Shop lighting

[cecil dean]

How do you calculate the (fc) foot-candles power of a light? I have read that at my age I should have at least 80-100 fc but can not find how you measure the fc output of a light.
Thanks
Cecil

[David Parker]

See this discussion: http://www.sawmillcreek.org/showthread.php?t=77936

Particularly the posts by Jack Lindsey, who wrote a nice article on the topic in FWW.

I have 5 fixtures in my 12' by 15' shop. Each fixture holds four 4' T8 tubes. I find the lighting to be about perfect. Not too bright, not too dim. I wanted to use 6 fixtures, but overhead ductwork prohibited the 6th one.

[Robert Eiffert]

If you have a camera that allows settings and shows meter readings, you can use this process:

Set the ISO=320 and the aperture to 5.6 and point camera at white paper in the area you want to measure (top of workbench). The shutter speed is pretty close to foot candles; i.e. 1/60 = 60 foot candles.

Meters can be off a bit, white paper has variations in reflectivity, bulbs may change brightness through their lives, reflectors get dirty......

[Jack Lindsey]

Cecil,
You can approximate the average footcandle level in your shop by the following method:

1. Multiply the lumen output rating of the lamp by the total number of lamps in your shop. You probably don't have the rating info but if you use the following it will be close enough: 4' lamp use 2800; 8' 60 watt lamp use 5800; 75 watt lamp use 6300
Example: assume thirty 4' lamps

30 x 2800 = 84000

2. Divide the answer from step 1 by 2: 84000/2 = 42000

3. Divide the answer from step 2 by the square footage of the lighted area. Assume a 500 sf shop
Example: 42000/500=84 footcandles.

This isn't precise but it's close enough.

You could also obtain a light meter and measure the illuminance at a few locations in the shop. A photographic meter won't work, though, since it lacks both color and cosine correction. Color correction accounts for the sensitivity of the human eye to variations in color, and cosine correction compensates for the angles at which light enters the meter.

[Wilbur Pan]

Or, if you want to figure out your lighting requirements in the opposite direction, here's what I did.

1. My goal was 75-100 foot-candles of light at my workspace. My shop is 10'x20', or 200 square feet.
2. Multiply 75-100 foot-candles by 200 square feet gives me 15000-20000 lumens of light total in my shop.
3. Throw in some fudge factors to account for the distance from the fixture to the work surface and loss of efficiency over time due to dust and aging bulbs, and that essentially doubles to 30000-40000 lumens.
4. I wanted to use 4' "Natural Daylight" T-8 tubes which put out about 2200 lumens apiece, this means I'll need 13-19 fluorescent tubes in my shop, which works out to about 6-10 2 bulb fixtures.

So, my plan is to put in a total of 8 4' 2-bulb fluorescent fixtures in my 10' x 20' shop.

As it turned out, I could only install 5 fixtures, due to low ceilings, and the way that ducts and plumbing ran across the ceiling of my basement workshop. I wish I had room for three more, so this method seems like a really good predictor of your lighting requirements.

[William Hutchinson]

Once you calculate your main lighting requirements, you may wish to consider installing a circuit of low level lighting. It's for those times when you don't need the full blast of T8's. Two sets at each end of my small (24x14) shop did the trick.

http://i3.photobucket.com/albums/y66/wlhutch/shopceiling1.jpg http://i3.photobucket.com/albums/y66/wlhutch/shopceiling-1.jpg