Light source

The invention relates to a light source for example for use in a display device such as a light box, e.g. for an illuminated advertisement, a traffic sign, a signpost, an illuminated ceiling, a TFT screen, an LCD screen, a clock face or a measuring device which device includes : housing; a light source housed in this housing; and at least one scattering translucent display surface illuminated by light from this light source.

BACKGROUND OF THE INVENTION [0002] Such a display device is known from many embodiments.

[0003] A device of the kind specified in the preamble is known, for example, from US-A-5 457 615. In the display device known from here, a number of omnidirectional light sources have been used to illuminate the dispersing semi-transparent display surface. The internal surfaces take on a reflective form. By means of a diffusingly translucent plate, referred to as a diffuser, the light emitted by the lamp towards the display surface is captured and scattered by this diffusingly translucent plate.

[0004] The description in US-A-5 457 615 does not provide guidance as to the degree of diffusivity of light transmission through the diffusingly translucent plate. However, it should be assumed that the light sources are visible to the observer on the outside of the device, which is considered undesirable according to the invention. Thus, a significant part of the light emitted by the light sources reaches the display surface directly through the diffusingly translucent plate. The remaining part of the light emitted by the light sources is mirrored by the inner surfaces of the housing with a strong mirror reflection and after a certain number of reflections reaches the display surface in a way not further defined. Because of this mirror reflection, it is not possible to make the light sources not visible on the external display surface, which creates darker and brighter zones. The human eye can easily detect transitions between them. If the contrast transition exceeds the set value on a short stretch, this can be manifested in a very annoying way. This aspect will be discussed below.

[0005] Furthermore, the construction according to US-A-5 457 615 is such that the light from the end zones of the elongated light sources can reach the display surface directly.

At this point, it is necessary to briefly devote attention to a variety of possibilities, known from literature or within the reach of an expert, in illuminating the translucent display surface so that the luminance is as uniform as possible on the external surface of the display:

1. A diffusely translucent display surface with a significant thickness and / or very limited light transmission can be used. It should be understood that in this way a certain homogenisation can be achieved for a material such as white glass or the correspondingly dissipative translucent plastics in terms of functionality. The disadvantage of this solution lies in the fact that the loss of light energy can be very large. This would mean that in order to achieve a fixed luminance, use a light source consuming a relatively large amount of energy.

2. Alternatively, a plurality of translucent plates may be provided that are spaced apart and which are, for example, the same or similar to the display surface. A better degree of homogeneity of the luminance on the outer display surface is obtained by the mutual distance between such plates. This good solution as such also has the disadvantage that it leads to a large loss of light energy.

3. As the distance between the source and the display surface increases, the uniformity of the luminance on the external surface of the display surface will improve. The disadvantage of this solution lies in the fact that the casing takes on great dimensions, and in particular gains a large depth, which is undesirable and sometimes even unacceptable for specific applications. In addition, it should be assumed that light that is not directed directly to the display surface by the light source must be absorbed because otherwise unpredictable reflection effects may occur, which will increase the luminance, but will have an adverse effect on the homogeneity.

4. A light source with a large radiation area can be used. For example, you can imagine a surface equipped with a grid pattern of small light sources such as halogen bulbs or light emitting diodes. While with a good design, such a solution can provide a reasonable degree of luminance homogeneity on the external display surface, high costs are a deterrent.

5. Optical means may be provided, in particular lenses, mirrors or ieh configurations that will be designed in such a way that the luminance on the outer display surface will be constant within a certain tolerance. However, the design of such systems is complicated, as it depends entirely on the appropriate dimensioning and is accordingly costly.

SUMMARY OF THE INVENTION [0007] Furthermore, a display device for advertising purposes comprising one or more fluorescent lamps as a light source is commonly known and used. Such a display device has the obvious advantage of a large luminous flux. This is accompanied, however, by a disadvantage which in practice prevails in that the dispersive, translucent display surface has a high luminous intensity in the area at a relatively short distance from the fluorescent lamp. However, this brightness decreases rapidly as the distance of the illuminated surface of the display surface relative to the fluorescent lamp decreases. Oddly enough, the result is the phenomenon of

[0008] Attempts have been made to overcome this known drawback by using more than one fluorescent lamp or other light source, but this defect can not be satisfactorily solved with a highly illuminated contrasting area with less illuminated areas. Advancing the description of the figures below, we now refer to Figures 1, 2, 3, 8A and 13 and the description accompanying the said figures.

