First attempts to produce light using electric
discharge were made at the end of 19th century and both Tesla and Edison
In 1901, Peter Cooper Hewitt obtained a patent for his
low pressure discharge mercury vapour tube.
There has been enormous progress in the technology of phosphors since the
first fluorescent tubes appeared in nineteen-thirties. The development of
colour TV and the discoveries of lanthanides, or rare earth elements - not much
know before then, had a great effect on this.
There are about 10 different basic types of phosphors. The desired spectrum
of a lamp can be obtained by mixing these powders in the right quantities. A
CRI of about 90 can be achieved in this way.
Using special phosphors and techniques, partly patented and partly kept
secret by their producers, a CRI of up to 98 can be achieved.
Phosphors are powder mixtures of complex salts of elements such as (for example):
aluminium, antimony, barium, calcium, cerium, europium, gadolinium, germanium,
magnesium, strontium, terbium, tin, yttrium and chlorine, fluorine and
Thicker layers of phosphors are sometimes used to reduce the peak light of
the mercury discharge to further improve colour rendering, for the sake of a
bit of efficiency.
A great invention in this field was the “hot spot/cool spot” arrangement.
The temperature difference works to prevent the migration of the mercury ions
caused by direct electric current, keeping the lamp in a good shape
even when operated on steady or pulsating DC (Direct Current).
The smaller the diameter of the tube, the better is the efficiency, and
less material is needed to build a lamp. Between the 30's and 80's, T12 (1
1/2") tubes dominated the market. The 80's brought T8 (1") to the mainstream
and T5 (5/8") is the current trend today.