LIGHT EXTRACTOR FOR A LIGHT GUIDE LAMP

Abstract: There is described a light extractor (8) for a light guide lamp (1) whose useful surface, that is to say, the surface that, in use, faces the emission surface (5) of the light guide lamp (1), comprises a predetermined distribution of diffusing points (10) and a complementary distribution of reflecting points (9). The light extractor (8) allows obtaining an illumination having preselected features. There are also described methods for determining such a distribution of points so as to obtain a highly uniform illumination.

Description: Light extractor for a light guide lamp * * * * * DESCRIPTION The present invention relates to a light extractor for a light guide lamp. Moreover, the invention relates to a diffuser and a light guide lamp comprising it, as well as methods for determining a distribution, along a main propagation direction, of diffusing points on a light extractor, and a computer program suitable to perform the steps of one such method. The invention also relates to a method for manufacturing the light extractor and a silk- screen. In the present description and the attached claims, as it will be clear hereinafter: - "diffuser" indicates a light guide structure provided with a light extractor, so that the light is partly guided and partly exits, - "diffusing point" indicates a point of a surface having such optical characteristics that a ray incident onto it generates a bundle of light rays reflected within the local bending angle of the surface, thus being comparable to a point source; thus, in principle, also a hole could be used, - the term "direction" is used so as to include curvilinear directions . Light guide lamps are known, and they provide for the light generated by one or more sources to be directed into a light guide, that is to say, a closed structure having a surface at least partly transparent, and at least partly reflecting on the internal side. The light rays propagate within the structure through a series of successive reflections, and exit therefrom in a more or less gradual manner. The light guides obtained through the use of total internal reflection films (or TIR films) are especially effective. Said TIR films are known, for example, from European patent no. EP 0 225 123, to which reference shall be made for a detailed description of their features; an example of said TIR films are those produced and marketed by Minnesota Mining and Manufacturing Company under the brand name of OLF - Optical Lighting Film. They are shaped as flexible sheets or tapes, exhibiting a surface with a series of parallel micro-relieves with a substantially triangular section; said films can be applied onto the surface of a transparent carrier material, with the micro-relieves oriented in the propagation direction and usually facing outwards, thus creating an effective light guide. In fact, thanks to the optical characteristics of said films and as described, for example, in document US 4 805 984 - to which reference shall be made for further details - the light forming with the main propagation direction a smaller angle than a critical angle θ rnax is always internally reflected, whereas the light forming with the main propagation direction a greater angle θ than angle θ max is internally reflected if it is incident on the TIR film with an angle, with respect to the normal, that is smaller than a certain angle, depending on angle θ. Thus, contrary to the current definition, in the following description and in the attached claims, the incidence angle of a light ray on a surface shall indicate that formed by the ray with the plane tangent to the surface. Angle θ max depends on the characteristics of the TIR film and, for the OLF film mentioned above, it is. of 27.6°. Another material used in light guide lamps is the so-called multilayer optical film, described for example in document US 5882774. Special elements called light extractors are inserted into the light guide to cause a controlled diffusion of a portion of the light travelling within the light guide, thus making some of the light rays deviate and be incident onto the TIR film with such angles as to exit from the light guide. Known light extractors comprise, for this purpose, a typically white diffusing surface, and they consist of tridimensional bodies arranged in axial position in the light guide (reference can be made, for example, to Italian patent application TO98A000513) , or of bidimensional bodies applied to the internal surface of the light guide (reference can be made, for example, to European patent application EP-A-1006312) . The technical problem at the basis of the present invention is that of providing a light extractor for a light guide lamp which should allow obtaining an illumination with preselected features, for example a highly uniform illumination. Thus, in a first aspect thereof, the invention relates to a light extractor for a light guide lamp, characterised in that its useful surface comprises a predetermined distribution of diffusing points and a complementary distribution of reflecting points. In the present description and attached claims, the expression "useful surface" indicates the surface that, in the use of the light extractor, faces the emission surface of the light guide lamp. Since the light incident on the reflecting points propagates through reflection, whereas the light incident on the diffusing points is deviated at such angles as to partly exit from the lamp, by controlling the distribution of diffusing points - and thus the complementary distribution of reflecting points - it is possible to control, point by point, the light diffusion ' properties, on the basis of the desired result when the light extractor is used in the light guide lamp. Advantageously, the useful surface comprises a reflecting base layer and the predetermined distribution of diffusing points on the reflecting base layer. In this way, the manufacture of the light extractor is simplified. Even more preferably, the reflecting base layer comprises the side without micro-prisms of a TIR film. In particular, the reflecting base layer can consist of the above cited OLF films or optical multilayer films. In an embodiment, adjacent diffusing points of the predetermined distribution are at a decreasing distance along a main propagation direction from a light inlet end to an opposed end. Such an extractor, used in a light guide lamp having a single illumination system, provides an uniform illumination along the main propagation direction. In another embodiment, the distribution of diffusing points is symmetrical with respect to a median axis perpendicular to the main propagation direction. Such an extractor is particularly useful in light guide lamps having two symmetrical illumination systems at the two ends . In this case, in a particularly preferred way, adjacent diffusing points of the distribution of points are at a decreasing distance from each end of the extractor to the median axis perpendicular to the main propagation direction. Such an extractor provides an uniform illumination along the main propagation direction. Advantageously, the predetermined distribution of diffusing points is imprinted on the reflecting base layer through silk-screen printing. Silk-screen printing allows easily achieving any distribution of points also on non-absorbing surfaces, with an acceptably small distance between the diffusing points. Moreover, in a second aspect thereof, the invention relates to a diffuser for a light guide lamp comprising a light extractor as said above, and an emission surface transparent for rays forming, with a main propagation direction, a greater angle than a predetermined critical angle, and internally reflecting for rays incident with a smaller angle than the predetermined critical angle. Typically, the emission surface comprises a transparent support layer and a total internal reflection film applied on the transparent support layer. Preferably, the total internal reflection film has micro- relieves facing outwards, and oriented in parallel to the main propagation direction. Since the light emitted by the TIR film, in particular by the above-cited OLF, has a fixed output angle with respect to the longitudinal direction of the micro-relieves, their arrangement as indicated allows minimising the visual perception of the shadows caused by said angle. Preferably, the light extractor has a cross section selected from the group consisting of rectilinear, semicircular, semi-elliptical, parabolic, semi-parabolic, or according to higher-order curves, and the emission surface has a cross section selected from the group consisting of rectilinear, semicircular, semi-elliptical, parabolic, semi-parabolic, or according to higher-order curves . Preferably, moreover, the maximum distance between extractor and emission surface in a plane perpendicular to the main propagation direction is a preselected fraction of the length of the extractor in the main propagation direction, even more preferably, it is equal to about a twentieth part. In this way, the light propagates with a sufficiently small number of incidences, and thus, with few losses. Moreover, in another aspect thereof, the invention relates to a light guide lamp comprising a diffuser as said above, and at least an illumination system suitable to project the light into the diffuser with smaller angles than the critical angle of the emission surface. Those rays projected into the diffuser which are incident first of all onto the emission surface, are thus totally internally reflected, whereas those incident first of all onto the extractor are reflected or diffused according to whether they are incident on a diffusing point or on a reflecting point. Moreover, in another aspect thereof, the invention relates to a method for determining a distribution along a main propagation direction of diffusing points on a useful surface, elsewhere reflecting, of a light extractor intended to be coupled to an emission surface, transparent for rays propagating with a smaller angle than a predetermined critical angle, and internally reflecting for rays propagating with a greater angle than the critical angle, in a diffuser for a light guide lamp having an illumination system suitable to project the light of a light source into the diffuser with smaller angles than the critical angle, the method comprising the steps of: (a) setting a first point of the distribution as the point of final incidence on the extractor closest to the illumination system of a light ray propagating according to a preselected type of trajectory, (b) setting a second point of the distribution as the point of final incidence on the extractor farthest from the illumination system of a light ray propagating according to the preselected type of trajectory, (c) recursively determining, for each point of the distribution of points, a new point of the distribution as the point of the extractor equally illuminated by a first point source arranged at the preselected point of the distribution, and by a second point source arranged at a next adjacent point of the distribution, the intensities of the point sources being a function of the intensity of the light source and of a characteristic distance of the preselected type of trajectory. A light extractor having the light distribution thus determined allows obtaining, in use, a sufficiently uniform illumination along the main propagation direction. Preferably, in steps (a), (b) , (c) , the trajectory type is classified on the basis of the number of reflections within the diffuser. More in particular, in step (c) , when the preselected type of trajectory consists of the rays that, from the light source of the light guide lamp directly strike the extractor, the intensity of each point source is given by the intensity of the light source, multiplied by an absorption-in diffusion coefficient of the extractor, and divided by the squared distance of the point of the source point from the light source. Thus, the introduced schematisation is that each diffusing point is deemed to have a constant emission at each angle, and independent of the incidence angle of the light ray on it. When, on the contrary, in step (c) the preselected type of trajectory consists of the rays that, from the light source, strike the extractor after undergoing at least one reflection by the emission surface, and optionally by the extractor, the intensity of each point source is given by the intensity of the light source, multiplied by an absorption-in diffusion coefficient of the extractor, multiplied, for each reflection onto the emission surface, by an absorption-in reflection coefficient of the emission surface, multiplied, for each reflection onto the extractor, by an absorption-in reflection coefficient of the extractor, and divided by the squared projection, along the main propagation direction, of the portion of the ray under consideration from the light