How many light fittings

[chessp]

Can anybody advise as to how many GU10 CFL lamps I would need to light a room of approx 30m2 and having cathedral ceilings up to 5m high. I understand that it would be subject to preferential brightness the wife likes bright white light (4000 per globe) rooms.

[Smurf]

Can anybody advise as to how many GU10 CFL lamps I would need to light a room of approx 30m2 and having cathedral ceilings up to 5m high. I understand that it would be subject to preferential brightness the wife likes bright white light (4000 per globe) rooms.

What is the room to be used for?

Kitchen?
Bathroom?
Office / Study?
Lounge?
Bedroom?
Something else?

They all have different light level requirements.

[chessp]

the room I mention is lounge. Not sure whether there is some simple guidelines to calculate lighting in room

[Smurf]

the room I mention is lounge. Not sure whether there is some simple guidelines to calculate lighting in room

Depends how bright you want it.

TV manufacturers work on lounge rooms being 50 lux. That's pretty dim but would be fairly typical for a longe room.

Offices are normally lit to about 400 lux. From memory (I have the books but they're in a box somewhere - I'm moving house...) it's around 240 for the kitchen.

There are two ways to calculate lighting. First and by far the most commonly used is the lumen method. That is, total light output and assuming it is evenly distributed in the room.

The other method is the point method. That is, the light intensity of any particular light source measured at the appropriate location (eg on the kitchen bench top). Then you work out how many lights are needed, and where, to maintain the required brightness.

The point method is useful for spot lights of any type. Also for street lighting and some other applications. But as soom as you mention fluoresent lights or any other diffuse source, the lumen method is the way to go unless you really do like lots of math.

So I'll assume you're aiming for at least 50 lux and we'll use the lumen method.

Next thing is lumen output from a light source. A 60 watt bulb (or a 50 watt halogen) puts out around 600 lumens in round figures. I'm assuming that your CFL's will be around this level. (Be warned that a 40 watt bulb is only half as bright as a 60 watt - the relationship between power in and light out is NOT linear).

Next is the efficiency. This can be properly calculated with reference to the colour of the walls, type of light fitting and so on. In the absence of a better figure, 57% tends to be about right and will do for a non-critical application.

So, 30m2 lit to 50 lux. 30 x 50 = 1500 lumens assuming 100% efficiency of light falling on the horizontal.

But we only have (assumed) 57% efficiency. So we want about 2630 lumens in total (1500 / 0.57)

Now, roughly 600 lumens (could be anywhere 450 - 700) per 60 watt equivalent CFL. So we'll want to use 5 of them to get around 2630 lumens in total. Actually it will be around 3000 but the trend is to brighter rather than dimmer so that's the way to go.

The above is a VERY crude approximation but it will be good enough unless there's something unusual (like painting the walls black) in the room.

The method used can be compared to putting 1000 litres of water (or 1000 lumens of light in this case) on a lawn with a sprinkler. Assuming the water is evenly distributed, and allowing some loss for evaporation and overspray, you could work out that this is equivalent to x mm of rain falling on the lawn. The above calculations are doing much the same but with light instead of water. Produce x lumes, lose some due to various reasons, and then you end up with y amount per m2.

[chessp]

many thanks in deed for the guidance. I will now calculate how many we require and let you know how this works.

[OBBob]

I have a 50m2 room with 6m high cathedral ceilings. I came up against the same issue when I got to that stage. I couldn't get a clear answer so I hung 6 pendant lights with CFL globes (100w equivalent) and then put about 10 downlights around the perimeter. It's important to keep them all at aheight that allows you to change globes without scaffold!

Anyway ... the downlights are never used and two of them use almost as much power the 6 CFLs! Also the downlights give very little light in this sort of area compared to the CFLs.

Anyway ... just for your info.

[MrFixIt]

Hi

FWIW Light falls off, or fades away at a ratio of the square of the distance. That means that if you double the distance you only get a quarter of the light.

So in your case, a 5m high ceiling would require FOUR times the amount of light fittings to retain the same amount/level of brightness at floor level.

Floor level illumination (from ceiling lights) is of course not a really useful criteria as we don't "live" at floor level. Essentially the level of illumination can be subjective and depend on the "view" of the observer.

Personally I don't think ceiling mounted lights at 5m is very effective. I would consider pendant light fittings or possibly wall mounted light fittings. This way you will retain the "normal quantity" of light fittings. Pendant and/or wall mounted lights can also create a great illumination effects.

[Smurf]

I overlooked the bit about high ceilings.

Yes, you'll certainly need extra light due to that as the other poster said.

If you're going for a reasonably high light level (ie more than 50 lux or so) in a lounge room then you want some way to control it. Dimmers for incandescent lights are fine. So too are separate switches for fluoros etc.

What you don't want is to put in, say, 400 lux and be stuck with that all the time - the novelty of having your lounge room look like an office will wear off VERY quickly.