FR2482701A1 - Lens for high intensity lamp - has dropped square pyramid with triangular lower panels and vertical side walls - Google Patents

Abstract

The lighting system for a building interior and exterior includes lighting fixtures including a high intensity discharge lamp and lens. The lens is formed as a dropped square pyramid with four triangular lower panels and four vertical side walls. Prisms on the inner and outer faces of the lens produce a higher luminous intensity at an angle of about 30 degrees to 50 degrees than at nadir and project light at higher angles toward the corners of the lens than perpendicular to the sides of the lens, producing a square, widespread light distribution pattern. The lens cuts off high angle light reducing glare.

Description

 The present invention relates to a lighting device comprising a lens of a new type. The invention relates more particularly, but not exclusively, to a lens for a high intensity light source, such as a high intensity discharge lamp (HID), to ensure better illumination of the interior of buildings.

The invention is also applicable to other light sources.

 To light indoor or outdoor areas, one should strive to produce the necessary levels of lighting using as little energy as possible and as little lighting equipment as possible. Consequently, it is entirely desirable to use light sources having a high efficiency, expressed in lumens per watt. For this reason, fluorescent lamps have been used more and more instead of incandescent lamps. However, fluorescent lamps have two drawbacks compared to high intensity discharge (HID) lamps. Firstly, although their efficiency is high, it is not as large as that of a large number of available HID lamps. Secondly, fluorescent lighting lamps are bulky and, due to the poor efficiency in lumens of these lamps, many devices must be provided to light an area with the levels of lighting commonly required.

Consequently the use of high intensity discharge lamps seems preferable but two disadvantages of said lamps must be remedied before their use is generalized. The first difficulty concerns glare.

Due to the very large amount of light emitted from a relatively small area, the degree of glare can be extremely high if not controlled by suitable optical means. The second difficulty consists in that, because of the very great power of illumination, it is enough for a small number of apparatuses to establish normal levels of illumination. And consequently these apparatuses are very spaced apart others. Large spacing between lighting fixtures tends to create large dark areas between fixtures, while below them there are very high levels of illumination, so the light emitted should be spread out to cover the surface. but it should not be excessively spread out because concentrated light would then be produced close to the horizontal, causing glare. Lighting fixtures with high intensity discharge lamps which are used at present do not solve the above problems adequately and do not provide acceptable interior lighting.

 The object of the invention is therefore to provide a lighting device comprising a lens which ensures a relatively uniform distribution of light in a space lit by several devices.

 Consequently, the invention relates to a lighting apparatus comprising a housing, means placed in the housing for supporting a high intensity lamp, and a lens extending below the housing, characterized in that the lens comprises a rectangular pyramid directed downwards and comprising four side panels oriented essentially vertically and four lower panels of triangular shape, all the panels comprising prisms on their internal faces and on their external faces, said prisms comprising means making it possible to produce, in all the lateral positions, a much greater light intensity in a direction away from the vertical or nadir than along the vertical or nadir and to produce a maximum light intensity at an angle to the vertical or nadir which is smaller in a perpendicular lateral direction alongside the lens only in a lateral direction oriented towards the co lens ins.

The invention also relates to a lens usable in a lighting device below a high intensity lamp, said lens comprising a rectangular pyramid directed downwards and comprising four vertically oriented side panels and four triangular shaped bottom panels, all the panels comprising prisms on their inner faces and on their outer faces, said prisms comprising means for producing a much higher light intensity in directions distant from the vertical or nadir than along the vertical or nadir and for producing a maximum light intensity according to an angle with respect to the vertical or nadir which is smaller in a direction perpendicular to the side of the lens than in a direction oriented towards the corners of the lens, the prisms placed on the outside faces of the lower panels being generally oriented parallel to the outer edges of the panels and const using means used to produce a much greater light intensity in directions distant from the vertical than along the vertical or nadir, and the prisms placed on the inner faces of the lower panels being oriented generally perpendicular to the outer edges of the lower panels and constituting means for increasing the light intensity in a lateral direction towards the corners of the lens.

