Basic  Principles of Light and luminance

It can be daunting trying to figure out what all the different terms of lighting mean
and how to use them to devise a lighting plan. Here are some common terms and
their meanings.

Beam Angle or spread - This is the shape of the light emitted from a bulb with
reflective properties. The beam expressed in the form of an angle measurement
can be wide, normal or narrow.

Color Rendering Index (CRI) - This is a scale of 1 to 100 to determine how the
light will show the color of an object. With 100 being sunlight, that is the reference
point. An object will appear as the color it should to the human eye. A lower CRI
number will distort the color of an object.

Color Temperature - This is used to measure the color appearance of light. It is
measured in units called Kelvin or K. Light sources below 3200K are considered
warm and have reddish overtones. 4000K and above are considered cool and
have bluish overtones. For reference a normal home will be in the 3000K area
while Offices and retail establishments will be in the 4000K area. 5000k is reserved
for such areas as an operating room or jewelry store.

Compact Fluorescent (CFL) - This is a term used for fluorescent bulbs that are
made to take the place of incandescent lamps and are manufactured in shapes
and sizes to accomplish this task. They outlast an incandescent about 10 times and
use energy much more efficiently.

Foot candle - This is was originally the unit of measurement based on how much
light will reach the surface of an object one foot away from a candle. It is now
considered equivalent to a lumen which is the illumination of one square foot.

Work Plane - The work plane is considered to be an area about 30" off the floor.
Lighting this plane should be your goal as this is where the majority of tasks are

I know you can read all these definitions and many more on the Internet, and still
be confused. When it comes to recessed lighting which is when the layout is usually
in question, it's all about spacing. Once you can visualize what pattern the light
form you are using has, you can then accurately lay out the lighting properly.

The three factors that are most important when designing a lighting plan that works
are the type of light, the color of the light and the spacing. The type of light fixture
will determine the light pattern. Once a pattern is established the spacing can be
figured out. Several factors can help when choosing the color of the light.
Fluorescents tend to be colder or emit more of a blue light. Today there are some
very good full spectrum bulbs that make fluorescent light a little warmer.
Incandescent are the friendliest of all the colors and provide a welcoming and warm
feeling. They do however, cast a yellow tone over everything in the room. In some
cases this is a good thing. In other situations this may not be the best solution.
Some wood cabinets, such as a bleached birch, tend to look yellow under
incandescent light. Using a halogen bulb in these cases solves the problem.  Below
is a basic room lighting plan to give you ideas.
Discover how
helps you
shape a home
environment to
support your
Design with
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This is a 12 x 12 room, a typical bedroom. By
placing these lights about 3 feet out from the
corners we get a nice even distribution of
light in the room. Using this plan we are also
able to add a ceiling fan in the room with no
adverse effects on the lighting plan
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