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Title:
LIGHTING APPLIANCE HAVING A REDUCED ENCUMBRANCE WITH EFFECTIVE CONTROL OF THE PHOTOMETRIC DISTRIBUTION OF EMITTED LIGHT
Document Type and Number:
WIPO Patent Application WO/2009/044269
Kind Code:
A2
Abstract:
The present invention relates to a lighting appliance having reduced encumbrance with effective control of the photometric distribution of the emitted light which comprises at least one light source (13) interposed between a reflector (11) and an optical screen (14) and is characterized in that it comprises at least one light divergence limiting element (15a, 15b, 15c, 18) suitable for reducing the divergence angle of the light beams directed towards said optical screen (14).

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Inventors:
GUZZINI, Adolfo (Contrada Duomo, 33/A, Recanati, I-62019, IT)
GIGLI, Giuseppe (Via Nostra Signora di Lourdes, 135, Rome, I-00167, IT)
MANCA, Michele (Via Adelaide Ristori 2, Nardo', I-73048, IT)
Application Number:
IB2008/002629
Publication Date:
April 09, 2009
Filing Date:
September 29, 2008
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
iGUZZINI ILLUMINAZIONE S.P.A. (Via Mariano Guzzini 37, Recanati, I-62019, IT)
GUZZINI, Adolfo (Contrada Duomo, 33/A, Recanati, I-62019, IT)
GIGLI, Giuseppe (Via Nostra Signora di Lourdes, 135, Rome, I-00167, IT)
MANCA, Michele (Via Adelaide Ristori 2, Nardo', I-73048, IT)
International Classes:
F21V7/00; F21V5/02; F21V5/04; F21V13/12
Domestic Patent References:
WO1998053137A11998-11-26
Foreign References:
US5161880A1992-11-10
US20060203493A12006-09-14
GB2130353A1984-05-31
DE952918C1956-11-22
Attorney, Agent or Firm:
DE GREGORI, Antonella (Barzano' & Zanardo Milano S.p.a, Via Borgonuovo 10, Milan, I-20121, IT)
Download PDF:
Claims:

CLAIMS

1. A lighting appliance (10) with reduced encumbrance comprising at least one light source (13) interposed between a reflector (11) and an optical screen (14) characterized in that it comprises at least one light divergence limiting element (15a, 15b, 15c, 18) suitable for reducing the divergence angle of the light beams directed towards said optical screen (14) .

2. The lighting appliance (10) with reduced encumbrance according to claim 1, characterized in that said light divergence limiting element (15a, 15b, 15c, 18) is a divergence limiting element (15a, 15b, 15c) of reflected light suitable for reducing the divergence angle of the light beams reflected by said reflector (11) . 3. The lighting appliance (10) with reduced encumbrance according to claim 2, characterized in that said at least one divergence limiting element of reflected light (15a, 15b, 15c) comprises a plurality of fins (15a) arranged on a portion of surface (26) of said reflector (11) facing said at least one light source (13) , said fins (15a) having surfaces made of a material suitable for at least partially absorbing the light beams striking them. 4. The lighting appliance (10) with reduced encumbrance according to claim 3, characterized in that the angle formed between the tangent to the profile of said fins

(15a) and the perpendicular to the plane of said optical screen (14) ranges from 0° to 45°.

5. The lighting appliance (10) with reduced encumbrance according to claim 3 or 4 , characterized in that the distribution of said fins (15a) on said surface (26) of the reflector (11) ranges from 0.2 to 6 fins per cm.

6. The lighting appliance (10) with reduced encumbrance according to any of the claims from 3 to 5, characterized in that the height (h) of said fins (15a) ranges from 0.2 cm to 5 cm.

