The dark-sky movement is a campaign by people who want to reduce light pollution so people can see the stars, to reduce the effects of unnatural lighting on the environment, and to cut down on energy usage.
The movement started with professional and amateur astronomers alarmed that nocturnal glow from urban areas was blotting out the sight of stars. For example, the Griffith Observatory in Los Angeles is useless for astronomy, because of daytime smog and nighttime light pollution, while world-famous Palomar Observatory is threatened.
The movement has since spread with groups like the International Dark-Sky Association. In addition, other concerns have been raised. For example, nocturnal animals can be harmed by light pollution.
The dark-sky movement's main activity is to encourage the use of full cutoff fixtures, that cast little or no light upward, in public areas and generally to encourage communities to adopt lighting regulations.

Ballast - Digital (DHID)
Digital High-Intensity Discharge

DHID lamp ballasts are electronic ballasts that use a microprocessor to control and regulate a High-Intensity Discharge (HID) lamp. The Ballast can provide varying lamp currents during start-up and during lamp operation for the most efficient burning of the lamp.
The main benfits of the Digital HID ballast are: saving of electrical energy and maintenance costs over magnetic ballast HID lamps; automatic loss-of-lumen compensation as the lamp ages; produces up to 40% more lumens per watt than magnetic ballasts; increased life expectancy of the lamp compared to magnetic ballast; DHID ballast generates less heat, reducing air conditioning energy cost.
DHID ballasts can be used to upgrade existing lamp installations. In a retrofit, the DHID ballast can replace the existing magnetic ballast. The rest of the fixture is reused, and the magnetic ballast can be recycled.

Electronic Ballast

Electronic ballasts usually change the frequency of the power from the standard mains 60 Hz to 20,000 Hz or higher, substantially eliminating the stroboscopic effect of flicker (a product of the line frequency) associated with fluorescent lighting. In addition, because more gas remains ionized in the arc stream, the lamps actually operate at about 9% higher efficiency above approximately 10 kHz. Lamp efficiency increases sharply at about 10 kHz and continues to improve until approximately 20 kHz.[1] Because of the higher efficiency of the ballast itself and the improvement of lamp efficiency by operating at a higher frequency, electronic ballasts offer higher system efficiency.

HID
High-Intensity Discharge

Gas-discharge lamps are a family of artificial light sources that generate light by sending an electrical discharge through an ionized gas, i.e. a plasma. The character of the gas discharge critically depends on the frequency or modulation of the current.Such lamps use a noble gas (argon, neon, krypton and xenon) or a mixture of these gases.
Most lamps are filled with additional materials, like mercury, sodium, and/or metal halides. In operation the gas is ionized, and free electrons, accelerated by the electrical field in the tube, collide with gas and metal atoms. Some electrons circling around the gas and metal atoms are excited by these collisions, bringing them to a higher energy state. When the electron falls back to its original state, it emits a photon, resulting in visible light or ultraviolet radiation. Ultraviolet radiation is converted to visible light by a fluorescent coating on the inside of the lamp's glass surface for some lamp types. The fluorescent lamp is perhaps the best known gas-discharge lamp.
Gas-discharge lamps offer long life and high light efficiency, but are more complicated to manufacture, and they require electronics to provide the correct current flow through the gas.

HPS
High-Pressure Sodium

A Sodium vapor lamp is a gas discharge lamp which uses sodium to produce light. There are two varieties of such lamps: low pressure and high pressure. Because sodium vapor lamps cause less light pollution than mercury-vapor lamps, many cities that have large astronomical observatories employ them.
HPS lamps contain elements such as mercury, and produce a dark pink glow when first struck, and a pinkish orange light when warmed. Some bulbs also briefly produce a pure to bluish white light in between. This is probably from the mercury glowing before the sodium is completely warmed. The sodium is the main source of light from the HPS lamp, and due to the emissions from mercury, they are used in areas where good color rendering is important.

LPS
Low-Pressure Sodium

A Sodium vapor lamp is a gas discharge lamp which uses sodium to produce light. There are two varieties of such lamps: low pressure and high pressure. Because sodium vapor lamps cause less light pollution than mercury-vapor lamps, many cities that have large astronomical observatories employ them.
Low-pressure sodium (LPS) lamps have a borosilicate glass gas discharge tube (arc tube) containing solid sodium and a small amount of neon and argon gas Penning mixture to start the gas discharge. When the lamp is turned on it emits a dim red/pink light to warm the sodium metal and within a few minutes it turns into the common bright yellow as the sodium metal vaporizes.