The light intensity produced by the lens, and measured in a horizontal plane below each luminaire, reaches a maximum value for an angle to the vertical or nadir which is larger along a directed line toward each corner of the rectangular lens only along a line perpendicular to an outer edge of the lens. As a result, the light pattern produced by each light fixture is generally rectangular; the fixtures are particularly well suited for illuminating interior volumes but can also be used to illuminate outdoor areas, such as parking areas and work stations. service stations, where it is desirable to have uniform lighting
The rectangular lighting pattern makes it possible to space the devices on a rectangular grid and establish generally uniform lighting levels even when the ratio of the spacing to the mounting height is large enough, namely of the order 1.8: 1 or more in both directions.

 Preferably, the maximum light intensity is obtained at an angle between 25 and 45 "relative to the vertical or nadir in a lateral direction perpendicular to each outer edge of the lens, and the maximum light intensity in a directed lateral direction towards the corners of the lens is obtained at an angle greater than S to 15 with respect to the vertical or nadir. The angle of the beam of maximum intensity is kept less than approximately 55 "in all lateral directions in order to prevent a direct glare.The maximum light intensity in a lateral direction towards the corners of the lens is preferably produced at an angle between about 40 and 500 and it is at least 50% greater than the value of the light intensity produced according to the vertical or nadir.

 The lens preferably has an essentially square shape. The prisms placed on the outer faces of the lower panels are generally pure alleles at the outer edges of the panels while the prisms provided on the inner faces of the lower panels are generally perpendicular to the outer edges of the lower panels. The prisms placed on the inner faces of the lower panels include a central strip of prisms much narrower than the length of the outer edges of the lower panels, and the areas of the inner faces of the lower panels which are located outside the strip are generally clear. The prisms located on the outer faces of the lower panels constitute means making it possible to produce a much higher light intensity in directions distant from the vertical or nadir than along the vertical or nadir while the prisms placed on the inner faces of the lower panels constitute means for increasing the light intensity in a lateral direction oriented towards the corners of the lens.

The prisms on the outside faces of the side walls are generally horizontal and divide the light emitted by them into an upper beam directed upwards from the lens and into a lower beam directed downwards from the lens, according to an angle of less than about 500 from the vertical or nadir. This technique is known per se from US Pat. No. 3,647,148. The prisms placed on the inner faces of the side walls are essentially vertical and decompose the image of the lamp.

One or more of the lens surfaces may be provided with a light matte finish to soften the images of the lamp.

 It should be noted, with regard to the design, that the total height of the lighting fixture may be small. Because the lens has the shape of a pyramid directed downwards, and because the optical components are designed so as to obtain a satisfactory distribution of the lighting power by placing the lamp in a low position in the device, there is no need to give these devices as great a depth as in commonly used devices. This overcomes a major disadvantage of other forms of high intensity discharge (HID) lamp apparatus, the great depth of which normally results in restrictions on their locations, due to the limited height available. Due to the fact that the lens according to the invention makes it possible to place the lamp in a low position in the lighting apparatus, a greater number of light rays emitted by the lamp arrive directly on the lens and the efficiency of the apparatus. is therefore extremely large despite its excellent light control.

The actual lighting fixture may have any suitable design for placing the lamp close to the lens. The lamp can be horizontal, vertical or tilted and, if horizontal, it can be oriented perpendicular or oblique to one side of the fixture. An advantageous property of the apparatus is that it is possible to vary the vertical position of the central point of illumination in order to produce different light patterns. A change in the light center in a horizontal plane can also produce interesting variations in the light distribution. The interior surface of the luminaire may be provided with a white light scattering finish or an aluminum reflector may be interposed. The reflector design may be conventional or it may be designed to reduce the light towards the vertical; it can also be designed so as to compensate for an asymmetry in the distribution of the light of an HID lamp, as demonstrated in the patent for
United States of America 3,259,739.

