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#21
05-12-2011, 04:52 PM
 Unregistered Guest Posts: n/a
Re: Comparing lumens and candlepower?

Not as easy as it looks.
Here's my Excel sheet

HID lamps in spotlights
watts W Lumens L million candlepower mcp L/W mcp/W cp/L
25 3200 25 128 1.00 7813
35 3200 15 91 0.43 4688
50 4500 15 90 0.30 3333
50 3200 25 64 0.50 7813
so your lumens per watt varies from 128 to 64, your million cp per W goes from 1 to 0.3 and your cp per lumen varies from 7800 to 3300.
#22
05-13-2011, 08:34 AM
 Unregistered Guest Posts: n/a
Re: Comparing lumens and candlepower?

Sorry for the runaround on this can of worms.

50 W HID gives you 20 million candlepower and 4500 HID lumens gives 15 million candlepower, so your light puts out somewhere between 20 and 15 million cp. Let's say 17 million, but because there is a wide tolerance on this value.

I have learned from this exercise that the correlation between the listed watts, lumens and candlepower for HID spotlights is probably inaccurate or untruthful or both.
#23
05-20-2011, 01:57 PM
 Unregistered Guest Posts: n/a
Re: Comparing lumens and candlepower?

ANSWER: 1 candlepower equivalent equals 12.57 lumens
----------------------------------------------------------------
First of all nowadays we use the term CANDELA instead of candlepower.
otherwise "Luminous Intensity" - (aka CandlePower)
= 12.57 Lumens

Details:

We understand a candle radiates light equally in all directions, its output, in this consideration is not focused by any mechanical means (lenses or reflectors). Pretend for a moment that a transparent sphere one meter in radius surrounds your candle. We know that there are 12.57 square meters of surface area in such a sphere. Remember your Solid Geometry classes?

That one candle (1 Candlepower/Candela) is illuminating equally the entire surface of that sphere. The amount of light energy then reflected from that surface is defined thusly:

The amount of energy emanating from one square meter of surface is one lumen. And if we decrease the size of the sphere to one foot radius, we increase the reflected energy 12.57 times of that which fell on the square meter area.

LUX is an abbreviation for Lumens per square meter.
Foot-candles equal the amount of Lumens per square feet of area.

So, that one candlepower equivalent equals 12.57 lumens.

And for you figuring out LED equivalents, first you must know how many lumens your LED's each produce. Then divide that value by 12.57 and you have candlepower of the LED. You don't have foot-candles, remember foot-candles are illuminance.

--------------------------------------------------------------

SI photometry units
Quantity ; Symbol ; SI unit ; Abbr.; Notes
Luminous energy ; Qv ; lumen second ; lm·s ;units are sometimes called talbots
Luminous flux ; F ; lumen (=*cd·sr) ; lm ;also called luminous power
Luminous intensity; Iv ; candela (=*lm/sr); cd ; an SI base unit
Luminance ; Lv ; candela per square metre; cd/m2; units are sometimes called "nits"
Illuminance ; Ev ; lux (= lm/m2) ; lx ; Used for light incident on a surface
Luminous emittance; Mv; lux (= lm/m2) ; lx ; Used for light emitted from a surface
Luminous efficacy ; lumen per watt ; lm/W; ratio of luminous flux to radiant flux

A LIght Converter to play here here:
www (dot)convertunits(dot)com/type/luminous+intensity

www(dot)theledlight(dot)com/lumens.html[/url]
#24
05-21-2011, 11:00 AM
 Unregistered Guest Posts: n/a
Re: Comparing lumens and candlepower?

Quote:
 Originally Posted by Unregistered ANSWER: 1 candlepower equivalent equals 12.57 lumens [B]
How do you reconcile advertised watts, lumens and candlepower, either from the Web or from store product packaging?
#25
05-21-2011, 04:30 PM
 JohnS Moderator Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,445 Rep Power: 19
Re: Comparing lumens and candlepower?

Quote:
 Originally Posted by Unregistered How do you reconcile advertised watts, lumens and candlepower, either from the Web or from store product packaging?
Poorly. To relate lumens and candela, you need to know the beam angle of the light. The relationship of 1 CP = 12.57 lumens applies only to spherically uniform light patterns, not to flash lights. If you are not told the beam angle, it is hopeless.

Watts and lumens will only be relatable if you know the technology of the lamp.
#26
05-22-2011, 02:42 PM
 HerrWarum Supreme Member Join Date: May 2011 Location: MD, USA Posts: 303 Rep Power: 3
Re: Comparing lumens and candlepower?

