I think you are thinking about mercury vapor lamps.
These were widely used for motion picture phtography in the
silent era. Some they were also used for plate burning for
half-tone plates and in some large format enlargers such as
the old Saltzman units. These lamps must be run continuously
and exposure controlled with a shutter. The output is mostly
near UV.

Cold light heads are fluorescent lamps. They are actually
similar to a low pressure mercury lamp inside but the inside
surface of the envelope is coated with a fluorescent
material which converts the UV to visible light. The mix of
materials in the coating determines the spectral output.
There are low equivelent color temperature cold-lights which
will work fairly well with variable contrast filters. I
believe the Ilford VC lamp is composed of two fluorescent
lamps, one concentrated in the green range. However, most of
the old cold-lights were very blue intended to be used with
graded paper.

  While any diffusion head tends to blur certain kinds of
negative blemishes the increased paper contrast needed to
print with them sometimes brings them right out again. Even
with a cold light one must be careful of dust:-)

  I don't know when cold light was introduced but I think
it may have been as early as the late 1930's.  They
certainly became popular in the late 1940's.