[0009] Furthermore, attempts have been made to reduce the differences in light by laying a net with a defined progression that locally covers the light from the light source to a greater or lesser extent. Even thanks to this, the desired effects have not been achieved.

The object of the invention is to provide a suitable light source for the display device such that the display surface will be illuminated uniformly within relatively strict tolerances. As research has shown, it is not only luminance, but more and more the homogeneity of luminance that determines the visibility of information appearing on the display surface, for example the readability of the texts appearing on it. For light coffers, for example, luminance in the range of about 10-5000 lux can be predicted. It will be obvious that this is essential, especially in some of the listed applications such as advertising signs and road signs.

[0011] On the basis of the foregoing, the invention provides a light source of the type defined in the preamble, which is characterized in that the light source comprises at least one longitudinal lamp made up of a plurality of groups of at least three light emitting diodes extending in substantially the same direction and for each emitter has been added with individual directing means so as to obtain a directional characteristic of the light emitted by the light source such that the light source illuminates at least one wall almost exclusively, e.g. in more than 75%; and that at least one wall is diffusely reflecting such that a part of the light incident on it is reflected on the display surface.

[0012] In advance of the description of figures shown below, we refer to Fig. 24 in terms of the term "diffusingly reflecting". In this figure, a substantially spherical directional characteristic corresponding to the diffuse reflection is shown in two dimensions. The beginning of the drawn coordinate system, i.e. [x = 0; y = 0], is the point where light falls on the diffusingly reflecting surface. The light will be reflected in accordance with the drawn directional scheme in principle irrespective of the direction in which the light will fall. The largest part will bounce off the surface perpendicular to it, thus in the direction of the y axis. The part in the direction of the x-axis will be essentially equal to 0. Relative intermediate values ​​represent the distance between the beginning and the sphere, which is illustrated in two dimensions in the form of a circle.

[0013] A light source according to the invention is preferred, in which the proportion of light energy from light that is directed directly to the display surface of the display device by the light source (= LED module) and escaping through the display surface is less than 10% of the total energy light from the light emitted by the light source. Thanks to this, the light source is not visible to the observer.

[0014] An embodiment is highly recommended, in which the first derivative for a luminance position on the outer display surface divided by a local value of 4 luminance has a maximum value of about 1.0-1.2 m '¹ in every direction. Research has shown that this value represents a transition between the good homogeneity perceived by the human eye and the perceived luminance difference. As the value of said normalized first derivative increases, the contrast is increased, which will eventually increase to an irritating and even unacceptable degree.

[0015] It is noted here that luminance is a measure that takes into account the subjective properties of the human eye.

[0016] It is important that the guide means ensure that as much light emitted by the light source reaches the scattering reflecting wall. The direction means can be separate means that are added to the light source. Alternatively, the display device may have a particular feature in that the direction means are integrated into the light source.

[0017] According to a particular aspect of the invention, the display device has the particular feature that the forwarding means comprise optical means from the group to which the mirror means and the lens means belong.

[0018] It is pointed out that known display surfaces for prior art display devices are made in the form of plates, e.g. made of dairy plastic or glass, which are completely smooth on both sides. As we know from the optics, a beam of light falling on a smooth surface at an angle significantly different from the normal incidence will not get into the discussed center in the event that the so-called Brewster angle. In the present case it can be used, allowing some of the light from the light source to fall directly onto the inner surface of the display surface, but at an angle such that the light is theoretically completely reflected and reaches after reflection to the reflecting wall. This aspect will be discussed further with reference to Fig. 9C.

