ELECTRIC LAMPS

ELECTRIC LAMPS:

There are 3 types of electric lamps:

1. Filament Lamps (Incandescent lamps):

Filament lamps may be

(a) Vacuum type
(b) Gas filled.

Both vacuum and gas filled lamps use tungsten filaments the life of such lamps largely depends on the rate of filament evaporation. To reduce the rate of filament evaporation, inert gas like argon or nitrogen is filled inside the shell. For the same power output and the same filament size the temperature attained by the filament of the gas filled lamp would be less than that of a vacuum lamp. The filament lamps are normally available in the wattage range of 25, 40,60,100, 200,500, 750,1000 and 1500. Single coiled filament is used on low wattage lamps and coil filament is used on high wattage lamps.

2. Electric discharge lamps:

The operation of these lamps is. based on the phenomenon of excitation and ionization in a gas or vapor. The colors given by various vapors are given below:

Vapor or gas	Color	Lumens / Watt
		Theoretical	Actual
Neon	Red	190	15-40
Sodium vapor	Yellow	475	40-50
Mercury vapor (at low pressure)	Bluish green	245	15-20
Mercury vapor (at high pressure)	Bluish white	300	20-30

They are of two types :

(a) Cold cathode lamp. No filament is used to heat the electrode for starting e.g. neon lamp. Sodium vapor lamp etc.
(b) Hot cathode lamp. A filament is used to heat the main electrode at the time of starting e.g. mercury vapor lamp, fluorescent tube etc.

3. Carbon arc lamp :

Here 2 carbon electrodes are placed in contact end to end in which DC is flowing. By separating apart by about 0.6 cm, gives out a luminous arc as shown in the figure given below. The operating voltages of these lamps varies between 40 to 60 V.

FLUORESCENT LAMP :

Fluorescence is used to correct the deficiency of red color in the spectra of mercury lamp. Further fluorescent powders convert ultra-violet radiations of the mercury discharge at 3654 A and 2537 A into visible light particularly at the red end of the spectrum These powders are usually white in color and are coated on the inside surface of the glass tube. The tubes are usually 30 cm (1 ft). 45 cm (1 1/2 ft), 60 cm (2 ft.) (120 cm, 4 ft.) in length and are provided with two oxide coated electrodes mounted on glass pinches and sealed into the ends. The ends of the electrodes are separately brought out independently as they have to be separately heated during starting. The starting is effected through a starter switch (alternatively a transformer can be used).

Various phosphors have been developed which when coated on the lamp surface, give different colors:

Phosphor	Color
Zinc silicate	Green
Calcium tungstate	Blue
Magnesium tungstate	Bluish white
Calcium borate	Pink
Calcium halo phosphate	White

When the main switch is put on, starter being closed initially, the current runs through the choke and (hen through the two electrodes. As a result electrodes get heated up and emit copious supplies of electrons. After a predetermined time, the starter switch opens automatically causing the choke to induce a momentary high voltage surge. This surge is sufficient to start the discharge. Once the discharge is started the electrodes require no other heat provided by ionic bombardment and the starter switch remains open during operation.

Starter switches used may be

(i) Glow type Switch:

In this type of switches, contacts are normally open. As the lamp is put into the circuit almost entire main voltage is across the switch electrodes thus causing a glow discharge to take place in the gas with which the switch is filled. This discharge heats a bimetal strip in the electrodes and causes the contact points to bend over and finally close. Now when the contacts are closed the current flows through the electrodes of the lamp and heats them. In the mean time the starter switch discharge is short circuited and therefore the bimetal stop gets cooled down thereby causing the contacts to part-open out thereby breaking the circuit and starting the main discharge. It may also be noted that the voltage across the lamp under normal operation is net sufficient to restart the glow discharge while switch remains open.

(ii) Thermal switch:

in thermal type switch, the contacts are closed when the lamp is not operating. As the tube light circuit is switched on the current flows through starter (and of course through choke and electrodes) and a special heater filament of the switch. This heater warms a bimetal electrode and after a few seconds during which the current is heating the lamp's electrodes the contacts part and start the discharge while the tube light is operating the contacts remain in the open position as the current still flows through the heater coil. The amount of electrical energy dissipated in the form of heat in maintaining the open position of the bimetal elements is small as compared to the tube light rating thus not affecting overall efficiency of the unit.

ILLUMINATION LEVELS :

The factors on which the degree of illumination necessary for a specific task depends are:
(i) Size of the object,
(ii) Speed of movement,
(iii) Duration of task,
(iv) Brightness contrasts,
(v) Color.


The deleterious effect on the observer of the brighter portion of the visual field is called glass Glare in its mildest form results in the distraction of the workers away from his task. However, when glare is intense physical discomfort is experienced. Excessive glare causes fatigue and reduces the ability to perceive. Adequate care must be taken while deciding about illumination level at a place to avoid glare.