Lux, Lumens and Watts: Our guide

Introduction to this guide

Here at Green Business Light, we must ensure that our energy efficient industrial and commercial lighting installations provide the necessary illumination levels for an end-client's building (such as a warehouse). The specified illumination or brightness levels usually expressed as an amount of 'lux' (e.g. - 150 lux, 400 lux)….but what does this actually mean?

This guide helps you to understand what the expressions "Lux" and "Lumens" mean and how they relate to the energy consumption & lighting output levels of commercial & industrial lighting technologies (including the latest LED lighting fixtures).

Definition of Lux (with examples)

Lux is a standardised unit of measurement of light level intensity (which can also be called "illuminance" or "illumination")

So what does 1 lux equal? - > a measurement of 1 lux is equal to the illumination of a surface one metre away from a single candle.

Some further examples of average light levels (measured in lux) are shown below:

  • Outdoor average sunlight ranges from 32,000 to 100,000 lux
  • Moonlight represents an illumination level of approximately 1 lux
  • At sunset and sunrise (with a clear sky), ambient outdoor light is about 400 lux
  • Warehouse aisles are lit to approx 100-200 lux
  • Warehouse loading bays need to be higher at approx 300-400 lux
  • A bright office requires about 400 lux of illumination
  • An industrial factory building (where metal or glasswork takes place) can require illumination as high as 500-750 lux

Definition of Lumen output (with examples)

The lumen is a standardised unit of measurement of the total "amount" of light (packets or quanta if you want to get really technical!) that is produced by a light source - such as a lamp or tube (or LED chip). This total measured light may also be referred to by lighting engineers as "luminous flux".

Some examples of total light output (as measured in lumens) from common commercial & industrial light sources are given below (NOTE: these are sample figures for example purposes only – the actual output can vary):

  • A 400W Metal Halide lamp - high bay factory or warehouse lighting: 38000 Lumens
  • A 100W Incandescent bulb – for general domestic & task lighting applications: 1700 Lumens
  • A 32W T5 or T8 Fluorescent tube – used in office ceiling lighting panels 1600 Lumens
  • A 150W High pressure sodium bulb – for street/outdoor area lighting 12000 Lumens
  • A 200W LED array in a high bay fitting – an energy efficient replacement for 400W metal halide and sodium high bays - 20000 Lumens

The relationship between Lumens & Lux

One (1) lux is defined as being equivalent to one lumen spread over an area of one square metre. To put it another way:

A measurement in lux tells you how many Lumens (total light output) you need given the measured area you are trying to illuminate.

So 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 Lumens, spread out over ten square metres, produces an illuminance of only 100 Lux.

Therefore, lighting larger areas to the same necessary lux levels will requires a larger measured level of Lumens – this is usually achieved by increasing the number of light fixtures (and hence the power consumed). Industrial buildings (such as factories and industrial units) and warehouses have large open spaces so a large number of high power light fittings are generally required.

Efficacy - the relationship of Lumens & Watts (with examples)

The power required to operate a light fitting (or luminaire) is measured as a rated Wattage (Watts being Joules of energy per second). The rated wattage of a light source refers to the entire power consumed in creating the light Lumens and includes:

  • The energy required in creating "visible" light emitted from the lamp
  • The heat output generated (including the infra-red part of the lighting spectrum)
  • Other parasitic power losses (such as inefficiencies of the control gear/ballast) of the light fixture

A light engineering term exists 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" (or just efficacy) – and is expressed in Lumens per watt (LPW) or Lumens per circuit Watt

Some examples of luminous efficacy in common commercial & industrial light sources are given below (NOTE: these only refer to the light sources and not the light fitting):

  • A 400W Metal Halide lamp - high bay lighting in industrial factories & warehouses: 90-95 Lumens/Watt
  • A 100W Incandescent bulb – general task lighting applications: 17 Lumens/Watt
  • A 32W T5 or T8 Fluorescent tube – general office ceiling lighting: - 50 Lumens/Watt
  • A 150W high pressure sodium bulb – street lighting: 80 Lumens/Watt
  • A 200W LED array in a LED high bay fitting – an energy efficient replacement for 400W metal halide and sodium high bays: 100 Lumens/Watt

NOTE: All of the measurements above relate to light sources which are new and have not dropped in efficiency - as discussed below - the gradual decay of lighting levels must be taken into account when performing lux calculations for buildings such as warehouses, factories etc..

The "real" Lumen output of lamps and light fittings

Up to now this article has covered the technical definitions of Lux, Lumens & Watts but this is only part of the story.

In the specification of lighting for real-world industrial & commercial applications it cannot be assumed that 100% of the lamp output will be emitted from the fitting - or the light output will be constant over its operational lifetime - the concepts of Light Output Ration and Lumen Depreciation are explained further below.

The Light Output Ratio of a light fitting

The actual total illumination levels that can be provided by a light fitting will depend on the Light Output Ratio – this term is defined as:

The LOR is the ratio of the total amount of measured lumen output of a light fitting (containing a lamp) to that of just the lamp in isolation. As an example - taking an industrial or warehouse light fitting with a LOR of 70% - this indicates that 30% of the lamp's light output is lost due to the design of the fitting

This term is required in lighting engineering as when a lamp is positioned in a light fitting (such as an industrial 400W metal halide high bay) losses of light occur will occur within the fitting itself. Usually light needs to be directed towards the working area (e.g. - downwards from roof to floor) but light radiates from lamps and bulbs in all directions (upwards, sideways etc.)

The use of highly polished reflectors (aluminium etc.) will redirect most of the light downwards but a proportion always will be 'lost' in the fitting. Its worth noting that direction light sources (such as LED chips) do not suffer from this problem to the same extent as light is emitted as a beam in a singular direction - therefore the LOR will typically be high.

Lumen depreciation from light fittings

The LOR of a light fitting will also be affected over time as debris builds up on reflectors as well as protective covers in the case of fittings with an 'IP' rating. This will particularly be the case in industrial factory buildings which have many different processes being undertaken

Lumen depreciation from lamps & light sources

Lumen depreciation is a process of the gradual decline in light output that is observed from most light sources over time. This includes (but is not limited to):

  • Gradual light filament/electrode deterioration
  • Blackening/discolouration of the lamp surface

For LED technology - the process of Lumen depreciation is slightly different. The lifetime of a LED light is defined as the time the Lumen output reaches 70% of the initial output. This is also called L70. In other words, the module does not die instantly as most conventional light sources do, it slowly dims down. However - it should be noted that lower cost high power LEDs (such as those required for large industrial buildings) can suffer a rapid initial loss of lumens which may pass unnoticed by the building users.

Both Light Output Ratio and Lumen Depreciation must be taken into account when calculating the required number of light fittings to maintain a desired lux levels for a warehouse or industrial setting.

1.5 m3 m4.5 m6 m556 lux2,222 lux8,889 lux988 lux20,000 lux
Lux deteriorates with distance from the light source according to the "Inverse Square Law"

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