What is a Lumen?
As ‘light artists’ (pronounced in a bad French accent…) we use light as our paint. The light sources that we use are rated in Lumens, but what really is a Lumen? I mean, no-one really knows what a TOG is when it comes buying a new duvet...
Let’s start with SI Units…
Lumens are an SI derived unit, SI standing for the “International System of Units” which means that they can be expressed as a product of one of the 7 base internationally recognised units. These base units are:
Seconds (s) which measures time
Metre (m) which measures length
Kilogram (kg) which measures mass
Ampere (A) which measures electrical current
Kelvin (K) which measures thermodynamic temperature
Mole (mol) which measures the amount of a substance
Candela (cd) which measures luminous intensity
Calculating a Lumen (Prior to 20/05/2019)
For Lumens (lm), the base SI unit it uses is Candela (cd) in the following formula. This is where the science comes in (and I got make a large cup of tea to continue writing this blog post!)….
1 Lumen = 1 Candela x Steradian
Now to define the components of that equation!
LUMEN : a measure of the total quantity of light emitted by a source per unit per unit of time, the emission is called Luminous Flux (not to be confused with the Flux Capacitor…)
CANDELA : a measure of the luminous intensity in a given direction
STERADIAN : (a.k.a square-radian) imagine you shine a torch inside a large sphere from the centre, the beam creates a cone shape inside. The torch will also create a circle on the surface of the sphere. A Steradian is a cone/circle of fixed proportions in relation to the spheres radius.
Now let’s go back to the formula….
1 Lumen = 1 known standard of light intensity (Candela) x 1 known sized cone shape like a torch beam
Essentially, the Lumen is defining the total amount of visible light in a defined beam cone
Calculating a lumen (Post 20/05/2019)
So, the Bereau International des Paid et Mesures convened in 2018, and from appendix 3, in resolution 1 of the 26th CGPM we can see that a number of SI units got a new definition. Including the Candela, which forms part of the Lumen formula. (LINK HERE)
The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, to be 683 when expressed in the unit lm W–1, which is equal to cd sr W–1, or cd sr kg–1 m–2 s3, where the kilogram, metre and second are defined in terms of h, c and ΔνCs.
Yea… good luck with that!
A paper called “An Introduction to the New SI” better explains the change in section III / H.
For light, the candela, abbreviated cd, is the unit of luminous intensity, which is the power emitted by a light source in a particular direction multiplied by a factor Kcd, which takes into account the sensitivity of the eye to various colors of light.
One candela is approximately the luminous intensity observed for a candle, where it gets its name. If the source has a luminous intensity that is the same in all directions, then the total luminous flux is 4π lumen.
In the New SI, the candela is defined by specifying that the factor Kcd for green light at 540 THz is exactly 683 lumen per watt.
In other words… the definition now uses a specific wavelength of light to define what a Candela is BUT the value of a 1 Candela is the same as it ever was.
What does this mean when buying a torch for light painting?
If you managed to follow the above, then we can now define a lumen a universally standard measure of brightness in a standard shape light beam, that hasn’t changed since Candela’s were redefined in 2019. It’s not an arbitrary marketing tool.
Assuming that all manufactures are sticking to the correct measurements, then a lumen should be as standard across all devices as other units of measure like a metre, a second, or a kilogram. (Sorry USA, Liberia and Myanmar… we’re talking metric system here!!)
But again, that’s assuming you are buying from a reputable manufacturer who hasn’t boosted their Lumen ratings for marketing purposes!
What is Lux?
Lux takes into account the area over which the luminous flux is spread and is a calculation based on lumens.
I will cover Lux, Lumens, Distance (from Camera!) in a future article which I will write once my brain recovers from researching this blog post!
