An optical system for illumination

The present invention relates to an optical system for illumination, said optical system having rotational symmetry about a main axis, and said optical system comprising a first reflecting part and a second reflecting part for reflecting light from a light source.

When illuminating workspaces such as tabletops below a lumi- naire, it is of interest to have a luminaire with a good light distribution. This inter alia means having a high degree of efficiency in illuminating the workspace, where most of the light hits the workspace area and an minor part of the light illuminates the room around the workspace area. Moreover, the luminaire should not glare in the surroundings of the workspace.

From EP-A-337351 a reflector/refractor comprising prismatic elements, which may reflect or refract the light depending of the incidence angle of the light from the light source located within the reflector/refractor, is known. The reflector/refractor is somewhat bowl-shaped and has an overall rotational symmetry about a main axis. Along the main axis the overall contour of the bowl-shaped reflector/refractor is composed of a number of contiguous segments. These segments are frustro-toroidal at the bottom of the bowl shape and frustro-conical at the light exit opening at the top of the bowl. The light distribution of un- scattered light over the workspace below the reflector/refractor is however somewhat narrow or localized. That is to say the exit angle for un- scattered light, i.e. the angle between the light rays and the main axis, is not as high as could be desired. Outside the somewhat narrow area the workplace is only lighted by scattered light, which has passed the reflector/refractor.

On this background it is the object of the invention to provide an optical system according to the opening paragraph, having an improved light distribution.

According to the present invention this object is achieved by an optical system according to the opening paragraph, where said first reflecting part has a curvature corresponding to a segment of the peri-

phery of a first ellipse, and wherein the second reflecting part has a curvature corresponding to a segment of the periphery of a second ellipse, wherein the second reflecting part comprises prismatic elements providing total internal reflection. Thereby the good light distribution mentioned above is achieved, where a high degree of efficiency because a workspace with a large area is lit by unscattered light, which then again illuminates the room as scattered light reflected from the workspace. This scattered light will not glare in the surroundings of the workspace. The remainder of the light, e.g. penetrating the second reflecting part because the total internal reflection is not perfect, also illuminates the room as scattered light without glare in the surroundings of the workspace.

According to a preferred embodiment, at least one of said first and second reflecting parts comprises a sector of an ellipsoid having an axis coinciding with the main axis of the light control device.

According to a further preferred embodiment, at least one of said first and second reflecting parts comprises a sector corresponding to the revolution of a segment of the periphery of an ellipse having both of its major axes inclined at an angle with respect to the main axis of the optical system and having a focal point coinciding with the focal point of the respective other one of said first and second ellipse.

According to yet a further preferred embodiment, at least one of said first and second reflecting parts comprises a further sector corresponding to the revolution of a segment of the periphery of an ellipse having both of its major axes inclined at an angle with respect to the main axis of the optical system and having a focal point coinciding with the focal points of said first and second ellipses.

According to another preferred embodiment, said second reflecting part comprises a number of sectors, each corresponding to the revolution of a segment of the periphery of an ellipse having its major axis inclined at an angle with respect to the main axis of the optical system and having a focal point coinciding with the focal points of said first and second ellipses, and wherein said angle increases with the eccentricity of said ellipse.

According to yet another preferred embodiment, said first and second ellipses have a coinciding focal point. Thereby an especially simple geometry is achieved in cases where the light source can be assumed to be a point source. The invention will now be explained in greater detail based on non-limiting exemplary embodiments illustrated in the figures. In the figures,

Fig. 1 schematically shows a first embodiment of an optical system according to the invention with indication of the directly emitted light form the light source,

Fig. 2 shows the optical system of fig. 1, with indications of the light reflections from the first reflector,

Fig. 3 shows the optical system of fig. 1, with indications of the light reflections from the second reflector, Fig. 4 illustrates the total internal reflection of a light ray in the second reflector of the first embodiment,

Fig. 5 shows a diagram of the light distribution from the optical system according to the first embodiment of the invention,

Fig. 6 shows a second embodiment of an optical system accord- ing to the present invention with indications of the light reflections from the first reflector, and

Fig. 7 shows the optical system of fig. 6 with indications of the light reflections from the second reflector.