 The lamp is preferably a form of high intensity discharge lamp but it can also be constituted by another compact source such as an incandescent lamp or else a halogen-tungsten lamp. Although the embodiment described above cannot use a lamp with a power greater than 400 watts for thermal reasons, all the principles of the invention also apply to large lamps and could use for example a 1000 watt lamp by increasing the overall dimensions of the device.

 The lens is preferably formed from an acrylic plastic, but it goes without saying that other types of plastic or glass can be used. Its thickness, kept within reasonable limits, has no significant influence on optical performance.

Other advantages and characteristics of the invention will be highlighted in the following description, given by way of nonlimiting examples, with reference to the appended drawings in which FIG. 1 is a schematic plan view of a lighting system comprising the present invention, FIG. 2 is a side elevational view of a luminaire according to the invention, provided with a lens according to the invention, FIG. 3 shows a diagram of the light distribution (light intensity) for the lighting fixture of FIG. 2 this distribution being measured in a lateral direction perpendicular to an edge of the lens transverse direction), further in a direction perpendicular to an adjacent edge of the lens (longitudinal direction) 9 and finally in a lateral direction towards a corner of the lens (direction d 450) g. 4 is a somewhat schematic plan view of the lens of FIG. 2, fig. 5 is a somewhat schematic plan view of the lens of FIG. 4, FIG. 6 is a sectional view taken along line 6-6 of FIG. 5, fig. 7 is a detailed view of FIG. 6 showing a cross section of linear prisms located on the rear face of the lower panels, FIG. 8 is a sectional view, taken along line 8-8 of FIG. 5 and showing prisms located on the inside of the bottom panels, FIG. 9 is a detailed view of FIG. 8 showing a central prism forming part of the prisms placed on the inner face of a lower panel, FIG. 10 is a detail view corresponding to FIG. 9 and showing an intermediate prism, FIG. 11 is a detail view corresponding to FIGS. 9 and 10 and showing an external prism, FIG. 12 is a detailed view of FIG. 6, showing in cross section linear prisms located on the outside of a side panel, FIG. 13 is a detail view showing sections of prisms located on the inside of a side panel of the lens.

 With reference to the drawings, and in particular to FIG. 1, a lighting system according to the present invention comprises several lighting devices 1 which are distributed in the form of a rectangular grid The devices 1 are spaces from each other of 3.6 m in a parallel direction to the axis of the lamps mounted inside (longitudinal direction) and 4.5 m in a direction perpendicular to the axes of the lamps (transverse direction). The devices are installed 3 m above the floor and 2.25 m above the level of work surfaces such as office furniture placed on the floor. The relationship between the spacing and the installation height is Therefore 1.6 in the "longitudinal" direction and 2.0 in the "transverse" direction and the average spacing / installation height ratio is 1.8 in both directions.

As shown schematically in fig. 1, the light distribution 3 of the devices 1 is generally rectangular. Consequently, the amount of light obtained at the working level in the area illuminated by the apparatus 1 is relatively uniform.

 As shown in fig. 2, each lighting device 1 consists of a housing 5 in which is mounted a socket 7 intended to receive a high intensity discharge lamp 9, a ballast 11 and a lens 13 directed downwards. The device 1 can be mounted in a ceiling structure 15 of a building or in another interior volume. The sides of the housing 5 are inclined upwards and inwards. As indicated, the housing 5 has a depth of 25 cm, it has a square shape of 59 cm side at its lower end and a square shape of 24 cm side at its upper part. The interior of the housing 5 is coated with a highly reflective white paint.

 The lens 13 is in the form of a pyramid with a square base, directed downwards and comprising a peripheral rim 17, four vertical side panels 19 of identical shape and four triangular lower panels 21 of identical shape. The lens 13 is produced by injection molding of an acrylic plastic material and has an average thickness of 4.9 mm. The lens 13 has a square shape with a side length of 56 cm and a depth of 9.7 cm. The rim 17 is horizontal and has a width of 12.5 mm. Each side panel 19 is inclined inward at an angle of 14.5 relative to the vertical and has a width of 44 mm. Each of the bottom panels 21 is inclined downward relative to the horizontal of an angle of 10.890 and it has a width of 26 cm, measured along a central line22 extending from its outer edge to its top (bottom).