[0019] It will be evident that, according to the invention, it is necessary to prevent as much as possible the light from light sources corresponding to, for example, the lateral flap of the directional characteristic, directly falling on the display surface in an uncontrolled manner. To prevent this phenomenon, which occurs in most publicly available light sources, the device according to the invention comprises, in a particular embodiment, a screening means arranged between the light source and the display surface to shield the light directed directly to the display surface by the light source. In this way, imperfections in the light source are completely eliminated.

[0020] In order to ensure the best homogeneity of lighting within strict limits, the display device may have the particular feature that the light source extends over a distance of at least 70% of the linear dimension of the display surface.

[0021] Very simple is an embodiment in which the light source comprises at least one elongated lamp, e.g. made of light emitting diodes arranged side by side, in a row.

[0022] It has been found that in the case of a luminous range of approximately 10-5000 lux, often used in light coffers in practice, the absolute light intensity of the emitted light only plays a subordinate role in the visibility of the presented information, and the homogeneity of luminance on the external display surface is a measure of the obtained quality. In this regard, the display device may have a particular feature according to a preferred embodiment in that the light source comprises a light emitting diode or at least one group of electroluminescent diodes extending in substantially the same direction. A certain number of light emitting diodes may be arranged in such a way that they ensure the required homogeneity of the wall illumination within certain established standards.

[0023] An embodiment in which the light source extends over a distance of at least 70% of the linear dimension of the display surface may further have the particular feature that a group of electroluminescent diodes are arranged side by side in a row.

The display device may have the particular feature that this or any directly illuminated first wall extends approximately in a direction transverse to the display surface.

Even better homogeneity of the light incident on the scattering display surface is achieved by a display device comprising at least one second dispersing reflecting wall that receives a portion of the light reflected by the first reflecting reflecting wall and directs its part to the display surface.

The light source according to the invention may also have the particular feature that the directional characteristic is such that the light source illuminates this or any directly illuminated wall at least more or less homogeneously or ensures that the directly illuminated wall is at least approximately homogeneous. line source. Optical means may be used for this purpose, including lens means and / or mirror means, which provide the required directional characteristics. In this regard, we refer in advance to, inter alia, Fig. 4, and the accompanying description.

The display device can have the particular feature that at least one wall has a slightly colored, in particular substantially white, covering layer consisting of a material of the group that includes: matt paint, satin gloss paint, white paper, film LEF (trademark of the 3M company). Thanks to these materials, a sufficiently high reflection coefficient can be obtained in combination with a small color dependence on light absorption.

[0027] In addition, attention is drawn to WO-A-99/67663. As described above, the uniformity of the luminance on the outside of the display surface can be improved by using a relatively large thickness and / or low light transmission in the dispersive translucent display surface. The prior art document uses this knowledge. In Fig. 5, among others, and in the accompanying description, mention is made of materials that can be used in the light box to be discussed. It has been found that the luminous flux is significant for a material such as 3635-70, but there are lighter and darker areas that combine in such a way that the transitions will be visible in an annoying manner. Only when using other materials, especially materials P645 and P945, is the permissible homogeneity of luminance achieved.

[0028] US20040255497A1 discloses an elongated light source according to the preamble of claim 1 and which comprises at least three light emitting diodes extending in the same direction and each light-emitting diode is provided with respective directing means.

[0029] US6249375B1 discloses a light source according to the preamble of claim 1, which comprises a plurality of light emitting diodes and wherein each light-emitting diode is provided with respective directing means.

BRIEF DESCRIPTION OF THE DRAWINGS [0030] The invention will now be elucidated with reference to the accompanying drawings.

