In 1852, Sir George Stokes had discovered transformation of the ultra violate ray radiation into visible radiation.
From this time to 1920 various kinds of experiments had been done to develop the low and high pressure electric discharges in the mercury and sodium vapor. But all those circuitry developed were inefficient to transform the ultra violate ray to the visible ray. It was because; the electrodes could not emit sufficient electrons to establish arc discharge phenomenon. Again many of the electrons collided with the gas atoms and that was elastic. So the excitation did not create spectral line to be utilized. But very little work was done on fluorescent lamps.
But in 1920s, a major breakthrough happened. The fact has been discovered that the mixture of the mercury vapor and inert gas at low pressure is 60% efficient to convert the electrical input power into a single spectral line at 253.7 nm.
Ultra violate ray gets converted into the visible light rays by using appropriate fluorescent material inside the lamp. From this time fluorescent lamp got paved to be introduced in the people’s daily life.
Later, Dr. W. L. Enfield in 1934 had received a report from Dr. A. H. Crompton about the use of fluorescent coated lamp. Immediately a research team had been created by Enfield and started to create commercial fluorescent Lamp. In 1935 their team had produced a prototype green fluorescent lamp that had efficiency about 60%.
After two and half years later, fluorescent lamps have been introduced in white and six other colors in the market. Various mixture of phosphor powder is used to produce various colors from the fluorescent lamps. The first lamp had been introduced with 15, 20 and 30 W in 18 inch, 25 inch and 36 inch lengths.
Sooner after 40 W T12, 4-ft lamp had been introduced and used widely in office, school, industry lighting. The early lamps gave the light somewhat yellowish to a 3500K. Later on, 6500K daylight lamps are developed in such a way that it produces a light to simulate an average north sky light on an overcast sky.
Generally the 4 ft lamps, with 1.5 inch in diameter, 40 W were available in the market in 1940. But gradually design has been changed to its better utilization. In the arc, discharge portion of the lamps has been changed. But argon is still used though the pressure that is somewhat less than the previous pressure. The mercury vapor is maintained at that same pressure like previous. This lamp requires 425 mA with 100 to 105 V voltage drop.