Referring first to fig. 1, an optical system 1 according to a first embodiment of the invention is shown. The optical system is symmetrical with rotational symmetry about a main axis 2 of the optical system 1. The optical system comprises a first reflecting part in the form of a first reflector 3 and a second reflecting part in the form of a second reflector 4. The first reflector 3 and the second reflector 4 are interconnected by an interconnection means 5. The interconnection 5 means is preferably a frustro-conical screen of an opaque or translucent material. The material, however, may also be transparent. Also, if desired, the interconnection means 5 could be an open structure e.g. a number of struts.

The first reflector 3 is generally cup-shaped, with an opening 6, through which light from a light source 7 may exit as direct light or as reflected light. In fig. 1 the arrows 8 indicate light rays of direct, i.e. non-reflected light from the light source 7. In the following description it is assumed that the light source 7 is a point light source corresponding to the optical centre of a lamp 11. Though this approximation does not necessarily hold true for all applications, it is sufficient for the considerations made here in connection with the optical system 1 according to the first embodiment of the invention. For situations where the light source cannot be considered to be a point source the second embodiment according to the invention is preferred. This embodiment will be discussed further below after the discussion of the first embodiment. The first reflector 3 is preferably of a reflective metal or of a material such as glass or plastic with a reflective metallic coating. The reflectance is preferably more than 0.85.

The second reflector 4 has an overall annular shape. The second reflector 4 is made of a transparent material such as plastic or glass and has a number of prismatic elements 9 projecting externally and thus forming the outer surface. These prismatic elements 9 provide total in- ternal reflection of incident light as illustrated in figs. 3 and 4. The inner surface of the second reflector 4 is preferably smooth. The shaded area in fig. 1 illustrates the light emanating directly from the light source 7, i.e. as the light rays 8, and passing unhindered and without reflection through the second reflector 4. Also, in fig. 1 an encapsulation 10, for e.g. a socket for the lamp

11 providing the light source 7, is shown. This encapsulation 10 does not form part of the optical system 1 as such, and will not be discussed any further.

Fig. 2 shows examples of light rays 27 emanating from the light source 7 located in the first focal point F and reflected by the first reflector 3. The first reflector 3 is preferably composed of several sectors 12, 13, 14 as indicated by the interrupted lines 15, 16, 17, 18. In the illustrated example, the number of sectors is three, but there could of course be more or less. Each of these sectors 12, 13, 14 have a different ge-

ometry, but they all have rotational symmetry about the main axis 2. They are all formed by rotation of a segment of the periphery of a respective ellipse about the main axis 2. The respective ellipses, however, differ in eccentricity, dimensions of their major axis and the inclination of their major axis with respect to the main axis 2. In particular the inclination of the major axis of at least one of the ellipses may be zero degree with respect to the main axis 2, in which case the sector in question is a sector of an ellipsoid. The respective ellipses, however, have one thing in common, a focal point. As it will be known, ellipses have two focal points. Thus, the ellipses forming the sectors 12, 13 and 14, respectively, all have as a common focal point the focal point F.

In the illustrated embodiment, the first sector 12 of the first reflector 3 is formed by revolution of a segment of the periphery of an ellipse with a low eccentricity of approximately 0.25 and a high inclination of the major axis of 70°, thus having a first focal point in F and second focal point in Fl.1. Since, however, the ellipse is rotated, the focus Fl.1 of the first sector 12 of the first reflector 3 is actually ring-shaped. For typical dimensions in practical use, where the diameter of the output opening 6 is 120 mm, the distance between F and Fl.1 in the embodi- ment illustrated is then approximately 20 mm. As can be seen from fig. 2, the low eccentricity in combination with the high inclination is advantageous in the sense that the light reflected by the first sector 12 is not reflected back into the lamp 11 providing the light source 7, and thus is not wasted. The second sector 13 of the first reflector 3 in the illustrated embodiment is also formed by revolution of a segment of the periphery of an ellipse. Here, however, the inclination of the major axis is 0° and the sector is thus an ellipsoid. The eccentricity of the ellipse, and hence the ellipsoid, is approximately 0.5. The second focal point Fl.2 of the el- lipse is also the focal point of the ellipsoid. With the 120 mm output opening 6 mentioned above, the distance between F and Fl.2 is approximately 60 mm.

Like the first sector 12 of the first reflector 3, the third sector 14 of the first reflector 3 is formed by revolution of a segment of the pe-

riphery of an ellipse. This ellipse, however, has a higher degree of eccentricity that the ellipses forming the first and second sectors 12, 13. The third sector 14 of the first reflector has an eccentricity of approximately 0.7 and an inclination of the major axis of 17°, thus having a first focal point in F and second focal point in Fl.3. Since, however, the ellipse is rotated, the focus Fl.3 of the third sector 12 of the first reflector 3 is also ring-shaped. For the above typical dimensions, where the diameter of the output opening 6 is 120 mm, the distance between F and Fl.3 is then approximately 120 mm. It should be noted that the optical centre of the lamp 11 corresponding to the light source 7 lies entirely within the cup-shaped first reflector 3, i.e. above the interrupted line 18 in fig. 2, where the output opening 6 of the first reflector 3 faces downward. Having the light source 7 within the first reflector 3, provides good lighting on the workspace, because the light is reflected only once from the reflecting surface, the light thus neither being scattered nor being absorbed. Moreover, this geometry of the first reflector matches very well the geometry of the second reflector 4, to be described in details below, and thus contributing to the overall good lighting quality of the optical system 1 according to the first embodiment of the invention.

It should be noted that lamps with integrated ellipsoidal reflectors exist. The first reflector 3 could thus be constituted by the integrated reflector of such a lamp. Examples of such lamps are Philips Masterline ES low voltage halogen reflector lamps, and Master Colour CDM-R PAR20 and 3OL, having a light distribution angles in the interval from 10° to 60°.

Fig. 3 shows examples of light rays 28 emanating from the light source 7 located in the first focal point F and reflected by the second reflector 4. The second reflector 4 is preferably also composed of several sectors 19, 20, 21 as indicated by the interrupted lines 22, 23, 24, 25. In the illustrated example the number of sectors are three, but there could of course be more or less. Each of these sectors 19, 20, 21 has a different geometry, but they all have rotational symmetry about the main axis 2. They are all generally formed as a rotation of a segment of

the periphery of a respective ellipse about the main axis 2. Since however the external surface of the second reflector 4 comprises a number of prismatic elements 9, the rotational symmetry is an n'th order symmetry, where n is the number of prismatic elements 9. The respective ellipses, from which the sectors of the second reflector 4 are formed, differ in eccentricity, dimensions of their major axis and the inclination of their major axis with respect to the main axis 2. In particular the inclination of the major axis of at least one of the ellipses may be zero degree with respect to the main axis 2, in which case the sector in question is a sector of an ellipsoid. The respective ellipses, however, have one thing in common, a focal point. This focal point is the very same focal point F, which the sectors 12, 13, 14 of the first reflector 3 also share.

The dimensions of the second reflector are larger than those of the first reflector 3. In particular the light output opening 26 of the sec- ond reflector 4 is approximately 380 mm in the illustrated embodiment, where the light output opening 6 of the first reflector 3 is approximately 120 mm.

In the preferred embodiment, the three sectors of the second reflector 4 increase in both inclination with respect to the main axis 2 and in eccentricity.

Thus, the first sector 19 of the second reflector 4 in the illustrated embodiment is formed by revolution of a segment of the periphery of an ellipse, where the inclination of the major axis is 0°. The sector is thus an ellipsoid. The eccentricity of the ellipse, and hence the ellip- soid, is approximately 0.6. The second focal point F2.1 of the ellipse is also the focal point of the ellipsoid. With the 380 mm output opening 26 mentioned above, the distance between F and F2.1 is approximately 300 mm.

