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.