At the launch of the XM, great claims were made for its "complex surface" headlights:-
lighting is an area where technology is constantly advancing and
improving. It is also an area where French manufacturers are among 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 CitroŽn with the "complex surface" headlamps
fitted to the XM.
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.
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.
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
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.
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.
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.
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.
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.