 Linear prisms are provided on the interior and exterior faces of the side panels 19 and of the bottom panels 21. Each linear prism has a uniform cross section over its entire length and it extends from one edge to the other of its panel. corresponding. These prisms together create the rectangular distribution of light, with the wide spreading which is quite desirable, which constitutes the characteristic of the lighting devices according to the invention.

The external prisms 23 provided on the bottom panels 21 are indicated in FIG. 7. These prisms are oriented parallel to the perimeter of the lens 13 and they each have a width of 3.2 cm. Each prism has a face 25 directed towards the periphery of the lens and which makes an angle A with respect to the normal to the plane of its panel 21, as well as a face 27 oriented towards the center of the lens and which makes an angle B with respect to the normal to the plane of its panel 21. The faces 27 are slightly rounded in a crown (plus and minus 1 "), and the corners are slightly rounded to avoid a notching effect in the optical system. The values of the angles A and B are shown in the following Table 1 for each prism 23, starting with the half-prism located in the center of the lens
TABLE 1
Prisms nO Angle A Angle B
1 to 15 12.8 65.0
16 12.8 66.2
17 12.8 68.0
18 12.8 69.8
19 12.8 70.4
20 12.8 73.0
21 12.8 71.7
22 12.8 76.5
23 12.8 78.6
24 12.8 80.5
25 12.8 82.5
26 12.8 84.7
27 12.8 86.6
28 12.8 88.4
29
30 88.5 19.4
31 86.5 19.4
32 84.5 20.7
33 82.5 20.7
34 80.5 21.7
35 78.5 21.7
36 76.7 23.1
37 75.0 23.1
38 73.4 23.1
39 71.8 24.7
40 70.3 24.7
41 38.8 24.7
42 37.4 25.9
43 66.1 25.9
44 64.9 25.9
45 63.7 27.0
46 62.6 27.0
47 61.3 27.0
48 60.2 28.3
49 59.2 28.3
50 58.2 28.3
51 & 52 57.0 29.9
53 to 55 55.0 29.9
56 to 58 53.2 31.3
59 & 60 51.8 31.3
61 & 62 50.5 32.6
63 to 65 48.7 32.6
66 to 68 46.9 32.6
69 to 71 45.2 33.6
72 to 74 44.0 33.6
75 to 77 42.7 34.3
78 to 80 42.0 34.3
81 to 83 41.3 35.1
84 & 85 40.4 35.1
The prisms 23 located on the outer faces of the lower panels 21 form the main lighting beam. The incident ray arriving on a given prism makes a different angle from that of the other prisms. The angle of the prism changes to compensate for this effect so that the emitted ray always comes out at the same general vertical angle. There is a difference from this principle for prisms 1 to 15, which have a constant face angle. At these points, refractive effects are unable to create a sufficient deflection of the incident rays (which are close to the vertical) to place them in the main beam, without creating a high-gloss area due to the inter-reflection producing in deep prisms.The constant angle and value
moderate of these prisms eliminates this shiny area for
improve lighting conditions, while preventing
the emission of rays directly vertically
For aesthetic purposes, we create a slight
diffusion or light texturing on part of the
surface of the exterior bottom panel 21. Near the
lower vertices of the panels 21, the outer surface
is optically clear and the texturing gradually increases
towards the perimeter of the lens 13. This creates
uniform shine and attractive appearance.
is created in addition only in the desired areas,
but not in areas where diffusion could cause
a wide angle and dazzled light emission
sement.

