In conventional control panels for installations of the type mentioned above, the front face of the front wall of the light box is opaque except in predetermined lighting zones, which are for example disposed around the periphery of holes formed through the front wall for receiving control members such as rotary knobs or push buttons.  These control members, whether of the mechanical or electrical type, are arranged for controlling particular functions of the installation.  The lighting zones mentioned above are intended to enable the driver and/or a passenger in the vehicle to locate the appropriate control members and their respective positions, especially during night driving.  The lighting zones include indicia in the form of symbols, characters or other marks, of predetermined colours such as white, orange, red and/or blue for example.  It is usual generally to provide at least two different colours, red and blue being the commonest. 
With these conventional arrangements, several problems arise.  A first of these lies in the fact that the distribution of light behind the front wall of the control panel is never as uniform as would be obtained with an ideal light box.  In this connection, it is found that the luminous intensity available at any point behind the front wall that is lit directly by the light source decreases substantially as the distance separating that point from the light source, or the closest light source, increases. 
In addition, the axes of the rotary knobs, the means for guiding the push buttons, and more generally the whole of the means contained within the housing of the control panel and enabling it to function, create shadow zones behind the front wall, these being so called because they do not receive direct lighting from the light source or sources.  They only receive indirect lighting produced by the rays issued from the light source after being reflected at least once from the walls of the light box (the coefficient of reflection of these walls being less than unity). 
As a result, variations in luminance occur on the front face of the front wall, in particular from one point to another in the lighting zones corresponding to indicia of the same colour.  It is known that luminance is a physical quantity that characterises luminous intensity per unit surface area of any light source, being expressed in candela per square metre (cd/m.sup.2).  In the present document, reference will sometimes be made to the luminance of the symbols considered as light sources on the front face of the front wall of the control panel. 
Variations in luminance which are found to exist in practice can be as much as fivefold within a lighting zone corresponding to symbols of the same colour.  Such variations in luminance are considered to be a manufacturing fault, because they detract from the appearance of the styling component of the control panel within the fascia panel of the vehicle in which it is to be mounted. 
A second problem arising in conventional control panels is that lighting zones can include indicia of a first colour, for example white, while other zones contain indicia of a second colour, for example red, or a third colour, for example blue.  In this connection it is well known that, in order to be visually comfortable to the user, indicia of various colours have to have different luminances at the front of the control panel. 
For example, white symbols should have a luminance of the order of 8 cd/m.sup.2, red symbols have a luminance of the order of 3 cd/m.sup.2, and blue symbols a luminance of the order of 0.3 cd/m.sup.2.  The difficulty of obtaining such values of luminance at the front of the front wall of the control panel increases according to the extent to which the layers of paint used have different transmittivities, determined by their colour. 
There is therefore a need to find ways, firstly to compensate for imperfections in the light box generating differences in the lighting of the front face of the front wall of the control panel, and in consequence variations in luminance in front of the front wall for lighting zones having indicia all of one colour; and secondly, if necessary, to introduce voluntarily a difference in luminance in the front of the front wall between light zones which have indicia of different colours.  In other words, there exist lighting zones of a first type, the illumination of which by the light box needs to be improved, and also, light zones of a second type in which, by contrast, illumination by the light box needs to be attenuated. 
In the present state of the art, it has been proposed to make use of optical bars made in a material having a refractive index greater than that of air, for guiding the light from the light source, where it is collected by a first end of the bar, to a lighting zone of the first type in which it is restituted by a second end of the bar. 
Control panels have also been proposed in which the front element of the housing comprises, besides the front wall itself, an element having transmittivity which is locally variable, being for example greater in those portions which are designed to be in line with lighting zones of the first type, and smaller in those portions which are intended to be in line with lighting zones of the second type.  This element may for example consist of a silk screen printed film applied on the front of the front wall. 
However, these known solutions have the disadvantage that they call for additional components, which increases the cost of the control panel and makes its assembly more complex.  In addition, these components cannot be used in different versions of a control panel corresponding to different levels of apparatus being controlled, and it is necessary to design them especially for each different version of the control panel. 
It has also been proposed to deposit on the rear face of the front wall element a number of layers of translucent paint, this number of layers being greater in the lighting zones of the second type than in those of the first type, thus locally modifying the transmittivity of the front wall, However, this method is found to be expensive in quantity production applications, because an operation of applying paint is complex and somewhat delicate to perform.