Here is an experiment on the ‘Sources of Light’ especially written for school and college students.

Aim of the Experiment:

Study of different types of sources of light and make connections, and measure intensity of light with lux meter.


To measure the intensity of light with lux meter of:

(i) Fluorescent lamp


(ii) H.P. mercury vapour lamp

(iii) H.P. sodium vapour lamp

(iv) Compact fluorescent lamp (CFL).

Apparatus Required:

Luxmeter, fluorescent lamp, H.P. mercury vapour lamp, H.P. sodium vapour lamp, and compact fluorescent lamp, connecting wires etc.



Luxmeters (Light Meters):

The meters or light meters measure illumination in terms of luxes (lx) or foot candles (fc). A lux is equal to the total intensity of light that falls on a one square meter surface that is one foot away from the point of source of light. A foot candle is equal to the total intensity of light that falls on a one square foot surface that is one foot away from the point source of light. Most lux meters or light meters consist of a body, photocell or light sensor, and display. The light that falls on to the photocell or sensor contains energy that is converted to electric current. In turn, the amount of current depends on the amount of light that strokes the photocell or light sensor.

Luxmeters read the electrical current, calculate the appropriate value, and output the results to an analog digital video display. Since light usually contains different colours at different wave lengths, the reading represents the combined effects of all the wave lengths. Typically, standard colors or color temperatures are expressed in degrees kelvin (K). The standard colour temperature for the calibration of most lux meters is 2856°K, an amount that is more yellow than pure while.

Selecting luxmeters or light meters requires certain performance specifications include photo cell, illumination range. Lux resolution, operating temperature and foot candle resolution.


Analog devices display values on a dial usually with a needle or pointer. Digital devices display values as numbers letters. Lux meter may be portable or designed to sit atop a desk. Special feature includes low battery indicators, low voltage, alarms, remote light sensors etc.

Lux meters are used to measure levels of light in schools, hospitals, production areas, laboratories. They are also used to monitor light sensitive displays in museums, art galleries etc., special features include backlit displays, low battery indicators, low voltage alarms, built in memory. Auto powers off zero function.

Measurement of Light Intensity by Digital Luxmeter

Analog Luxmeter:


Analog Luxmeter display values on a dial usually with a needle or pointer it is operated with low voltage battery the intensity of light of any lamp can be measured by connecting loads from luxmeter to the lamp terminal when it is glowing.

Measurement of Light Intensity by Analog Luxmeter

Specifications of Luxmeter:


(i) Digital Luxmeter:

a. Compact and rugged design with large display

b. Widest range to 40,000 Fc/400,000 Lux is ideal for outdoor applications

c. Data Hold and Min/Max readings

d. Auto power off, zero function.  

(ii) Analog Luxmeter:

a. Widest range of measurement upto 10,000 Lux with accuracy of ± 10% operating voltages of upto 300 V ac.  

Procedure: (To measure Intensity of Light of CFL):

1. Make the connections as shown in fig. 1.

2. Keep the source of light one feet away from the light sensor.

3. Now switch on the luxmeter.

4. Note down the reading from Luxmeter and record in observation table.

5. Repeat the above procedure for the fluorescent lamp, mercury vapour lamp, sodium vapour lamp.



The different lamps have different intensity of light.

Curve between I and r

Fluorescent Tube (Lamp):

It is a low pressure mercury vapour lamp. Due to low pressure the lamp is in the form of a long tube. In this type of lamps, fluorescent powders are used to coat the inside of the lamp. The tube is filled with small quantity of mercury with small amount of argon gas. The electrodes are oxide coated filaments in the form of spiral.

A starter ‘S’ connected in between the filaments which is a small cathode glow lamp having bimetal strip at the electrodes. The choke is connected in series with the tube filament as well as supply.

The choke acts as a blast and gives 1000 V while the starter is open. It gives low voltage of about 110 V while working. The starter completes a series circuit of tube, choke and supply. Then it opens and interrupts the current in the circuit while the tube is running. Capacitor of 4 µF is connected across the supply to improve the power factor.

Connection Diagram for a Fluorescent Tube

Different sizes of lamps are available from 2 ft – 20 W, 4 ft – 40 W and about 5/8” dia to 1½” dia with Bipin caps.

Lamps are also available in different colours like red, green, blue and tri-colour. Parts of a fluorescent lamp are shown in Fig. 5.

Parts of a Fluorescent Lamp


When the starter is not connected in the circuit, the electrodes are open. As the lamp is switched on, the current passes through the filaments that are heated and emit large amount of electrons. The current also passes through the normally close contact ‘CC’ of the starter and heated it, thereby contacts are separated after a few seconds. This sudden opening of contacts ‘CC’, due to action of choke, high voltage surge of about 1000 V is induced across the electrodes and discharge starts and the lamp lights up immediately.

The lamp emits good quality light close to daylight and free from colour distortion. Its efficiency is between 30 – 40 lumens/watt. The initial cost is high but the life of lamp is much more about 3000 working hours.

Tube Starter:

Tube starters are of two types:

(i) Glow type

(ii) Thermal type but former is commonly used.

(i) Glow Type Starter:

This starter is enclosed in a bulb with an inert gas, containing two contacts, one of which mounted on a bimetal strip. The contacts are normally open.

When the circuit is closed, full voltage appears across the switch contacts which sufficient to strike the glow discharge. The heat of discharge warms the bimetal strip and closes the contact. This allows preheating current to pass through electrodes. The closing of starter contacts extinguishes the discharge between them.