There is shown in Figs. 8 to 11 prisms
interiors 29 located on the bottom panels. These prisms
are oriented perpendicular to the perimeter of the
lens 13 and they each have a width of 2.5 mm. The
prisms 29 are arranged symmetrically to the axis
central 22 of each panel 21. These prisms 29 are of
three types. As shown in fig. 9, from
the central axis 22 of each panel 21 the first seventeen
prisms 29 are in the form of lenticles having a
radius of curvature Ce The radius C, expressed in millimeters, of
each of the first 17 prisms has been indicated in the Table
2 below
TABLE 2
Prisms nO Ray C
1 to 10 4.87 mm
11 4.58
12 4.34
13 399l
14 3.55
15 3.25
16 and 17 3.12
As shown in fig. 10, the prisms 29 between the eighteenth and the twenty-fifth have a curved face 31 facing the central axis 22 and a flat face 33 facing the perimeter of the lens. The face 31 has in cross section the shape of a segment of a circle whose center 35 is spaced internally by a distance D, in the plane of the face 21, from the outer edge of the prism 29, said center 35 being spaced, perpendicularly in the plane of the panel 31, by a distance E with respect to the interior edge of the prism 29. The angle formed between the line passing through the center 35 and the interior edge of the prism 29 on the one hand and the line passing through the center 35 perpendicular to the plane of the panel 21 on the other hand, is equal to 24 "'for all the prisms going from the eighteenth to the twenty-fifth. The faces 33 of these prisms 29 are all inclined 1.50, towards the interior from a perpendicular to the plane of the panel 21. The values of the variables D and E, expressed in millimeters, are indicated in table 3 according to each of the prisms 29 ranging from the eighteenth to the twenty-fifth.

TABLE 3
Prisms nO 'D "" E "
18 & 19 0.94 mm 3.55 mm
20 & 21 0.63 4.26
22 & 23 0.31 4.98
24 & 25 0.00 5.70
The prisms 29 going from the twenty-sixth to the fiftieth are arranged in a similar way to the preceding plums and they have curved inner faces 37 and flat outer faces 39. These prisms 29 have been highlighted in FIG. 11. On the inner edge of each curved face 37, the face 37 makes an angle F with the normal to the plane of the panel 21. The center of curvature 40 of the face 37 is located on a straight line perpendicular to the plane of the panel 21, which passes through the outer edge of the prism 29, and it is spaced from the face 37 by a distance equal, in millimeters, to one tenth of the secant of the angle F. The faces 39 are inclined towards the inside of a angle G with respect to the normal to the plane of the panel 21. The values of the variables F and
G, expressed in degrees.

TABLE 4
PRisms nO "F""G"
26 & 27 68.0 1.50
28 & 29 70.0 1.5
30 & 31 72.0 1.5
32 & 33 74.5 1.5
34 & 35 77.0 2.5
36 & 37 79.5 4.5
38 & 39 82.0 6.5
40 & 41 84.0 8.5
42 & 43 86.0 10.5
44 & 45 87.5 12.5
46 to 50 88.5 14.5
The function of the interior bottom prisms 29 is to create the necessary lateral spreading of the light rays and to facilitate the creation of a uniform light pattern in a lighting system. To produce the light distributions of FIGS. 1 and 3, the lens must produce the maximum illumination in a direction which cuts the work plan approximately 2.25 m from the luminaire, this distance being measured horizontally in the plane passing through the transverse axis, and 1.8 m in the passing plane along the longitudinal axis, to fill the otherwise dark area. However, in relation to the light distribution in a plane oriented diagonally to the lighting fixture, the effective spacing is 4.86 m
The maximum intensity of illumination along the horizontal must therefore be directed towards points located approximately 2.9 m from the device. This requires that the angle corresponding to the maximum intensity of illumination in the diagonal plane is increased compared to that corresponding to the transverse plane or to the longitudinal plane.

 The four bottom panels used for the formation of the light beam are roughly oriented in the directions corresponding to the transverse axis and to the longitudinal axis and the panels producing the most powerful light are those which are provided with external prisms oriented parallel to the lamps, because the lamps produce their greatest light intensity in a direction perpendicular to their axis. Consequently, we must remove some of the light from the plane passing through the transverse axis and transfer it in the diagonal plane in order to fill the existing void in the center of the square formed by four lighting fixtures.