7. The lighting appliance (10) with reduced encumbrance according to claim 2, characterized in that said at least one divergence limiting element of reflected light (15a, 15b, 15c) is at least one plate element (15b) which covers at least a portion of a surface (26) of said reflector (11) facing said at least one light source (13) , said at least one plate element (15b) being made of a material suitable for at least partially absorbing the light beams striking them. 8. The lighting appliance (10) with reduced encumbrance according to claim 2, characterized in that said at least one divergence limiting element of reflected light (15a, 15b, 15c) is at least one plate element (15b) which covers at least a portion of a surface (26) of said reflector (11) facing said at least one light source

(13) , said at least one plate element (15b) consisting of a portion of reflector (11) with reduced reflectance.

9. The lighting appliance (10) with reduced encumbrance according to claim 2, characterized in that said at least one divergence limiting element of reflected light (15a, 15b, 15c) comprises a plurality of grooves (15c) situated on at least a portion of a surface (26) of said reflector (11) facing said at least one light source (13) .

10. The lighting appliance (10) with reduced encumbrance according to any of the previous claims, characterized in that said light divergence limiting element (15a, 15b, 15c, 18) is a divergence limiting element of direct light (18) suitable for reducing the divergence angle of the light beams coming directly from said at least one light source (13) .

11. The lighting appliance (10) with reduced encumbrance according to claim 10, characterized in that said at least one divergence limiting element of direct light (18) comprises at least one lens having a first curved surface (19) for the inlet of light beams facing said at least one light source (13) and a second curved surface (20) for the exit of the light beams.

12. The lighting appliance (10) with reduced encumbrance according to claim 11, characterized in that said first curved surface (19) has a concavity facing said at least

one light source (13) .

13. The lighting appliance (10) with reduced encumbrance according to claim 11 or 12, characterized in that the profile of said first curved surface (19) is defined by a section of circumference.

14. The lighting appliance (10) with reduced encumbrance according to any of the claims from 11 to 13, characterized in that said first curved surface (19) has a protuberance matrix having nanometric dimensions arranged according to a moth's eye scheme.

15. The lighting appliance (10) with reduced encumbrance according to any of the claims from 11 to 14, characterized in that said second curved surface (20) has a plurality of prisms (21) . 16. The lighting appliance (10) with reduced encumbrance according to any of the claims from 13 to 15, characterized in that said at least one light source (13) has a development along a curve (C) , the joining line of the centre of this circumference with the point corresponding to said curve C forming, with respect to the vertical passing through this point, an angle χ ranging from 0°to 60°

17. The lighting appliance (10) with reduced encumbrance according to any of the claims from 13 to 16, characterized in that said profile of said first curved

surface (19) extends along a section which is such that the shielding angle φ, defined by the joining lines of the extreme points of this section with said curve (C) of said light source 13, ranges from 20° to 90°. 18. The lighting appliance (10) with reduced encumbrance according to any of the claims from 13 to 17, characterized in that the joining line of the lower extreme of the profile of said first curved surface (19) with a point corresponding to said development curve (C) of said light source (13) forms, with respect to the vertical passing through this point, an angle ξ ranging from 15° to 60°.

19. The lighting appliance (10) with reduced encumbrance according to any of the previous claims, characterized in that said optical screen (14) has a plurality of refracting elements (23) on its emission surface (22) of light beams.

20. The lighting appliance (10) with reduced encumbrance according to the previous claim, characterized in that said plurality of refracting elements (23) consist of revolution solids whose generatrix is defined by at least a first (24) and a second (25) curvilinear section with a concavity facing the interior of revolution solid (23) , said second curvilinear section (25) having a curvature radius (Rl) less than or equal to the curvature radius

(R2) of said first curvilinear section (24) .

21. The lighting appliance (10) with reduced encumbrance according to claim 19, characterized in that said generatrix of said protuberances (23) comprises a straight section (26) terminating at the base plane of said protuberance (23) .

22. The lighting appliance (10) with reduced encumbrance according to any of the previous claims, characterized in that said reflector (11) has at least one lobe (12) with a parabolic profile, at least one light source (13) being substantially positioned in correspondence with the focal axis (A) of each parabolic lobe (12) .

