A gaseous discharge lamp is essentially a glass or quartz envelop containing two electrodes and a small quantity of gas or vapour at a low pressure. In these lamps light is obtained by applying an electric potential difference to a gas or vapour contained by the lamp under a suitable pressure. Due to application of electric potential difference gas gets ionised and an electric current flows; the tube is filled with a luminous discharge. The light spectrum obtained is however discontinuous (i.e. it consists of one or more coloured line). The colour of the light obtained depends upon the nature of the gas or vapour used. Discharge lamps are of two types:
1) Those which give the light of the same colour as potential as produced by the discharge through the gas or vapour such as sodium vapour lamp, mercury vapour lamp, neon gas lamp. 2) Those which use the phenomenon of fluorescence and are known as fluorescent lamps. In these lamps, the discharge through the vapour produces ultraviolet waves which cause fluorescence in certain materials called the phosphorus. The inside of lamp is coated with phosphor which absorbs invisible ultra-violet rays and radiates visible rays. Example: Fluorescent mercury-vapour lamp.
High pressure mercury vapour lamp:
It consists of a discharge envelope enclosed in an outer bulb of ordinary glass. The discharge envelope may be of hard glass or quartz. The space between the bulb is partially or completely evacuated to prevent heat loss by convection from the inner bulb. The outer bulb absorbs harmful ultra rays. The inner bulb contains argon and a certain quantity of mercury. In addition to two main electrodes a starting (auxiliary) electrode connected through a high resistance. The main electrodes are made of tungsten wire in shape of helices. In this case no separate heater is required for the cathode which is heated by the constant bombardment of the heavy mercury ions. The choke is provided to limit the current to safer value. This choke lowers the power factor, so a capacitor is connected across the circuit to improve the power factor. These lamps must be operated vertically, since if they are used horizontally convection will cause the discharge to touch the glass bulb, which will fail. When the supply is switched on, full mains voltage is applied between the auxiliary electrode and neighbouring main electrode; this breaks down the gap and a discharge through the argon takes place. This enables the main discharge to commence. As the lamp warms up, mercury is vapourized, including increasing the vapour pressureand the luminous column becomes brighter and narrower. The lamp requires 4 or 5 minutes to attain full brilliancy. The temperature of operation inside the inner bulb is about 6000C. The efficiency is about 30-40 lumens/watt. Once ionisation has started in the gas, it has a tendency to increase continuously accompanied by a fall in the circuit resistance, i.e. a gas discharge lamp possess a negative resistance characteristic. To prevent current from attaining an abnormally high value, it is necessary to limit the current to a safe value which is accomplished by the use of choke or ballast in a.c. circuits. The choke performs the dual functions of providing the ignition voltage initially, and limiting the current subsequently. Since the power factor of choke due to self inductance is low (it is of the order of 0.3-0.4 only), the power factor of gas discharge lamps is improved by the use of a condenser.
Fluorescent tube is just a low pressure mercury vapour lamp. In these lamps considerable amount of radiation is in the ultra-violet range. By coating the inside of the tube by phosphor these uv heat radiations are converted into visible light. Phosphors have the property of emitting visible light radiations are converted into visible light. Phosphors have the...