LIGHT OUTPUT TERMINOLOGY

The most common measure of light output is the lumen (luminous flux). Light sources (lamps) come labeled with an output rating measured in lumens. For example, a T12 40-watt fluorescent tube typically has a rating of 3050 lumens. Also, the output of an entire light fixture can be expressed in lumens. As lamps and fixtures age, their lumen output decreases; that is, the amount of light they put out goes down. This is called lumen depreciation. Most lamp ratings are based on initial lumens – the output of new lamps.

LIGHT LEVELS

The intensity of light is called illuminance, and is measured in foot-candles (lumens per square foot). Footcandles are measured with standard light meters. Hold them on the work surface where tasks are performed, and keep the meter away from your body or other reflective surfaces. For planning purposes, use a manufacturer’s photometric data to reliably predict foot-candle levels in a defined area.

ELECTRICITY-TO-LIGHT EFFICIENCY

Different types of light production are more or less efficient in converting energy into visible light than others. This is referred to as the efficacy of the light source, which compares the number of lumens leaving the lamp versus the number of watts required. Efficacy is measured in lumens per watt. For example, H10 lamps put out far more lumens per watt than incandescent lamps, and thus have a higher efficacy. 1-2

LIGHT COLOR TEMPERATURE

This is a measurement of “warmth” or “coolness” provided by the lamp. Color temperature is expressed in degrees Kelvin. For example, an electric stove element changes in color as its temperature increases; first red, then orange, yellow, and finally bluish white. A “warm” light source refers to a more yellow or orange tint of light, even though it has a lower color temperature. Likewise, a “cooler” light source has more of a bluish tint, even though the color temperature is higher. For example, a cool-white fluorescent lamp appears bluish in color with a color temperature of around 4100 K. A warmer fluorescent lamp appears more yellowish with a color temperature around 3000 K.

COLOR RENDERING INDEX

Color rendering index (CRI) is a relative scale, ranging from 0-100, indicating how well perceived colors match actual colors. It measures the degree that perceived colors of objects, illuminated by a given light source, conform to the colors of those same objects when they are lighted by a reference standard light source. The higher the color rendering index, the less color shift or distortion occurs. The CRI number does not indicate which colors will shift or by how much; it is rather an indication of the average shift of eight standard colors.Two different light sources may have identical CRI values, but colors may appear quite different under these two sources. So, the CRI is a useful guideline, but is not a perfect scientific indicator. CRIs in the range of 75-100 are considered excellent, while 65-75 are good. The range of 55-65 is fair, and 0-55 is poor. Under higher CRI sources, surface colors appear brighter, creating the illusion of higher illuminance levels. DETERMINING TARGET LIGHT LEVELS The Illuminating Engineering Society of North America (often called the IES) publishes recommended light levels which include the following factors:

The task(s) being performed (contrast, size, etc.).1-3

The ages of the occupants.

The importance of speed and accuracy.

Once the lighting level has been determined, you can select the best combination of lamps and fixtures based on the following criteria:

Fixture efficiency.

Lamp lumen output.

The reflectance of surrounding surfaces.

The effects of light losses from lamp lumen depreciation and dirt accumulation.

Room size, shape and availability of natural light.

When you design a lighting system (new or upgraded), you must be careful to avoid overlighting a space. This wastes energy and may not provide any better results. Years ago, some offices were designed for 100 footcandles or more, where 50 foot-candles were adequate or even superior. For example, to light a space that uses computers, the overhead fixtures should provide up to 30 fc (foot-candles) of ambient lighting. Task lights should provide the additional foot-candles needed to achieve a total illuminance of up to 50 fc for reading and writing. (Task lights are small, direction light fixtures such as reading lamps and under-cabinet fluorescent strips.)

LIGHTING QUALITY

In addition to the color rendering index, there are other important ways to measure lighting quality. Visual comfort probability (VCP) indicates the percent of people who are comfortable with the glare from a fixture. Spacing criteria (SC) refers to the maximum recommended distance between fixtures to ensure uniformity of light and to avoid glare.

LIGHT SOURCES

Selecting the appropriate source depends on installation requirements, life-cycle cost, color qualities, dimming capability and the effect wanted. Electric light sources have three characteristics: efficacy, color temperature and color rendering index.