 The interior bottom prisms therefore fulfill two functions: firstly to rotate certain rays laterally to move them away from the plane passing through the transverse axis and secondly to raise these rays before their emission in angles close to the diagonal.

It can be seen that the prisms 29 constitute a range extending from uniformly rounded grooves which are situated close to the central axis so as to produce a diffusion and to decompose the lamp image without causing a notable redirection of light up to to prisms that rotate the spokes to the diagonal direction.

The prisms are rounded, both to create a lateral angular offset that may be necessary and also to spread outgoing rays over a large angular range in order to improve the aesthetic effect. The prisms are gradually reduced until their effects are completely canceled at a distance of 126 mm from the central axis 22.

 An important operating characteristic of the prisms 29 is that an observer can look directly at the lens from different lateral angles and notice similar brightnesses. As a result, when moving around a lighting fixture, the effect of "next" glare is minimized.

 A slight texturing is provided on all the surfaces of the panels 21.

 We have highlighted in fig. 12 the prisms 41 placed on the outside of the side panels 19. These prisms are oriented parallel to the perimeter of the lens 13 and they each have a width of 2.5 mm. The prisms 41 are identical and symmetrical. The faces of the prisms are inclined by 270 relative to the normal to the plane of the side panel. The faces have a rounded and convex profile corresponding to plus and minus 10, in order to spread the light emitted by them. These prisms divide the light passing through them so as to form a lower beam which enlarges the main beam of the lens and an upper beam which illuminates the ceiling structure above the apparatus 3. The light directed upwards attenuates the contrast between the apparatuses and the surrounding ceiling and causes a reflection of useful light from the ceiling.

 The exterior surfaces of the side walls 19 are textured to produce diffusion of the lamp image.

 There is shown in FIG. 13 the prisms 43 located on the inner side of the side panels 19. These prisms are oriented perpendicular to the perimeter of the lens 13 and they each have a width of 2.5 mm. The prisms 43 are identical and symmetrical. Each face is in cross section in the form of a circle segment having a radius of 5.6 mm and a center of curvature located on a straight line perpendicular to the plane of the panel 19 and passing through the top of the adjacent prism. These rounded and deep prisms are used to completely decompose the images of the lamp in order to improve the appearance obtained.

It has been found that the lighting apparatus corresponding to the preferred embodiment of the invention had an efficiency greater than 75% when the reflective paint placed in the housing of the apparatus has a reflection capacity of 8, 85 and when the lamp had a power of 250 watts, this lamp was constituted by a high pressure sodium lamp producing an illumination power of 27,500 lumens and whose axis is spaced 21.5 cm from the upper part of the 'apparatus. The distribution of light emitted by the lighting fixture has been highlighted in fig. 3 and also in the following table 5
TABLE 5
LIGHT INTENSITY (LUX) AND POWER VALUES
ILLUMINATION (LUMENS)
POWER PLAN
ANGLE
TRANSVERSE to 67.5 to 45 to 22.5 "LONGITUDINAL LUMENS 0 56,600 lux 56,600 56,600 56,600 56,600
5 55,209 55,436 56,095 56,827 - 57,099 498
10 54 529 54 659 56 311 58 482 59 453
15 57 100 55 587 56 538 60 825 62 424 1523
20 61 171 57 456 55 230 61 212 63 828
25 67 543 60 120 55080 63 439 67 362 2644
30 75 945 65 005 54 853 64 627 70 146 35 102 146 78 300 58 557 63 215 70 146 4211 40 112 827 102 125 76 647 60 069 66 266 45 74 002 89 111 100 175 50 241 50 372 5127
50 40 995 54 410 84 802 36 429 32 767
55 25,689 31,428 47,595 25,252 22,464 2,882 60 20,313 21,546 25,900 18,792 17,280 65 16,513 16,580 18,712 15,012 14,105 1,539
70 13,867 13,845 14,540 12,515 12,450
75 11 296 11 372 11 320 10 206 9655 1067 80 9 115 9 110 8 800 8056 7668 85 7 106 7200 7051 6070 5810 678 90 5346 5605 5340 4579 4244 95 3809 3574 3359 2872 2948 347 100 2289 2171 2095 1652 1706 105 1771 1728 1846 1242 1220 159 110 1706 1458 1648 951 1123 115 1455 1339 1242 778 951 104 120 1415 1123 756 605 799 125 1123 831 54 442 637 46 130 658 311 0 173 367 135 76 0 0 0 22 4 140 O- 0 O 0 O 145 0 0 0 0 0 0 150 0 0 0 0 0 155 0 0 0 0 0 0 160 0 OO 0 0 165 0 0 0 0 0 0 170 0 0 0 0 0 175 0 0 0 0 0 0 180 0 0 0 0 0
In a lighting system such as that of FIG. 1, where the average ratio between the spacing and the installation height is equal to 1.8, the ratio between the maximum and the minimum horizontal illumination at the working level is less than 1.5.