Fig. 1 is a partial perspective view of a prior art display device;

Fig. 2 is a partial perspective view of another prior art display device;

Fig. 3 is a partial front view of the display device of Fig. 2;

Fig. 4 shows a view corresponding to Fig. 1 of a display device equipped with a light source according to the invention;

Fig. 5 shows a partial front view of the display device according to Fig. 4;

Fig. 6 shows a view corresponding to Fig. 5 of the variant;

Fig. 7 shows a view corresponding to Fig. 6 modifying the embodiment of Fig. 6;

Fig. 8A is a cross-sectional view of the display device according to Figs. I;

Fig. 8B is a graphical representation of the distribution of luminous intensity on both display surfaces in the embodiment of Figs. 1 and 8A;

Fig. 9A shows a cross-section through the display device of Fig. 4;

Fig. 9B shows the distribution of light intensity on both display surfaces of the display device of Figs. 4 and 9A;

Fig. 9C shows a view corresponding to Fig. 9A of an embodiment in which the light sources have an aperture angle such that the dispersing semi-transparent display surfaces are partially illuminated directly by the light sources;

Fig. 10 is a partial view of yet another embodiment;

Fig. 11 shows a part of the light source according to Fig. 10;

Fig. 12A shows a longitudinal section through a mold for producing a light source with a shutter against scattered light;

Fig. 12B shows a cross section through the mold according to Fig. 12A;

Fig. 13A shows a partial front view of the prior art display device with one fluorescent lamp;

Fig. 13B shows a cross-section through a display device as in Fig. 13 A;

Fig. 13C shows the distribution of light on both translucent surfaces by the display device according to Figs. 13A and 13B;

Fig. 14A is a view corresponding to Fig. 13A of a display device according to the invention in Fig. 10;

Fig. 14B shows a cross-section through a display device according to Fig. 14A;

Fig. 14C shows the distribution of light intensity on the display surface of the display device of Figs. 10, 14A and 14B;

Fig. 15 is a graphical representation of the spectral distribution of a red, green and blue light-emitting diode;

Fig. 16 shows the spectral distribution of cold white light emitted by a specific electroluminescent diode;

Fig. 17 shows the spectral distribution of cold white light emitted by another type of electroluminescent diode;

Fig. 18 shows the spectral distribution of light emitted by a warm white LED;

Fig. 19A shows a cross-section through a light box according to the invention, which is provided on both sides with a row of light sources;

Fig. 19B is a graph of measured luminance as a function of the position on the outside of the display surface, measured in a direction that is horizontal in the drawing;

Fig. 19C is a graph according to Fig. 19B, which also includes hints for calculating a first derivative of normalized luminance;

Fig. 20A is a view corresponding to Fig. 19A of a prior art light box in which four fluorescent lamps are used as the light source;

Fig. 20B is a graphical mapping corresponding to Fig. 19B of luminance in a position function;

Fig. 20C shows the mapping corresponding to Fig. 19C of the graph of Figure 20B;

Fig. 21A shows in a horizontal direction, respectively, a front view of the light box according to Fig. 20A and a partial side view, on which the display surface is represented from different distances, for introducing measurements, the results of which are shown in Fig. 2IB;

Fig. 21B shows six diagrams on which the luminance is plotted on the outer surface of the display light box according to Fig. 21A, the distance of the display surface from the initial position being a parameter;

Fig. 21C shows a graphical representation of the luminance as a function of the position in case the display surface is placed at a distance of 0.5 m, there being indications there for calculating the first derivative of normalized luminance;

Fig. 21D shows a graphical mapping corresponding to Fig. 21C when the display surface is placed at a distance of 0.6 m;

Fig. 22A shows a front view, a side view and a plan view of a light box, whose inner surfaces are covered with LEF film (trademark of 3M);

Fig. 22B shows the luminance distribution on the outer surface of the display light box according to Fig. 22A;

Fig. 22C is a graph according to Fig. 22B, in which all data necessary to calculate the first derivative of normalized luminance is included;

Fig. 23 is a graphical representation showing all of the luminance diagrams of Figs. 19A, 19B, 20A, 20B (in two versions), 21A, 21B, 22A and 22B to illustrate the first photographic panel features of the invention; and

Fig. 24 shows the directional characteristics of a scattering reflecting surface in the form of two-dimensional mappings.

