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Comparing lumens and candlepower?

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  • #31
    Re: Comparing lumens and candlepower?

    Originally posted by JohnS View Post
    There is no way to answer without knowing the beam angle, or the spot size at some distance. Each gives you half the answer.

    For a lightbulb that radiates in all directions 1 cd (candlepower) emits 12.57 lumens (4*pi) because that is the solid angle of a complete sphere). For a light that emits a moderately small angle of light, any type of spotlight or flashlight, you need the solid angle to relate the two units.

    Candle power (properly called candela) measures the brightest part of the beam. Lumens integrate the candlepower at each angle and give the total light emitted. If you are trying to light up a room, lumens are what counts. If you are trying to throw light a long way, and small spot size or beam angle is acceptable, candlepower is what counts.
    Okay. If I want my LED spot light to project a beam of light the same distance and the same intensity as a 2,000,000 candle power spot light, What lumens mesurement would I look for??

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    • #32
      Re: Comparing lumens and candlepower?

      Originally posted by Unregistered View Post
      I am looking at basically two of the sme type of lights. They are the remote contolled spots that has magnetic mount to put on top of truck cab. One says 500,000 candle power, the other says 4400 lumens. Which is the best as far as light goes. Thanks
      from my spreadsheet
      One says
      500000 candlepower
      the other says
      4400 lumens =
      350.1408748 cp

      the ratio of the surface area of a sphere
      to the surface area of the spherical cap
      that this beam illuminates has to be
      1427.996661

      for these two spots from these two spotlights to be equal in brightness, a
      6 degree beam [actually, field] angle comes very close

      You won't believe the work that went into this spreadsheet, because of me misunderstanding definitions, making arith. errors and general screwups.

      The moral of the story is, if you derive equations then put them onto a spreadsheet and plug in numbers and check your answers. Also change the input numbers and see if the output numbers go in the right direction.
      Last edited by HerrWarum; 10-23-2011, 12:54 PM.

      Comment


      • #33
        Re: Comparing lumens and candlepower?

        Originally posted by Barndoor View Post
        Just as a practical comparison my section engineer recently bought a 'handlamp' with 2,000,000 candlepower.

        but it's good up to 90 metres.

        it illuminated deer at 400 metres.
        The 'throw' is the distance that the original intensity is down to an illumination of 1 lux or 1/4 (actually 0.27) lux, the 'moonlight standard'.

        For a 1 million cp light this would be 1000 meters or 2000 meters.

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        • #34
          Re: Comparing lumens and candlepower?

          From watts into the lamp to lux on the target
          The formulas are below.

          25 >enter lamp watts, LW
          68 >enter luminous efficacy, LE
          in lumens/watt
          LE is less than 200 lumens/watt at present
          1700 =calc'd lumens = LWxLE
          Divide by 4Pi to get candlepower in all directions, CPALL.
          135.2817016 =CPALL

          If you have lamp cp in all directions CPALL
          and you know beam angle BA
          then the multiplier to find beam cp BECP is
          MULT
          so CPALL x MULT = BECP

          a = sin(BA/2), b = sqrt(4-4(a^2)),
          then MULT = 4/(2-b)
          then CPALL x MULT = BECP
          135.2817016 >enter CPALL
          20 >enter BA in degrees
          0.173648178 =calc'd a
          1.969615506 =calc'd b
          131.6460956 =calc'd MULT
          17809.30783 =calc'd BECP

          lamp with
          17809.30783 candles [I] luminous intensity
          illuminates a spot with
          1 lux, E
          How far away is it?
          R=sqrt(I/E)
          133.4515187 =calc'd distance in meters, R

          Comment


          • #35
            Re: Comparing lumens and candlepower?

            OK here is the problem, the newer flash lights are rated in candle power and the newer ones are rated in lumens. Why can't the malefactors put both in the box like some lumens equal to candle power. They put Spanish and press 1 for English. I WON'T TO KNOW HOW MANY LUMENS ARE 1800 COMPARED TO CANDLE POWER CAN ANYONE PLEASE HELP A OLD GUY OUT. THANK YOU WILLIE.

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            • #36
              Re: Comparing lumens and candlepower?

              Originally posted by Willie View Post
              OK here is the problem, the newer flash lights are rated in candle power and the newer ones are rated in lumens. Why can't the malefactors put both in the box like some lumens equal to candle power. They put Spanish and press 1 for English. I WON'T TO KNOW HOW MANY LUMENS ARE 1800 COMPARED TO CANDLE POWER CAN ANYONE PLEASE HELP A OLD GUY OUT. THANK YOU WILLIE.
              CP = lumens/4Pi.

              For beam candlepower vs. beam angle you need to
              solve for the area of a spherical cap as a function of
              beam angle. This is best done on Excel

              Below, I used a 10 degree beam angle.

              1800 >enter lumens

              Divide by 4Pi to get candlepower in all directions, CPALL.
              143.2 =CPALL

              If you have lamp cp in all directions CPALL
              and you know beam angle BA
              then the multiplier to find beam cp BECP is
              MULT
              so CPALL x MULT = BECP

              The equation for MULT is pretty messy so
              I've broken it up into a and b components.
              But, going backwards from BECP to MULT
              requires solving a cubic equation.
              Just fuss with values in Excel until you
              get close enough [+/- 10%].


              a = sin(BA/2), b = sqrt(4-4(a^2)),
              then MULT = 4/(2-b)
              then CPALL x MULT = BECP
              143.2 >enter CPALL
              10 >enter BA in degrees
              0.087155743 =calc'd a
              1.992389396 =calc'd b
              525.5824763 =calc'd MULT
              75284 =calc'd BECP

              And if you look up the 'luminous efficacy' for the bulb type you can judge whether they are lying about the lumens.