source to the first reflection point. Thus, to simplify the calculations, the emission surface is deemed as arranged at the extractor. Preferably, the method further comprises the steps of: (d) determining at least a second distribution of points through steps (a) , (b) , (c) , for a different type of trajectory, (e) adding each second distribution of points to the distribution of points. The result is increasingly precise as the types of trajectory considered increase, so the obtainable illumination is increasingly uniform. Moreover, if the extractor is intended to be inserted into a light guide lamp having a second illumination system that is specular to the first illumination system and arranged at the opposed end, it comprises the step of: (f) adding to the distribution of points the specular points of each point of the distribution of points. Preferably, moreover, the method comprises the step of: (g) repeating the distribution of points at constant intervals along a direction perpendicular to the main propagation direction of the extractor. In this way, the extractor exhibits a distribution of stripes of diffusing points, and the illumination is sufficiently uniform also in planes perpendicular to the main propagation direction. More preferably, the method comprises the further step, during the repetitions of step (g) , of: (gl) alternately staggering the distribution of points forwards and backwards in the main propagation direction by a constant interval. In this way, it is possible to reduce the distance between points below the minimum distance imposed by the physical implementation of the extractor. Preferably,, the method further comprises the step of: (h) rejecting, from the distribution of points, those points exhibiting a distance that is greater than a preset minimum distance from the immediately adjacent points. In this way, it is possible to prevent obtaining a distribution having a higher precision than the physical implementation of the extractor. Moreover, in another aspect thereof, the invention relates to a computer program comprising program code means suitable to perform the steps of the above method when the program is run on a computer. In an advantageously parametric implementation, the computer program comprises means for receiving in input one or more of the following parameters: - critical angle of the emission surface to which the extractor is intended to be coupled, - shape and/or size of the extractor, - shape and/or size of the emission surface to which the extractor is intended to be coupled, - maximum distance of the extractor from the emission surface to which the extractor is intended to be coupled, - number, type, intensity and nature of the light sources of the illumination system to which the extractor is intended to be coupled, - minimum distance between the points of the distribution of points, - absorption-in reflection coefficient of the extractor, - absorption-in diffusion coefficient of the extractor, absorption-in reflection coefficient of the emission surface to which the extractor is intended to be coupled, - type or types of trajectories to be considered or maximum number of reflections to be considered. To facilitate the management of multiple simulations, the computer program can comprise means for saving the parameters entered into a file and means for loading the parameters from a file. Preferably, the computer program generates one or more outputs, among: - the coordinates of the points of the distribution, - the percent density of the points of the distribution in elementary intervals, - a report of the preset parameters, - a graphical representation of the distribution of points, - a file of the points of the distribution in a format readable by a printing machine, in particular a silk-screen printing machine, - the geometry of each illumination system of the light guide lamp for which the extractor is intended, as a function of a preset maximum distance between the extractor and the emission surface, and of a preset critical angle of the emission surface. The table format, the graphical representation and the report of the set parameters are useful for multiple simulations; the file in a format readable by a printing machine, for example Autocad™ and Macintosh™, allows the integration with the extractor manufacturing step, while the determination of the geometry typically coincides with determining the focus of a reflecting parabola of the illumination system, which thus meets the requirement of projecting the light into the diffuser at smaller angles than the critical angle of the emission surface. Typically, moreover, the computer program can be embodied into a computer readable means . In another aspect thereof, moreover, the invention relates to a method for determining a distribution along a main propagation direction of diffusing points on a useful surface, elsewhere reflecting, of a light extractor intended to be coupled to an emission surface, transparent for rays propagating with a smaller angle than a predetermined critical angle (θ max ) an d internally reflecting for rays propagating with a greater angle than the critical angle (θ max ) , in a diffuser for a light guide lamp having at least one illumination system suitable to project the light of a light source into the diffuser with smaller angles than the critical angle (θ max ) , the method comprising the steps of: (a) defining, for each interval of a preselected length along the main propagation direction of the useful surface of the extractor, a diffusing-points-density variable, (b) defining, for each interval of a preselected length along the main propagation direction of the emission surface, a luminosity variable; (c) expressing the value of the luminosity variable of each interval of the emission surface as a function of the diffusing-points-density variables of the extractor, and (d) calculating the values of the diffusing-points-density variables of the extractor such as to equal with one another the values of the luminosity variable of all intervals of the emission surface. As for the first method illustrated above, a light extractor having the distribution of light thus determined allows obtaining, in use, a sufficiently uniform illumination along the main propagation direction. In a preferred embodiment of the method illustrated above: - step (a) comprises the sub-step (al) of presetting to zero the values of the diffusing-points-density variables, - step (b) comprises the sub-step (bl) of presetting to zero the values of the luminosity variables, - the calculation step (d) is carried out through the steps of : (dl) evaluating the values expressed in step (c) of the luminosity variable of each interval of the emission surface on the basis of the current values of the diffusing-points-density variables of the extractor, (d2) individually incrementing the values of the diffusing- points-density variables of the extractor and (d3) iteratively repeating steps (dl) and (d2) until the values of the luminosity variable of all intervals of the emission surface evaluated in step (dl) are equal to each other. Preferably, moreover, the step (c) of expressing the luminosity variables comprises the steps of: (cl) splitting the light projected by the at least one illumination system into a finite number of rays, each having an initial luminosity value, (c2) incrementing the luminosity variable of each interval of the emission surface ' by the value of the final luminosity of each ray incident on it with a greater angle, with respect to the main propagation direction, than its critical angle (θ max ), wherein: at each incidence of a ray in an interval of the extractor, an evaluation of whether the ray is- reflected or diffused is made on the basis of the value of the respective diffusing-points-density variable, — if the ray is reflected, the luminosity value of the reflected ray is obtained by multiplying the luminosity value of the incident ray by an absorption-in reflection coefficient of the extractor, — if the ray is diffused, the luminosity value of each diffused ray is obtained by multiplying the luminosity value of the incident ray by an absorption-in diffusion coefficient of the extractor, - at each incidence on the emission surface with a smaller angle, with respect to the main propagation direction, than its critical angle (θ max ) , the luminosity value of the reflected ray is obtained by multiplying the luminosity value of the incident ray by an absorption-in reflection coefficient of the emission surface. Preferably, the method further comprises the step of: (e) repeating the distribution of points at constant intervals along a direction perpendicular to the main propagation direction of the extractor. In this way, the extractor exhibits a distribution of stripes of diffusing points and the illumination is sufficiently uniform also in the planes perpendicular to the main propagation direction. More preferably, the method comprises the further step, during the repetitions of step (e) , of: (el) alternately staggering the distribution of points forwards and backwards in the main propagation direction by a constant interval. In this way, it is possible to reduce the distance between points below the minimum distance imposed by the physical implementation of the extractor. Preferably, the method further comprises the step of: (f) rejecting, from the distribution of points, those points exhibiting a distance that is greater than a preset minimum distance from the immediately adjacent points. In this way, it is possible to prevent obtaining a distribution having a higher precision than the physical implementation of the extractor. Moreover, in another aspect thereof, the invention relates to a method for manufacturing a light extractor as said above, comprising the steps of: (a) providing a reflecting base layer; (b) silk-screen printing diffusing points onto the reflecting base layer according to a predetermined distribution of points. Although such a light extractor can be manufactured through other methods, such as printing or differently applying on a neutral substrate the distribution of diffusing points and the complementary distribution of reflecting points, silk-screen printing on a reflecting base layer allows easily obtaining, also on non-absorbing • surfaces, any desired distribution of points, with an acceptably small minimum distance between the diffusing points. Preferably, the printing step (b) is carried out by printing the distribution of points obtained according to one or the other of the above methods. Moreover, in another aspect thereof, the invention relates to a silk-screen embodying a distribution of points obtained with one or the other of the above methods. Features and advantages of the invention shall now be illustrated with reference to embodiments represented by way of a non-limitative example in the attached drawings, wherein: - Figure 1 schematically shows, out of scale-, a light guide lamp embodying the present invention, in a first embodiment; - Figure 2 shows a plan view of the lamp of Figure 1, useful to explain a first method for determining the distribution of diffusing points according to the present invention; - Figure 3 shows a block diagram relating to the method illustrated in Figure 2; - Figures 4 to 6 are views similar to Figure 2, useful to explain said method more in detail; - Figures 7 to 9 show further block diagrams relating to said method; Figures 10 and 11 schematically show preferred embodiments of an extractor according to the invention; Figures 12 to 14 schematically show alternative embodiments of light guide lamps embodying the present invention; and - Figure 15 shows a block diagram of a computer program suitable to perform the first method of the invention for determining the distribution of diffusing points. Figure 1 illustrates a light guide lamp 1 comprising two illumination systems 2, 2a, and a parallelepiped-box-shaped diffuser 3, coupled to them through respective glasses 4, 4a. The light guide lamp 1 may alternatively comprise a single illumination system, for example only illumination system 2. Diffuser 3 comprises an emission surface 5 and a light extractor 8, described more in detail hereinafter. In turn, the emission surface 5 comprises a transparent support layer 6 made of a rigid material, such as polycarbonate or the like, internally (as shown) or externally coated with a TIR film 7, for example the OLF film by Minnesota Mining and Manufacturing Company. The thickness of diffuser 3, that is to say, the distance h between extractor 8 and emission surface 5, preferably is a preselected fraction of length / of the extractor in the main propagation direction, for example 1/20. Observing such a ratio, the light propagates with a sufficiently small number of incidences, and thus with few losses. The TIR film 7 is preferably arranged so that its micro- relieves, schematised as lines, are horizontally