MH

Metal halide lamps, a member of the high-intensity discharge (HID) family of lamps, produce high light output for their size, making them a compact, powerful, and efficient light source. By adding rare earth metal salts to the mercury vapor lamp, improved luminous efficacy and light color is obtained.
Like most HID lamps, metal halide lamps operate under high pressure and temperature, and require special fixtures to operate safely.
Since the lamp is small compared to a fluorescent or incandescent lamp of the same light level, relatively small reflective luminaires can be used to direct the light for different applications (flood lighting outdoors, or lighting for warehouses or industrial buildings).

MT
Multi-Tap

Many ballasts have multi-tap inputs, allowing for different voltages to be applied. Typically, the voltage inputs range from 110VAC to 240-277VAC.
Although multi-tap electrical devices are slightly more expensive to produce, they eliminate the need for seperate devices to handle each type of input, resulting in a cost savings for the end user.

PS
Pulse-Start Ballast

High-Intensity Discharge lamps require electrical ballasts to regulate the arc current and deliver the proper voltage to the arc. Like high-pressure mercury vapour lamps, some metal halide bulbs contain a third electrode to initiate the arc when the lamp is first lit.
Pulse-start ballasts eliminate the need for a starting electrode. Instead, they use an ignitor to generate a high-voltage (1–5 kV on cold strike, over 30 kV on hot restrike) pulse to start the arc. Because PS ballasts have less loss than a line-frequency "iron" ballast, they are more energy efficient. In addition, they can relight a lamp immediately, without waiting for cool-down.

Colour Temperature of Common Light Sources

Colour Rendering Index (CRI)

• Colour rendering is a general expression for the effect of a light source on the colour appearance of objects, compared with the effect produced by a reference or standard light source of the same correlated colour temperature.
• The colour rendering properties of a light source are expressed by the (CRI).
• The CRI is obtained as the mean value of measurements for a set of eight test colours.
• The CRI has a value between 0 and 100.
• A CRI of 100 indicates a light source, which renders colours as well as the reference source.
• The CRI is used to compare light sources of the same chromaticity (or colour temperature).
• The CRI is used as a general indicator of colour rendering: a higher CRI means a better colour rendering.
• It is essential to understand that the CRI value has no reference to ‘natural’ light, although colours under a high CRI lamp will appear more natural.
• The most important characteristic of a lamp, from an energy viewpoint, is its ability to convert electrical energy into light. This measure is referred to as efficacy, in lumens per watt or light output per watt input.

The chart below shows the general range of lumens per watt and the CRI for various light sources.

Colour Rendering Index and Efficacy of Common Light Sources

Colour Rendering Description

Technology and Performance

• Incandescent lamps produce smooth, even SPD curves and outstanding CRI values.
• Halogen versions of incandescent lamps produce whiter light with +95 CRI.
• With gaseous discharge technology, colour characteristics are modified by the mixture of gases and by the use of phosphor coatings.
• HID lamps are chosen mostly for their exceptional energy efficiency; metal halide versions have acceptable CRI levels.

Application Notes

• Warm colour light is associated with indoors, nighttime and heat, and fits better indoors and in cool environments.
• Warm colour light makes warm colour objects (red–yellow colours) look richer.
• Cool colour light is associated with outdoors, daytime and cold, and fits better in warm environments.
• Cool colour light mixes better with daylight (daytime lighting).
• Cool colour light makes cool colour objects (blue–green colours) look refreshing.
• Match light source colour with room objects’ colour (interior decoration).
• Sources with high CRI cause the least emphasis or distortion of colour.

Visible light comprises only a tiny fraction of the entire electromagnetic radiation spectrum, yet it contains the only region of frequencies to which the rods and cones of the human eye will respond. The wavelengths that humans are typically able to visualize lie in a very narrow range between approximately 400 and 700 nanometers.

Humans can observe visible light because the eyes contain specialized nerve endings that are sensitive to this range of frequencies. However, the remainder of the electromagnetic spectrum is invisible.

The chart below shows spectral distribution curves for several different sources of light.

The red curve represents the relative energy of tungsten light over the entire visible spectrum. As you can see, the energy of tungsten light increases at longer wavelengths. This influences the average color temperature of the resultant light, especially when it is compared to that of natural sunlight and fluorescent light.

The yellow curve shows the visible light distribution from the natural sunlight at noon. The dark blue spectral curve is characteristic of a mercury arc lamp, and exhibits differences from the tungsten and natural sunlight spectra. Several energy peaks are present in the discharge arc lamp spectrum that show individual line spectra originating from the mercury vapor.