23. The lighting appliance (10) with reduced encumbrance according to any of the previous claims, characterized in that said at least one light source (13) is at least a tubular fluorescent light source (13) .

24. The lighting appliance (10) with reduced encumbrance according to any of the previous claims, characterized in that said at least one light source (13) is at least a circular fluorescent light source (13) .

Description:

LIGHTING APPLIANCE HAVING A REDUCED ENCUMBRANCE WITH EFFECTIVE CONTROL OF THE PHOTOMETRIC DISTRIBUTION OF EMITTED LIGHT

The present invention relates to a lighting appliance having a reduced encumbrance with effective control of the photometric distribution of the emitted light.

For the lighting of work environments or areas containing video terminals, on the basis of predefined ergonomic vision criteria regulated by the regulations in force, the luminance must be limited to a value of lOOOcd/m 2 for emission angles greater than 65°.

It is also necessary, for projectual and aesthetical reasons, for the lighting appliances used in these environments to have reduced encumbrances, in particular in the direction perpendicular to the light emission surface i.e. to the screen of said lighting appliance.

For this purpose, the lighting appliances generally used in these environments contain tubular fluorescent lamps and are normally provided with a reflector having a maximum height ranging from 50 m to 100 mm and preferably having two lobes with a parabolic profile. This configuration allows the encumbrance of the appliance to be reduced in the direction orthogonal to the plane of the front screen, but it generally does not allow the

beams reflected to be effectively collimated.

Limiting the transversal encumbrance, in fact, also implies limiting the focal distance of the reflector, and consequently the fact that the light sources must be positioned at a short distance from the reflector itself.

This distance is in the same order of magnitude with respect to the dimensions of the diameter of the light sources themselves. Consequently, as in the very frequent case in which these are light sources with Lambertian emission, it has been observed that most of the light beams which affect the reflector do not pass through the focal axis of the paraboloid and cannot therefore be collimated by this, they are consequently diverted to angles different from zero with respect to the perpendicular to the plane of the screen of the light appliance.

They therefore have a certain divergence angle with respect to the diffusion direction of the beams which would be emitted by a hypothetical filiform source superimposed on the focal axis of the paraboloid.

The effect of this divergence is particularly significant with reference to those beams which strike the upper part of the reflector.

Figure 1 schematically indicates the path of reflected light beams in a lighting appliance 101

belonging to the state of the art.

Due to the reduced distance between the light source 102 and the reflector 103, the non- focalized beams 104 which strike the upper part of the reflector 103 have a much higher divergence angle with respect to the non- focalized beams which strike the more extreme areas of the profile.

As a result of the high divergence, some of the beams 104 are reflected by the inner surface of the screen 105 and others by its outer surface.

In the case not illustrated, in which the screen 105 has refracting elements which emerge from the outer surface, some of the beams can actually be incorrectly diverted by these elements, causing undesired light emission effects with almost grazing angles.

It can be verified that the divergence of the beams

104 coming from the upper part of the reflector 103 has a considerably negative influence on the effectiveness of the front optical screen 105 in controlling the luminance of the appliance at high angles.

In other words, the upper part of each lobe 106 of the reflector contributes to generating a large number of beams 104 with a high divergence which cannot be correctly redirected by the transparent optical screen 105 even if equipped with refracting elements, and which

consequently contribute to creating undesired dazzling effects.

An objective of the present invention is to overcome the drawbacks described above and in particular to provide a lighting appliance having a reduced encumbrance which substantially does not create undesired dazzling effects.

A further objective of the present invention is to provide a lighting appliance having a reduced encumbrance which offers an effective control of the photometric distribution of the emitted . light .

These and other objectives according to the present invention are achieved by providing a lighting appliance having a reduced encumbrance as specified in claim 1. Further characteristics of the device are object of the dependent claims.

