General: Lighting Calculation from first principals

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- CommentAuthordanbates
- CommentTimeJun 28th 2012
Hello, I'm new to this forum but thought I'd dive right into unknown territory as this seems like the place to do it!

I'm wondering on the required lighting levels in dwellings and how to calculate the light levels from first principals. Perhaps even there's a spread sheet out there based on SAP or otherwise that I can reverse engineer as it were?

If anybody could illuminate (OMG he didn't) this topic or point me in the right direction I'd be happy.

All the best,Dan
- CommentAuthortony
- CommentTimeJun 28th 2012
use the inverse square law,

bottom line is dont use recessed downlighters.

Depends a lot on what you want to do, there are no minimum levels for homes!, there are regulations about using low energy bulbs though

the nearer the light source is to what you are doing the more light you get, use daylight wherever possible, examples of some activities below:-

Activity: Illumination(lux, lumen/m2)
Public areas with dark surroundings 20 - 50
Simple orientation for short visits 50 - 100
Working areas where visual tasks are only occasionally performed 100 - 150
Warehouses, Homes, Theaters, Archives 150
Easy Office Work, Classes 250
Normal Office Work, PC Work, Study Library, Groceries, Show Rooms, Laboratories 500
Supermarkets, Mechanical Workshops, Office Landscapes 750
Normal Drawing Work, Detailed Mechanical Workshops, Operation Theatres 1,000
Detailed Drawing Work, Very Detailed Mechanical Works 1500 - 2000
Performance of visual tasks of low contrast and very small size for prolonged periods of time 2000 - 5000
Performance of very prolonged and exacting visual tasks 5000 - 10000
Performance of very special visual tasks of extremely low contrast and small size 10000 - 20000
- CommentAuthorCWatters
- CommentTimeJun 28th 2012
Very hard to work it out from first principles..

The data above gives you the light level required at the target (eg the work surface in office = 500 lumens/sqm). Then you can in theory work back to the source using geometry..

Imagine you had a bulb that radiated uniformly in all directions. In that case all the light (lumens) produced would all pass through an imaginary sphere around the bulb. So if the bulb emitted 1000 lumens was mounted 2m above the target work surface you could work out the lumens per square meter arriving at the target by dividing 1000 by the area of a sphere radius 2m.

The problem is slightly more complicated if the bulb does not have a spherical pattern. Suppose the beam angle was 180 degrees. In that case all the light would pass through half a sphere. The rest is all geometry.

Well it would be except for the issue of reflections. Light bouncing off walls might also arrive at the target. Horribly complicated to work out from first principles. I suspect there is some expensive software available for architects and the like.
- CommentAuthorPeter_in_Hungary
- CommentTimeJun 29th 2012
And then of course there is the colour temperature of the light which will affect the perceived usefulness of the light, And any lampshade will generate a loss (more over time as the thing gathers dust) and the colour of the walls will attenuate the amount of useful light reflected
- CommentAuthorbarney
- CommentTimeJun 29th 2012
If you just want to get an idea of how much light is in a space you could apply:

Elux - (LL x mf x Uf)/area

Where

LL is the total lamp lumens connected - ie the sum of all the lamps in all the fittings

MF is a maintenace factor to allow for lamp ageing, dirt on luminaires and room surfaces - (usually about 0.8)

UF is a "factor of utilization" - a measure of how much of the total lamp lumens escapes the fitting and how it is reflected around the space base on the room and luminaire geometry. UF can be fearsomely difficult to calculate unless you have photometric data - as a first pass you could use about 0.35.

That will give you an average illuminance in the space on a horizontal plane.

If you want the illuminance at a point then if you have knowledge of the luminaire intensity for given directions, you can estimate it from ;

Epoint - (Intensity at angle theta x Cos3 angle theta)H2

Where Intensity in candelas is at the angle between the source and the point and H is the height of the luminaire above the point in consideration.

Failing that, download a copy of Relux - it's free, has access to a massive database of manufacturers data and is easy to use

Regards

Barney
- CommentAuthorCWatters
- CommentTimeJun 29th 2012
I bet he's sorry he asked :-)
- CommentAuthorSteamyTea
- CommentTimeJun 29th 2012
Colin, was it you who fitted some LED downlighters in your kitchen and worked out what the wattage was? Where is that thread?
To jog your memory I said that my lighting was adequate and of lower wattage.
- CommentAuthorCWatters
- CommentTimeJun 29th 2012 edited
Yes I've done my kitchen with an array of LED downlights. Thread here..

http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=7769&page=1

The prices seem to have come down since that thread was started. Looks like about 20-30% since last year.
- CommentAuthorowlman
- CommentTimeJun 30th 2012
Posted By: tony...............bottom line is dont use recessed downlighters.

Why?
- CommentAuthorSteamyTea
- CommentTimeJun 30th 2012 edited
There is a bit in my favourite comic about lighting this week. More about LEDs downside than anythign else.
http://www.newscientist.com/article/mg21428710.300-better-than-sunshine-see-life-in-an-improved-light.html
- CommentAuthortony
- CommentTimeJun 30th 2012
Downlighters give very different type of light -- it always seems a lot darker than the lumens suggest, also often cause air leakage problems, high electricity costs if tungsten and high bulb costs too.
- CommentAuthorCWatters
- CommentTimeJun 30th 2012
I've recently put a GU10 LED "downlight" bulb into an adaptor and plugged it into a regular pendant light socket (downstairs loo). The lamp shade is kinda redundant as the beam angle is only 180 compared to nearer 360 for a regular bulb but we've got used to any differences.
- CommentAuthortony
- CommentTimeJul 1st 2012
There was a difference then, from the same power of bulb?
- CommentAuthorTimSmall
- CommentTimeJul 1st 2012
Posted By tony: bottom line is dont use recessed downlighters.

