How far can fluoresc lights be from floor w/o losing effectiveness?

Question

We have an old bookstore with a 16 foot ceiling in a 40x68' room. We now have 6 1x8' flourescents with 2 lamps in each. The fixtures are hanging 5 feet from the ceiling. We are thinking of adding new fixtures and mounting them on the ceiling.
How many fixtures would do the job?

Answer 1

it depends on the lights' power.

Answer 2

The way to approach this is to think in terms of watts of lighting per square foot of floor illuminated.

Lights that are way up high will illuminate many square feet. But if the ceiling has lights installed at intervals, the lighting will be the same whether a high ceiling or a low ceiling.

It also depends what is meant by effectiveness. Some situations require dim levels of lighting and some require bright levels.

You don't provide any numbers so there can be no answers with numbers.

You could go to a store with flourescent lighting and count the light fixures in an area of the store, and count floor tiles to get square feet. That will give you an idea. (All eight-foot tubes will give the same illumination)

Answer 3

A point source of light (light bulb?) loses effectiveness with distance in accordance with the inverse square law. A flourescent light fixture (usually two flourescent tubes to reduce stroboscopic effects?) is like two lines of points and loses effectiveness less. A wall (ceiling?) of light would avoid the inverse square law to an extreme. Therefore a flourescent light fixture likely performs somewhere between a light bulb and a wall of light. A reflector also helps throw more light. Hope that helps a little.