The characteristics and advantages of a lighting appliance having a reduced encumbrance according to the present invention will appear more evident from the following illustrative and non-limiting description, referring to the enclosed schematic drawings, in which: - figure 1 is a sectional schematic view of a lighting appliance of the known art; figure 2 is a partial sectional view of a first embodiment of the lighting appliance according to the

present invention;

- figure 3 is a partial perspective view of the lighting appliance of figure 2;

- figure 4 is a sectional view of a second embodiment of the lighting appliance according to the present invention; figure 5 is a partial sectional view of a third embodiment of the lighting appliance according to the present invention,- - figure 6 is a partial perspective view of a fourth embodiment of the lighting appliance according to the present invention,- figure 7 is a sectional view of a detail of the embodiment of figure 6; - figure 8 is a partial perspective view of a fifth embodiment of the lighting appliance according to the present invention,-

- figure 9 is a schematic view of a protuberance present on an optical screen used in the lighting appliance of the present invention.

With reference to the figures, these show a lighting appliance with a reduced encumbrance, indicated as a whole with 10.

Said lighting appliance 10 with a reduced encumbrance is provided with a light source 13 enclosed

between a reflector 11 and an optical screen 14.

If the reflector 11 has a longitudinal development, as shown in figures 2-7, said reflector 11 has at least one cylindrical lobe or concentrator 12, for example with a parabolic profile, which collects the light coming from one or more light sources 13 substantially positioned in correspondence with the focal axis of the same 12.

According to a preferred embodiment of the present invention, the profile of each lobe 12 of the reflector can be defined as the combination of two different branches belonging to two different parabolas, joined in correspondence with the vertex or any other point of the profile.

In an alternative embodiment, not illustrated, the two branches of the profile of each lobe 12 consist of different kinds of curves such as, for example, a hyperbola, an ellipse or generally all curves described in Cartesian coordinates by a polynomial of order higher than two. The configuration with two lobes 12, is capable of collimating the reflected beams more effectively, but, above all, it reduces the encumbrance of the appliance 10 in a direction orthogonal to the plane of the optical screen 14. According to the invention, the lighting appliance

10 is advantageously provided with at least one divergence limiting element 15a, 15b, 15c, 18.

According to the particular embodiment, the divergence limiting elements 15a, 15b, 15c, 18 act as divergence limiters of reflected light 15a, 15b, 15c, i.e. they act on the light beams 104 coming from the reflector 11, or as divergence limiters of direct light 18, i.e. they act on light beams 104 coming directly from the source 13. A common prerogative regarding all divergence limiter elements 15a, 15b, 15c, 18 is to reduce the divergence angle of the light beams 104 which are diffused towards the inner surface of the optical screen 14. According to a first embodiment of the invention shown in figures 2 and 3, divergence limiting elements of reflected light are envisaged on the reflector 11 and consist of a plurality of fine lamellas 15a or fins having surfaces which partially or totally absorb the light striking them.

These fins 15a are arranged so as to absorb the light of those beams which are diffused towards the screen 14 at angles greater than a maximum divergence angle α defined by the geometry and arrangement of the fins 15a themselves.

The fins 15a are situated on the surface of the reflector 11 facing the light source 13 and extend in a direction substantially parallel to the focal axis A over the whole length. With respect to any sectional plane orthogonal to the focal axis A, these fins 15a are distributed over a portion of each lobe 12 of the reflector 11 having dimensions ranging from 20 mm to 100 mm and preferably from 30 mm to 50 mm. The fins 15a can indifferently have a section with a rectangular, triangular, trapezoidal or curvilinear profile .

The relative distance between two adjacent fins 15a and their extension h in a direction orthogonal to the surface 26 of the reflector 11 facing the light source 13 create the maximum divergence angle α of the light beams 17 reflected by the reflector 11 in each portion of the reflector delimited by two fins 15a.

