Lumens vs candela/m^2

Thank you Crabtree that was exactly what I was looking for.

Another good resource for lighting information is the www.IESNA.org website for the Illuminating Engineering Society of North America.

Rorschach,

You can view a free online unit conversion here (and download a desktop version which you will need to pay for):

http://www.unitconversion.org/index.html

It has all types of conversion from general (time, power, temperature, currency etc), engineering (velocity, acceleration, torque, force etc), heat (thermal, heat density etc), fluid (mass flux, flow rate, viscosity etc), sound, light (luminence, digital image etc), electricity (charge, surface current, inductance etc), magnetism, radiology and miscellaneous (typography, data transfer etc).

Or you can download (free of charge) a program called 'Convert' - written by Joshua Madison from here:

http://joshmadison.com/software/convert/

Hope thats of help to you.

Kind Regards

Kev Brown

Kev, thank you, I have the Madison program already, and there are multiple sites on the web that will do similar conversions as well. My issue was one of definitions. The two terms are not apples and apples conversions, there is a directional component for one whereas there is no directional component for the other. It is not at all like converting Farenheit to Celsius or KW to HP, it is more like trying to convert KWH to HP. they aren't directly comparable.

No problem.

I just re-read your first post and realised I got the complete wrong end of the stick! Should learn to read more carefully first thing in the morning!

Kev Brown

Hi Rorschach,

It's funny that I happen to be holding a light meter in my hand while reading your question. The units which I am used to are Lumiens and foot Candles. Basically a foot Candle is the intensity of light from a candle at a distance of 1 foot. I don't know where the metre squared is coming from that you are referiing to.

On my meter right now, I see that 38.8 Ft-cd is roughly equivalent to 418 Lux. I hope this helps.

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A lumen can be thought of as a quantity of light energy.

If you spread it equally over a square meter, it will illuminate the surface to a value of 1 lux

This will happen if the (curved) surface is at a distance of 1 meter...

...and the intensity of the source (almost brightness, but not quite) is 1 candela.

A square meter on the surface of a sphere will subtend a solid angle (think of a cone) of 1 steradian.

There are 4 pi steradians in a sphere (roughly 12,5) and so the final crunch is that, from the 1cd source, there will eminate a total of 12,5 lumens.

A LED usually has a cosine related drop-off in intensity as you get further off-axis. Therefore, to calculate how much the intensity (Cd) will increase as you squish the lumens into an increasingly tighter solid angle, you need to do some intergration (calculus) which I am far too rusty to even attempt!

The reason for a square in the distance relationship with respect to lux and cd, is that the light expands in two directions as it travels from the source, thereby diluting both in an x and y direction.

So, our 1m distant radius of the sphere, if it were increased to 2m, would then be illuminated to a value of not 1 over 2 lux, but 1 over 2 x 2 = 1/4 lux.

This gent has an interesting website,

http://led.linear1.org/lumen.wiz

and this calculator looks like a reasonable tool, but, as he says, the larger angles can be inaccurate because just considering the total solid angle as subtending equally spaced lumens is not a real-life scenario - LEDs do deliver most of their light to the front.

In fact, the angle as quoted is always the total angle (i.e. 2 half-angles, each side of centreline) at which the intensity is half the maximum.

There is still a fair amount of lumens outside of that, even right up to 90 degrees each side.

Ciao,

RobG