Discharge sources
cold cathode (fluorescent)
mercury fluorescent (high and low pressure)
low pressure sodium
high pressure sodium
high pressure mercury
metal halide (inc. ceramic arc)
Fluorescent lamps
halophosphor – tubular fluorescent triphosphor
compact fluorescent
induction lamps
Fibre optics (remote source) At its simplest, it is the transfer from a light
source placed in one position to light emitted in another, by means of
glass fibre or polymer strands.
Fluorescent phosphors The internal coatings on surfaces of mercury
discharge lamps which produce ‘visible’ light when excited by the ultra-
violet rays emitted by the discharge. The phosphors determine the colour
of the visible light.
Gaslight The light derived from burning coal gas, developed in the late
eighteenth century, and widely used during the nineteenth century both
for domestic and industrial use.
Oil lamps These together with firelight are the earliest forms of artificial
light source, the oil being derived from animals, birds or fish. Hollowed
out stone dishes and later clay pots were used with some form of wick.
Oil lamps survived until the nineteenth century with the development of
the ‘Argand’ lamp.
- LIGHTING TERMINOLOGY
Angle of separation The angle between the line of sight and the light
fitting. The smaller this is, the more likely the light will be glaring.
Brightness The subjective appearance of a lit surface; dependent upon
the luminance of the surface and a person’s adaptation.
Bulk lamp replacement The replacement ‘en masse’ of the lamps in a
lighting system when it is calculated that a percentage of the lamps will
fail, and that the light output of the system will fall below the design
level.
Colour We accept that we only see true colour under daylight, despite
the fact that this varies considerably throughout the day. All artificial
sources distort colour in one way or another.
Colour renderng A comparison between the colour appearance of a
surface under natural light and that from an artificial source.
Efficiency/efficacy The ratio of the light output from the lamp, toe
nergy consumed in lumens/watt.
Flicker The rapid variation in light from discharge sources due to the 50
Hz mains supply, which can cause unpleasant sensations. With the devel-
opment of high frequency gear the problem is overcome.
Glossary 203