oriented. In fact, the optical characteristics of the OLF film are such that the light rays exit from it at an angle α of about 15° with respect to the direction of the micro- relieves, and their horizontal arrangement minimises the visual perception of the shadows caused by said output angle. As in the traditional applications of said OLF film, the micro-relieves preferably face outwards diffuser 3. Each illumination system 2, 2a comprises a light source 21, 21a. The light source is a fluorescent tube having for example a 36-W power, arranged at the focus of a reflecting parabola 22, 22a. The electrical power supply and control components of the illumination system 2, 2a are not shown in detail since they are totally conventional. Each illumination system 2, 2a is designed so as to project the light into diffuser 3 with smaller angles than the critical angle θ max of the emission surface 5, or of its TIR film 7 (θ max = 27. β° in the case of the OLF film mentioned above) . For the known geometrical properties of the parabola, wherein all rays exiting from the focus are reflected in parallel to the axis, arranging the light source 21 at the focus the above condition is obtained when distance δ between the focus and the glass 4 is given by: δ = ^*tgθ max (1) The light extractor 8 consists of a substantially bidimensional body having a reflecting base layer 9 and a plurality of diffusing points 10 arranged on the reflecting base layer 9 according to a predetermined distribution. The reflecting base layer 9 can itself be the body of extractor 8, or it can be provided as a reflecting coating onto a substrate (not shown) , for example polycarbonate or glass. Preferably, the reflecting base layer 9 consists of the side without micro-prisms of the above-cited OLF film. The diffusing points 10 preferably are white points, but they can be points of another colour, . and they are preferably silk-screen printed on the reflecting base layer 9. A light extractor 8 thus made performs as a plurality of elementary extractors arranged on a reflecting surface. In fact, a light ray incident onto the reflecting base layer 9 is reflected with an angle equal to the incidence angle, whereas a light ray incident on a diffusing point 10 is diffused, that is to say, reflected in a bundle of light rays distributed over the internal solid angle of extractor 8 at the diffusing point 10 under consideration, in particular over the plane angle in the illustrated case of plane extractor 8. In a first approximation, the diffused rays can be regarded as equipotent since the differences between their intensity are infinitesimal. Thus, with reference to Figure 1, the three following situations occur: - if a light ray, as ray R]_, is incident on the reflecting base layer 9 of extractor 8 with an angle β^ with respect to the main propagation direction indicated by axis x, it is internally reflected within diffuser 3 with an angle that is equal to the incidence angle, that is, it further propagates within diffuser 3 with the same angle β]_ with respect to the propagation direction x, directed towards the emission surface 5; - similarly, if a light ray, such as ray R2, is incident on the emission surface 5 with an angle β2 with, respect to the main propagation direction x, it is internally reflected within diffuser 3 with an angle that is equal to the incidence angle, that is, it further propagates within diffuser 3 with the same angle β2 with respect to the propagation direction x, directed towards the light extractor 8; - if a light ray, such as ray R3, is incident on a diffusing point 10 of extractor 8 with an angle β3 with respect to the main propagation direction x, it is internally diffused within diffuser 3. In the last cited case, the incident ray R3 originates two groups of secondary rays: - rays, such as rays R31, forming a smaller angle than the critical angle θ max with respect to the main propagation direction x: they further propagate within diffuser 3, generically directed in the propagation direction or, respectively, in the direction opposed to the propagation direction; - rays, such as ray R33, forming a greater angle than θ max with respect to the main propagation direction x. The rays of the last considered type, such as ray R33, when arriving at the emission surface 5 are not internally reflected, but they exit from it and contribute to the illumination of the light guide lamp 1. Moreover, it is worth noting that, since the rays are projected by the illumination system 2, 2a into diffuser 3 with angles that are always smaller than the critical angle θrnax' an due to the above characteristics of transmission and reflection, the light rays incident onto extractor 8 always form smaller angles than the critical angle θ max with the main propagation direction x. Thanks to this, it is possible to make the reflecting base layer 9 as the side without micro-prisms of the same OLF film mentioned above, as previously indicated. From what set forth it must be understood that the particular light extractor 8 of the invention provides an ideally unlimited flexibility of control of the light rays exiting the light guide lamp 1, that is to say, of its illumination. In the practice, this flexibility is restricted only by the resolution of the diffusing points, in turn dictated by the method used for physically implementing the light extractor 8, and by the difficulty of the calculations needed for determining the suitable distribution of diffusing points to provide the desired illumination. For this purpose, hereinafter there is illustrated a first method for determining a distribution of the diffusing points 10 of the light extractor 8 such that the light emitted by the light guide lamp 1 is as uniform as possible . The method will be illustrated with the aid of Figure 2, which shows a plan view out of scale of the light guide la p 1. In fact, considering the geometry of the light guide lamp and since the fluorescent-tube light sources 21, 21a emit in a substantially equipotent way along their length, the optical system can be deemed to be a bidimensional system, neglecting in a first approximation the rays that, from the light source 21, 21a of the illumination system 2, 2a, propagate along directions having a component along axis y, perpendicular to the main propagation direction x. Let us consider, first of all, a light guide lamp 1 comprising only the illumination system 2, whose light source 21 can be regarded as a point source and has been designated with S in Figure 2. For what said, the luminosity at the emission surface 5 is generated only by the rays as ray R33 of Figure 1, that is, it is due to the plurality of point sources resulting at the diffusing points 10 on extractor 8. By fixing the origin of axis x at the end of extractor 8 close to the illumination system 2, the distribution of the diffusing points 10 on extractor 8 along the main propagation direction x can be expressed as W(x) = xl, x2, , xn where xl, x2, ... xn are the coordinates of each diffusing point 10 of extractor 8. In the method of the invention, to obtain the desired distribution of diffusing points, the following recursive method is applied. Firstly, it is assumed to have only two diffusing points 10 on extractor 8, arbitrarily selected or, preferably, in the way that shall be illustrated after, having coordinates X]_ and X2, that is, W(x)=xχ,X2 is set. This is represented by blocks 32 and 34 of the diagram of Figure 3. Then, as represented by block 35 of Figure 3, it is determined whether there exists a point X3 on extractor 8 equally illuminated by the point sources with respective intensity I (x]_) , I(X2) arranged in the diffusing points having coordinates X]_ and X2 • If it exists, point X3 is added to the distribution of diffusing points W(x), as represented by block 36 of Figure 3. In this respect, it should be noted that since the luminous intensity is an inverse function of the squared distance from the light source, if X]_<X2 then I (x]_) >I (X2) and therefore, point 3 will be arranged between points X]_ and X2, closer to X2, that is, W (x) =X]_, X3, 2 assuming that the distribution W(x) is kept ordered. Continuing in the iteration, point 4 equally illuminated by the point sources with respective intensity I(xχ), I(X3) arranged in the diffusing points having coordinates x^_, X3, and point X5 equally illuminated by the point sources with respective intensity I(x3), I(x2) arranged in the diffusing points having coordinates X3, X2, are calculated, thus obtaining distribution W(x) = ,X4,X3,X5,X2, afterwards distribution W(x)= X_,X5, 4,X7, 3,xg,X5,xg,X2 etcetera . Thus, blocks 35 and 36 of the block diagram of Figure 3 represent the recursive calculation, for each preselected point of the distribution of points, of a new point of the distribution as the point of the extractor equally illuminated by a first point source arranged at a preselected point of the distribution, and by a second point source arranged at an adjacent point of the distribution. Said recursive calculation ends when it is not possible to determine a new point, so the cycle must be understood as being repeated for each index i of the distribution of diffusing points W. Nevertheless, the index increment has not been shown since the order in which the various pairs of adjacent points are considered is unimportant. For example, instead of considering every time in succession the two new pairs of values resulting upon adding the new value, it would be possible to consider every time only the new pair of smaller coordinates up to the end of the recursion, considering then the following pair and ending the recursion again, etcetera, so that the points of Figure 2 would be determined in the following order: The luminous intensity I (x) of the point source in any diffusing point 10 of distribution W(x) depends on all the light rays that, through a series of different trajectories optionally comprising reflections within diffuser 3, arrive at the diffusing point 10 under consideration. The light rays arriving at a diffusing point 10 of extractor 8 are preferably classified, from a qualitative point of view, in the following types of trajectory: (a) rays, such as ray SX of Figure 4, which from the light source S directly strike extractor 8 in point X, (b) rays, such as ray SX of Figure 5, which from the light source S strike the emission surface 5 in a point XQ and afterwards, extractor 8 in point X, (c) rays, such as ray SX of Figure 6, which from the light source S strike extractor 8 in a point XQ of the reflecting base layer 9, that is, in a non-diffusing point, and afterwards, the emission surface 5 in a point X]_ and afterwards, extractor 8 in point X, (d) rays that, from the light source S alternately strike several times the emission surface 5 and extractor 8 in a point of the reflecting base layer 9 before striking extractor 8 in a diffusing point 10; (e) rays that are emitted by a diffusing point 10 of extractor 8 at smaller angles than the critical angle θ max , such as rays R31 and R32 of Figure 1, strike the emission surface 5 and afterwards, strike extractor 8 in a diffusing point 10, immediately or after one or more successive reflections on points of the reflecting base layer 9 of extractor 8 and on the emission surface 5. In practice, the above classification is based on the number of reflections within diffuser 3. The effect of the walls of diffuser 3 on the luminous intensity can be modelled by introducing the following coefficients : absorption-in reflection coefficient of the emission surface 5, Rifs, for example « 0.98 for the OLF film, - absorption-in reflection coefficient of the points of the reflecting base layer 9 of extractor 8, Rifg, for example « 0.98 for the side without micro-prisms of the OLF film, or « 0.95 for a mirror, absorption-in diffusion coefficient of the diffusing points 10 of extractor 8, Difg, for example « 0.80»^7 for white, silk-screen printed points, where Yl indicates the number of rays exiting from the diffusing point, for example selected, in the case of plane extractor, as π/10, π/100, π/1000 etcetera according to the desired precision. It follows that the contribution to the luminous intensity I(x) of the point source in any diffusing point 10 of extractor 8 due to any ray incident thereon decreases from the rays of (a) type to the rays of (e) type, and it decreases with increasing number of reflections of the rays within the various types of trajectory summarised under letters (d) and (e) . Thus, in a first approximation, it is possible to consider the rays of (a) type only. Thus, with reference to Figures 3 and 4, in the method according to the present invention the procedure is as follows. As illustrated in block 31 of Figure 3, the first point X]_ of distribution (x) of diffusing points 10 (point A in Figure 4) is set as the incidence point on extractor 8 closest to the illumination system 2, of a type (a) ray, that is, of