ISO 4182-1999FMVSS, 1998. FMVSS 108. Lamps, reflective devices and associated equipment. Federal Motor Vehicle Safety Standards and Regulations, U.S. Department of Transportation, National Highway Traffic Safety Administration, Safety Assurance, Office o
适用范围
A precise objective definition of a low beam visual vehicle headlamp cut-off has become necessary,
since a requirement for the correct aiming of these beams is specified within all existing regulations.
Until recently, only European countries had regulations for cut-off based on visual aiming. The
Northern American countries used to require geometric aiming of totally standardised devices, but since 1997 the USA has also introduced visual aiming. More recently, the evolution of the shape of
the front of vehicles has led these countries to consider new headlamp regulations, which could lead
them to prescribe an initial vertical aiming on the vehicles using other methods than the geometric
method prescribed until now. Discussions regarding the definition of a worldwide low beam pattern
have also stimulated a requirement for a visual aiming procedure.
These issues mean that it is now necessary to determine a clear and acceptable definition of
headlamp cut-off. The visual definition currently used in Europe is not sufficiently accurate, mainly
because it calls upon visual perception, which can be very different from one observer to another.
A good level of accuracy could be obtained by accepting the mean value of several aiming
operations. Nevertheless, this could not be done in practice under the conditions normally
encountered when performing aiming operations in car factories or repair shops, since it would require
a considerable amount of time. For the same reason, it could not be carried out during routine
operations in laboratories.
The only method that can provide, in one determination, the required level of accuracy in the
objective determination of the cut-off, is one using physical measurements and based on good
knowledge of the visual perception.
The difficulty in obtaining a cut-off model lies in the fact that it should represent the visual
perception of the projection of a luminous phenomenon on a screen. Consequently, the model has to take into account the luminous beam itself, the observers (who will be presented as a mean value),
the kind of screen on which the beam is projected and the distance at which the screen is situated
from the device emitting the light. Discussions and studies have been undertaken during the last 25
years, at first mainly in Europe, and later mainly in North America where the cut-off system is
prescribed. All or parts of these parameters have been taken into account during these studies or
discussions.
The following text (i) sets out the definitions that could be given to the "cut-off" according to the
above-cited parameters, and (ii) gives a short summary of the discussions held on the subject, and
the results of the studies conducted (including models, if any). Finally, in Clause 6 some conclusions are given. These conclusions indicate that a number of methods are acceptable, and it is still necessary for the responsible bodies to decide which parameters should be chosen and which value
should be given to set an internationally accepted standard.