Getting enough light in a shop

It's Hard

When you're working, It's hard to every get enough light. The recommended light levels of the Illuminating Engineering Society are a straight forward delineations of what it takes to get productive lighting. So I've covered how to calculate how bright work lights have to be in other sections of this website.

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Unfortunately, IES standards are rarely met for a number of reasons:

1. Battery Powered Lights Can't Store Enough Energy You can't violate the laws of physics. For instance if you tried to run our Super Bright 11 way work light off of a car battery, you'd have to recharge the battery every 3 hours!

2. Energy Conservation Codes limit the amount of general illumination that can be installed and tacitly presume that occupants will supplement overhead lighting with work lights or dedicated task lights.

3. Light Levels have been reduced to save Energy. Most newer light fixtures produce less light than the fixtures they are replacing.

Furthermore, light spectrums that are closer to natural daylight will normally appear to be the brightest. That is lumens of daylight appear brighter than lumens of cool white or warm white light. This is known as the Perkinje Effect and is covered in the article, Daylight is Best for Work Lighting.

Using the Rules and Formulas Given- Optional Technical Paper

So to help you judge how much light you need, I've given you formulas in the website. So in the interest of completeness, I am using this page as an "appendix" to let you know where these formulas come from and how you can change them if needed. This is an optional, technical presentation.

High Intensity Task and Work Bench Lighting Formulas

If you spread 1 lumen of light over 1 square foot, you will get 1 foot-candle of illumination. Each lumen of light coming from a work light has to cover a larger and larger area as it leaves the light fixture. The simple formula I gave you idealizes a light source as a point source that emits light in either spherical, hemispherical, or conic shells. Therefore, if you can concentrate light in a cone, the foot-candles will be higher.

If a light fixture is long, it will actually spread light underneath it in the directions of it's length. But assuming light is coming from a point is reasonably accurate unless a long light is really close to the work piece. You get the most illumination on a surface if the light rays from a work light directly strike the work piece at a normal, 90 degree angle. If the light rays only make a glancing strike at a low angle, the illumination will be higher. The formula does not take this into account, but if your work piece is no more than 30 degrees off to the side from the light, the error is less than 13%.

The light coming off of a light will bounce around a room several times and may by chance land back on a work piece. This isn't likely to happen in a large room. So I ignored this indirect lighting effect because most of the indirect light in a typical room comes from the overhead lights anyway, so the indirect lighting from the work light is ignored.

In the sections on temporary area light and shop lighting and , both the direct and indirect lighting is calculated.

Temporary Area Lighting including Illumination of Vertical Surfaces.

For painting and many construction tasks, you need all of the walls, ceiling, and floor illuminated. So I give you a formula for the average illumination on all of the surfaces of a room. Since this is an average you will get more illumination closer to the work light than further away.

Average illumination= M x Fixture Lumens ➗ Surface area of Room in Square Feet

M is a function of the average reflectance of a room given by the following table:

Average Room Reflectance M

.1 1.1

.2 1.23

.3 1.42

.4 1.66

.5 2

.6 2.5

.7 3.3

The Average room reflectance is given by:

Average room reflectance = (Rw x Wall Area + Rc x Ceiling Area + Rf x Floor Area)➗

Surface Area of Room

Rw, Rc, and Rf is the reflectivity of the walls, ceiling and floor respectively

The reflectivity of ordinary white paint or light colored paint is .5; for bright white paint, .8; for dark paint, .1; for wood and concrete, .2. The reflectivity of an unsheathed 2x4 wall is almost zero, about .02, ie most of the light escapes the room.

Shop Lighting

Overall Light Levels in a shop are normally calculated on an average basis for a horizontal plane at waist level. I've given you formulas to use with our high output LED shop lights.

The general formula for any shop light is

Lumens Required= Foot Candles Desired x Floor Area ➗ Fixture Coefficient of Utilization

The "Coefficient of Utilization" comes from a table for each kind of light fixture. You read the coefficient from a table based upon wall and ceiling reflectivity, and a ratio called "RCR". The "RCR" (Room Cavity Ratio) is equal to 2.5 x Fixture height above work plane x Room Perimeter ➗ Room Area.

The RCR compares the wall area to the floor area. The smaller the relative wall area, the easier it will be to light up a horizontal plane above the floor. Here is a table that you can use: