The operation of vehicles is prohibited if:

Explanation

In both cases; both the absence of bulbs or diffusers and the use of bulbs and diffusers that do not match the type of lamp.

A vehicle lamp is not a simple bulb but an optical system: the reflector bowl, a bulb of a specific shape and wattage, and a diffuser lens that turns the rays into the designed pattern. If one of these three elements is missing or does not belong with the others, the output is no longer "headlamp light".

A lamp without a diffuser scatters light in all directions. The result is two simultaneous losses: the road ahead of you is less well lit, because the light disperses in the air instead of reaching the road surface; and the oncoming driver is dazzled, because the upper cut-off of the dipped beam has disappeared. An unsuitable bulb has the same effect: the filament does not sit at the reflector's focus and the light–dark cut-off is displaced, even though the lamp looks intact from outside.

The real danger is that a dazzled driver is effectively blind for a few seconds, and in those few seconds he covers tens of metres. Head-on collisions on two-way roads and running over a pedestrian at the roadside are the well-known consequences of this very defect.

Exam note: changing the type of bulb is not an "upgrade" but a defect listed among the grounds for prohibiting operation; the criterion is conformity with the type of lamp provided for in the vehicle's structure.

• The absence of a bulb or diffuser is the most obvious form of defect and is chosen for that reason, but it sets aside the second, less obvious case. • An unsuitable bulb or diffuser prohibits operation on its own too; so this option is likewise only half the answer.

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