Re: how do you figure lighting coverage
What you need to do is a simple calculation. Well, simple to some anyway. The basic calculation to determine the number of light fixtures needed in a room is called the Zonal Cavity Method. You need the following information:
Room Width - W, Room Length - L, Room Area - A, Ceiling Height - CH, Room Cavity - RC, Room Cavity Ratio - RCR, Lumens per Lamp- Lu, Coefficient of Utilization - CU, Lamps per Fixture - LPF, Maintenance Factor - MF.
Calculate the Room Cavity First. It is the difference between the height the fixture is mounted in the room and Horizontal plane in the room at which you want to calculate your footcandles. Usually we want to calculate the number of footcandles at a point 2.5' above the finished floor. That is where we do most of our work at desks, tables, counter tops, etc.,. So, if your fixture is mounted directly on the ceiling or recessed in the ceiling just subtract 2.5' from the ceiling hieght to get RC. If you have a fixture that is suspened 12" below the ceiling then you subtract 12" from this number. In other words instead of subtracting 2.5' from the ceiling height you decrease the ceiling height by 12" and then subtract 2.5'.
Calculate your Room Cavity Ratio:
RCR = (5 x RC x L x W)/(L x W)
With this done, you now need a spec sheet of the fixture that gives you what is called Coefficients of Utilization. This may be a little confusing but light fixtures have a chart on their spec sheets for CU's. The RCR is listed in a column on the far left of the chart. Choose a number close to the one you calculated above and come across and find your CU. Now, you will notice at the top of the chart that it is divided into sections that depend on what Ceiling Reflectance you are using. I would always use 70% to be conservative so come all the way across to the section of the chart that is for 70% ceiling reflectance. You will then also notice that each Ceiling Reflectance section is divided according Wall Reflectance. Use the 50% column for that. Now you have your CU.
Finally, here is the formula for how many fixtures you need in the room:
No. of Fixtures = (A x FC)/(LPF x Lu x CU x .8)
FC is the footcandle level you desire to have in the room. The last value that I used (.8) is a maintenance factor. What this does is put in a factor that makes allowance for fixtures getting dirty, lamp output deteriorating over time and other factors. You can make it smaller but .8 is good enough for our purposses.
If you search around you can probably find some free software that will do most of the work for you. Maybe others can suggest a source.