Since the batteries don't last long for handheld spotlights so I'm going to buy Grainger catalog part number 3JK36 tomorrow. The beam angle is 10 degrees, lumens = 1500, w = 100, halogen.
Most of my usage is indoors.

I guess I can then use a camera to measure the Exposure Value of a sheet of white paper lit by this thing at some distance away and then use formulas to convert to watts, lumens, lux and cp.

The Grainger catalog values for spotlights don't seem to correlate well either and I think these guys are pretty credible so I think we're missing something in addition to beam angle. BTW, they also specify the distance the beam is usable.
#27
06-03-2011, 01:42 PM
 Unregistered Guest Posts: n/a
Question

Looking for a good wide angle portable LED flood to use as a continous lightsource for photography. relatively expensive at a camera store - it'd be cake to use a bright task light with a very wide angled beam - thus having something that cost a lot less with a lot more utility as it could be used elsewhere. Any leads on buying something like this?
#28
06-03-2011, 03:56 PM
 HerrWarum Supreme Member Join Date: May 2011 Location: MD, USA Posts: 303 Rep Power: 3
Re: Question

Quote:
 Originally Posted by Unregistered Looking for a good wide angle portable LED flood to use as a continous lightsource for photography. relatively expensive at a camera store - it'd be cake to use a bright task light with a very wide angled beam - thus having something that cost a lot less with a lot more utility as it could be used elsewhere. Any leads on buying something like this?
What about the spectrum you'd get?

Post your desired and minimum specs.
LEDs give about 5x the light per watt as incand.

If you have a light meter you could run tests on Radio Shack LEDs or LED flashlights for minimal bucks, and then scale up.

BTW, my 'spot' light seems to have a beamspread considerably wider than 10 degrees.

Last edited by HerrWarum; 06-03-2011 at 06:25 PM.
#29
08-23-2011, 04:02 AM
 Barndoor Guest Posts: n/a
Re: Comparing lumens and candlepower?

Just as a practical comparison my section engineer recently bought a 'handlamp' with 2,000,000 candlepower.

Mostly, in my opinion, bazillions of candle power is too often used to draw in the gullible buyer. This lamp takes close to 24 hours charge for less than 1 hour of use. It also requires the use of a shoulder strap to carry for any length of time. It looks like a regular handlamp only much much bigger. It does provide a good floodlight but only for a short period of time. In our case furnace inspections.

Alternatively, I have a rifle lamp of 200 lumens. It fits into my trouser pocket. It lasts for up to 7 hours on a very short charge. It does the same job. There is only a narrower beam than the big one but it's good up to 90 metres.

I am expecting delivery any day of an 800 lumens rifle lamp. One review by a shooter stated it illuminated deer at 400 metres. It also has a wide beam setting, reported as 100 metres wide, but I can't remember at what range.

Alan
UK
#30
08-23-2011, 09:45 AM
 HerrWarum Supreme Member Join Date: May 2011 Location: MD, USA Posts: 303 Rep Power: 3
Re: Comparing lumens and candlepower?

Quote:
 Originally Posted by Barndoor This lamp takes close to 24 hours charge for less than 1 hour of use. It also requires the use of a shoulder strap to carry for any length of time. It looks like a regular handlamp only much much bigger. It does provide a good floodlight but only for a short period of time. In our case furnace inspections. Alternatively, I have a rifle lamp of 200 lumens. It fits into my trouser pocket. It lasts for up to 7 hours on a very short charge. It does the same job. There is only a narrower beam than the big one but it's good up to 90 metres. I am expecting delivery any day of an 800 lumens rifle lamp. One review by a shooter stated it illuminated deer at 400 metres. It also has a wide beam setting, reported as 100 metres wide, but I can't remember at what range. Alan UK
If AC is typically available I'd buy a spot lamp from Grainger. You may also be able to adapt low voltage lamps to 120 vac using a transformer. A 100 w, 120:12 volt 'frmr may not be too costly.

I assume the range is at half-power [?], which is determined by intensity and beam width.

Statistics can detect lies, and we have some idea in which direction each vendor is lying.
Game Theory predicts that if one vendor is lying then all will have to lie to get market share. All vendors lying is a stable equilibrium.
If one vendor declares that he will tell the truth, the other vendors "will openly praise him for being a good citizen and privately condemn him for being a simpleton." The "system" will not go back to everyone being truthful; that is an unstable equilibrium.
It's called a Social Trap or The Tragedy of the Commons.

Last edited by HerrWarum; 08-23-2011 at 09:50 AM.

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