Posted By owlman: Why?

Cos they look really terrible, and will soon be the equivalent of the avocado bathroom suite? Only joking.

Actually, a down-lighter fitting is a really bad place to put an LED, because it keeps them too warm - they then:

1. Aren't as bright from day 1 (efficiency drops as temperature rises).
2. "Wear out" a lot quicker than they would be if they were kept cool in a better-suited fitting.

e.g. check out the first and second graphs in this article:

http://www.designworldonline.com/lighting-the-way-for-led-development/
- CommentAuthorowlman
- CommentTimeJul 1st 2012
Posted By: TimSmall.......................Cos they look really terrible,

That's a very subjective view. and many would put form before function. The idea of a single bulb light source hanging from the centre of a room is, from a design point of view, a retrograde step, however environmental it may be, and doesn't suit all homes.

Posted By: tony,........ also often cause air leakage problems,

Not a problem in most homes I would suggest.

............and high bulb costs too.

I've got 5 MR16 downlight fittings in my kitchen which are subjected to more use, and "on and off" cycles than any other room in the house. I think I've only replaced 2 maybe 3 of the Osram lamps once in the last 13 years. the other two are still going. efficient when compared to LED not, but cheap, yes. The single toroidal transformer is still OK too.
- CommentAuthorCWatters
- CommentTimeJul 1st 2012
Posted By: tonyThere was a difference then, from the same power of bulb?

Yes. Previous bulb was a spiral CFL. The shade blocked some of the light but with the CFL there was still some light bounced off the ceiling. With the 180 degree LED the top of the room is darker. But heck it's a loo. Don't want to make it too cozy.
- CommentAuthorSteamyTea
- CommentTimeJul 2nd 2012
Nothing like having a spot light aimed at you when you are thrutching is there
- CommentAuthordanbates
- CommentTimeJul 6th 2012
Thanks for the replies!
Second line of the first comment - "bottom line is dont use recessed downlighters." genius ;)

Seems like the 180degrees rule of thumb (effectively doubling the lumens from the lamp?) and inverse square law are the most useful starting points.. Would all depend on the light fitting though wouldn't it.. A few factors I can think of..
Wattage
Luminous Efficiency
Colour Temp
Colour Rendering Index
Colour
Beam angle
Distance to incident surface
Intended effect..
Product of the the LL (total lamp lumens), UtilisationFactor (headache), MaintenanceFactor (roughly 0.8) divided by the distance squared seems like a good approximation..
There's probably a million and one things inside the bulb, housing, shade and room to bounce the light all over the place!
- CommentAuthorCWatters
- CommentTimeJul 7th 2012
Posted By: danbatesSeems like the 180degrees rule of thumb (effectively doubling the lumens from the lamp?)

No. It's best to think of changing the beam angle as focusing the light emitted onto a different size spot.
- CommentAuthorsnyggapa
- CommentTimeJul 7th 2012
I had to use downlighters pretty much everywhere in my new build (ceiling heights less than 2m) apart from one room that has a vaulted ceiling where I have LED tape running over the top of the oak beams and illuminating the ceiling, which looks fantastic.

Wary of the overheating problem, I got a about 35 of these downlight cages : http://www.icage.co.uk/

they're open frame so air can move about in the ceiling void to aid cooling the LEDs but have an intumescent ring that closes the void off in event of fire.

managed to get an air test result of 3.65 m3/h/m2 which considering the windows and plug sockets leaked most of that wasn't a bad result

-Steve

P.S. first principles for me involved lashing a load of lights up on temporary wires and asking the misses if it was adequate. any other method is doomed to fail
- CommentAuthorSteamyTea
- CommentTimeJul 7th 2012 edited
Posted By: snyggapaP.S. first principles for me involved lashing a load of lights up on temporary wires and asking the misses if it was adequate. any other method is doomed to fail

Not a bad way to do it.
I think working from 1st Principles involves two lots of calculations.
Pick a light source and work out what it does (that inverse square law from point source, integrate for angle).
Then change that source to different types and then work out the lumen's per watt.
Make a list and compare.
- CommentAuthorManninwood
- CommentTimeJul 7th 2012 edited
I just got some top info from this forum, so I hope this is my chance to re-seed.

This may be of use, or may be entirely useless for what you're trying to do....

A very good friend of mine works for a practice that has developed their own Daylight Analysis plug-in for sketch up. Even if you're not using sketchup you should be able to knock up a basic model pretty quick, and locate it using google earth, giving fairly accurate sunlight plotting info. Fairly easy to model your lighting too. So far as I'm aware this plug-in has been developed for use with BIM.

http://manascisaac.com/our_work/our_research

Hope it's of some use.

J.
- CommentAuthordjh
- CommentTimeJul 9th 2012 edited
Posted By: SteamyTeaNothing like having a spot light aimed at you when you are thrutching is there

I find having a cat patiently staring at me as it sits on the end of the bath waiting for me to finish and wash my hands so it can then lick the tap (and the bowl if it's given a chance) has a greater effect than a spotlight.
- CommentAuthorDamonHD
- CommentTimeJul 9th 2012
TMI
- CommentAuthorSteamyTea
- CommentTimeJul 9th 2012
Remember Cat in Red Dwarf, the show when everything was backwards sorts a cat out

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