Page 82 - Between light and shade

between light and shade, TRANSPARENCy and reflection

decreases with the colour temperature 90. Artists
and craftsmen are called upon to spread the light
emitted by these low-power but vibrant sources 91.
They create crystal chandeliers and play with
mirrors beneath very high ceilings.

18th century oil lamps, gas lamps and kero-
sene lamps, and other flammable sources marke-
ted in the 19th century offered better efficacy and
more significant luminous flux. In the towns
and cities, buildings were gradually connected
to gas distribution networks 92 for lighting and
heating. This network joined those for sewage 93
and water 94 and was followed by electricity
and telephone networks at the end of the same
century, and then by fibre optic networks at the
end of the 20th century. Street lighting was born,
improving not only the safety of city streets but
also that of buildings, whose intrusion protection
mechanisms were reduced. Major urban centres

90 This is why, according to Kruithoff, 15 Lux is sufficient for
candle light (Tc=2000°K), and 50 Lux for incandescent light
(Tc=2400°K), while we need 300 Lux for a colour tempera-
ture (Tc) of 3000°K (halogen) and more than 500 Lux for a
colour temperature (Tc) of 5000° K. The eye’s sensitivity to
light, which reaches its maximum in broad daylight for the
wavelength corresponding to yellow (555nm) shifts, at night,
to that of blue (470 nm) and is zero for wavelengths below 380
nm (ultraviolet) and above 770 nm (infrared).
91 The luminous efficacy of a candle is only 0.3 Lm/W.
92 In London in 1812, in Brussels in 1818.
93 Vaulting of the Senne in Brussels was only completed in 1871.
94 The supply of water to each building is relatively recent in
Belgium where it only became organised from 1860.

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