Lighting Intro

This is our introduction to lighting technology.
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Basic Principles of Electric Lighting

In general, all electric lighting relies on the transition of electrons from a high energy state to a lower energy state. When the electron drops from one state to another, the difference in energy is emitted as light.

The actual difference between the many different kinds of electric lighting is the method that is used to excite the electrons to the higher energy level in the first place. 

 

Incandescence

The simplest form of electric lighting is the incandescent light. The electricity is used to heat up a tungsten filament until it gets so hot it glows, emitting what is known as "black body radiation".

The term comes from the theoretical behavior of an "ideal" material. When held at the temperature of absolute zero (0°K), the material emits no radiation. Since it is completely dark, it is known as a "black body". When the material is heated, it starts to glow with emitted "black body radiation". It turns out that a hot tungsten filament behaves very much like such an ideal material.

The color of the light emitted corresponds to the temperature of the heated body. In fact, the color of a light is sometimes specified by its "color temperature", in degrees Kelvin. The common expression "red hot" comes from a simple, every-day observation of black body radiation.

One important benefit of light production via black body radiation is that the light is composed of a continuous spectrum of colors. This gives good color rendition when the light is used to illuminate objects.

Planck's Constant can be used to calculate the wavelength of a photon with a given amount of energy. The Planck law for blackbody radiation gives the intensity radiated by a blackbody as a function of frequency (or wavelength). The Stefan-Boltzmann law gives the total flux integrated over all frequencies (or wavelengths).

For details, please see: 

 

Arc

A slightly more complex form of electric lighting excites electrons by passing an electric arc through a gas. The gas forms a luminous plasma, as the arc kicks the electrons up to a higher energy level and they fall back down. This is known as "arc" or "gas discharge" lighting.

The light emitted by this form of excitation generally is composed of discrete spectral lines. This gives poor color rendition when the light is used to illuminate objects. The situation can be improved by generating the arc under high pressure gasses, which tends to blur the lines, making more of a continuous spectrum of colors.

High pressure arc lights tend to use physically small bulb envelopes. Low pressure arc lights tend to use physically large envelopes. 

 

Fluorescence

Another way to excite electrons is to hit them with light. While it might sound strange to use light to generate light, this technique is quite useful. For example, a light source might contain large amounts of ultraviolet light, which is useless for visible illumination and dangerous to the eye. But the ultraviolet can be used to excite phosphors, which then emit visible light.

Usually, fluorescence takes the form of higher energy photons (shorter wavelength) being converted to lower energy photons (longer wavelength), as is the case of invisible black light causing minerals to visibly glow.

Under certain exotic circumstances, several low-energy photons can be used to kick an electron high enough that it emits a higher-energy photon when it returns to its resting state. This phenomenon is used to produce green solid-state LASERs by shining an infrared LASER through a frequency-doubling crystal.

Fluorescence is an extremely versatile tool, since phosphors exist to generate a wide variety of colors, and they can be blended to produce shades and tints. 

 

Efficiency Of Lighting Technology

The numerous lighting technologies differ in how efficiently they convert electricity into light.

technology lumens per Watt efficiency
Tungsten lamp 17
Tungsten halogen lamp 20-25 10%
Mercury lamps 55-60 20%
Low pressure sodium lamp more than 220 80%
High pressure sodium lamp 80-140 50%
Fluorescent lamp 65-80 40%
Compact fluorescent lamp 45-60
Metal halide lamp 85-115 50%
HMI lamp 100-110
Sulfur lamp 120 40%
[The lumens per Watt and efficiency numbers are collected from various sources and might not match. These figures include losses in the system, like ballast losses for fluorescents and light pipes for sulfur lamps.] 

 

General Lighting Terminology

 

 

Assorted Lamp Tips

 

Measuring Light Intensity

There are several ways to measure the intensity of light.

Please note that simple intensity is not the only factor affecting visibility. One must also consider contrast, glare, and the color of the light (the eye is more sensitive to some colors than others).

The SI (metric system) units are:
unit name measure of symbol derivation
Candela luminous intensity cd
luminance cd/m2
Lumen flux of light lm cd·sr
Lux illumination lx lm/m2

Footcandle

The "Footcandle" (abbreviated "fc") is the unit of intensity of light falling on a surface, when the foot is taken as the unit of length. [As opposed to the Lux, which is used in the metric system.]

The "Footcandle" is defined as the illumination on a surface, one square foot in area, on which there is a uniformity distributed flux of one Lumen. In other words, one Footcandle is one Lumen per square foot. [It can also be defined as the illuminance on a surface that is everywhere 1 foot from a point source of one Candela.]

To convert from Lux to Footcandle:

fc = lux x .0929

Sunlight on a summer day may be higher than 10,000 fc. Moonlight might go as high as .002 fc.

Lux

The "Lux" is the unit of intensity of light falling on a surface, when the metric system is taken as the unit of length. [As opposed to the Footcandle, which is used in the English measurement system.]

It is defined as the illumination produced on a surface, one square meter in area, at a distance of one meter from a uniform point source of one Candela. In other words, one Lux is one Lumen per square meter.

The Lux is the international standard unit of illumination, and should be preferred to the Footcandle.

To convert from Footcandle to Lux:

lux = fc x 10.76

Sunlight on a summer day may be higher than 100,000 lux. Moonlight might go as high as 0.2 lux.

Lumen

The "Lumen" (abbreviated "lm") is a measure of luminous flux in the metric system. It is defined as the amount of light given out through a solid angle of one steradian by a source of one Candela intensity radiating equally in all directions.

Candela

The "Candela" (abbreviated "cd") is the modern metric unit of luminous intensity, adopted in 1948. One candela is equal to 1/60 of the luminous intensity per square centimeter of a blackbody radiating at the melting point of platinum (2046°Kelvin).

Candlepower

"Candlepower" is an obsolete lighting measurement, defined in 1860. It was defined as the light emitted by a standardized candle - made from a particular kind of material, of a particular weight, burning at a particular rate.

Sometimes "Candlepower" and Candela are used interchangeably. This is imprecise and can lead to confusion. Don't do it. 

 

A Brief History Of Lighting Technology

If you think that the electric light was invented by Edison, you need a history lesson.

Here is a brief history of artificial lighting through the ages. 

 

Related Pages

You may be interested in these related pages:
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