ELECTRIC LAMPS – PART – 08 - OPERATION OF METAL HALIDE (MH) LAMPS

INVENTOR OF METAL HALIDE (MH) LAMPS

In the year 1912 Charles P. Steinmetz was the first to use halide salts in a mercury vapor lamp. In the year 1962 Robert Reiling developed the first reliable MH lamp. Reiling built on the work of Steinmetz to complete his work. The MH lamp became more popular, and decades later the price of the lamp became more affordable. MH is very popular due to its good quality white light and good lamp efficacy.

METAL HALIDE (MH) LAMPS

MH lamps are similar in construction to HPMV lamps but in addition to mercury, a number of iodides [Iodide means a salt or ester of hydriodic acid] are added which fill the gaps in the light spectrum and these lamps are operated at higher temperatures and pressures. Iodides are used to improve the colour characteristics of light.

HALIDE

A Halide is a chemical compound of a halogen combined with an electro-positive element. A halogen is a monovalent element which readily forms negative ions. There are five halogens: fluorine, chlorine, bromine, iodine and astatine.

CONSTRUCTION OF MH LAMPS

1. It consists of two glass tubes, two main electrodes, one

    auxiliary electrode, a ballast and a capacitor. 

2. Outer glass tube is made up of Borosilicate glass which is     

    used for insulation as well as to block the UV radiation

    coming from the arc.

3. Inner tube (discharge) is made up of quartz or hard glass and

    contains a starting gas (usually argon), mercury and MH

    salts.

4. Filaments are made up of tungsten treated with radioactive

    Thorium.

5. Molybdenum is used in sealing discharge tube as it does not

    expand even under extremely high temperatures.

6. Molybdenum is highly resistant to corrosion and is also used

    in high strength steel, armor and electrical contacts.

7. MH lamps have a coated finish on the inside of the bulb that

    diffuses the light.

8. Often a phosphor coat is used in diffusing light as well as

    changing the lamp's colour properties.

9. Choke is provided to develop a high voltage at the time of

    starting and to limit the current after discharge.

10. Capacitor is connected across the supply to improve the

    power factor.                                                                                                                                                                       
HALIDE 'SALTS' USED IN THE MH LAMP

AgCl (Silver chloride)       – white light

AgF (Silver fluoride)         - No color

AgBr (Silver bromide)       - pale yellow

Agl (Silver iodide)            - green yellow

OPERATION OF MH LAMP

1. When the lamp is cold, the halides and mercury are

    condense on the fuzed quartz tube (discharge tube).

2. When supply is switched ‘ON’, current passes through the

    starting electrode and jumps to a short distance leading to

    the main electrode.

3. Argon is used to start discharge in the lamp and argon gas

    strikes and initiates an arc at low temperature.

4. After the arc initializes, the tube heats up and the mercury

    is vaporized.

5. The initialized arc activates to work through the resistance of

    the gas, during this period, more molecules of the gas

    become ionized.

6. This creates more electric current to pass through the tube,

    allowing the arc to become wider and hotter.

7. This heat produced by the arc in the tube vaporizes the solid

    mercury and travels through the mercury vapor to

    reach the other main electrodes

8. There is less resistance on this path now and current stops

    flowing through the starting electrode.

9. After a mercury vapor arc strikes and heats, the halides

    vaporize and gets separated.

10. The metal atoms diffuse away from the arc to cooler areas

    and recombine with the halogen before they damage any

    part of the electrodes and the lamp is now fully warmed up

    and produce its white light.

TYPES OF METAL HALOIDE LAMPS

A variety of shapes are available  

(i) Elliptical                    
(ii) Tubular

(iii) Double Ended and   (iv) Compact Pin ended and reflector.

ADVANTAGES

1. More pure white light than the popular HPS lamps.

2. More energy efficient than mercury vapor lamp.

3. Lumens per wattage range from 65 to 115.

4. Life time is 20,000 hrs for mounted base up and 10,000 hrs for

    horizontal mounted lamps.

5. Colour Rendering Index is 60 - 90 (depends on brand and

    chemical compositions)

DISADVANTAGES

1. Very high manufacturing cost.

2. The light is very bright and it produces much more light

    pollution than HPS lamps, since the whites from an MH lamp

    are closer to daylight in frequency.

3. Warm up time is 1 - 15 minutes.

APPLICATIONS

1. These lamps are suitable for flood-lighting, industrial

    lighting and public lighting.

2. Outdoor lighting where good colour rendering is needed.

3. Television/film lighting, sports fields, car headlights, heavy

    flashlights and green house applications.