Lumens, Lux and Watts

Measurements of Light

When lighting an area for night time activity you may hear the following expression used “lumens”, “Lux” and “watts. These terms are used to help describe and measure the quality, quantity and power consumption of lighting. They are a part of a group of standard measurements known as the photometry group which measure different aspects of light. This group also includes such units as the foot –candle, candela and Steradian.

While those in the industry should have a good understanding of these measurements, the way light is calculated is actually rather technical and can often be very confusing. This post is designed to provide a basic understanding of the different measurements especially “lumens”, “Lux” and “watts and their relationship with each other.

Candela

The candela (cd) was originally called the foot-candle and simply referred to the amount of luminous energy emitted by a known type of candle. Candela is the non-standard measure of illuminance, basically defined as how much light would strike the inner surface of a sphere one foot in radius if it were lit by a single candle in the centre of that sphere. The candela is the base unit in light measurement. A common candle emits light with roughly 1 cd luminous intensity.

Lumens, Lux, Watts-01When it was standardized, a definition was chosen that would approximate the older definition, so that existing equations could remain the same. The modern definition of the candela has to do with the radiation emitted by 1/60 of a single square centimetre of platinum when it is at its melting point. The definition describes how to produce a light source that (by definition) emits one candela. Such a source could then be used to calibrate instruments designed to measure luminous energy.

Steradian

A Steradian (sr) is a standard unit of measurement used to define a solid angle which, having its vertex at the centre of the sphere, cuts off a spherical surface area equal to the square of the radius of the sphere.

Lumens, Lux, Watts-02A steradian is defined as conical in shape, as shown in the illustration. A solid angle projecting from the centre of a sphere and cutting its surface has a measure of s / r² steradians, where s is the surface area of the sphere cut out by the solid angle, and r is the radius of the sphere.

Because the surface area of a sphere is 4πr2, the definition implies that a sphere measures 4π ≈ 12.56637 steradians.

1 sr =r2 x 4 π

Definition of Lumens

Lumen (lm) measures the “luminous flux” or the total amount of light that is produced by a light source in all directions. Strictly speaking, a lumen is defined as candela multiplied by steradian, which can be expressed as:  lm = cd x sr

 

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Lumens are great to measure light from the source of an object. But brightness has one important factor: Distance this is where Lux is measured. Sometimes it is much better to know how much light is hitting a particular spot (light intensity), rather than know what the light is outputting.

 

Definition of Lux

Lux is a scale used to measure light intensity or the illumination that is produces by a light source at a distance. The standardized unit of measurement of Lux is; the amount of lumens per square meter (the illumination of the light)

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Lumens and Lux come together.

So when you put Lumens and Lux together to describe light illumination and the amount of light produced; a measurement of lux (light intensity) tells you how many lumens (total light output) you need given the area you are trying to illuminate or to put it another way –. One Lux is defined as being equivalent to one lumen spread over an area of one square metre.

lux=  lumens X meter² so 1000 lumens, concentrated into an area of one square metre, lights up that square metre with a light intensity (illuminance) of 1000 lux. The same 1000 lumens, spread out over ten square metres, produce a dimmer illuminance of only 100 lux.

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This image above shows 1 steradian has a projected area of 1 square meter at a distance of 1 meter.  Therefore, a 1 candela (1 lm/sr) light source will similarly produce 1 lumen per square foot at a distance of 1 foot, and 1 lumen per square meter at 1 meter.

The relationship of lumens & Watts       

Wattage (measured in Watts) is the power required to operate a light fitting.

Lumens, Lux, Watts-09It refers to the total electricity that is consumed which will include:

– The heat generated by the light source

– Operational aspects depending on the type of lighting device (such as gear control)

– The energy of the light consumed from the bulb or tube

 

So the rated wattage of a light source refers to the entire power consumed to drive the source.

As we have worked out Lumens refer to the light output of that source. A lumens per watts is a light engineering term for the measurement of the rate at which a lamp is able to convert electrical power (watts) to light (lumens) – this is referred to as “luminous efficacy”.

Lumens, Lux, Watts-10Light lumens divided by the wattage of the light equals the luminous efficacy

Lumens per watt = Lumens ÷ watts

KONICA MINOLTA DIGITAL CAMERAWriten By Rachel McCann

Project Manager Photovoltaic Lighting Group

 

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Categories: Lighting Technology

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