The second sector 20 of the second reflector 4 is also formed by revolution of a segment of the periphery of an ellipse. The major axis of the ellipse however is in this case inclined with respect to the main axis 2, at an angle of 8°. The eccentricity of the ellipse from which the second sector 20 of the second reflector 4 is approximately 0.7, i.e. slightly higher than the eccentricity of the ellipse, from which the first sector 19

of the second reflector 4 is formed. Consequently, the distance between the first focal point F and the second focal point F2.2 is larger, viz. approximately 340 mm. Since, however, the ellipse is rotated, the focus F2.2 of the second sector 20 of the second reflector is actually ring- shaped in a manner similar to the foci Fl.1 and Fl.3 of the first sector 12 and third sector 14 of the first reflector 3.

Likewise, the third sector 21 of the second reflector 4 is also formed by revolution of a segment of the periphery of an ellipse. The major axis of the ellipse however is in this case inclined with respect to the main axis 2, at an angle of 25°. The eccentricity of the ellipse from which the second sector 21 of the second reflector 4 is approximately 0.8, i.e. higher than the eccentricity of the ellipses, from which the first sector 19 and the second sector 20 of the second reflector 4 are formed. Consequently, the distance between the first focal point F and the third focal point F2.3 is also larger, viz. approximately 450 mm. Since this ellipse is also rotated, the focus F2.3 of the second sector 21 of the second reflector is also ring shaped in a manner similar to the foci Fl.1 and Fl.3 of the first sector 12 and third sector 14 of the first reflector 3 as well as the focus F2.2 of the second sector 20 of the second reflector 4. The dimensions mentioned above are of course only examples.

Depending on the desired lamp power the output opening 6 of the first reflector 3 could typically lie in the interval from 50 mm to 150 mm and the output opening 26 of the second reflector could typically lie in the interval from 200 mm to 600 mm. Though there are geometrical similarities between the first reflector 3 and the second reflector 4, there is a major difference between them in the way they reflect the impinging light. The first reflector 3 is a metallic or metal-coated reflector and relies on the light reflecting properties of the metal. Ideally, it should reflect all impinging light, i.e. have a low absorbance and a low transmittance. The reflectance is preferably above 85%.

The second reflector 4 is of a transparent material such as plastic, and relies instead on a prismatic surface geometry providing total internal reflection properties. These properties are provided by means of a

number of elongate prismatic elements 9, each comprising two flanks 29 and 30. The angle between the flanks 29 and 30 is preferably 90°, as seen in the plane perpendicular to a tangent to the elliptical curvature, which the prismatic elements 9 follow, in any given sector 19, 20, 21. The total internal reflection of a light ray 28 in this surface geometry is illustrated in fig. 4. Here a light ray 28 from the light source 7 impinges on the internal surface 31 of the second reflector 4 at a point 32. The light ray 28 is refracted at the point 32 and continues inside the transparent material, of which the second reflector 4 is made. The light ray 28 continues within the transparent material and hits the external surface at the flank 29 in the point 33, where it is totally internally reflected due to the difference in refractive index of the transparent material, of which the second reflector 4 is made, and the surrounding atmospheric air. The light ray thus continues inside the transparent mate- rial, of which the second reflector 4 is made, until it hits the other flank 30 of the respective prismatic element 9 in the point 34, where it is again totally internally reflected, this time back towards the surface 31 again. When the light ray hits the surface 31 again, the angle is such that it is not totally internally reflected, but refracted. The light ray then leaves the surface 31 at a desired angle. The refractive material, of which the second reflector is made, preferably has a refractive index in the interval from 1.4 to 1.5

Having carefully considered the curvatures of the sectors 19, 20, 21, this results in most of the light from the light source 7 being to- tally internally reflected in the second reflector 4. Only a small portion of the overall light emitted from the light source 7 hits the prismatic elements 9 at an angle not giving total internal reflection. This portion, which is approximately 4%, illuminates the exterior of the second reflector in a pleasant and non-blinding way. The rest of the light from the light source hits the workspace in the following proportions. 9% directly from the light source 7. 45% from the light source 7 reflected by the first reflector 3 and exiting through the output opening 26 of the second reflector 4. 22% from the light source 7 reflected by the second reflector 4 and exiting through the output opening 26 of the second reflector 4.

This yields a high efficiency of approximately 80% of the light flux, where approximately 76% hits the workspace, whereas 4% directly illuminates the room around the workspace. As to the light hitting the workspace the distribution is good over the entire area which itself is quite large, depending of course of the distance of the workspace, from the optical system.

