This article presents a glossary of common lighting terminology and their meaning. Don’t be confused by the terms lumen or candlepower after reviewing some of the more common lighting terminology!
The human eye is a very complex device. The manufacturers of light bulbs have developed means of measuring light that are meaningful to them, but are not intuitively meaningful to laymen.
Brightness – See also “Lumens” and “Candlepower.”
The term “brightness” is understood intuitively, but it is a complex phenomenon. What we perceive as being brighter or dimmer can be affected by a lot of things. Rather than delving into the numerous scientific complexities of this term, it will suffice for this section to allow the reader to make use of the intuitive meaning of brightness.
Just as for “lumens,” candlepower is a measure of brightness that is adjusted to the selective sensitivity of the human eye. (See “Lumens.”) A beam pattern is never completely equally bright in all directions. The brightness of light at a specific point is measured in candlepower. The total amount of light present in a beam pattern is measured in lumens.
Center Beam Candlepower. Some data sheets for light bulbs list the CBCP. This is the brightness, measured in candlepower, at the center of the beam pattern. A spot light and a flood light can emit the same total light (the same lumens), but the spot light will have a much higher CBCP than the flood light.
Color Rendering Index. An important factor about color from light bulbs is how other colors look when illuminated by the light bulb.
An incandescent bulb has all the colors of the rainbow, so all colors will be rendered properly if viewed under an incandescent light bulb. The Color Rendering Index (CRI) for incandescent lamps is a perfect 100. Other lights do not fair so well.
One of the worst is a low pressure sodium bulb, which emits a nearly pure yellow color. Some well designed fluorescent bulbs have a CRI of 80, which is considered to be very good.
The sun emits a wide range of frequencies of radiation. Those frequencies that can be detected by the human eye we call light. Infrared radiation and ultraviolet radiation are not visible to the eye. Most light sources produce both light and invisible infrared and/or ultraviolet radiation.
The term “light bulb” is used here to refer to the light source, which is the glass envelope and all its innards.
The term “lamp” is used here to refer to the light bulb plus all of the hardware surrounding the light bulb that positions, reflects, and focuses the light.
In the light bulb manufacturing industry, however, the light bulb is called a “lamp,” and the assembly that holds the light bulb is called a “fixture.”
Lumens Per Watt. When any device uses electrical power, that electrical power is converted to some other form.
An electric light converts some of the electric power into light. The rest of the electric power is converted to heat (infrared light and the warming of the light bulb’s parts) and some ultraviolet light.
Different light bulb technologies have different efficiencies. The efficiency (the term “efficacy” is used by scientists) is measured in lumens per watt, or LPW. (See “Lumens.”) For two bulbs with the same brightness (lumens), the one that uses the least power (watts) will cost less to use.
The human eye is more sensitive to some colors than it is to others. To account for this variation, scientists measure light in “lumens,” which takes the amount of radiation (in watts) that is present and adjusts it to the human eye’s selectiveness. For example, if we were in a room illuminated by 100 watts of invisible radiation (such as ultraviolet), the light bulb’s output is zero lumens.
The human eye is more sensitive to green and yellow than to blue and red. Five watts of pure green light has more lumens (appears brighter) than five watts of pure red light.
Mean Spherical Candlepower. Some light bulb data sheets list the MSCP. This is a way to represent the total light in the beam pattern by assuming that the beam pattern is equally bright in all directions. If you multiply the MSCP by 12.57, you have the lumens. (See “Lumens.”)
The human eye can distinguish fine variations in color, but it cannot distinguish fine variations in brightness.
An incandescent light bulb that is rated as 1000 lumens looks to be about the same brightness as one that is 850 lumens even if the two bulbs are in the same room. The dimmer bulb would need to be about 800 lumens (a 20% difference) before we would perceive that one is dimmer than the other.
If the two bulbs are in two different rooms, then the dimmer bulb would need to about 700 lumens (a 30% difference) before we would perceive that one is dimmer than the other.
The sun produces a continuous spectrum of light that spans infrared, visible, and ultraviolet light. What our eyes perceive as white sunlight is actually a mixture of all the colors of the rainbow.
Different kinds of light bulbs produce different colors. There are no light bulbs that produce the same true white color we see in sunlight, though they are moving towards it.
An important thing to understand concerning light bulb design is that there is no such thing as a free lunch. Light bulb manufacturers design light bulbs by selecting a compromise between three factors: lumens, watts, and life (in hours).
A light bulb can be re-designed so that it produces more light for the same watts, but it will have a much shorter life. Some long-life bulbs have lower brightness, with the company taking advantage of the fact that you can’t see the difference. The same game can be played with bulbs that use less electricity. A 52 watt bulb uses less electricity than a 60 watt bulb, but your eyes can’t see that one is dimmer than the other.
Most light bulbs are sold by their wattage, not their lumens nor their efficiency. This is practical when comparing two light bulbs of the same technology.
An incandescent bulb at 100 watts will produce more light than an incandescent bulb at 75 watts. The comparison is impractical, however, when comparing two light bulbs of different technologies. A halogen bulb at 75 watts may produce as much light as an incandescent bulb at 100 watts.
Thanks to our guest lighting expert – Lance Kaczorowski, who brings a wealth of expertise to the site.
Kaczorowski, a native of New York City, now resides in Fort Wayne, IN. Kaczorowski has a 4-year degree in Mechanical Engineering from the University of Texas at Austin, and also a 2-year degree in Electronics Engineering Technology from the Community College of the Air Force. Kaczorowski’s broad work history includes (chronologically): Three years as a Mercedes-Benz mechanic; six years as an electronics technician with the Air Force; three years as a new product development engineer with General Electric Lighting in Cleveland; seven years as a new product development engineer and an engineering analyst with Grote Industries in Madison, IN; and currently as an engineering analyst with International Truck and Engine Corporation in Fort Wayne.
The first two years of Kaczorowski’s employment with General Electric consisted of extensive training in light source sciences and engineering under GE’s Edison Engineering Program. Kaczorowski’s experience with lighting was broadened at Grote Industries, which is a supplier of vehicle lighting for heavy duty trucks.