DETAILED DESCRIPTION OF EMBODIMENTS [0031] The figures described below schematically show, among others, measuring systems with associated measurement results. All measurements were carried out under exactly the same conditions and as such are completely comparable. Thus, the same display surfaces are used for all measured display devices. For the measurements described below, the display surface used, in each case, a die-cast milk acrylic plate 100-27006 with shimmering surfaces, from Vink in the Netherlands. The light transmission factor of this material is 29%. This material is widely used in light coffers in the Netherlands.

[0032] FIG. 1 shows a prior art display device 1. It comprises a housing 2, only the upper and lower walls of which are shown. In this housing there is a light source comprising three fluorescent lamps 3, 4, 5. Both the front and the back have a display surface, 6, 7, respectively. They may include translucent or opaque optical information patterns, e.g. signpost or advertising information.

[0033] The display surfaces 6, 7 are illuminated directly by fluorescent lamps 3, 4, 5.

[0034] Fig. 8A shows a display device in cross-section.

[0035] Figure 8B graphically depicts the distribution of light on the outer surfaces 61, 62 of the respective display surfaces 6, 7. As clearly seen in the figure, the center value of luminosity is around 1,000 lux, with fluctuations of + 30% and -30%. Large fluctuations in the short stretch are often perceived as irritating.

In order to reduce these large fluctuations, the system of figure 2 is sometimes used. In this embodiment, the display device 8 shown here comprises four fluorescent lamps 9, 10, 11, 12, which are arranged so that the distribution of light gives a slightly more homogeneous image. This solution was not found to work well in practice. In the middle there is an area in which more or less the luminous intensity of the lamps accumulates, which gives the peak value of the light in the middle, while the edges have a relatively underexposed area, according to the disappearance at the ends, which is plotted in Fig. 8B.

[0037] Fig. 3 shows a complete display device 8 with fluorescent lamps 9, 10, 11, 12, 13, 14. The said overlap region is numbered 15. The relatively undersized end edge region is indicated by the reference numeral 16.

[0038] Fig. 4 shows an example of a display surface 19 with a light emitting diode module according to the invention. On the bottom wall 17 there is a row of substantially equal-spaced electroluminescent diodes 20 that together form a light source (= LED module) for the display device 19. Lenticular agents have been added to the light-emitting diodes 20 that ensure that the light-emitting diodes are drawn in a manner illustrated , have an aperture angle of about 25 ° in the longitudinal direction of the opposite upper wall 18 and an aperture angle of about 4 ° in the transverse direction. This choice and dimensioning of the housing makes

[0039] To achieve the required, approximately elliptical, directional characteristics of the light-emitting diode 20 with an aperture angle of about 25 ° in the main direction and an aperture angle of about 4 ° in the perpendicular direction, for example, Carclo Precision Optics optical agents (<a href="http://www.carclo-optics.com">www.carclooptics.com</a>), part number 10049.

[0040] It is pointed out that the translucent rear wall 7 can be replaced with, for example, a non-transparent wall. This wall could also be diffusely reflective and thus contribute to some degree to the luminosity and homogeneity of the light falling on the wall 6. The other display devices shown and described in the present specification can also have only one or two display surfaces, as desired.

[0041] FIG. 5 shows a complete display device 19.

[0042] Fig. 6 shows a display device 21 in a variant in which a row of light emitting diodes 23 is placed on the top wall 18 directly opposite the light emitting diodes 20 on the bottom wall 17. Thus, the light output will increase twice. In addition, these light emitting diodes improve the homogeneity of the luminance on the outside of the surface-display.

[0043] Fig. 7 shows a display device 22 in which the light-emitting diodes 23 are positioned on the top wall 18 by moving a half of the divisional section against the light-emitting diodes 20 on the bottom wall 17. [0044] Fig. 9A shows a display device 19.

[0045] Figure 9B shows the distribution of light intensity on the outer surfaces 61, 62 of the respective display surfaces 6 and 7. It is about 140 lux with a maximum variation of +/- 10%. It is noted here that even smaller, even much smaller luminosity can give the desired effect. You can, for example, imagine a light of 50 lux and less.