According to a preferred configuration of the fins 15a, this angle is between 10° and 45°.

According to this preferred configuration, in each point of the profile of the fins 15a, the angle formed between the profile tangent and the perpendicular to the plane of the optical screen 14 can vary from 0° to 45°. The distribution of the fins 15a on the surface of

the reflector 11 preferably ranges from 0.2 to 6 fins per cm, and in particular from 1 to 3 fins per cm.

The height h of the fins 15a preferably ranges from 0.2 cm to 5 cm, and in particular from 0.5 to 1.5 cm. According to a second embodiment of the invention shown in figure 4, the light divergence limiting elements 15a, 15b, 15c, 18 are divergence limiting elements of reflected light consisting of plate elements 15b which cover at least a portion of the surface 26 of the reflector 11 partially or totally absorbing the light striking them.

These plate elements 15b are preferably arranged on the portion of surface 26 of the reflector 11 which is closest to the light sources 13 where the divergence effect of the light beams is considerable, and they extend in a substantially parallel direction to the focal axis A over the whole length of the reflector 11.

With respect to any sectional plane orthogonal to the focal axis A, these plate elements 15b are preferably distributed over a portion 26 of surface of each lobe 12 of the reflector 11 having a width ranging from 20 mm to 80 mm and in particular from 30 mm to 50 mm.

These plate elements 15b preferably consist of thin films applied in various ways to the surface of the reflector 11. The plate elements 15b are glued by

adhesives, for example, fixed with screws or interference couplings after the positioning of specific housings on the reflector unit 11.

According to advantageous embodiments, the plate elements 15b are integrated directly in the reflector 11, as they consist of a portion of reflector 11 with reduced reflectance.

The portion 15b with reduced reflectance can be obtained with suitable varnishing treatment or in general by suitably modifying the angular distribution profile of the reflected beam intensity by means of physico-chemical treatment suitable for effecting a controlled modification of the morphological characteristics of the surface of said reflector portion 15b. According to a third embodiment of the invention shown in figure 5, the light divergence limiting elements 15a, 15b, 15c, 18 are divergence limiting elements of reflected light obtained directly on the reflector 11 by suitable modifications of the profile in some areas of the reflector 11 itself.

In particular, in the upper area of each lobe 12 of the reflector 11, there is a plurality of grooves 15c whose profile is such as to divert the light beams coming from the light source 13 towards the optical screen 14, redirecting them so that the maximum divergence angle α

they form with the perpendicular to the plane of the screen 14 ranges from 20° to 45°.

Said angle α is determined by the form of the profile and position of the single grooves 15c. Each groove 15c has a first surface 16 suitable for appropriately redirecting the light beams 104 and at least a second surface 17, arranged so as to define with the first surface 16, in any sectional plane orthogonal to the focal axis A, a substantially triangular or trapezoidal profile (not illustrated) .

In the case of a substantially triangular profile of the section, the second surface 17 intersects the first surface 16 along a line parallel to the focal axis A.

The trace of the surface of each lobe 12 of the reflector 11 on which said grooves 15c are situated, has a length preferably ranging from 20 mm to 80 mm and in particular from 30 mm to 50 mm.

For each groove 15c, the profile of the first surface 16 suitable for reflecting the light beams 104 can be curvilinear or straight and in each point of the profile the line perpendicular to the surface 16 forms, with the line that joins said point to the focal axis A, an angle which can range from -30° to + 30°.

In a particular embodiment, the first surfaces 16 of the grooves 15c form adjacent sections belonging to a

same continuous curve, similar to the geometry- characterizing a Fresnel lens.

In alternative embodiments, on the other hand, the grooves 15c are characterized by surfaces 16 having the same profile.

The grooves 15c closest to the vertex of each lobe 12 of the reflector 11 preferably have first surfaces 16 suitable for redirecting the light beams adjacent and symmetrical to each other, so that their profile defines a single curvilinear section, preferably parabolic.