a ray that, from the light source 21 of the light guide lamp 1, indicated as point S in Figure 4, directly strikes extractor 8. Said first point corresponds to the end close to the illumination system 2 of extractor 8, that is, to point A having coordinate xχ=0. As illustrated in block 33, moreover, the second point X2 of distribution W(x) of diffusing points 10 is set as the point of final incidence on extractor 8 farthest from the illumination system 2 of a type (a) ray. Said point corresponds to the end of extractor 8 opposed to the illumination system 2, that is, to point B having coordinate Indicating with K the intensity of the light source 21, the luminous intensity in a generic point X haying coordinate x due to the type (a) ray is given by the expression: where the Pythagorean theorem has been applied to triangle SXX' of Figure 4, where X' represents the projection of point X on the longitudinal axis of the light guide lamp 1. In block 35 of Figure 3, coordinate x of point X of the diffuser which is equally illuminated by the point source of intensity 1(0) arranged in point A and by the point source of intensity I(/) arranged in point B, is given by expression: More in general, coordinate x of a point X equally illuminated by a point source of intensity I(XJ_) arranged in a point Xj_ of the distribution of points (x) and by a point source of intensity I ( i + i) arranged in an next adjacent point i + i is given by expression: The solutions of said equation can be expressed as follows or, by applying expression (2), as: x = x. + 1 + l ! (5) SX i+l The recursive application of the formulas with the criteria set forth above provides the desired distribution of points W(x) =X]_, 2, ... T that is, such that the illumination of the light guide lamp 1 is uniform within the limits of the simplifying assumptions formulated. For the purposes of improving the approximation, the method of the present invention proposes, as illustrated in the block diagram of Figure 7, to determine (block 41) at least a second distribution W]_ (x) , W2 (x) , ... of points in the manner indicated above, but for a different type of propagation, and to add (block 42) every second distribution ^ (x) , 2 (x) , ■ • • of points to the distribution (x) of points determined above (as recalled in a block 40) . For example, with reference to Figure 5, it ' is possible to consider the type (b) rays according to the classification shown above, that is to say, the rays that from the light source 21 (S in Figure 5) of the light guide lamp 1 strike the emission surface 5 in a point Q and afterwards, extractor 8 in point X. In this case, in the step represented by block 31 of Figure 3, the first point A of distribution Wτ_ (x) has a coordinate xi given by expression: xl = —^— (6) tg oθ w max obtained observing that the light ray under consideration is reflected according to angle θ max at the end point An of the emission surface 5 close to the illumination system 2. The second point B of distribution W]_ (x) , calculated in the step represented by block 33 of Figure 3, has, as for distribution W(x), a coordinate The luminous intensity in a point X having coordinate x can be deemed to be given by the expression: I (x) = if 5 * Dif 8 * -I j - = Rif 5 * Dif 8 * K (7) SX 0 (δ + x 3 where X Q ' is the projection of point XQ on the longitudinal axis of the light guide lamp 1. In fact, indicating with X' the projection of point X on the longitudinal axis, of lamp 1 and with X Q ' ' the intersection of said axis with the reflected ray XQX , from the equality of triangles SXQ O ' / XnXn'Xo'' and X 0 ' ' XX ' it follows that SX 0 '=SX'/3. In fact, it is more practical to use distance SXn ' instead of SXn and neglect the length of portion XQ / without inserting significant errors. The coordinate x of a point X equally illuminated by the two point sources considered in turn in the recursive calculation of block 35 of Figure 3 is of course still given by equations (3) and (4) . To further improve the approximation, with reference to Figure 6 , it is possible to consider type (c) rays according to the classification given above, that is to say, the rays that from the light source 21 (S in Figure 6) of the light guide lamp 1 strike extractor 8 in a point XQ, the emission surface 5 in a point X]_ and afterwards, extractor 8 in point X. In this case, in the step represented by block 31 of Figure 3, the first point A of distribution W2 (x) has a coordinate ]_ given by the expression: xl = -* _ (8) tgθ max obtained from the observation of triangles An ^A^ ' e AA1 'A. The second point B of distribution ^ (x) , calculated in the step represented by block 33 of Figure 3, has, as for re¬ distributions W(x) and V1 1 (x) , a coordinate xg=/. The luminous intensity in a point X having coordinate x can be deemed to be given by the expression: I (x) = Rif g * Rif 5 * Dif g * 0) obtained from the equality of the various triangles that the longitudinal axis of the light guide lamp 1 forms with the ray under consideration. Coordinate x of a point X equally illuminated by the two point sources considered in turn in ' the recursive calculation of block 35 of Figure 3 is, also in this case, given by equations (3) and (4) . Generalising what said above, it is easily understood that in the case of type (d) rays of the classification given above, that is, of rays that from the light source 21 strike the extractor after undergoing a reflection within diffuser 3, namely after undergoing at least one reflection by the emission source 5, and optionally by extractor 8, the intensity I (x) of each point source is given by intensity • K of the light source multiplied by the absorption-in diffusion coefficient of the extractor Difβ/ multiplied, for each reflection on the emission surface 5, by the absorption-in reflection coefficient of the emission surface Rifs, multiplied, for each reflection of extractor 8, by the absorption-in reflection coefficient of the extractor Rife, and divided by the squared projection, along the main propagation direction, of the portion of the ray under consideration from the light source S to the first reflection point X - Finally, if one wants to consider also type (e) rays of the classification given above, for each incidence on extractor 8 in a diffusing point 10 it will be necessary to multiply the intensity I (x) of the point sources by the absorption- in diffusion coefficient of the extractor, Difg. In case the light guide lamp 1 in which the extractor is intended to be inserted comprises both illumination systems 2, 2a, specular with one another, it would be possible to take into account the two light sources 21, 21a during all of the previous steps of the method of the invention. However, according to the method of the invention and as represented in the block diagram of Figure 8, if two illumination systems 2, 2a are provided ' (decision block 51) , it is preferably provided to determine (block 52) a second distribution of points W' = (-x n , ... -X 2 , /-xi) that is specular to the distribution of points W(x) determined as indicated above (as schematised in a block 50) and to add (block 53) to the distribution of points W=(X]_, 2 , ... x n ) the second distribution of points W' = (/-x n , ... - 2 , I- χ ι). In virtue of the symmetry of the light guide lamp 1, the second distribution of points W 1 is such as to provide a uniform illumination in a lamp 1 comprising the single illumination system 21a. Moreover, it is easily understood that the illumination due to the illumination system 21 in the presence of the points of the second distribution of points W' is equal to the illumination due to the illumination system 21a in the presence of the points of the first distribution of points W. Having obtained the distribution of points W(x) along the main propagation direction x, the bidimensional distribution of diffusing points 10 on the reflecting base layer 9 of extractor 8 can be obtained by simply repeating the distribution of points W(x) at constant intervals Δy along direction y perpendicular to the main propagation direction x. This is represented in the left path of the block diagram of Figure 9, wherein after having determined in a block 60 the distribution of points (x) , a cycle is entered, comprising a block 61 of increment of coordinate y and a block 62 wherein the distribution of points W(x, y) at the current value of coordinate y is set as equal to the distribution of points W(x) . Thus, as represented in Figure 10, extractor 8 exhibits a distribution of stripes of diffusing points 10 and the illumination is sufficiently uniform in the planes that are perpendicular to the main propagation direction x. Moreover, in Figure 10 it can be noted that in extractor 8, the distribution of diffusing points 10 is symmetrical with respect to the median axis perpendicular to the main propagation direction x. An extractor provided with such a symmetry is particularly suitable to be used in a light guide lamp 1 having the two illumination systems 2, 2a. Moreover, when the distribution of points is determined by applying the above method, as in the illustrated case, adjacent diffusing points 10 of the distribution of points have a decreasing distance from each end of extractor 8 to the median axis A-A perpendicular to the main propagation direction x and the extractor provides a uniform illumination along the main propagation direction x. Analogously, if extractor 8 is intended to be used in a light guide lamp 1 having only illumination system 2, adjacent diffusing points 10 are at a decreasing distance along the main propagation direction x from a first light inlet end, that is, intended to be coupled to the illumination system 2, to the opposed end. Such an extractor is represented by extractor 8 ' bounded by the dashed line in Figure 10. As an alternative, the distribution of points W(x) can be alternately staggered forwards and backwards in the main propagation direction x during its repetition along direction y. In this case, the diffusing points 10 on the base layer 9 of extractor 8 are arranged as rhombuses, as represented in Figure 11. This exhibits the advantage of reducing the distance between diffusing points below the minimum distance imposed by the physical implementation of the extractor. This is represented in the right path of the block diagram of Figure 9, wherein after having determined in block 60 the distribution of points W(x) a cycle is entered, comprising block 61 of increment of coordinate y and a block 63 wherein the distribution of points W(x, y) at the current value of coordinate y is set as equal to the distribution of points (x ± Δx) . For what said, this must be understood as meaning that at successive iterations, the + sign and the - sign are used alternately. Preferably, moreover, the stagger interval Δx is equal to the interval Δy of repetition of the distribution W(x) . To avoid obtaining a distribution W(x) with a higher precision than the limits of the physical implementation of the extractor, it is preferable to reject from the distribution of points W (x) those points exhibiting a greater distance than a minimum distance Dmi rom immediately adjacent points, that is, to impose condition x i+ . "Xi ≤D min 00) Similarly, in determining the bidimensional distribution W(x,y) as set forth above, the following conditions will be imposed Δy<D min (11) Δx<D min (12) The minimum distance O m ± n can, for example, correspond to the resolution obtainable through silk-screen printing. Condition (10) could also be imposed upon the calculation of the coordinate of the point equally illuminated by the two point sources in adjacent points of the distribution according to expressions (3) and (4) given above, but in any case its validity must be checked again after any optional step of addition of second distributions, that is to say, after block 42 of Figure 7 and/or after block 53 of Figure 8. In an alternative method for determining a distribution of diffusing points 10 of the light extractor 8, in particular such a distribution that the light emitted by the light guide lamp 1 is as uniform as possible, the procedure is as follows . Firstly, the useful surface of extractor 8 is divided along the main propagation direction x, into intervals as little as desired, defining for each interval a diffusing-points- density variable D]_, D2 , ..., D n . Similarly, the emission surface 5 is divided along the main propagation direction x, into intervals as little as desired, defining for each interval a luminosity variable N 1# N 2 , ..., N m . Then,, the value of the luminosity variable N^, N , ..., N m of each interval of the emission surface 5 is expressed, for example in the way described hereinafter, as a function of the diffusing-points-density variables D]_, D 2 , ..., D n of extractor 8. Finally, the values of the diffusing-points-density variables D^, D 2 , •••, D π of extractor 8 are calculated, such as to provide the