[0046] FIG. 9C shows a display device largely corresponding to FIG. 9A. Due to the orienting means (e.g. lenses) added to it, the light source 20 has in this embodiment a larger aperture angle such that a part of the light emitted by the light source 20 falls on the smooth inner surface of the display surfaces 6 and 7 at an incidence angle y against the normal N Due to the fact that the angle in question is larger than the Brewster angle, the appropriate light will be reflected and will fall on the reflecting wall 18 in accordance with the formula indicated by the arrows. This wall 18 then begins to act as a second diffuse source for illuminating the diffusely translucent display surfaces 6 and 7 so that they thus exhibit a very constant luminance on their surface,

[0047] Fig. 10 shows a display device 125 with an elongated carrier 25 having electroluminescent diodes 26, 27, respectively, alternately pointing up and down. These light emitting diodes illuminate the curved, opaque, reflecting rear wall 28 and its part edges, i.e. the bottom edge 29 and the upper edge 30. The respective radiation patterns of the light emitting diodes 26 and 27 are indicated with ellipses 31, 32, respectively.

In order to prevent direct light from the light-emitting diodes 26, 27 to the display surface 33, a longitudinal screen 41 is used in this embodiment which captures and absorbs or reflects the light arriving directly from the light-emitting diodes 26, 27, in which case preferably reflects them with a diffusing effect. .

[0049] It is important to note that as can be seen in particular on the radiation patterns 32, no light arriving directly from the light-emitting diodes 27 falls on the display surface 33. This could have a very adverse effect on the homogeneity of the light distribution.

[0050] Fig. 11 shows an elongated carrier 25. Light-emitting diodes 26, 27 have terminals, all designated for convenience by number 35, connected by soldering, welding or other electrically conductive to conductive slats on the sides of the insulating carrier 25. Copper strips 36 on the side of the carrier 25 directed to the display surface 33 are insulated from each other and offset with respect to the corresponding functional strips 37 on the rear side of the carrier. The clamps (invisible) on the back side are connected with these slats 37. It will be clear that in this way a serial connection of light emitting diodes was made.

[0051] The carrier can be manufactured in a known manner as a printed circuit board, for example epoxy glass. The connection of the LED terminals and strips can be made by soldering, spot welding with disk electrodes or other suitable means.

[0052] Figure 12A schematically shows how a carrier 25 with light-emitting diodes 26, 27 can be placed in a form 38, 39 for proper alignment.

[0053] Fig. 12B shows that the mold 38, 39 can include an additional cavity that can be filled with, for example, epoxide 40. After making the electrically conductive connections mentioned, said cavity can be filled with epoxide, thus forming the structure clearly visible in Fig. 12B . The epoxy mass, which in this embodiment is more or less cylindrical, provides effective coverage of the lateral scattered light, whereby the screen 41 according to FIG. 10 is no longer needed.

[0054] Fig. 13A schematically shows a display device 42 with one fluorescent lamp 43.

[0055] Fig. 13B is a cross-sectional view.

[0056] Fig. 13C depicts the distribution of light. As you can see clearly, it is very heterogeneous. Light intensity varies within the relevant values ​​from 100 lux in edge zones to approximately 400 lux in the central zone. Thus, this prior art display device has exactly the disadvantages for which the invention intends to provide a solution.

[0057] In Fig. 14, a display device 125 according to Fig. 10 and the results obtained therewith are shown by way of comparison.

[0058] Fig. 14A shows a partial view of display device 125.

[0059] Fig. 14B is a cross-sectional view. As shown in this figure, the aperture angle of the light-emitting diodes 26, 27 can in this case be in the order of 8 °.

[0060] Fig. 14C depicts the distribution of light on the display surface 33. It will be clear that as long as the light is much smaller than the light that can be obtained by direct lighting with the fluorescent lamp of Fig. 13, it has radically better uniformity. This uniformity is a measure of the quality of the display device.

[0061] Figure 15 shows the relative spectral distribution of the energy of three light-emitting diodes of different colors. The curve 44 corresponds to the light of blue; curve 45 corresponds to the green light and curve 46 corresponds to the red light.

