Metal Halide Lamp

A specific type of high-intensity discharge (HID) lamp known as a metal halide lamp produces light by using a combination of metal halide salts.

The metal halide salts and other gases are contained in a tube that the lamp employs to generate light by circulating an electric current through it.

Metal Halide Lamp: Working Principle

💥A metal halide lamp can be considered as an extension of HPMV lamp.

💥In HPMV lamp visible radiations are at 3 spectral areas ; Blue (436nm), Green(546nm) & Yellow(578nm). Red region is absent. It causes poor colour renditionproperty and moderate luminous efficacy.

💥By adding suitable suitable metals with lower excitation excitation potentials potentials to the arc tube, thecolour property i.e. CRI can be improved.

💥Metals may be of different types – Single Line radiators ( Sodium – Yellow, Indium – Blue), Multi Line Radiators (rare earth maetals like Scandium, Thorium, etc), Molecular Radiators for wide band light emissions.

💥 But they are generally highly reactive and would damage the quartz arc tube.

💥By adding the metals in the form of halides these problems can be eliminatedto a satisfactorily good extent.

💥Iodides (less reactive) are mostly used & Fluorides never used (Strong reactive)

Working Process

≽ At first the output spectrum is only due to Hg Vapour, because Halides remainsolidified.

≽ On further heating Halides dissociate in the hot central regionof thedischarge tube allowing their metal atoms to make a contribution to the lightoutput by their characteristic spectral emission mostly in the Red region.

≽ In the relatively cool region near arc tube wall, metal and halogen atomsrecombine to form the halides again. These halides are much less chemicallyreactive than the metal ions and hence cause less damage to the fused silica(quartz) glass of arc tube.

≽ This cyclic process results efficient lamp operation with better CRI.

≽ The radiation Characteristics of MH lamps are strongly dependent of thevapour pressure of the halides and hence the Cold and Hot spot temperaturesof the arc tube

Optical Characteristics and Applications

👉 MH lamps can be made in various size and rating and with different colour output – from Daylight to even UV.

👉 With varying composition of Metal Halides and inside pressure, MH lamps are available with variable optical properties -

👉 Luminous Efficacy – 70 -100 Lm/W

👉 CRI – 70 -90  CCT – 3600 K – 5600 K

👉 Life – 6000 – 15000 working hours

👉 Application – Quality Interiors, Wide Area Floodlighting, Stage and Studio Light, Sports Light particularly for Colour TV Broadcasting.

Ceramic Discharge Metal Halide lamp (CDMH)

🧿Major criticism to the MH lamp is - poor lamp to lamp colourconsistency (less uniformity) and changes in colour through out the life(lack of stability)

🧿Ceramic Discharge Lamp is recent development to solve the issue to asatisfactory extent.

🧿Ceramic Ceramic can withstand withstand higher temperatures temperatures than quartz glass. Thispermits higher wall temperatures thereby evaporating more of themetal halide salts into the gas arc and allowing for more efficient use ofthe chemicals. Ceramic lamps offer improved luminous efficacy andcolour rendering as a result.

🧿Normally Poly Crystalline Alumina (PCA) is used.

🧿However, in contrast to silicon, aluminum hardly dissolves in tungstenand the reaction with rare earth elements are much more self limiting.

🧿But there is a colour shifting in output from Quartz to Ceramic Tube