The first surface 16 suitable for redirecting the light beams 104 consists of the same material as the reflector or is alternatively covered with highly reflectant coatings deposited with various possible techniques.

The second surface with a preferably straight profile 17 is preferably covered by coatings capable of totally or partially absorbing the light beams 104 striking it . Alternatively on each second surface 17 an absorbing platelet is applied (not illustrated) to limit the divergence of the light beams 13 produced according to the portion 15b with reduced reflectance described above in relation to the second embodiment. According to a fourth embodiment of the present

invention shown in figures 6 and 7, the light divergence limiting elements 15a, 15b, 15c, 18 are direct light divergence limiting elements 18, i.e. they act on light beams striking the optical screen 14 which come directly from the light sources 13.

These direct light divergence limiting elements 18 comprise at least one collimating lens 18 capable of shielding the emitted light beams at angles greater than a certain divergence angle defined by the geometry of the lenses themselves and by their relative position with respect to the light source 13.

In the embodiment illustrated, the form of each lens 18 is defined by a solid generated by extruding the profile of the generatrix along a direction substantially parallel to the curve C along which the particular light source 13 develops .

Direct light divergence limiting elements 18 consisting of two collimating lenses are preferably associated with each light source 13, as illustrated in figure 6.

Each lens 18 has a first curved surface 19 for the entrance of the light beams facing the light source 13 and a second curved surface 20 for the exit of the light beams . The generatrix of each lens 18 has a profile with a

concavity facing the light source 13 corresponding to the trace of the inlet surface of the light beams in any- sectional plane orthogonal to the curve C along which the light source 13 develops. The profile of the first inlet surface 19 of the beams is preferably defined by a section of circumference, and more generally by a curve described in Cartesian coordinates by a polynomial of order equal to two or more . If it is defined by a section of circumference, the joining of the centre of this circumference with the point corresponding to the curve C along which the light source 13 develops, forms, with respect to the vertical passing through this point, an angle χ ranging from 0° to 60° and preferably from 20° to 45°.

The profile of the inlet surface 19 of the beams extends over a section whose length is such that the shielding angle φ, defined by the joining lines of the extreme points of this section with the curve C along which the light source 13 develops, ranges from 20° to 90°, and preferably from 30° to 60°.

The angle ξ, defined by the joining line of the lower extreme of the profile of the inlet surface 19 of the beams with the point corresponding to the curve C along which the light source 13 develops, with respect to

the vertical passing through this point, determines the extension of the portion of surface of the non-shielded source. This angle ξ preferably ranges from 15° to 60°, and in particular from 25° to 45°. The inlet surface 19 of the light beams can also have a protuberance matrix with nanometric dimensions arranged according to the moth's eye scheme, which give it the characteristic of transmitting almost all the inciding beams, including the beams which are transmitted with high angles of incidence.

The profile of the second surface 20 for the emission of beams can be defined by a circumference arch, or parabola, or hyperbole or more generally a curve described in Cartesian coordinates by a polynomial of order equal to two or more.

The second emission surface 20 of light beams is also characterized by the presence of a plurality of micro-prisms 21 which extend for the whole length of the lens 18 on a section of the profile of the generatrix or on the whole profile.

These prisms 21 preferably have different heights and profiles depending on their position along the profile .

Each lens 18 is alternatively made of glass or a transparent thermoplastic material, preferably

polycarbonate or acrylic.

In embodiments not illustrated, the lighting appliance 10 according to the present invention is provided with both divergence limiting elements 15a, 15b, 15c of reflected light and direct light divergence limiting elements 18.

According to alternative embodiments of the lighting appliance 10 object of the present invention, the reflector 11 is produced so that each lobe has a main inertia axis perpendicular to the plane of the optical screen 14 and which has one or more symmetry planes all passing through the above main inertia axis. According to this scheme, the appliance can carry one or more light sources 13 consisting of circular or compact fluorescent lamps, gas-discharge lamps or electro-luminescent sources made of organic or inorganic .