desired values of the luminosity variables N ] _, N 2 , ••• / N m of each interval of the emission surface 5, in particular such as to equal with one another the values of the luminosity variables N]_, ' N 2 , •••, N m of all intervals of the emission surface 5. More in particular, similarly to the first method described, the expression of the luminosity variables N^, N 2» • • • ' N m can De obtained as follows. Firstly, the light projected by the, or by each, illumination system 2, 2a is divided into a finite number of rays, each emitted according to a respective angle. For example, the number of considered rays can be selected as 2π/10, 2π/l00, 2π/1000 etcetera, according to the desired precision. Each emitted ray is given an initial luminosity value, which for simplicity can be value L=l . Then, following the path of each ray, the luminosity variable N]_, N 2 , ..., N m of each interval of the emission surface 5 is incremented by the value of the final luminosity L' of each ray incident within it with a greater angle, with respect to the main propagation direction, than its critical angle θ max , that is to say, such as to be transmitted outside the light guide lamp 1. The luminosity value of each ray is changed according to the following criteria. At each incidence of a ray within an interval of extractor 8, it can be reflected or diffused according to whether it is incident on a diffusing point 10 or on a reflecting point 9. The diffusing-points-density O of the interval under consideration can be deemed to express the probability that the ray is reflected or diffused. Thus, if the ray is reflected, the luminosity value of the reflected ray is obtained by multiplying the luminosity value of the incident ray by the absorption-in reflection coefficient of extractor, ifs whereas if the ray is diffused, the luminosity value of each diffused ray is obtained by multiplying the luminosity value of the incident ray by the absorption-in diffusion coefficient of extractor, Difg. At each incidence on the emission surface 5 with a smaller angle, with respect to the main propagation direction, than its critical angle θ max , the luminosity value of the reflected ray is obtained by multiplying the luminosity value of the incident ray by the absorption-in reflection coefficient of the emission surface, Rif5. By equalling the expressions of the luminosity variables N]_, 2 , ..., N m to the respective desired values, in particular by equalling them to one another, an equation system is obtained, its unknown values being the diffusing points densities D]_, D 2 , ••-, D n of extractor 8. Such a system can be mathematically solved, for example using numerical methods. However, since the mathematical solution is not immediate, the following recursive method can be used. The values of the diffusing-points-density variables D^, D 2 , •-., D n and the values of the luminosity variables N_, N 2 ' ••■/ N m are Preset to zero at the beginning. At each iteration, the values of the luminosity variable N^, N 2 , ... , N m of each interval of the emission surface 5, expressed as said above, are evaluated on the basis of the current values of the diffusing-points-density variables D_, D 2 , ..-, D n of extractor 8, namely on the basis of the current probability that the rays incident on extractor 8 are reflected or diffused. At each iteration, the values of the diffusing-points- density variables D^, D 2 , •••, D n of extractor 8 are then individually incremented by a respective sufficiently small quantity. The iteration ends when the values of the luminosity variables N]_, N 2 , ••-, N m are those desired, in particular when the values of the luminosity variables N]_, N 2 , •■•/ N m of all intervals of the emission surface 5 are equal to one another. For example, for an extractor 8 intended to be used in a light guide lamp 1 having two illumination systems 2, 2a, the increment of the values of the diffusing-points-density variables D ] _, D 2 , •••, D n can occur starting from the centre of extractor 8. Similarly to the first method described, once obtained, the distribution of diffusing points along the main propagation direction can be repeated at constant intervals Δy along direction y perpendicular thereto, optionally alternately staggering the distribution of points forwards and backwards in the main propagation direction by the preset minimum distance Δx. Moreover, similarly to the first method described, it can be suitable to reject from the distribution of points those points exhibiting a greater distance than a preset minimum distance D m -L n from the immediately adjacent points. Figures 12 to 14 schematically show other embodiments of light guide lamps comprising a light extractor according to the present invention. In the light guide lamp 121 shown in Figure 12, the light extractor 128 and the emission surface 125 are both curved with the concavity towards the longitudinal axis of diffuser 123. More in particular, the cross section of each of them can be semicircular, semi-elliptical, semi- parabolic, parabolic or according to higher-order curves. The projection system 122, or each of the two projection systems 122, 122a in the illustrated embodiment, comprises a respective point light source 1221, 1221a. In the light guide lamp 131 shown in Figure 13, the emission surface 135 is plane and the light extractor 138 is curved towards the longitudinal axis of diffuser 133, more in particular according to a semicircular, semi- elliptical, semi-parabolic, parabolic cross section, or according to higher-order curves. As an alternative, it could be possible to provide a plane extractor and an emission surface curved towards the longitudinal axis of diffuser 133, more in particular according to a semicircular, semi-elliptical, semi-parabolic, parabolic cross section, or according to higher-order curves. Also in this case, the projection system 132, or each of the two projection systems 132, 132a in the illustrated embodiment, comprises a respective point light source 1321, 1321a. Finally, in the light guide lamp 141 shown in Figure 14, both the light extractor 148 and the emission surface 145 are curved, more in particular according to a semicircular, semi-elliptical, semi-parabolic, parabolic cross section, or according to higher-order curves, but they are arranged with the concavities facing the same direction. That is to say that the emission surface 145 is arranged inside extractor 148 or vice versa, so that the cross section of the light guide lamp 141 is horseshoe-shaped. In this case, the projection system 142, or each of the two projection systems 142, 142a in the illustrated embodiment, comprises a respective elongated tubular light source 1421, 1421a. It is worth noting that the methods for determining the distribution of diffusing points illustrated with reference to the lamp having the parallelepipedal configuration shown in Figure 1 are applicable also to the other described configurations of light guide lamp, any changes to be made being within the abilities of one skilled in the art. Moreover, the first method described is particularly suitable to be performed through an electronic computer. Figure 15 illustrates a block diagram representing a particularly preferred embodiment of a computer program comprising program code means suitable to perform the steps of the above method when the program is run on a computer. No description of the code means relating to the various steps of the method is provided hereinbelow because deemed within the abilities of any programmer, as practically it is a matter of cycles, recursive routine calls, and application of numerical formulas. The distribution of points W(x) or each second distribution j_ (x) , W (x) , can for example be stored in a unidimensional array or in a list. The management of the recursive calls of the block diagram of Figure 3, whereby all value pairs must be considered, could for example occur managing a pointer to the matrix (index) , or through queue (FIFO) or stacked (LIFO) structures. As shown at block 70, the computer program comprises means, such as a form, for receiving in input one or more of the following parameters: - critical angle θ max of the emission surface 5, - shape and size of extractor 8, the shape for example being selectable from a list comprising the alternatives: rectangular, with semicircular section, with semi- elliptical section, with semi-parabolic section, with parabolic section, - shape and size of the emission surface 5, the shape for example being selectable from a list comprising the alternatives: rectangular, with semicircular section, with semi-elliptical section, with semi-parabolic section, with parabolic section, - maximum distance of extractor 8 from the emission surface 5, - number, type, intensity K and nature of the light sources 21, 21a of the illumination system 2, - minimum distance D m j_ n between the points of the distribution of points, - absorption-in reflection coefficient of extractor Rifβ/ - absorption-in diffusion coefficient of extractor Difg, absorption-in reflection coefficient of the emission surface Rifs, - maximum number of successive reflections, on the emission surface 5 and on extractor 8, to be considered. In this connection, respective default values can be • provided. Moreover, there can be- provided means .for controlling that the values entered are acceptable, in particular, that they are positive numbers, ' that the size of the extractor 8 and of the emission surface 5 are compatible with one another and preferably, that the ratio between length / in the main propagation direction and distance h between the extractor and the emission surface is equal to 20. In fact, such a ratio is preferred since in this way the light propagates with a sufficiently small number of incidences, thus with few losses. The values entered can be saved in a file, as shown in block 71, and be retrieved from the file, as shown in block 72, to be changed or to repeat the processing. Then, in a block 74, the program performs the various -steps of the method of the invention, preferably subject to receiving a start signal, as shown in block 74. For example, said start signal can be the pressure of a keyboard key, of a button of a graphical interface or other. Then, at a block 75, the program generates one or more outputs, represented by block 75', among: - the coordinates of the points of the distribution W(x), for example in a spreadsheet, - the percent density of the points of the distribution in elementary intervals, - a report of the preset parameters, for example as a spreadsheet, - a graphical representation of the distribution of points W(x) or W(x,y), for example with a graph with linear functions showing the gradual increase in points, - the geometry of each illumination system of the light guide lamp for which the extractor is intended, as a function of the preset average distance h between extractor 8 and emission surface 5 and of the preset critical angle θ max of the emission surface 5, in particular the focus δ of the reflecting parabola 22, 22a. Preferably, moreover, at a block 76, the program generates an export output, that is to say: - a file 76' of the points of the distribution W(x) in a format readable by a printing, in particular a silk-screen printing machine. Practically, such a computer program can be disseminated on an information network, in particular on the Internet, or embodied in a computer readable means, such as a floppy disk, a CD-ROM, a CD-R and the like. It is manifest that several changes, modifications, replacements and integrations can be made to the previously described embodiments without thus departing from the scope of the invention, as defined by the following claims. In particular, it must be understood that the special "dotted" useful surface, that is, consisting of diffusing points and of complementary reflecting points, can be useful also in tridimensional-body extractors, intended to be used in axial position in the light guide lamp. In particular, it is worth noting that the diffusing points and the reflecting points can be applied according to the predetermined and respectively complementary distribution on a neutral substrate, or by applying the reflecting points on a diffusing base layer. Thus, any reference - in the present description and attached claims - to a "point of the reflecting base layer" must be interpreted as equivalent to the expression "reflecting points ". Moreover, in place of the illumination systems with point source or fluorescent tube light source provided with reflecting parabola, plane illumination systems can be used in the various embodiments, obtained through a plurality of LEDs. Moreover, those skilled in the art will understand that both as the reflecting surface and as the emission surface, among the other materials, the multilayer optical film described in the above-mentioned patent US 5882774 could advantageously be used.

Claims: CLAIMS 1. Light extractor (8; 128; 138; 148) for a light guide lamp (1) , characterised in that its useful surface comprises a predetermined distribution of diffusing points (10) and a complementary distribution of reflecting points (9) . 2. Extractor (8; 128; 138; 148) according to claim 1, characterised in that the useful surface comprises a reflecting base layer (9) and the predetermined distribution of diffusing points (10) on the reflecting base layer (9) . 3. Extractor (8; 128; 138; 148) according to claim 2, characterised in that the reflecting base layer (9) comprises the side without micro-prisms of a TIR film. 4. Extractor (8 1 ) according to any one of the previous claims, characterised in that adjacent diffusing points (10) of the predetermined distribution are at a decreasing distance along a main propagation direction (x) from a light inlet end to an opposed end. 5. Extractor (8; 128; 138; 148) according to any one of claims 1 to 3, characterised in that the distribution of diffusing points (10) is symmetrical with respect to a median axis perpendicular to the main propagation direction (x) • 6. Extractor (8; 128; 138; 148) according to claim 5, characterised in that adjacent diffusing points (10) of the distribution of points (W(x)) are at a decreasing distance from each end of the extractor (8; 128; 138; 148) to the median axis perpendicular to the main propagation direction (x) . 7. Extractor (8) according to one of claims 2 to 6, characterised in that the predetermined distribution (W(x)) of diffusing points (10) is imprinted on the reflecting base layer (9) through silk-screen printing. 8. Diffuser (3; 123; 133; 143) for a light guide lamp (1) comprising a light extractor (8; 128; 138; 148) according to one of claims 1 to 7 and an emission surface (5; 125; 135; 145) transparent for rays forming, with a main propagation direction (x) , a greater angle than a predetermined critical angle (θ max ) and internally reflecting for rays incident with a smaller angle than the predetermined critical angle (θ max ) . 9. Diffuser (3; 123; 133; 143) according to claim 8, characterised in that the emission surface (5; 125; 135; 145) comprises a transparent support layer (6) and a total internal reflection film (7) applied on the transparent support layer. 10. Diffuser (3; 123; 133; 143) according to claim 9, characterised in that the total internal reflection film (7) has micro-relieves faced outwards and oriented in parallel to the main propagation direction (x) . 11. Diffuser (3; 123; 133; 143) according to any one of claims 8 to 10, characterised in that the light extractor (8; 128; 138; 148) has a cross section selected from the group consisting of rectilinear, semicircular, semi- elliptical, parabolic, semi-parabolic, or according to higher-order curves, and the emission surface (5; 125; 135; 145) has a cross section selected from the group consisting of rectilinear, semicircular, semi-elliptical, parabolic, semi-parabolic, or according to higher-order curves. 12. Diffuser (3; 123; 133; 143) according to any one of claims 8 to 11, characterised in that the. maximum distance between extractor (8; 128; 138; 148) and emission surface (5; 125; 135; 145) in a plane perpendicular to the main propagation direction (x) is a preselected fraction of the length (/) of the extractor (8) in the main propagation direction (x) 13. Light guide lamp (1; 121; 131; 141) comprising a diffuser (3; 123; 133; 143) according to any one of claims 8 to 12 and at least one illumination system (2, 2a; 122, 122a; 132, 132a; 142, 142a) suitable to project the light into the diffuser (3; 123; 133; 143) with smaller angles than the critical angle (θ max ) of the emission surface (5; 125; 135; 145) . 14. Method for determining a distribution (W(x)) along a main propagation direction (x) of diffusing points (10) on a useful surface, elsewhere reflecting (9) , of a light extractor (8; 128; 138; 148) intended to be coupled to an emission surface (5; 125; 135; 145), transparent for rays propagating with a smaller angle than a- predetermined critical angle (θ max ) and internally reflecting for rays propagating with a greater angle than the critical angle (θ max ) , in a diffuser (3; 123; 133; 143) for a light guide lamp (1; 121; 131; 141) having an illumination system (2; 122; 132; 142) suitable to project the light of a light source (21; 1221; 1321; 1421) in the diffuser (3; 123; 133; 143) with smaller angles than the critical angle (θ max ) , the method comprising the steps of : (a) setting a first point (A, x^_) of the distribution ( (x) ) as the point of final incidence on the extractor (8; 128; 138; 148) closest to the illumination system (2; 122; 132; 142) of a light ray propagating according to a preselected type of trajectory, (b) setting a second point (B, 2 ) of the distribution ( (x)) as the point of final incidence on the extractor (8; 128; 138 " ; 148) farthest from the illumination system (2; 122; 132; 142) of a light ray propagating according to the preselected type of trajectory, (c) recursively determining, for each point (XJ_) of the distribution of points, a new point (X, x) of the distribution as the point of the extractor (8; 128; 138; 148) equally illuminated by a first point source arranged at the preselected point (XJ_) of the distribution, and by a second point source arranged at a next adjacent point ( χ i +l ) °f the distribution, the intensities (I(x)) of the point sources being a function of the intensity (K) of the light source and of a characteristic distance of the preselected type of trajectory. 15. Method according to claim 14, characterised in that in steps (a) , (b) , (c) the trajectory type is classified on the basis of the number of reflections within the diffuser (3; 123; 133; 143) . 16. Method according to claim 14 or 15, characterised in that in step (c) , when the preselected type of trajectory consists of the rays that, from the light source (21; 1221; 1321; 1421) of the light guide lamp (1; 121; 131; 141) directly strike the extractor (8; 128; 138; 148), the intensity (I (x) ) of each point source is given by the intensity (K) of the light source, multiplied by an absorption-in diffusion coefficient of the extractor (Difg) and divided by the squared distance of the point of the source point from the light source (21; 1221; 1321; 1421) . 17. Method according to claim 14 or 15, characterised in that in step (c) , when the preselected type of trajectory consists of the rays that, from the light source (21; 1221; 1321; 1421) strike the extractor (8; 128; 138; 148) after undergoing at least one reflection by the emission surface (5; 125; 135; 145) and optionally by the extractor (8; 128; 138; 148), the intensity (I(x)) of each point source is given by the intensity (K) of the light source, multiplied by an absorption-in diffusion coefficient of the extractor (Difβ) , multiplied, for each reflection onto the emission surface (5; 125; 135; 145), by an absorption-in reflection coefficient of the emission surface ( ifs) , multiplied, for each reflection onto the extractor (8; 128; 138; 148), by an absorption-in reflection coefficient of the extractor (Rifβ) , and divided by the squared projection, along the main propagation direction (x) , of the portion of the ray under consideration from the light source to the first reflection point. 18. Method according to any one of claims 14 to 17, characterised in that it comprises the steps of: (d) determining at least a second distribution of points (W-L (x) ) through steps (a), (b) , (c) for a different type of trajectory, (e) adding each second distribution of points (W- j _ (x) ) to the distribution of points (W(x)) . 19. Method according to any one of claims 14 to 18, wherein the extractor (8; 128; 138; 148) is intended to be inserted into a light guide lamp (1; 121; 131; 141) having a second illumination system (2a; 122a; 132a; 142a) that is specular to the first illumination system (2; 122; 132; 142) and arranged at the opposed end, comprising the step of: (f) adding to the distribution of points (W(x)) the specular points (W (x) ) of each point of • the distribution of points (W(x) ) . 20. Method according to any one of claims 14 to 19, comprising the step of: (g) repeating the distribution of points (W(x)) at constant intervals (Δy) along a direction (y) perpendicular to the main propagation direction (x) of the extractor (8; 128; 138; 148) . 21. Method according to claim 20, characterised by the further step, during the repetitions of step (g) , of: (gl) alternately staggering the distribution of points (W(x)) forwards and backwards in the main propagation direction (x) by a constant interval (Δx) . 22. Method according to any one of claims 14 to 21, characterised in that it further comprises the step of : (h) rejecting, from the distribution of points, those points exhibiting a distance that is greater than a preset minimum distance (D m j_ n ) from the immediately adjacent points . 23. Computer program comprising program code, means suitable to perform the steps of the method of one or more of claims 14 to 22 when the program is run on a computer. 24. Computer program according to claim 23, characterised in that it comprises means for receiving in input one or more of the following parameters : - critical angle (θ max ) of the emission surface (5; 125; 135; 145) to which the extractor (8; 128; 138; 148) is intended to be coupled, - shape and/or size of the extractor (8; 128; 138; 148), - shape and/or size of the emission surface (5; 125; 135; 145) to which the extractor (8; 128; 138; 148) is intended to be coupled, - maximum distance of the extractor (8; 128; 138; 148) from the emission surface (5; 125; 135; 145) to which .the extractor (8; 128; 138; 148) is intended to be coupled, - number, type, intensity and nature of the light sources (21, 21a; 1221, 1212a; 1321, 1321a; 1421, 1421a) of the illumination system (2, 2a; 122, 122a; 132, 132a; 142, 142a) to which the extractor (8; 128; 138; 148) is intended to be coupled, minimum distance (D m _ n ) between the points of the distribution of points (W(x)), absorption-in reflection coefficient of the extractor ( Rif 8 ) / - absorption-in diffusion coefficient of the extractor (Dif 8 ), absorption-in reflection coefficient of the emission surface (Rifs) to which the extractor (8; 128; 138; 148) is intended to be coupled, and - type or types of trajectories to be considered or maximum number of reflections to be considered. 25. Computer program according to any one of claims 23 and 24, characterised in that it comprises means for saving the parameters entered into a file (72) and means for loading the parameters from a file (71) . 26. Computer program according to any one of claims 23 to 25, characterised in that it generates one or more outputs (75' , 76' ) , among: - the coordinates of the points of the distribution (W(x)), - the percent density of the points of the distribution (W(x)) in elementary intervals, - a report of the preset parameters, - a graphical representation of the distribution of points ( (x)), - a file of the points of the distribution (W(x)) in a format readable by a printing machine, in particular a silk-screen printing machine, - the geometry of each illumination system (2; 122; 132; 142) of the light guide lamp (1; 121; 131; 141) for which the extractor (8; 128; 138; 148) is intended, as a function of a preset maximum distance between the extractor (8; 128; 138; 148) and the emission surface (5; 125; 135; 145) and of a preset critical angle (θ max ) of the emission surface (5; 125; 135; 145) . 27. Computer program according to any one of claims 23 to 26, embodied in a computer readable means. 28. Method for determining a distribution (W(x)) along a main propagation direction (x) of diffusing points (10) on a useful surface, elsewhere reflecting -(9) , of a light extractor (8; 128; 138; 148) intended to be coupled to an emission surface (5; 125; 135; 145), transparent for rays propagating with a smaller angle than a predetermined critical angle (θ max ) and internally reflecting for rays propagating with a greater angle than the critical angle (θ max ) , in a diffuser (3; 123; 133; 143) for a light guide lamp (1; 121; 131; 141) having at least one illumination system (2, 2a; 122, 122a; 132, 132a; 142, 142a) suitable to project the light of a light source (21, 21a; 1221, 1221a; 1321, 1321a; 1421, 1421a) into the diffuser (3; 123; 133; 143) with smaller angles than the critical angle (θ max ) , the method comprising the steps of : (a) defining, for each interval of a preselected length along the main propagation direction (x) of the useful surface of the extractor (8; 128; 138; 148), a diffusing- points-density variable (D , D2 , ...D n ) , (b) defining, for each interval of a preselected length along the main propagation direction (x) of the emission surface (5; 125; 135; 145), a luminosity variable (Ni , 2 , • • .N m ) , (c) expressing the value of the luminosity variable (Ni, N 2 , ...N m ) of each interval of the emission surface (5; 125; 135; 145) as a function of the diffusing-points- density variables (D , D 2 , ...D n ) of the extractor (8; 128; 138; 148) , and (d) calculating the values of the diffusing-points-density variables (Di, D 2 , ...D n ) of the extractor (8; 128; 138; 148) such as to equal with one another the values of the luminosity variables (N , N , ...N m ) of all intervals of the emission surface (5; 125; 135; 145). 29. Method according to claim 28, characterised in that: - step (a) comprises the sub-step (al) of presetting to zero the values of the diffusing-points-density variables (D 1# D 2 , ...D n ), - step (b) comprises the sub-step (bl) of presetting to zero the values of the luminosity variables ( ^, N2 , • - -N m ) , - the calculation step (d) is carried out through the steps of: (dl) evaluating the values expressed at step (c) of the luminosity variable ( , N 2 , • • .N m ) of each interval of the emission surface (5; 125; 135; 145) on the basis of the current values of the diffusing-points-density variables (D , D 2 , .. -D n ) of the extractor (8; 128; 138; 148), (d2) individually incrementing the values of the diffusing- points-density variables (D , D 2 , . ■ -D n ) of the extractor (8; 128; 138; 148) and (d3) iteratively repeating steps (dl) and (d2) until the values of the luminosity variable (Ni, N 2 , ...N m ) of all intervals of the emission surface (5; 125; 135; 145) evaluated in step(dl) are equal to one another. 30. Method according to any one of claims 28 and 29, characterised in that the step (c) of expressing the luminosity variables (N , 2 , ...N m ) comprises the steps of: (cl) splitting the light projected by the at least one illumination system (2; 122; 132; 142) into a finite number of rays, each having an initial luminosity value, (c2) incrementing the luminosity variable ( i, N2 , ...N m ) of each interval of the emission surface (5;.125; 135; 145) by the value of the final luminosity of each ray incident on it with a greater angle, with respect to the main propagation direction (x) , than its critical angle (θ max ) , wherein: at each incidence of a ray in an interval of the extractor (8; 128; 138; 148), an evaluation of whether the ray is reflected or diffused is made on the basis of the value of the respective diffusing-points-density variables (Di, D 2 , ...D n ) , -- if the ray is reflected, the luminosity value of the reflected ray is obtained by multiplying the luminosity value of the incident ray by an absorption-in reflection coefficient of the extractor (Rifg) , -- if the ray is diffused, the luminosity value of each diffused ray is obtained by multiplying .the luminosity value of the incident ray by an absorption-in diffusion coefficient of the extractor (Difβ) , - at each incidence on the emission surface (5; 125; 135; 145) with a smaller angle, with respect to the main propagation direction, than its critical angle (θ max ) , the luminosity value of the reflected ray is obtained by multiplying the luminosity value of the incident ray by an absorption-in reflection coefficient of the emission surface (Rifs) . 31. Method according to any one of claims 28 to 30, comprising the step of: (e) repeating the distribution of points at constant intervals (Δy) along a direction (y) perpendicular to the main propagation direction (x) of the extractor (8; 128; 138; 148) . 32. Method according to claim 31, characterised by the further step, during the repetitions of step (e) , of: (el) alternately staggering the distribution of points forwards and backwards in the main propagation direction (x) by a constant interval (Δx) . 33. Method according to any one of claims 28 to 32, characterised in that it further comprises the step of: (f) rejecting, from the distribution of points (W(x)), those points exhibiting a distance that is greater than a preset minimum distance (L>min) from the immediately adjacent points. 34. Method for manufacturing a light extractor (8; 128; 138; 148) according to any one of claims 1 to 7 , comprising the steps of : (a) providing a reflecting base layer (9) , (b) silk-screen printing diffusing points (10) onto the reflecting base layer (9) according to a predetermined distribution of points (W(x)) . 35. Method for manufacturing ' light extractor (8; 128; 138; 148) according to claim 34, characterised in that the printing step (b) is carried out by printing a distribution of points (W(x)) obtained according to the method of any one of claims 14 to 22 or according to the method of any one of claims 28 to 33. 36. Silk-screen characterised in that it embodies a distribution of points obtained with the method of any one of claims 14 to 22 or according to the method of any one of claims 28 to 33. AMENDED CLAIMS [received by the International Bureau on 11 January 2002 (11.01.02); original claims 1-36 replaced by new claims 1-35 (11 pages)] 1. Method for determining a distribution (W (x) ) along a main propagation direction (x) of diffusing points (10) on a useful surface, elsewhere reflecting (9) , of a light extractor (3; 128; 138; 148) intended to be coupled to an emission surface (5; 125; 135,- 145) , transparent for rays propagating with a smaller angle than a predetermined critical angle (θ max ) and internally reflecting for rays propagating with a greater angle than the critical angle ( θ mχ) ' i* 1 di fus r (3; 123; 133; 143) for a light guide lamp (1; 121,- 131; 141) having an illumination system (2 , 122; 132; 142) [Suitable to project the light of a light source (21; 1221; 1321; 1421) in the diffuser (3; 123; 133; 143) with smaller angles than the critical angle (θmax) ' the method comprising the steps of: (a) setting a first point (A, χ j _) of the distribution ( (x)) as the point of final incidence on the extractor (β; 128; 138; 148) closest to the illumination system (2; 122; 132; 142) of a light ray propagating according to a preselected type of trajectory, (b) setting a second point (B, x 2 ) of the distribution (W(x)) as the point of final incidence on the extractor (8; 123; 138; 148) farthest from the illumination system (2; 122; 132; 142) of a light ray propagating according to the preselected type of trajectory, (c) recursively determining, for each point (XJ_) of the distribution of points, a new point (X, x) of the distribution as the point of the extractor (8; 128; 138; 148) equally illuminated by a first point source arranged at the preselected point (x^) of the distribution, and by a second point source arranged at a next adjacent point ( χ i +l ) ° t e distribution, the intensities (I(x)> of the point sources being a function of the intensity (K) of the light source and of a characteristic distance of the preselected type of trajectory, 2 > Method according to claim 1, characterised in that in steps (a) , (b) , (c) the trajectory type is classified on the basis of the number of reflections within the diffuser (3; 123; 133; 143) . 3« Method according to claim 1 or 2, characterised in that in step (c) , when the preselected type of trajectory consists of the rays that, from the light source (21; 1221; 1321; 1421) of the light guide lamp (1; 121; 131; 141) directly strike the extractor (8; 128; 138; 148), the intensity (I(x)) of each point source is given by the intensity (K) of the light source, multiplied by an absorption-in diffusion coefficient of the extractor (Difg) and divided by the squared distance of the point of the source point from the light source (21; 1221; 1321; 1421) . 4. Method according to claim 1 or 2, characterised in that in step (c) , when the preselected type of trajectory consists of the rays that, from the light source (21; 1221; 1321; 1421) strike the extractor (3; 128; 138; 148) after undergoing at least one reflection by the emission surface (5; 125; 135; 145) and optionally by the extractor (8; 128; 138; 148), the intensity (I(x)) of each point source is given by the intensity (K) of the light source, multiplied by an absorption-in diffusion coefficient of the extractor (Difg) , multiplied, for each reflection onto the emission surface (5; 125; 135; 145) , by an absorption-in reflection coefficient of the emission sur ace (Hifs) , multiplied, for each reflection onto the extractor (8; 126; 138; 148), by an absorption-in reflection coefficient of the extractor ( ifg) , and divided by the squared projection, along the main propagation direction (x) , of the portion of the ray under consideration from the light source to the first reflection point. 5. Method according to any one of claims 1 to 4, characterised in that it comprises the steps of: (d) determining at least a second distribution of points (Wj_(χ)) through steps (a), (b) , (c) for a different type of trajectory, (e) adding each second distribution of points (Wi(x)) to the distribution of points (W(x)). 6. Method according to any one of claims 1 to 5, wherein the extractor (3; 128; 138; 148) is intended to be inserted into a light guide lamp (1; 121; 131 / 141) having a second illumination system (2a; 122a; 132a; 142a) that is specular to the first illumination system (2; 122; 132; 142) and arranged at the opposed end, comprising the step of: (f) adding to the distribution of points ( (x)) the specular points (W (x) ) of each point of the distribution of points (W(x)) . 7. Method according to any one of claims 1 to 6, comprising the step of: (g) repeating the distribution of points (W(x)) at constant intervals (Δy) along a direction (y) perpendicular to the main propagation direction (x) of the extractor (8; 128; 138; 14Θ) . a. Method according to claim 7, characterised by the further step, during the repetitions of step (g) , of: (gl) alternately staggering the distribution of points (W(x)) forwards and backwards in the main propagation direction (x) by a constant interval (Δx) . 9. Method according to any one of claims 1 to 8 , characterised in that it further comprises the step of *. (h) rejecting, from the distribution of points, those points exhibiting a distance that is greater than a preset minimum distance (D i ) the immediately adjacent points. 10. Computer program comprising program code means suitable to perform the steps of the method of one or more of claims 1 to 9 when the program is run on a computer. 11. Computer program according to claim 10, characterised in that it comprises means for receiving in input one or more of the following parameters; - critical angle (θ max ) of the emission surface (5,- 125; 135; 145) to which the extractor (8; 128; 138; 148) is intended to be coupled, - shape and/or size of the extractor (8; 128; 138; 148), - shape and/or size of the emission surface (5; 125; 135; 145) to which the extractor (8; 128; 138; 148) is intended to be coupled, - maximum distance of the extractor (8; 126; 138; 148) from the emission surface (5; 125; 135; 145) to which the extractor (8; 128; 138; 148) is intended to be coupled, - number, type, intensity and nature of the light sources (21, 21a; 1221, 1212a; 1321, 1321a; 1421, 1421a) of the illumination system (2, 2a; 122, 122a; 132, 132a; 142, 142a) to which the extractor (8; 128; 138; 148) is intended to be coupled, - minimum distance (E>mϊn) between the points of the distribution of points (W(x)), - • absorption-in reflection coefficient of the extractor - absorption-in diffusion coefficient of the extractor (Dif a ), - absorption-in reflection coefficient of the emission surface (Elf5) to which the extractor (8; 128; 138; 148) is intended to be coupled, and - type or types of trajectories to be considered or maximum number of reflections to be considered. 12, Computer program according to any one of claims 10 and 11, characterised in that it comprises means for saving the parameters entered into a file (72) and means for loading the parameters from a file (71) . 13. Computer program according to any one of claims 10 to 12, characterised in that it generates one or more outputs (75 ' , 76 ' ) , among-. - the coordinates of the points of the distribution (W(x)) r - the percent density of the points of the distribution ( (x)) in elementary intervals, - a report of the preset parameters, - a graphical representation of the distribution of points ( (x)), - a file of the points of the distribution (W(x)) in a format readable by a printing machine, in particular a silk-screen printing machine, - the geometry of each illumination system (2; 122; 132; 142) of the light guide lamp (1; 121; 131; 141) for which the extractor (8; 128; 138; 1 B) is intended, as a function of a preset maximum distance between the extractor (8; 128; 133; 148) and the emission surface (5; 12Ξ; 135; 145) and of a preset critical angle (θrcι ) σ£ tla& emission surface (5; 125; 135; 145) . 14, Computer program according to any one of claims 10 to 13 , embodied in a computer readable means > 15. Method for determining a distribution (W(x)) along a main propagation direction (x) of diffusing points (10) on a useful surface, elsewhere reflecting (9) , of a light extractor (8; 128; 138; 148) intended to be coupled to an emission surface (5; 125; 135; 145) , transparent for rays propagating with a smaller angle than a predetermined critical angle (θ max ) and internally reflecting for rays propagating with a greater angle than the critical angle (θ max ) , in a diffuser (3; 123; 133; 143) for a light guide lamp (1; 121; 131; 141) having at least one illumination system (2, 2a; 122, 122a; 132, 132a; 142, 142a) suitable to project the light of a light source (21, 21a; 1221, 1 a,- 1321, 1321a; 1421, 1421a) into the diffuser (3; 123; 133; 143) with smaller angles than the critical angle (θ^x) , the method comprising the steps of t (a) defining, for each interval of a preselected length along the main propagation direction (x) of the useful surface of the extractor (8; 128; 13S; 148), a diffusing- points-density variable (D]_, D2, .. -D n ) , (b) defining, for each interval of a preselected length along the main propagation direction (x) of the emission surface (5; 125; 135; 145), a luminosity variable ( , N , (c) expressing the value of the luminosity variable (N^, N 2 , .. -N m ) of each interval of the emission surface (5; 125; 135; 145) as a function of the diffusing-points- density variables (£>ι, D 2 , , ..D n ) of the extractor (8; 128; 138; 148), and (d) calculating the values of the diffusing-points-density variables (D., D 2 , ♦ . -Dn) of fc k extractor (8; 128; 138; 148) such as to equal with one another the values of the luminosity variables (N]_, N2, ...N m ) of all intervals of the emission surface (5; 125; 135; 145) . IS. Method according to claim 15, characterised in chats - step (a) comprises the su -step (al) of presetting to sero the values of the diffusing-points-density variables (Di, D 2/ .. ,D n ), - step (b) comprises the sub-step . (bl) of presetting to zero the values of the luminosity variables ( , 2 , ■ - -N m ) , - the calculation step (d) is carried out through the steps σf ; (dl) evaluating the values expressed at step (c) of the luminosity variable (N^, N2, ■ - *■ ) °^ each interval of the emission surface (5; 125; 135; 145) on the basis of the current values of the diffusing-points-density variables (Di, D 2 , . ■ -D n ) of the extractor (8; 128; 138; 148), (d2) individually incrementing the values of the diffusing- points-density variables (D^, D , ■ ■ -E> ) of the extractor (8; 128; 138; 148) and (d3) iteratively repeating steps (dl) and (d2) until the values of the luminosity variable (N^, N , ...N m ) of all intervals of the emission surface (5; 125; 135; 145) evaluated in step(dl) are equal to one another. 17. Method according to any one of claims 15 and 16, characterised in that the step (c) of expressing the luminosity variables ( , N , . - .N m ) comprises the steps of j (cl) splitting the light projected by the at least one illumination system (2; 122; 132; 142) into a finite number of rays, each having an initial luminosity value, (c2) incrementing the luminosity variable ( , N 2 , - - .N m ) of each interval of the emission surface (5; 125; 135; 145) by the value of the final luminosity of each ray incident on it with a greater angle, with respect to the main propagation direction (x) , than its critical angle Q^^) , wherein t - at each incidence of a ray in an interval of the extractor (8; 128 ? 138; 148), an evaluation of whether the ray is reflected or diffused is made on the basis of the value of the respective diffusing-points-density variables (D lr D 2 , ..-Dn), -.- if the ray is reflected, the luminosity value of the reflected ray is obtained by multiplying the luminosity value of the incident ray by an absorption-in reflection coefficient of the extractor (Rifg), -- if the ray is diffused, the luminosity value of each diffused ray is obtained lay multiplying the luminosity value of the incident ray by an absorption-in diffusion coefficient of the extractor (Difg) , - at each incidence on the emission surface (5; 125; 135; 145) with a smaller angle, with respect to the main propagation direction, than its critical angle (Θ ^ J , the luminosity value of the reflected ray is obtained by multiplying the luminosity value of the incident ray by an absorption-in reflection coefficient of the emission surface (Rif 5 ) . 18. Method according to any one of claims 15 to 17, comprising the step of s (e) repeating the distribution of points at constant intervals (Δy) along a direction (y) perpendicular to the main propagation direction (x) of the extractor (8/ 128; 138 ; 148 ) . 19. Method according to claim 18 , characterised by the further step, during the repetitions of step (e) , of: (el) alternately staggering the distribution of points forwards and backwards in the main propagation direction (x) by a constant interval (Δx) . 20. Method according to any one of claims 15 to 19, characterised in that it further comprises the step of (f) rejecting, from the distribution of points (W(x)), those points exhibiting a distance that is greater than a preset minimum distance (^min) from the immediately adjacent points. 21. Light extractor (8 ? 128; 138; 148) for a light guide lamp (1) , having an useful surface comprising • a predetermined distribution of diffusing points (10) and a complementary distribution of reflecting points (9) , characterised in that said distribution of diffusing points (10) is obtained with the method of any of claims 1 to 9 or the method of any of claims 15 to 20. 22. Extractor (3; 128; 133; 148) according to claim 21, characterised in that the useful surface comprises a reflecting base layer (9) and the predetermined distribution of diffusing points (10) on the reflecting base layer (9) . 23. Extractor (S; 128; 138; 148) according to claim 22, characterised in that the reflecting base layer (9) comprises the side without micro-prisms of a TIR film. 24. Extractor (8 1 ) according to any one of claims 21 to 23, characterised in that adjacent diffusing points (10) of the predetermined distribution are at a decreasing distance along a main propagation direction (x) from a light inlet end to an opposed end, 25. Extractor (3; 128; 138; 148) according to any one of claims 21 to 23, characterised in that the distribution of diffusing points (10) is symmetrical with respect to a median axis perpendicular to the main propagation direction (X) - 26. Extractor (8; 128; 138; 148) according to claim 25, characterised in that adjacent diffusing points (10) of the distribution of points ( (x) ) are at a' decreasing distance from each end of the extractor (8; 128; 138; 148) to the median axis perpendicular to the main propagation direction (x) . 27. Extractor (8) according to one of claims 22 to 26, characterised in that the predetermined distribution (W(x) ) of diffusing points (10) is imprinted on the reflecting base layer (9) through silk-screen printing. 28. Diffuser (3; 123; 133/ 143) for a light guide lamp (1) comprising a light extractor (8; 128; 138; 148) according to one of claims 21 to 27 and an emission surface (5; 125; 135; 145) transparent for rays forming, with a main propagation direction (x) , a greater angle than a predetermined critical angle (&maχ) anci internally reflecting for rays incident with a smaller angle than the predetermined critical angle (θ max ) . 29. Diffuser (3; 123; 133; 143) according to claim 28, characterised in that the emission surface (5; 125 / 135; 145) comprises a transparent support layer (6) and a total internal reflection film (7) applied on the transparent support layer. 30. Diffuser (3; 123; 133; 143) according to claim 29, characterised in that the total internal reflection film (7) has micro-relieves faced outwards and oriented in parallel to the main propagation direction (x) . 31. Diffuser (3; 123,- 133; 143) according to any one of claims 28 to 30, characterised in that the light extractor (8; 128; 138; 148) has a cross section selected from the group consisting of rectilinear, semicircular, semi- elliptical, parabolic, semi-parabolic, or according to higher-order curves, and the emission surface (5; 125; 135; 145) has a cross section selected from the group consisting of rectilinear, semicircular, semi-elliptical, parabolic, semi-parabolic, or according to higher-order curves. 32. Diffuser (3; 123; 133 / 143) according to any one of claims 28 to 31, characterised in that the maximum distance between extractor (8; 128; 138; 148) and emission surface (5,- 125; 135/ 145) in a plane perpendicular to the main propagation direction (x) is a preselected fraction of the length ( ) of the extractor (8) in the main propagation direction (x) 33. Light guide lamp (1; 121; 131; 141) comprising a diffuser (3; 123 / 133/ 143) according to any one of claims 28 to 32 and at least one illumination system (2, 2a,- 122, 122a; 132, 132a,- 142, 142a) suitable to project the light into the diffuser (3,- 123; 133; 143) with smaller angles than the critical angle (θ raax ) of the emission surface (5; 125; 135; 145) . 34. Method for manufacturing a light extractor (8; 128; 138; 148), comprising the steps of. (a) providing a reflecting base layer (9) , (b) silk-screen printing diffusing points (10) onto the reflecting base layer (9) according to a distribution of points (W(x) ) obtained according to the method of any one of claims 1 to 9 or according to the method of any one of claims 15 to 20. 35. Silk-screen characterised in that it embodies a distribution of points obtained with the method of any one of claims 1 to 9 or according to the method of any one of claims 15 to 20. STATEMENT UNDER ARTICLE 19(1) The amendments have been made in consideration of the International Search Report and of WO 01/7124Θ published on 27 September 2001,

Inventors: PESENTI MARCELLA, PESENTI, MARCELLA
Applicants: MINNESOTA MINING MFG ITALIA, PESENTI MARCELLA, 3M ITALIA S.P.A., PESENTI, MARCELLA
Application year: 2000
Main classification: F21V8
IPC codes: F21S2, F21V8, F21Y103, G02B6