[zuihoujueding]

ops so sorry, think I've put up this question at the engineering site. Sorry, couldn't find this (electrical and lighting) site until I put it up.

oh well, I have questions:

1) regarding the lighting design for an office. If I have decided that I be using a 2x36W fluorescent fixtures as general lighting for the office, how many and how do I determine the spacing between each other so there will be no dark spots?

I roughly have calculated the numbers of fixtures I need from formulas from books. But can't get the spacing right...

2) how do I calculate the illuminance which will land of the work desk to make sure it satisfy the recommended illuminance stated in the standards?

somebody help...

[tkrussell]

First you need the photometric specifications of the fixture. In this data you will find a value called Spacing Criteria. Each side of a fixture is given a multipier that is applied to the height of the fixture above a task level.

This spacing value is determined by testing done by the manufacturer and is affected by several characteristics of the fixture,such as, the reflectance of the interior, the type of lens,the orientation of the lamps inside, the type of lamp the fixtures holds.

Say for example, your fixture is a typical 2x4, you can imagine that more light is available from the 4 foot side or long way, than the short 2 ft side.

So a typical SC for the 4 ft is 1.43 and the 2 ft is 1.25.

Even thou your fixture may be a 2 x 2, both sides may not have the same value as you might expect, because of the linear lamp, more light will be at the length of the lamp side, rather than the ends of the lamp.

Next you need to measure the distance from task level, usually 30 inches, for the height of a desk. Say, you have an 8 ft ceiling, minus the 2.5 ft, equals 5.5 feet above task level.

So,
Spacing = SC x Hgt above task
Spacing between short side = 1.25 x 5.5Ft=6.875 ft center to center of fixture.
Spacing between long side = 1.43 x 5.5Ft=7.865 ft center to center of fixture.

Center your pattern in the room, and splitting the difference to have equal spaces along the walls.

Hope this helps.

[tkrussell]

Opps, I answered the first question and posted before going back to look at the second one.

To calculate the amount of light on a surface is much more involved, again needing the photometric data of the fixture, and the room size. The method found at:

http://www.ndlight.com.au/lighting_calculations.htm

And is called the Room Cavity Ratio, and is generally good for basic lighting calcs, without getting into the really deep calculations.

I am glad to find these already published for use.

By the way, a general office that average tasks are performed, with normal colors of floor,ceiling and walls, the amount of light that is an average recommend is 50-80 footcandles (5-8 Lux in metric).

Hmmm, did I just do someone's homework?