In conformance with the above alternative embodiments of the invention, the constructive details of the divergence limiting elements 15a, 15b, 15c, 18 are constrained to the specific geometry of the reflector 11 and light source 13 assembled on the lighting appliance 10.

In particular, in the case of the embodiment of figure 8, the lighting appliance 10 has a configuration with a circular symmetry and the reflector 11 has a

symmetry axis B perpendicular to the plane of the optical screen 14.

In the particular embodiment illustrated, the reflector 11 consists of a revolution solid having a section with a substantially parabolic profile whose symmetry axis B coincides with the symmetry axis of the light source 13, which, in this embodiment consists of a fluorescent lamp 13 with a circular development. In particular, the profile of each lobe 12 of the reflector 11 conforms with the profile already described with reference to the configuration with a longitudinal axis.

In each sectional plane orthogonal to the screen 14, the point corresponding to the focal axis A of each lobe 12 of the reflector 11 substantially coincides with the point corresponding to the circular development curve C of the lamp 13, i.e. the focal axis A of the reflector 11 being substantially superimposed on the curve C with a circular development of the lamp 13.

In conformance with this embodiment of the invention, the divergence limiting elements 15a, 15b, 15c, 18 have a sectional profile corresponding to that described in relation to the embodiments of the invention described above, but they do not develop along a straight axis as in these embodiments, but are preferably configured as revolution solids with an axis coinciding

with the axis B of the reflector.

Figure 8 shows an embodiment in which these divergence limiting elements are in the form of a lamella 15a. According to a further aspect of the present invention, the optical screen 14 of the lighting appliance 10 according to the present invention advantageously has a plurality of refracting elements 23 on its outer surface 22 for the emission of light beams. These refracting elements 23 alternatively consist of solids with a conventional geometry, such as pyramids or trunks of pyramids, cones or trunks of cones, spherical or aspherical lenses, or they are preferably produced in the form of axial-symmetrical protuberances which narrow in the direction perpendicular to the plane of the screen 14.

The axial-symmetrical protuberances 23 advantageously consist of revolution solids whose generatrix is defined by at least a first curvilinear section 24 which has a concavity facing the interior of the revolution solid 23, and at least a second curvilinear section 25 also having a curvature facing the interior of the revolution solid but a curvature radius Ri less than or equal to the curvature radius R 2 of the first section 24.

In addition or alternatively to the second curvilinear section 25, said generatrix comprises a straight section 26 terminating in correspondence with the intersection point with the base plane of the screen 14.

The sections 24 and 25 of the generatrix are preferably joined in a point with a common tangent.

Alternatively, in a variant embodiment not illustrated, the sections 24 and 25 of the generatrix are joined in a point which defines a discontinuity in the first derivative of the function describing the profile of the generatrix and consequently a cusp.

Each of the curvilinear sections 24, 25 consists of one or more curvilinear segments, each of which can be described in Cartesian coordinates by a polynomial of order greater than or equal to two .

The protuberances 23 are preferably arranged so as to form a matrix with hexagonal cells so that each protuberance 23 is defined by the intersection of the revolution solid and a parallelepiped with a hexagonal base inscribed in the base circle of said revolution solid 23. Analogously, the protuberances 23 can be arranged to form a matrix with square, pentagonal, octagonal cells and so forth. The particular conformation of the profile of the

generatrices of the revolution solids 23 provides an even more effective control of the photometric distribution produced by the optical screen 14.

The characteristics of the device, object of the present invention, as also the relative advantages, are evident from the above description.

Finally it is evident that the lighting appliance thus conceived can undergo numerous modifications and variants, all included in the scope of the invention; furthermore, all the details can be substituted by technically equivalent elements. In practice, the materials used, as also the dimensions, can vary according to technical requirements .