[0062] It is noted that in combination these three colors may give white light. You can use this property to obtain almost any desired color for the display device by switching and controlling the luminous intensity of the groups of red, green and blue light-emitting diodes.

[0063] Fig. 16 shows a relatively spectral energy distribution of a commonly available light-emitting diode that emits cold, slightly bluish white light.

[0064] Fig. 17 shows a relatively spectral energy distribution of a commonly available light-emitting diode, which also emits cold white light. This electroluminescent diode is available from Nichia and would be, for example, very suitable for use in road signs.

[0065] Fig. 18 shows a relatively spectral distribution of warm white light energy that is emitted by the Lumiled publicly available light diode. This color is often seen as very pleasant and contains a greater proportion of the red range relative to the blue range. The use of electroluminescent diodes of this type is particularly important for presenting information on a display surface containing a red color. By illumination with cold white light (Figures 16 and 17), you can ultimately weaken the effect of the red color due to metamerism and thus make the display less realistic.

[0066] In the case of universal applications, it is conceivable to compile an average ratio of two cold white light emitting diodes into one warm white light emitting diode.

[0067] For certain applications, it is of course also conceivable to use light emitting diodes with different colors or combinations of colors.

[0068] Fig. 19A illustrates an exemplary embodiment of a light box according to the invention. The external dimensions of the coffer are defined as 1.15 mx 0.7 m. Five light-emitting diodes with beam lenses are placed on both short sides so that the light from the light-emitting diodes 20 is directed to the relative opposite wall on which the second row of light-emitting diodes is placed. This wall is diffusely reflective.

[0069] Fig. 19B shows the luminance as a function of the position towards the long side of the light box according to Fig. 19A. As can be clearly seen, luminance varies from about 53 cd / m² up to 62 cd / m².

[0070] In Figure 19B, it is clear that the luminance varies little over the entire width of the light box.

[0071] In Figure 19C, this quantitative observation was justified.

[0072] In the region of the highest value of the first derivative to the position of normalized luminance, the difference in luminance ΔΙ1 is determined on a finite path and the associated Δχΐ is also defined herein. According to the invention, it is considered that the value of the first derivative to the position of normalized luminance is representative. According to FIG. 19C, this derivative is approximated by means of finite ranges, normalizing ΔΙ 1 first by dividing it by the mean luminosity 101 at the appropriate interval and dividing the obtained difference of normalized luminance by the associated interval Δχΐ.

[0073] In this case, the average luminance is 58.0 cd / m.

ΔΙ1 = 4.5 cd / m²Δχΐ = 0.105 m [0074] On this basis, the value of the valid value Q = Δ (Ιι / Ιι₀) / Δχ = 0.74 m "¹.

[0075] The latter value is lower, and even significantly lower, than the standard value of 1.1 m '¹ determined by orienting according to the invention. This means that the quality of the outgoing light must be considered very good. No luminance transitions will be visible to the eye.

[0076] It is noted that Figs. 19B and 19C show that there is a small increase in luminance at the edges. This should be attributed to a small part of the direct light from the light-emitting diodes. Even with a specific lens attachment serving as a directional means, it has been found that the used light-emitting diodes have a small side leaf, so that the luminance of the display surface in the immediate vicinity of LEDs increases by several percent. From a certain Q value, it will be clearly apparent that this small portion of the undesirable light itself is completely harmless. If necessary, even this small undesirable effect can be eliminated by covering the discussed side leaf.

[0077] Fig. 20A shows a light box with four fluorescent lamps. The internal dimensions of the coffer are 0.7 mx 0.7 m. Fluorescent lamps are placed at a mutual pitch distance of 0.2 m.

[0078] The cassette of Figure 20A has a depth of 0.2 m. The fluorescent lamps are arranged at a central distance of about 0.1 m to the display surface.

[0079] Fig. 20B shows the luminance as a function of the position. The chart marked with small circles refers to an essentially ordinary or standard light box, whose internal walls consist of plain unprocessed aluminum of ordinary quality. The indeterminate graph refers to an embodiment in which the backsheet is covered with LEF film (trademark of 3M).

