Equipment: Lamps and Gear - Road Lighting: Fundamentals, Technology and Application - page 119

Fig. 10.8 Spectral power
distribution of the
high-pressure sodium lamp

300

200

100

0

400 450 500 550 600 650 700

λ (nm)

10.2.3.2 Lamp Properties

Spectrum and Colour Properties As with all gas-discharge lamps, the high-
pressure sodium lamp spectrum is discontinuous (Fig. 10.8 ). The main part of the
spectrum is centred around the yellow sodium line. It provides yellow-white light
with a colour temperature of around 2000 K and a colour rendering index R a of 25.
The colour rendering for road lighting is quite acceptable, although, for indoor light-
ing applications it is, of course, far from adequate 3 . Since the spectrum is relatively
strong in the red wavelength area, the rendition of human faces is often experienced
as being somewhat flattering. The S/P ratio is around 0.65.

Lumen Range High-pressure sodium lamps are produced in the approximate range
of 4000 (50 W)-150,000 lm (1000 W).

System Efficacy The efficacy goes from 80 lm/W, for the lower wattages, to 140
lm/W for the higher wattages.

Lifetime High-pressure sodium lamps have a lifetime, based on L 80 , ofupto
20,000 h.
Run-Up and Re-ignition The nominal pressure and full light output is reached after
3-5 min. Re-ignition of the hot lamp requires the lamp to cool down for about 1 min
to allow the pressure to decrease far enough for the ignition pulse to again ionise the
sodium atoms.

Dimming Lower wattages (100-150 W) can be dimmed to 20 % of light output
with special electronic gear. Higher lamp wattages can be switched to some 50 % of

3 By further increasing the sodium pressure, the colour quality of the light improves so that white
light is obtained with a colour rendering index of 80. These lamps are suitable for indoor lighting
applications but are seldom used in road lighting because their efficacy decreases to less than 50
lm/W.