Characteristics of Common Light Sources

LIGHTING FACTS

The quantity and quality of light around us determines 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 impact.

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.

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 (HID)
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.