are in the hot spot. If you could measure the number of lumens in the hot spot then you could calculate a more accurate value for CP.
    I wonder which spot diameter spotlight manufacturers use? (probably the greatest CP, that is why we see adds for million CP spotlights). Also, atmospheric losses are not considered in the calculations.
    As you can see from my example a number for CP is subject wide variability while the number of lumens in a beam is much more accurate.

    Best regards, Your Mad Scientist
    Adrian Popa

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    Member Array mkeBob's Avatar
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    QK, What!!!!!
    The power of the future is in its ability to inspire the present.

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    Don't worry mkeBob...those Converted Lumens are Zoomin'...right over my head.

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    Thx to QK - I feel better i-lumen-ated

    No one can hold a candela to our QK
    Chris - P95
    NRA Certified Instructor & NRA Life Member.

    "To own a gun and assume that you are armed
    is like owning a piano and assuming that you are a musician!."


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    Member Array fhqwhgads's Avatar
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    That's greek to me. Time to go get a gyro.

  10. #10
    Member Array clipse's Avatar
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    The way I understand it, Candlepower is how bright the center of the beam is. Lumens is overall brightness.

    clipse
    MOΛΩN ΛABÉ

  11. #11
    Distinguished Member Array ArmyCop's Avatar
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    WOW.

    Reason for my question is seeing a Scorpion Mini LED Lithium flashlight listed as having 42 Lumens of light output and the other Scorpion Mini at 6,500 candlepower.
    I'd like to know which is stronger & if it's a big difference or just a little difference.
    For God, Family and Country!

  12. #12
    VIP Member Array Tubby45's Avatar
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    Correct. Candlepower is a measure of the brightest spot of the beam. Lumen is a measure of the overall brightness of the beam itself. The conversion from CP to L is crazy difficult, as proved in the previous post. For all intense and purpose, it is beyond layman's conversion.

    When you are shopping for a light, the lumens are a better indication of the quality and brightness of a given light source. The law of averages is working here. There is a larger discrepency between different points of a CP rated light than a lumen rated light. A lumen rated light is more consistent in light output and that is more meaningful than 'power'. I won't buy a light that is rated in CP. Too much variation in light output. A CP rated light can have 5 million candlepower in a tiny spot, but a lumen rated light can have a very bright light signature ALL OVER the entire beam. That is more important to me. I don't care about the brightness of a pin-sized dot, as I look at something in the dark with the WHOLE beam, not just part of it.
    07/02 FFL/SOT since 2006

  13. #13
    Member Array clipse's Avatar
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    Just a though, its like converting how fast a car can go to horsepower. Only more difficult.

    Quote Originally Posted by ArmyCop
    WOW.

    Reason for my question is seeing a Scorpion Mini LED Lithium flashlight listed as having 42 Lumens of light output and the other Scorpion Mini at 6,500 candlepower.
    I'd like to know which is stronger & if it's a big difference or just a little difference.

    I guess you talking about the Streamlight Scopion LED and regular Streamlight Scorpion.

    I always check www.flashlightreviews.com. Doug takes lux readings at the center and has an aperatus to take overall brightness.

    Lets take a look at the two you mentioned. :)

    First the Streamlight Scorpion.

    Lux at the center=4418
    Overall output=5800
    lumens= ~93.96 (according to a calculation doug posted on CPF)

    Now for the Streamlight Scorpion LED.

    Lux at the center=1867
    Overall output=2600
    Lumens= ~36.14

    Obviously the regular Scorpion is brighter (by more than twice as much) but runtime is what kills it. You get about 1 hour of light with the regular Scorpion while the LED Scorpion will last 2 hr. 49min. before you get to 50% brightness. Its always comes to runtime vs. output. Which one you need is dependant on how you are to use it. If it will be an EDC light/general purpose light then the LED Scorpion would be great with the longer runtime and being pretty bright still. If this is going to be a nightstand light and only used when things go bump in the night then have the extra brightness may be adventagous. :)

    clipse
    MOΛΩN ΛABÉ

  14. #14
    Member Array ordy1's Avatar
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    Holy smokes it's a flashlight turn it on shine in your eyes which one blinds you more.

  15. #15
    Distinguished Member Array ArmyCop's Avatar
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    Talking

    Quote Originally Posted by ordy1
    Holy smokes it's a flashlight turn it on shine in your eyes which one blinds you more.
    That's funny. Good reply.

    Thanks everyone for the reply's. I might buy both. I'd like one for when I pull Police Auxiliary time (probably the reg bright spot) and the other for all around the home use.
    For God, Family and Country!

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