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# What is "black current"?
“Black current” is a current that is sent through the series circuit although all lights are turned off.
# In what way does temperature affect an LED?
Temperature affects an LED in a couple of different ways:
• The higher the temperature the lower the light intensity (a.k.a. droop). This implies that there is a point beyond which an increase in the current through the LED will result in less light being output. This is what today limits the use of LEDs for high intensity applications.
• Wavelength of emitted light varies with temperature, about one nanometer [nm] for every 10°C. Hardly noticeable by the human eye.
• The higher the temperature, the lower the life time of the LED. When the temperature on the LED gets close to its maximum rating, life time may be below that of a halogen light bulb. Thermal management is key for long life time and high reliability. It is safe to assume that under normal operating conditions the life time of the LED itself will exceed that of the complete fixture.
# What about an Arctic Kit for LED?
Field tests indicate that such an Arctic kit may not be needed since the excess power from the light is low enough not to melt the snow on the fixture and hence the problem with black ice is less likely to occur with LED than with a halogen light. As a consequence, neoprene- and plastic plow blades as well as blowers and brushes may be used to clear the snow from the light (instead of steel blades) resulting in considerably less wear on the fixture as a whole.
# What are the characteristics of a Halogen lamp?
A halogen lamp is an incandescent lamp in which a tungsten filament is sealed into a compact transparent envelope filled with an inert gas, plus a small amount of halogen such as iodine or bromine. The halogen cycle increases the lifetime of the bulb and prevents its darkening by re-depositing tungsten from the inside of the bulb back onto the filament. The halogen lamp can operate its filament at a higher temperature than a standard gas filled lamp of similar power without loss of operating life. This gives it a higher efficacy (10-30 lm/W). It also gives light of a higher color temperature compared to a non-halogen incandescent lamp (Halogen Lamp, 2009).
# What are the characteristics of a Fluorescent tube?
A 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 (Fluorescent Lamp, 2009).
# What are the characteristics of a LED?
A light emitting diode is an electronic light source. The LED is constructed from a semiconductor with specific properties such that when current passes through the device light is emitted. Depending on the properties of the semiconductor the wavelength (color) of the light emitted will vary, although it will always be a discrete wavelength (monochromatic light). Visible colors like red, orange, green, blue, purple and yellow can be achieved this way while white light relies on slightly different technology. Basically two ways to produce white light with LED exists (at least outside the lab), either a phosphor material is used to convert monochromatic blue or ultra violet light to white light (similar to the way fluorescent tubes work) or light from red, green and blue (RGB) LEDs is mixed.
# What is lighting efficiency and how is it calculated?
To compare the efficiency of different light sources the term overall luminous efficacy is used. The overall luminous efficacy is the ratio between the total luminous flux emitted by the light source (within the visible spectrum) and the total amount of power it consumes. When expressed with reference to the maximum possible luminous efficacy the term lighting efficiency may be used instead.
For reference the theoretical maximum for efficacy is 683 lm/W for 555 nm monochromatic green light which corresponds to 100% efficiency. Light Source Lighting Efficiency [%] Luminous Efficacy [lm/W]
Incandescent Halogen 2-3% 10-30
Fluorescent Tube 8-15% 50-100
Light Emitting Diode 2-15% 10-1001