|
For the fluorescent material currently employed in virtually all mercury lamps still manufactured worldwide, the lighting industry is indebted to another closely related sector which also has a keen interest in phosphor development - the cathode ray tube industry.
Following the boom in the production of colour televisions, a phosphor was required which gave a deeper red emitting, and more efficient red phosphor than had ever been produced before. It was known that Yttrium oxide with an activator of Europium met the colour and stability criteria, but there existed tremendous difficulties in the adhesion strength of this phosphor to the glass substrate, owing to the poor control over the particle sizes that could be produced with this material.
A suitable alternative was invented during 1964 by Frank Palilla and Albert Levine at GTE Sylvania Laboratories in the USA, this material being Europium activated Yttrium Vanadate (Levine and Palilla, 1964). It offered equivalent performance to the oxide phosphor but without the adhesion problems. The technology was quickly transferred from the company's cathode ray tube division to the lamp department, where it was immediately adopted in mercury discharge lamps.
The new phosphor made the former Purple stained Colour /X American lamps obsolete literally overnight, and such was the increase in performance it delivered, that within two years practically every mercury lamp plant worldwide had changed over to this new material. For the first time a phosphor was available which matched every single ideal requirement for mercury lamps, i.e.
-
Strong red emission
-
High quantum efficiency
-
Sensitivity to both short and long-wave UV stimulation
-
Colourless, i.e. no longer was blue light absorbed by the coating
-
Efficient at high lamp operating temperatures
-
Stable throughout very long lamp lives
Despite the high cost of europium and yttrium the advantages of this phosphor were such that every customer cried out for the new colour-improved lamps, and its cost was easily justified.
The red emission peaks of this phosphor centre at around 615-619nm, much shorter wavelengths than the former deep red 658nm peak of the germanate and arsenate materials. But owing to the spectral sensitivity of the human eye, this is still seen as red, and it is a wavelength to which the eye is much more sensitive. A combination of this effect, and the fact that the new powder coating had a much lower absorption of other wavelengths, the total luminous efficacy of vandate lamps increased by around 10% at the same time.
Subsequently in later years, the problems with yttrium oxides were also overcome, and today it is an important fluorescent lamp phosphor. Despite its slightly lower cost, it has never become popular in mercury lamp use and all manufacturers today still continue with the vanadate material, even though the performance of both phosphors is substantially the same.
Improved Vanadate Coatings
|