 Of course, the invention is not limited to the embodiments described and shown above, from which other modes and other embodiments can be provided, without thereby departing from the scope of the invention. Simply by way of example, in addition to the previously mentioned modifications, the lighting fixtures can be spaced differently and installed at different heights. The device housing and the lamp can also be modified. You can change the shape of the lens as well as the dimensions and profiles of the prisms.

Claims (8)

1. Lighting apparatus comprising a housing, a device placed in the housing to support a high intensity lamp and a lens extending below the housing, apparatus characterized in that the lens comprises a rectangular pyramid (13) directed towards the bottom and provided with four side panels (19) oriented essentially vertically and with four triangular lower panels (21), all the panels comprising prisms (23) on their inner faces and on their outer faces, said prisms being arranged so as to produce a light intensity, in all lateral positions, which is much higher in a direction far from the vertical or nadir than with respect to the vertical or nadir, and to produce a maximum light intensity for a smaller angle with respect to the vertical , or nadir, in a lateral direction oriented perpendicular to the sides of the lens, than in a lateral direction oriented towards the co lens ins. 2. Lighting apparatus according to claim 1, characterized in that said maximum light intensity is produced at an angle between 25 and about 45 "relative to the vertical or nadir in a lateral direction oriented perpendicularly to each outer edge of said lens (13) and in that said maximum light intensity in a lateral direction oriented towards the corners of said lens (13) is produced at an angle which is between about 5 and 150 relative to the vertical or nadir. 3. Lighting apparatus according to claim 2, characterized in that said maximum light intensity produced in all lateral directions corresponds to angles between about 25 and 55. 4. Lighting apparatus according to claim 3, characterized in that the maximum light intensity produced in a lateral direction oriented towards the corners of the lens corresponds to an angle between about 40 and 50 and is greater by at least 50 % to the value of the luminous intensity corresponding to the vertical or nadir. 5. Lighting device according to any one of claims 1 to 4, characterized in that the lens has an essentially square shape and in that the prisms placed on the outer faces of the lower panels are generally parallel to the outer edges of the panels while the prisms placed on the inner faces of the lower panels are generally perpendicular to the outer edges of the lower panels. 6. Lighting apparatus according to claim 5, characterized in that the prisms placed on the inner faces of the lower panels comprise a central strip of prisms much narrower than the length of the outer edges of the lower panels, the zone corresponding to the inner faces lower panels placed outside said strip being generally clear. 7. Lighting apparatus according to any one of claims 1 to 6, characterized in that the prisms located on the outer faces of the side panels are generally horizontal while the prisms placed on the inner faces of said side panels are essentially vertical. 8. A lighting device according to claim 7, characterized in that the prisms placed on the outside faces of the side panels are arranged to divide the light which they emit into an upper beam, directed upwards from the lens, and in a lower beam directed downward from the lens at an angle of less than about 50 relative to the vertical or nadir and in that the prisms placed on the interior faces of said side panels are arranged to increase the light intensity - in a lateral direction oriented towards the corners of the lens.