85 percent of our perceptions come from our eyes

Brightness and Color of Light
How good or poor a given light appears can be described in three basic ways: how much light is present, the quality of the light (is it bright or dull or does it produce glare?) and what its color characteristics are.

Watts and Lumens: How much light?
Light output, measured in lumens, refers to how much light leaves a light source. Because we are so used to incandescent bulbs, we usually describe the amount of light with watts. The number of watts is actually the amount of power the bulb uses. For an incandescent bulb this can be a misleading number because 90 percent of the watts is wasted as heat instead of making light.

To compare brightness between incandescent bulbs and the more efficient compact fluorescents, the quantity of light is expressed in lumens as well as wattage. As with watts, a higher number of lumens means a stronger light. The chart shows lumen levels to look for when replacing a given bulb. Note also the cost and CO2 savings CF bulbs bring.

Incandescent CFL        Lumens  Cost Savings    Cost Savings   CO2 Saving
                                                        ($.10/kWh)   ($.20/kWh)
40W         11-14W >       490        $39-$44        $78-$88         507-572 lbs.
60W         15-19W >       900        $62-$68       $124-$136      806-884 lbs.
75W          20-25W >      1,200     $76-$83       $152-$166      988-1,079 lbs.
100W       26-29W >      1,750      $107-$112    $214-$224      1,391-1,456 lbs
150W       38-42W >      2,600     $163-$169   $326-$338      2,119-2,197 lbs.
Calculations for cost and CO2 savings assume 15,000-hour life for CFL.
If you don't want to print this page out, remember this rule of thumb: CFLs use about a quarter of the wattage to produce the same light. So to replace a traditional 60-watt bulb, look for a CFL that's about 15 watts.

Lumen output deteriorates over time from a number of factors, including the ballast's electronics, variations in supply voltage, dust and dirt.

Color
How does a light render an object's colors, and what color is the light itself? These characteristics are expressed through the Correlated Color temperature (CCT) and Color Rendering Index (CRI).

Correlated color temperature (CCT)
Light has color, whether it emanates from the moon, the sun or your favorite reading light. It might tend toward orange (think of parking lot lights illuminating the night), yellow (most reading lights) or white (daylight).

Imagine heating a piece of metal gradually. As it heats up, it changes color. At first, it appears orange, then yellow and then blue or blue-white. The Correlated Color Temperature indicates how warm or cool light is. For a light color close to incandescent bulbs, look for a CCT range between 2650 and 2800 degrees Kelvin. This range is referred to as warm white. Common CF bulb colors are 2700K, 3000K, 3500K and 4100K. (The Kelvin scale and its units refer to absolute temperature; 0 degrees Kelvin refers to the point at which no heat energy remains in a substance.)

CCT measurements:

2650-3200K – warm white (yellowish-white)
3200-4000K – neutral
Above 4000K– cool (bluish-white), also called "daylight"
Color Rendering Index (CRI)
This is a numeric representation of a light bulb's ability to show colors "realistically," compared to a standard incandescent. The CRI scale ranges from 0 to 100—the higher the number, the closer to what you're used to. For good color quality, look for compact fluorescents with CRIs of 80 or above.

Light Technology Basics
About 85 percent of our perceptions come from our eyes, so lighting is important to what we do whether we're aware of it or not. Light can be produced naturally (like sunlight) or artificially.

To produce artificial light, electrical energy is transformed into light. This generally occurs in one of two ways:

Incandescence, in which current passes through a filament, which heats and then glows, or
Gas discharge, in which current passes through a gas. The atoms glow, giving off ultraviolet light, which reacts with phosphor to produce visible light.
Energy-saving compact fluorescent light bulbs produce light using gas discharge.

Did you know? The average American household has 28 light sockets, and about 10-20 percent of your electricity bill is from lighting.

Sources


"Light Guides" at www.lightsearch.com/resources/lightguides/index.html.

"Light Color Characteristics": www.sylvania.com/LearnLighting/LightAndColor/LightColorCharacteristicf/

"High Color Rendering Bulbs": www.topbulb.com/find/cri.asp

"High Color Temperature ('Daylight') Bulbs": www.topbulb.com/find/colortemp.asp