              Since you bring up age, if you tell me how old you are I can tell you how long you will live, to some level of certainty. Not everyone wants to know this, though.
              Last edited by HerrWarum; 11-04-2011, 08:11 AM.

              Comment


              • #37
                Re: Comparing lumens and candlepower?

                That being the case, I consider a 1 million candle power fairly bright. By comparison though, what this is saying is that 1 million candle power light would be classified as 12,570,000 lumens. I have never heard of a light with so many lumens, not even close! Are you sure about these formula's?

                Comment


                • #38
                  Re: Comparing lumens and candlepower?

                  Originally posted by Unregistered View Post
                  That being the case, I consider a 1 million candle power fairly bright. By comparison though, what this is saying is that 1 million candle power light would be classified as 12,570,000 lumens. I have never heard of a light with so many lumens, not even close! Are you sure about these formula's?
                  The 12.6 Mlm would only be correct if the 1 Mcd light radiated equally in all directions. Lights this bright are always spotlights with a relatively narrow beam angle.

                  Imagine being at the center of a large sphere and the light having a "perfect" reflector so that illuminates all of the sphere within a conical beam angle theta and no light outside of this area. (The angle from the centerline to the edge of the beam is theta /2). Then the illuminated area is 4*pi*sinČ(theta/4).
                  The lumens will be 4*pi*BCP*sinČ(theta/4), where BCP is beam candlepower. If the beam angle is 6°, the 1 Mcd beam will be 8610 lm.

                  In the real world, the very brightest part of the beam determines BCP, and the light is not uniform to the edge of the beam, so the above is only an approximation, the real lumens will be less. If you don't know the beam angle, there is not even an approximate relationship between lumens and candlepower.

                  Comment


                  • #39
                    Re: Comparing lumens and candlepower?

                    How does 1000 candlepower compare to 135 lumens?

                    Comment


                    • #40
                      Re: Comparing lumens and candlepower?

                      Originally posted by RudyRanger View Post
                      How does 1000 candlepower compare to 135 lumens?
                      Unless the beam angle is defined, there is no comparison.

                      Comment


                      • #41
                        Re: Comparing lumens and candlepower?

                        I have a 5W LED lamp that is rated at 240 lumens with a 15 degree beam pattern so how many candlepower would this be? If possible could you show the formula?

                        I appreciate any help, thanks.

                        Comment


                        • #42
                          Re: Comparing lumens and candlepower?

                          5 >enter lamp watts [W]
                          240 >enter lumens [L]
                          48 =luminous efficacy in L/W
                          Divide L by 4Pi to get candlepower in all directions, CPALL.
                          19.1 =CPALL

                          If you have lamp cp in all directions CPALL
                          and you know beam angle BA
                          then the multiplier to find beam cp BECP is
                          MULT
                          so CPALL x MULT = BECP

                          let a = sin(BA/2) and b = sqrt(4-4(a^2))
                          then MULT = 4/(2-b)
                          then CPALL x MULT = BECP
                          19.1 >enter CPALL
                          15 >enter BA in degrees
                          0.130526192 =calc'd a
                          1.982889723 =calc'd b
                          233.7776262 =calc'd MULT
                          which is the ratio of a sphere surface area
                          to that of a spherical cap illuminated by the beam
                          4465 =calc'd BECP

                          Comment


                          • #43
                            Re: Comparing lumens and candlepower?

                            Originally posted by HerrWarum View Post
                            5 >enter lamp watts [W]
                            240 >enter lumens [L]
                            48 =luminous efficacy in L/W
                            Divide L by 4Pi to get candlepower in all directions, CPALL.
                            19.1 =CPALL

                            If you have lamp cp in all directions CPALL
                            and you know beam angle BA
                            then the multiplier to find beam cp BECP is
                            MULT
                            so CPALL x MULT = BECP

                            let a = sin(BA/2) and b = sqrt(4-4(a^2))
                            then MULT = 4/(2-b)
                            then CPALL x MULT = BECP
                            19.1 >enter CPALL
                            15 >enter BA in degrees
                            0.130526192 =calc'd a
                            1.982889723 =calc'd b
                            233.7776262 =calc'd MULT
                            which is the ratio of a sphere surface area
                            to that of a spherical cap illuminated by the beam
                            4465 =calc'd BECP
                            Eq in #38 gives the same answer but is easy to solve for any one of the variables, lumens, beam candlepower or beam angle.

                            Comment


                            • #44
                              Re: Comparing lumens and candlepower?

                              Even for beam angles above 15 degrees?

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                              • #45
                                Re: Comparing lumens and candlepower?

                                Originally posted by Unregistered View Post
                                Even for beam angles above 15 degrees?
                                For the ideal beam as defined in post#38, the equation is valid to 2*pi (360°), producing the 4*pi steradians relationship between lumens and candela for omnidirectional light, but the beam has to illuminate the sphere uniformly over the beam angle, and go to black immediately outside the beam angle. In the real world, beams aren't quite that well focused.

                                Also in the real world, you usually want to shine the beam on a rectangular wall, not the inside of a sphere.

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