Equipment: Lamps and Gear - Road Lighting: Fundamentals, Technology and Application - page 120

light output, by including an extra inductive coil (ballast) in the lamp circuit, Lamp
colour remains virtually constant and lifetime is not affected.

Bulb Shape The outer bulb is either tubular (T-shape) or ovoid in shape. The internal
wall of the ovoid bulb is usually coated with diffusing powder. As a consequence
of this, these versions have a lower luminance at a similar lumen output. They
were originally introduced so as to obtain the same light-emitting surface area as
high-pressure mercury lamps have (see Fig. 10.12 ). In this way the coated ovoid
high-pressure sodium lamps could be used with the same luminaire optics as those
developed for high-pressure mercury lamps.

10.2.4 Tubular-Fluorescent Lamps

Tubular-fluorescent lamps belong to the family of low-pressure mercury lamps. As
with all low-pressure gas-discharge lamps, they are long: the tubular ones up to 1.8 m
for the higher wattages. Their length makes good optical control in any plane through
the lamp axis, which in road lighting is across the road axis, impossible to achieve.
Given their moderate lumen output, there where moderate to high lighting levels
are required, the luminaire would need to house several lamps in order to achieve
that lighting level. So, the length of the tubular versions and their moderate lumen
output are decided disadvantages in so far as the use of these lamps in road lighting is
concerned. We will therefore not go into detail on these lamps, although some older
road-lighting installations do still employ tubular fluorescent lamps.
Compact fluorescent lamps may sometimes offer cost-effective solutions for use in
residential and pedestrian areas and are therefore dealt with in the following section.

10.2.5 Compact Fluorescent Lamps

Low-pressure mercury lamps of the compact type, normally referred to as compact
fluorescent lamps (CFLs), were originally developed at the beginning of the 1980s for
use in those applications where incandescent lamps were traditionally in use. Today,
the application of “twin-leg” compact fluorescent lamps has been widened to include
those residential streets where relatively low lighting levels suffice. Alternatives are
low wattage high pressure sodium, metal halide and to an increasing extent solid-state
lamps.

10.2.5.1 Construction

The operating principle of compact fluorescent lamps is exactly the same as that
in tubular fluorescent lamps. A tube made out of glass is filled with an inert gas