||The MAF lamp illustrated here represents the first application of a phosphor coating to the outer jacket of a mercury lamp. It was developed in 1937 at the Wembley laboratories of Osram-GEC in London, where the modern medium pressure mercury lamps had been invented just a few years earlier.
The luminescent material is zinc cadmium sulphide with an impurity of copper as the activator (Zn,Cd) S:Cu. This type of coating is sensitive to the longwave UV radiation which is able to escape through the hard glass arc tube. The great size of this lamp is necessary to ensure that the temperature of the phosphor does not exceed 150°C, above which it loses its efficiency for converting the UV into reddish-orange light. The special shape of the bulb has an isothermal profile, and when the lamp is operated in the cap up position, the phosphor temperature is maintained in the region 100-150°C.
This phosphor has such a high cadmium content that it shows a natural yellowish body colouration. This quite strongly absorbs the usual mercury blue lines, and despite the extra red radiation, it makes the green mercury lines relatively more powerful. The colour hue of the MAF lamp was thus not always considered to be so desirable. The problem in this lamp is partially overcome by the addition of cadmium to the arctube, which radiates extra blue lines, and some weak red radiation. Luminous efficacy falls as a result, but the red ratio is valuably increased from around 1% to approximately 4%.
It is interesting to note the conical form of the arc tube at the lower end. This serves to raise the wall temperature in this region, and assists in keeping the additional cadmium metal dose fully vaporised into the discharge.