At
the launch of the XM, great claims were made
for its "complex surface" headlights:-
Car
lighting
is an area where technology is constantly
advancing and improving. It is also an area
where French manufacturers are among the
leaders.
The
efficiency
of French lighting equipment is not new. The
tradition goes back to Bleriot, Auteroche,
Grebel, Marchal and Cibie, who constantly
experimented with new ideas and made
significant steps forward with their products.
Today Valeo, the inheritor of the traditions
of all these companies, continues along the
same road they trod. This is the company which
supplies Citroën with the "complex surface"
headlamps fitted to the XM.
The
output
and efficiency of headlamps is determined by
the manufacturing quality, shape and above
all, the size of their reflectors. Twenty-five
years ago, headlamps were round, as they had
been forty years before that. This was the
shape which was easiest to manufacture, gave
the most efficient reflector shape and the
best ratio of width to depth. Moreover the
round shape enabled standardisation of
manufacture, since the same headlamp could be
used for different models and marques.
Then,
however,
in response to the demands of stylists wishing
to give their cars a more distinctive touch
and of aerodynamicists wishing to reduce
frontal area and streamline shapes, the
manufacturers began to make head lamps which
blended with the wing, bonnet and front
grille. Before long we saw head lamps in the
form of ovals, oblongs, squares, rectangles,
trapezoids and so on.
One
of the first cars to be equipped as standard
with new generation headlamps was the
Citroen Ami
6 of 1961. Today, when most headlamps
are of varying shapes, one rarely sees the old
round type. However these different shaped
lights have inferior performance to round
parabolic ones. The halogen bulb came along to
replace the old filament-type bulb and
compensated for this loss in performance to
some extent. This bulb, already used in
aircraft landing lights and in competition
cars, gave a beam that was more piercing,
brighter and more uniform than that of a
filament bulb.
However,
it
took the manufacturers several years to
produce a halogen bulb combining the full and
dipped beam function. Today the facility
exists with the H4 type bulb which equips all
halogen type headlamps but which remains
expensive.
On
today's
roads the driver uses dipped beam most of the
time, since with the growth of road traffic it
is more and more difficult to employ full beam
without annoying other road users. Yet on
dipped beam the spread is notably diminished
and visibility is reduced. All manufacturers
have been aware of the problem but it was
Valeo who came up with the solution and
launched a new generation of head lamps:
"complex surface" lamps, from which the XM is
the first car in the world to benefit.
Complex
surface
headlamps consist of a reflector, elliptical
in shape, its surface defined point by point
(50,000 points) by means of a computer; so
that 100% of the light is reflected within the
limiting pattern laid down for dipped beams by
the regulations. The shape of this complex
surface gives much better performance than the
conventional metal shades which, even with
halogen bulbs, absorb up to 45% of the
reflected light. One can therefore say that
the complex surface lamp offers 45% better
performance than traditional types.
This
means
that the width and range of the headlamp beams
in the Citroen XM are much more powerful than
those in competing models, though the latter's
lamps may be appreciably larger.
The
complex
surface headlamp has other advantages. The
glass can be raked more steeply in order to
improve the Cd, something which was not
previously possible since an inclined glass
absorbs too much light (12% loss for a 20%
slope). The glass of the XM headlamp is fixed,
and the headlamp aim is achieved by moving the
reflectors. This was necessary because in
choosing to favour the aesthetics and
streamlining of the nose shape, the joints
between headlamps, front grille, bonnet and
wings have been reduced significantly. This
means there is insufficient area to allow the
aiming of the whole lamp unit as is the case
with the head lamps of other cars, which is
why the reflector is used to shift the beam
and adjust the lamp.
The
head
lamps fitted to the Citroen XM, shallow,
ellipsoidal and very powerful, may well
attract as much attention and be as much
acclaimed as the directional headlamps of the
DS were in their day.
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