Howdy.
I’ve found several old threads on this subject, but I’ve got a question that remains unanswered.
I am about finish off my 20x20 garage and set it up for auto detailing (part-time hobby, not professional), and I’m trying to come up with the best location for the lights.
For that application, you have essentially the same issues in play as would be the case if you were painting, if perhaps not to the same degree. You need to light up the WHOLE car, including areas which are normally mostly ignored, or deep in shadow, such as down by the rocker panels. The main difference is, you can actually benefit from light sources which are stronger in one direction than others, to aid in seeing streaks, swirl marks, etc., by checking out the reflections of the light sources in the finish.
There are normally two cars parked in there, but for detailing purpose, I’d work on one in the center. (Note that wall lights are not possible.)
I'll take your word for it that wall-mounting is a non-starter; but that remains something of a shame, as it WOULD potentially help. As a "poor man's substitute", you might consider some sort of cart- or stand-mounted portable lighting. I'd probably prefer a home-brew solution based on the same fluorescent tubes I was using for the permanently-installed main lighting (in part to maintain color consistency); but in a pinch, something like this:
http://www.lowes.com/pd_394366-40642-UT-1005_4294935637__?productId=3699574
could go a long way toward solving the side-illumination problem, for only about $35. One of the neater features is that the entire top section (with it's own handle & feet) can be quickly disengaged from the tripod/stand via one hand-knob, and then placed on the floor for low-angle illumination. Kind of a "best of both worlds" thing. But, as always, high-output halogen bulbs **** up a TON of power and get REALLY hot; so it remains only an "if nothing else" solution.
I presume I’ll be using 4’ fluorescent fixtures.
That's not an unreasonable choice, at least at first blush. But be aware that fluorescent tubes in general do a pretty poor job of providing FULL SPECTRUM light, regardless of any "high CRI" claims which may be bandied about. So if any of your work is really color-critical, an alternate solution would be worth pursuing.
My gut feeling is to put them long-ways, parallel to and over each of the two cars – that way, when only one car is in the center, I’d have decent light on both sides.
That (or at least "mostly" that) will work
IF "single car in the middle" is the ONLY way you will use the space (at least when working). But it puts most/all of the light directly over the center of the vehicles when you have two cars parked in there, which in that case is the worst possible location -- how often do you need to brightly illuminate the roofs of your cars?
Also, with the two main fore/aft runs located that far out into the space, they won't do a very stellar job of illuminating whatever shelving, cabinets, tool chests, or other storage you might have located on or adjacent to the side walls. Typically, you want those runs perhaps 2-3 feet (4 at the most) off the walls, at least presuming "normal"-height ceilings.
In any event, if you do take the approach you described, don't forget to ALSO put a few fixtures running the other way, particularly at/near the ends of the long-ways runs. The idea is to completely "ring" the car with light, if possible. Also, given the detailing application, you WILL want to be able to see the roof at least somewhat well; so one or two more fixtures in the middle of the space won't hurt.
Do you do any other sort of work in that space, besides detailing? That too can influence things.
Also, I’m not sure how many lights I really need.
In practice (and again assuming "normal" ceiling heights of perhaps 9-12 feet), this will be dictated more by getting the lights everywhere you need them to provide good even illumination, than by any simplistic "lumens/ft.^2" calculation. By the time you accomplish the former, the latter will take care of itself.
That said, at 400 ft.^2 of total shop area, and targeting 100 lumens/ft.^2 at working height, you'll want a minimum of 16-18 F32T8 tubes. Having more won't hurt, and may well be needed in order to fill out the lighting PATTERN you want; but especially if you go WAY beyond that figure, setting up the switching to provide variable-intensity illumination becomes even more important.
).