**Basis of LUX and Lumens**

The measurement for the light level encountered in a room or for an exterior space is LUX. The measurement of the light output from a lamp is lumens. Although 1 lumen will theoretically produce 1 LUX of light over an area of 1 square metre this is only in the perfect world as there are other factors to consider. For instance the decor and colouring in the space will undoubtedly result in some loss of light unless the room was had perfectly reflecting mirrored walls.

**The Calculation of LUX level in a room**

The LUX level in a room is calculated from the following formula

E(LUX) = F(lm) x UF x MF / A

Where

E is the the LUX level achieved

F is the average lumens value form the light source

UF is the utilisation factor for the space which takes into account the colouring of the surfaces in the space together with the geometry

MF is the maintenance factor for the lamp which allows for a level of light depreciation over time.

It is clear to see from this formula that the light level experienced is reduced by the 2 factors which are the maintenance factor of the lamp and the utilisation factor of the space.

Clearly these values are situation dependent but typical values for these would be 0.4 for the utilisation factor and 0.9 for the maintenance factor. This means that in a real situation you would achieve typically 35% of the light level that could theoretically be achieved in an optically perfect space.

** What LUX levels are needed in a Room?**

The level of LUX needed in a space is obviously subjective but here are some examples to provide some indication of what is needed.

Areas for relaxation for instance lounge or TV rooms would typically have 120 LUX. For an area which where it is necessary to read in it would be advisable to increase this to 200LUX. A domestic office environment of office for occasional use could be illuminated to a level of 250 LUX. It would be advisable to illuminate a commercial office area to a level of 400-500 LUX but where the majority of work is computer based a lower level is acceptable. For a retail environment where people are purchasing goods then a level of around 500 LUX is typically used. In a work shop area or area where detail work is undertaken then a LUX level of between 500 and 700 is desirable. Within an operating theatre in a hospital a LUX level or around 1000 would typically be used.

**Calculation of how many lights are needed in a room?**

Simply by turning the formula around it is possible to work out how many lights are needed for a room. This is particularly useful when working our how many spot lights or ceiling lights are needed in a space.

N = Ex A/(FxUFxMF)

So be example for a large space which is 10x 10m with 700 lumens ceiling lights , taking a maintenance factor of 0.9 and a utilisation factor of 0.7 then 45 lights would be needed to provide a LUX level of 200.

For a Kitchen which is 5 x 3 metre and using downlighters which produce 430 lumens output then for 180LUX you would need 14 downlighters taking the same factors as before.

I do hope that helps but of you have any questions please feel free to get in touch with us at SLB for some advise.

How does one calculate the UF and the MF?

The maintenance factor is now much light depreciation the lamp suffers. If you assume a figure or 0.8 this means that it allows for a loss of 20% which is reasonable for a good LED over a 5 year period. This is not calculated data but data which the LED supplier should be able to provide.

The UF would be 1 for mirrored walls and 0 for mat black no-reflective walls so typically for light coloured walls with some furniture and pictures the 0.4 would be a good guesstimation. This formula is a dead reckoning tool which gives a reasonable answer but if you need to be more precise then it is best to use light simulation software.

Hope that helps.

Paul SLB

I am trying to calculate how many LED lights and what type to best illuminate my workshop at night to between 500 & 700 Lux. Would you be able to give me any advice on this please? My tent/workshop is 6m x 12m with a height of 4m approx.

Matt

I need to achieve a lux of 800 to 1000 for a showroom, iam going to use metal halide of 70 and 150 watts, height of ceiling is 4 Metre, pls advice on the number of lamps to be used..

i need to achieve a lux of 450 to 550 for a laboratory. i am going to use the LED lamps 20Watts, heights of the ceiling is 3mtrs. pls advice the how to calculate the no of lights accordingly to room size..

Hi there

the best way is to simulate it with a programme like Dialux. The light level you achieve will also depend on the reflexivity of the ceiling and walls.

I found the best way to calculate the LUX level here. Excellent work. Thanks for sharing such an useful formula.

I need to calculate how many lamps (36 watt) needed for a cold-room with 200 lux lighting? The area is 5m*3m and the height is 2m.

I would be pleased if someone tell me what is the relation between surface area and height in calculating lux.

Hi there

this would require a Dialux simulation to work out accurately. If you need this we could do but would need to charge. If you want a rough approximation then space them at about 120cm spacing , assuming these are 36 watt halogens.

1K lux how many lux hours

1K lux means 1000 lux which is broadly equivalent to an old 100w filament light bulb