Light sources are described by the type of light source (bulb), the number of Watts, Lumens, Kelvin, and Footcandles:
WATTS: The amount of electricity used to generate light.
LUMENS: The amount of light generated by a light source, which determines the brightness of the
KELVIN: The rating for color temperature, which is sometimes described as the light appearance. A lower Kelvin rating will produce a more yellow, warm light while a higher Kelvin rating will produce a harsh white light, high in blue light waves. NOTE: In New York City, the most common type of streetlights, which are High Pressure Sodium, are rated 1800-2000 Kelvin; the newly installed LED streetlights are rated in excessive of 4000 Kelvin. LED streetlights are currently available between 2300 and 6000 Kelvin. The AMA recommends a maximum of 3000 Kelvin; and some cities have mandated a maximum of 2700 Kelvin due to complaints and health concerns about the excessive blue light waves in high Kelvin
FOOTCANDLES: The rating used to indicate the amount of light falling onto a surface. There are professionally recommended light levels appropriate for various tasks.
LIGHT POLLUTION: is an increasing environmental problem throughout the world. The three main sources are streetlights, lights from vehicles, and lights from buildings. See, for example, this BBC News summary of a June 2016 scientific study which concludes that light pollution now affects 80% of the world’s population: http://www.bbc.com/news/science-environment-36492596 There are three kinds of light pollution: excessive and unnecessary lighting, and lighting which produces glare, light trespass, and skyglow:
GLARE: (also referred to as Obtrusive light): Spill light which, because of quantitative or directional attributes, gives rise to annoyance, discomfort, distraction, or a reduction in ability to see essential
LIGHT TRESPASS: Illumination or glare cast unnecessarily on the property of another with the effect of reducing privacy, limiting use or enjoyment, hindering sleep and/or detracting from the appearance of the illuminated property without the permission of the owner, lessee or lawful occupant. Light trespass may be measured at any point on the property by a light meter.
SKYGLOW: The brightening of the night sky that results from the reflection of light (visible and nonvisible), scattered in the atmosphere (gas molecules, aerosols and particulate matter), and can obscure the stars in the sky and cast an urban overhead glow. Skyglow is increased when light sources have a high Kelvin rating due to the shorter length of blue light waves.
TYPES OF LIGHT SOURCES
Intelligent choices for lighting are based on several factors, such as efficiency, effectiveness, costs, and impacts on human health and the environment as well as the intended purpose and duration.
Traditional light sources are: incandescent (includes halogen), fluorescent (includes compact fluorescent lights i.e. CFL’s); sodium (high pressure and low pressure), mercury vapor; and metal halide. Mercury vapor is no longer manufactured due to inefficiency. LEDs are a new technology that is constantly being upgraded. Each light source has a different watt to lumen output ratio.
Fluorescent: A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light. Unlike incandescent lamps, fluorescent lamps always require a ballast to regulate the flow of power through the lamp.
Metal Halide: A high-intensity discharge lamp type that uses mercury and several halide additives as light-producing elements. Metal halide lamps are available with correlated color temperatures (CCTs) from 2300 to 5400 Kelvin and with CRI values from 60 to 93. Efficacies of metal halide lamps typically range from 75 to 125 lumens per watt (LPW).
Mercury Vapor: A high-intensity discharge lamp type that uses mercury as the primary light-producing element. Mercury vapor lamps produce light with a CCT from 3000 to 7000 Kelvin. Mercury vapor lamps are less efficacious than other HID lamp types, typically producing only 30 to 65 LPW. Energy codes prohibit the manufacture of mercury vapor ballast fixtures.
High Pressure Sodium: A high-intensity discharge lamp type that uses sodium under high pressure as the primary light-producing element. HPS lamps produce light with a (CCT) of approximately 2000 Kelvin. HPS lamps are among the most efficacious light sources, with efficacies as high as 150
LED: Light Emitting Diode: LED lights are a relatively new lighting technology based on light emitting diodes that are blue in color unless filters are applied.
Halogen: An incandescent lamp that uses a halogen fill gas. Halogen lamps have higher rated efficacies and longer lives than standard incandescent A-lamps.
Incandescent: An electric light in which a filament is heated to incandescence by an electric current. An electric current passes through a thin filament, heating it until it produces light.
HID: High-Intensity Discharge lighting is a family of light sources which use electrical arcs to create powerful illumination thereby reducing the watts to lumen ratio.