Characteristics of Common Light Sources

LIGHTING FACTS
The quantity and quality of light around us determine how well we see, work, and play. Light affects our health, safety, morale, comfort, and productivity. Lighting also directly affects our economy. As a nation, we spend about one-quarter of our electricity budget on lighting, or more than $37 billion annually. Yet much of this expense is unnecessary. Technologies developed during the past 10 years can help us cut lighting costs 30% to 75% while enhancing lighting quality and reducing environmental impacts.

EFFICACY This is the ratio of light output from a lamp to the electric power it consumes and is measured in lumens per watt (LPW). Note the differences in the LPW in the table above

Illumination A 'lumen' is a measurement of light output from a lamp, often called a tube, a bulb or more accurately as a lamp. The distribution of light on a horizontal surface is called its illumination. Illumination is measured in footcandles. A footcandle of illumination is a lumen of light distributed over a 1-square-foot area. The amount of illumination required varies according to the difficulty of a visual task.

Light Quality Light quality describes how well people in a lighted space can see to do visual tasks and how visually comfortable they feel in that space. Light quality is important to energy efficiency because spaces with higher quality lighting need less illumination. High-quality lighting is fairly uniform in brightness and has no glare. For example, direct intense sunlight streaming through the windows of a room with chocolate brown carpets and dark wall paneling will likely give too much contrast in brightness. The pupils of your eyes will constantly adjust to the differing brightnesses. Making this area visually comfortable would involve using lots of artificial lighting with a high illumination level. On the other hand, in a pale-colored room bathed in soft light, you can hardly tell where the light is coming from because no one area of the room appears much brighter than another. The walls, ceiling, floor, and work surfaces are relatively the same light hue. People can perform tasks faster and with fewer mistakes with this type of high-quality lighting. Also, lighting such a room requires far less artificial lighting than the previous example.

Glare Eliminating glare (i.e., excessive brightness from a direct light source) is essential to achieving good lighting quality. Types of glare include direct glare, reflected glare, and veiling reflections. Direct glare results from strong light from windows or bright lamps shining directly into your eyes. Reflected glare is caused by strong light from windows or lamps that is reflected off a shiny surface into your eyes. Veiling reflection is a special type of reflected glare that can obscure contrasts and reduce task clarity. Veiling reflections occur when light is reflected into your eyes from a work surface, such as a printed page or a computer screen.

Light Color and Color Rendering Lamps are assigned a color temperature (according to the Kelvin temperature scale) based on their "coolness" or "warmness." The human eye perceives colors as cool if they are at the blue-green end of the color spectrum, and warm if they are at the red end of the spectrum. Cool light is preferred for visual tasks because it produces higher contrast than warm light. Contrast is the brightness difference between different parts of the visual field, which is the expanse of space you can see at a given instant without moving your eyes. Warm light is preferred for living spaces because it is more flattering to skin tones and clothing.

Keep in mind, though, that artificial light sources vary widely in their color rendering indexes (CRI). The CRI is a measurement of a light source's ability to render colors the same as sunlight does. For example, incandescent lamps are rated at a CRI of 100�nearly equal to sunlight�while some high-pressure sodium lamps have a CRI of 22, which means they render colors very poorly. However, a light's color-rendering ability is not related to whether it is a cool or warm color. For example, blue light from the northern sky, white light at noon, and red light from a sunset all have perfect color rendering (a CRI of 100) because our eyes are designed to read the colors of objects illuminated by sunlight.

TYPES OF LIGHTING

Incandescent Incandescent lamps are the least expensive to buy but the most expensive to operate. Incandescent light is produced by a tiny coil of tungsten wire that glows when it is heated by an electrical current. Incandescent lamps have the shortest lives of the common lighting types. They are also relatively inefficient compared with other lighting types. However, significant energy and cost savings are possible if you select the right incandescent lamp for the right job. The three most common types of incandescent lights are standard incandescent, tungsten halogen, and reflector lamps.

Fluorescent The light produced by a fluorescent tube is caused by an electric current conducted through mercury and inert gases. Fluorescent lighting is used mainly indoors�both for ambient and task lighting�and is about 3 to 4 times as efficient as incandescent lighting. Fluorescent lamps last about 10 times longer than incandescents. But, to gain the most efficiency, you should install fluorescents in places where they will be on for several hours at a time.

High-Intensity Discharge
High-intensity discharge (HID) lamps provide the highest efficacy and longest service life of any lighting type. They are commonly used for outdoor lighting, in large indoor arenas , supermarkets and 'big box' retail stores. HID lamps use an electric arc to produce intense light. They also require ballasts, and they take a few seconds to produce light when first turned on because the ballast needs time to establish the electric arc. The three most common types of HID lamps are mercury vapor, metal halide, and high-pressure sodium. HID lamps and fixtures can save 75% to 90% of lighting energy when they replace incandescent lamps and fixtures. Significant energy savings are also possible by replacing old mercury vapor lamps with newer metal halide or high-pressure sodium lamps.

Mercury vapor�the oldest type of HID lighting�is used primarily for street lighting. Mercury vapor lamps provide about 50 lumens per watt. They cast a very COOL BLUE / GREEN WHITE LIGHT. Most indoor mercury vapor lighting in arenas and gymnasiums has been replaced by metal halide lighting, which has better color rendering and efficiency.

Metal halide lamps are similar in construction and appearance to mercury vapor lamps. The addition of metal halide gases to mercury gas within the lamp results in higher light output, more lumens per watt, and better color rendition than from mercury gas alone. Metal halide lamps are used to light large indoor areas such as gymnasiums and sports arenas, and for outdoor areas such as car lots or anywhere that color rendition is important. New 'pulse start metal halide' lighting is very efficient, produces WHITE LIGHT and long life. It is possible to retrofit first generation MH to PSMH with a fast payback period.

High-pressure sodium lighting is becoming the most common type of outdoor lighting. It provides 90 to 150 lumens per watt�an efficiency exceeded only by low-pressure sodium lighting. High-pressure sodium lamps are also reliable and have long service lives. HPS produces a YELLOW LIGHT, and their color rendition ranges from poor to fairly good depending on design and intended use.

Low-Pressure Sodium Low-pressure sodium lamps work somewhat like fluorescent lamps. They are the most efficient artificial lighting, have the longest service life, and maintain their light output better than any other lamp type. Low-pressure sodium lighting is used where color is not important because it renders all colors as tones of YELLOW or GREY LIGHT. Typical applications include highway and security lighting. This type of lighting is not very popular in the United States.