[0080] FIG. 20C again shows the graph of FIG. 20B, but in this figure, Q values ​​were calculated for both described embodiments.

[0081] For a standard light box, the average luminance is 103 925 cd / m².

ΔΙ3 = 106 cd / m²Δχ = 0.06 m [0082] Based on these values, Q = 1.91 m '¹. It will be clear that this value shows that great differences in light are visible to the naked eye. They are so large that in practice they are considered irritating and perhaps even unacceptable.

[0083] The average light intensity of the light box with the LEF film is 974 cd / m².

ΔΙ = 130 cd / m²Δχ = 0.09 m Q = 1.48 m '¹ [0084] FIG. 21A relates to the placement of a display surface at different positions. FIG. 21A shows a display surface in two positions such that the distance of the fluorescent lamps increases more and more, here every 0.1 m.

[0085] The results are shown in Fig. 21B.

[0086] The indeterminate figure refers to the situation illustrated by the solid lines in Figure 21A, in which the display surface is located at a distance of 0.1 m relative to its standard position. It will be obvious that a lot of light is lost at the edges. Therefore, edge zones will be heavily underexposed with respect to the central zone, which, however, itself shows significant variations in luminance in short sections.

[0087] The chart marked with circles refers to a distance of 0.2 m.

[0088] The square graph indicates a distance of 0.3 m.

[0089] The graph with triangles refers to a distance of 0.4 m.

[0090] The asterisk plot refers to a distance of 0.5 m.

[0091] The graph with full circles refers to a distance of 0.6 m.

[0092] Fig. 21C shows the determination Q for a distance of 0.5 m. It is 1.37 m ' ¹. This value for the gradient, although far from ideal in relation to the standards of the invention, is still acceptable for some applications.

[0093] Fig. 2ID refers to a distance of 0.6 m. In this arrangement, Q has a value of 0.75 m¹. It should be considered very good. However, it should be noted here that this high quality is obtained through the use of four fluorescent lamps, each with a power rating of about 30 W, which means a net energy consumption of 120 W. Then you must also add energy loss of chokes. Depending on the embodiment, it is, for example, 5 to 10 watt per fluorescent lamp. This very high energy consumption compared to LEDs (LED strip according to the invention with comparable results will consume about 15 to 30 W) may perhaps still be acceptable in certain applications, but it will be obvious that a light box with a depth of 0, 7 m is not suitable for practically any use.

[0094] Fig. 22A refers to a light box that is closely related to a light box according to WO-A-99/67663, which has already been discussed above.

[0095] Fig. 22B shows the luminance as a function of the position.

[0096] The calculation Q according to Fig. 22C gives the value Q = 1.75 m '¹. It will be clear that this result leaves much to be desired.

[0097] Fig. 23 shows a bundle of ten plots, i.e. a summary of the diagrams drawn above.

[0098] The plot drawn with black squares is the luminance according to Figure 19, thus a light box constructed in accordance with the technical knowledge of the invention.

[0099] A diagram marked with black triangles refers to a standard light box according to Fig. 20.

[0100] The black hourglass chart refers to a standard light box, in which the back wall is covered with the LEF film of Figure 20.

[0101] An unmarked figure refers to a situation with a display surface at a distance of 0.1 m according to Fig. 21.

[0102] A diagram marked with circles refers to a distance of 0.2 m.

[0103] The square plot is a distance of 0.3 m.

[0104] The graph with triangles refers to a distance of 0.4 m.

[0105] A star chart refers to a distance of 0.5 m.

[0106] A graph with a black circle refers to a distance of 0.6 m.

[0107] A black star chart refers to a standard light box according to Fig. 22.

[0108] From the above comparison and comparison of the magnitude values ​​Q selected as standard according to the invention it will be clear that only the graph marked with black squares satisfies the standard Q &lt;1.0 - 1.2 m '¹, also taking into account small depth and low energy consumption. It should be concluded that despite many well-known techniques and general knowledge, one skilled in the art is able to achieve the stated goal using simple means.