High Pressure Mercury Vapour Lamp (HPMV):

It is similar in construction to the sodium vapour lamp.

There is an inner tube of special quality glass containing argon gas and a little mercury at low pressure. It is enclosed in an evacuated outer tube in order to prevent loss of heat. Two oxide coated electrodes (electron emitting material) are enclosed in the inner tube.

The working temperature is about 600°C and the lamp should be operated in the vertical position. So that the mercury does not touch, the inner walls of the glass tube.

There is a starting electrode used in order to start the discharge. It contains a high resistance of the order of 50 kW in series with supply, the same end of which is connected to a main electrode. As the lamp is switched on, ionisation of argon between the main electrode and the starting electrode starts. Later on this process is extended between the two main electrodes. When the ionisation is completed, discharge starts in the tube and the mercury starts evaporating due to heat of discharge and in a minute or so the discharge passes through the vapour. Thus the lamp starts. At this time the current through starting electrode is reduced to small leakage current.

High Pressure Mercury Vapour Lamp (HPMV)

As the discharge has negative resistance characteristics i.e. the resistance falls down quickly as the discharge is completed, so a choke is added in series which acts as a stabiliser. The condenser ‘C’ improves power factor of the lamp. The heat of discharge is sufficient to extract electrons from the electrodes. The lamp takes about five minutes to be on to full light after switching. Once the lamp is switched off or supply fails, the mercury quickly condenses and restarting again takes about the same time.

The lamp works at a pressure of about one atmosphere.

The lamp is deficient in Red light. So it emits a characteristic greenish blue light (not white). Thus it produces colour distortion. It has an efficiency of about 40 lumens per watt. Its main use is on thorough fares, industrial lighting and in street lighting.

Sodium Vapour Lamp:

The major application of this type of lamp is for highway and general out­door lighting where colour discrimination is not required. The sodium vapour lamp is most efficient of discharge lamp, the efficiency being of the order of 60 to 70 lumens per watt. However, in terms, of input power and output, the overall efficiency is only about 20% [Rest being heat and other invisible radiation]. The sodium vapour lamp is only suitable for alternating current.

Sodium Vapour Lamp


It consists of a U-type of special resistant glass which is known as discharge tube. A small quantity of neon gas and sodium vapour, are introduced in the tube. The presence of neon gas serves to start the discharge.

In order to reduce the heat losses, this U tube is enclosed in a double-walled evacuated glass jacket known as outer tube. A high leakage reactance transformer is connected in the circuit as shown. It provides a high voltage (about 480 V) to start the discharge. Also, because of its high reactance, it works as a stabiliser.

A static capacitor C is used in order to improve the power factor.


As the lamp is switched on, electrons are emitted from cathode and attack the gas molecules. This starts the process of ionisation and the discharge commences. The sodium is vapourised due to heat of the discharge and the lamp assumes normal operation. The lamp will come up to its rated light output in about 15 minutes.

Since the discharge has a negative resistance characteristics, so a high leakage reactance transformer is used which stabiliser the current.

The lamp emits characteristic yellow high at a single wavelength of about 5980 A.U. Due to this reason; it is sometimes called a monochromatic lamp. The working temperature is about 300°C. The starting time of the lamp is from 5 to 6 minutes due to the time taken for sodium to vapourise. The lamp must be operated horizontally or nearby so as to keep the sodium well spread out along the tube.

Once the lamp goes out and sodium vapours condense, may be due to supply failure, on resumption, it will again take its normal time to start. Such lamps are manufactured is 45, 60, 85 and 140 watt ratings. The average life is about 3000 hours and is not affected by voltage variations. The major application of this type of lamp is for highway and general outdoor lighting where colour discrimination is not required. The luminous efficiency of the lamp is about 60 lumens/watt.

Compact Fluorescent Lamp (CFL):

A compact fluorescent lamp (CFL), are also known as a compact fluorescent light bulb. Compared to incandescent lamps of the same luminous flux, CFLs use less energy and longer rated life. The purchase price CFL is higher than of an incadescent lamp of the same luminous output, but this cost is recovered in energy savings and replacement costs over the bulbs time.

The modern CFLs typically have a life span of between 6000 and 15000 hours, whereas incandescent lamps are usually manufactured to have a life span of750 to 1000 hours.

The life time of any lamp depends on many factors including manufacturing defects, exposure to voltage spikes, mechanical shock and ambient operating temperature, among other factors.

CFL lamps give less light later in their life than they did at the start. CFLs are produced for both a.c and dc input d.c CFLs are operated with car batteries. CFLs can also be operated with solar powered street lights, using solar panels located on the top or sides of a pole and luminaires that are specially wired to use the lamps.

Different Type Lamp (CFL) Compact Fluorescent Lamp

Salient Features of CFL:

1. Voltage limits AC 220 V – 240 V, 50/60 Hz

2. Long life

3. High Brightness

4. Low power consumption

5. Quick starting

6. No interference to TV or Radio

7. Not fit for turning light.  


Never use with following fixtures:

1. Emergency light fixtures

2. Direct current fixtures

3. Never install where water may drop on the lamp

4. Never install where the ambient temperature may rise about 400°C

5. Unsuitable for applications where the lamp must be turned on and off frequently

6. Never use where there is frequent fluctuation of voltage.