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Dimensions in millimeters

Luminous Efficacy
Unit of measurement: lumen per watt [lm/W]: Luminous efficacy indicates the efficiency with which the electrical power consumed is converted into light.

The maximum that can be achieved in theory, with all the energy being converted into visible light, is 683 lm/W. In reality, the figures are much lower, between 10 and 150 lm/W.

Luminous efficacies of various light sources

Color Rendering
Depending on the location and the purpose, artificial light should enable colors to be perceived correctly, as though being seen by natural daylight. Such assessments are based on the color rendering properties of a light source, which are expressed in terms of categories of the general color rendering index, R a. The color rendering index is a measure of the correspondence between the color of an object under the light source being measured and its color under a reference light source.

Incandescent and Halogen Lamp Operation
Incandescent lamps are classic thermal radiators in which electricity flows through a Tungsten wire in an enclosed glass bulb filled with a vacuum or inert gas, heating it to approximately 2600˚C to 3000˚C and making it glow.  Most of the radiation emitted is at the infra-red end of the spectrum.

The main properties of an incandescent lamp, namely its luminous efficacy and life, are influenced largely by the filament temperature.

Operating principle of an Incandescent Lamp
The higher the filament temperature, the  higher the luminous efficacy, but the shorter the lamp life.

A reduction in lamp life is a consequence of the rapid increase in the rate of vaporization of the Tungsten atoms as the temperature rises; this process not only produces a black coating on the bulb but also ultimately causes the filament to break.

Blackening of the bulb can be effectively countered by using as heavy an inert gas as possible (Argon, Krypton or Xenon) and ensuring that this gas is at high pressure to reduce the rate at which the Tungsten vaporises.

The principal types of incandescent lamp are: general-purpose lamps, special-purpose lamps, decorative lamps and incandescent reflector lamps. The luminous efficacy of incandescent lamps in the 25 to 1000W range is between about 9 and 19 lm/W for lamps with an average life of 1000 hours.

Tungsten-Halogen Lamps
Tungsten-Halogen lamps operate in the same way as incandescent lamps and have a similar design. The small quantities of Halogens (Bromine, Chlorine, Fluorine and Iodine) and their compounds added to the filler gas, almost entirely prevent the bulb from blackening due to vaporizing Tungsten atoms within a particular temperature range, so there is no associated drop in luminous flux. The bulbs in Tungsten-Halogen lamps can therefore be made much smaller, which means the pressure of the filler gas can be increased and more economical use can therefore be made of expensive inert gases like Krypton and Xenon. The life of Tungsten lamps varies from 50 hours for photographic studio types, to over 2000 hours for commercial display lighting types. Automotive Halogen lamps are generally rated in the 300 hour range.

Halogen Cycle
As already mentioned, the main characteristics of an incandescent lamp, namely its luminous efficacy and service life, are determined to a large extent by the filament temperature. The higher the filament temperature, the higher the luminous efficacy but the shorter the life of the lamp.

This reduction in the life of the lamp is a consequence of the rate at which Tungsten vaporises from the filament. This rate increases rapidly with temperature. Not only does it blacken the inside of the bulb, it causes the filament ultimately to burn right through.

This blackening of the bulb can be effectively countered by adding halogens to the filler gas to keep the vaporized Tungsten away from the bulb wall in a cycle process . The Tungsten that vaporises from the filament during normal operation drifts towards the bulb wall by diffusion or convection in thermal regions (•1 <1400°C), where it forms stable Tungsten-Halogen compounds. Thermal currents take these compounds near the hot filament (•2 >1400°C)            where they disintegrate. Some of the Tungsten is transported back to the filament, but not to its original location. The normal Halogen cycle therefore merely prevents the bulb  from blackening, it does not extend the life of the lamp. The lamp comes to the end of its life when the filament breaks at one of  its hot spots.

Halogen Cycle
A regenerative cycle would be feasible with Fluorine but because this Halogen is so corrosive towards the quartz and hard glass used for the bulb, and resistant to the Halogen currently used, a satisfactory solution has not yet been found.

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