The light distribution is illustrated in fig. 5. Here the curve 35 illustrates the intensity of the light output through the output opening 26 of the second reflector 4 as a function of the angle to the main axis 2 of the optical system according to the first embodiment of the invention.

In respect of the light distribution it should be noted that the use of several sharing ellipsoids sharing a common focal point is known in the art from e.g. the documents JP-A-2005-309121 and EP-A-645648. These however aim at focussing the light in a narrow area in order to create a high intensity. None of these documents, however, suggest the use of ellipsoids to distribute the light over a large area. Moreover these are metallic reflectors and do neither suggest the use of elliptical surfaces in conjunction with total internal reflection, nor do they suggest the use of inclined major axes and increased eccentricity when forming the reflectors. These documents therefore have no teaching applicable to the present invention.

Also, it should be noted that other elliptical curvatures could be used, as long as the angle with respect to the main axis, the eccentricity, the overall dimensions, and the difference between the refractive indices of the material, of which the second reflector 4 is made, and the surrounding air, allow total internal reflection in the prismatic elements 9.

The second embodiment of the invention will now be discussed with reference to Figs. 6 and 7. In Figs. 6 and 7 similar elements have corresponding reference numerals. Where differences in the similar ele- ments have to be emphasized, this has been indicated by the addition of primes to the reference numerals.

Like in the first embodiment the optical system 1' of the second embodiment is symmetrical with rotational symmetry about a main axis 2 of the optical system 1'. The optical system 1' comprises a first reflect-

ing part in the form of a first reflector 3' and a second reflecting part in the form of a second reflector 4. The second reflector 4 may be, and preferably is, identical in construction to the second reflector 4 of the first embodiment and will not be discussed in further detail. In fact the second reflector 4 can be regarded as a unique optical element in its own right, which may be used in other applications than the embodiments of the invention described herein. The first reflector 3' and the second reflector 4 are connected by means of an interconnection means 5' of a construction similar to that of the interconnection means 5 of the first embodiment. The first reflector 3' is preferably of a reflective metal or of a material such as glass or plastic with a reflective metallic coating. The reflectance is preferably more than 0.85.

The difference between the first and second embodiments reside mainly in the first reflector, which is especially adapted to accommodate a lamp 11' which cannot be considered to be a point light source. Comparison between e.g. Fig. 1 and 6 reveals that for identical second reflectors 4 the first reflector 3' according to the second embodiment is somewhat larger than the first reflector 3 of the first embodiment.

The first reflector 3' according to the second embodiment is also generally cup-shaped, with an opening 6, through which light from a the lamp 11' forming the light source may exit as direct light or as reflected light. The first reflector 3' is preferably composed of several sectors 12', 13' as indicated by the interrupted lines 15', 16', 17'. In the illustrated example, the number of sectors is two, but there could of course be more or less. Each of these sectors 12', 13' have a different geometry, but they all have rotational symmetry about the main axis 2. They are all formed by rotation of a segment of the periphery of a respective ellipse about the main axis 2. The respective ellipses, however, differ in eccentricity, dimensions of their major axis and the inclination and/or offset of their major axis with respect to the main axis 2. In particular the upper sector 12' is formed as a circular arc segment offset from and rotated about the main axis 2. Here it should be noted that a circle is considered only to be the special case of an ellipse, where the major axes are equal, and the two focal points thus merge in the centre of the circle. The upper

sector 12', thus does not have a focal point as such, but rather a ring- shaped focus indicated by Fl.1' in Fig. 6. This ring-shaped focus lies outside the lamp 11'.

The lower sector 13' is also formed by rotation of a segment of the periphery of an ellipse. This ellipse has an inclination with respect to the main axis 2. It furthermore has one focal point coinciding with the focal point Fl.1' of the circle segment from which the first sector 12' is formed. The other focal point Fl.2' of the ellipse lies at the lower edge of the second reflector 4. Thus the second sector 13' has two ring-shaped focuses Fl.1' and Fl.2'. This geometry ensures that those portions of the light reflected from the first reflector, which hit the second reflector 4, does this under an angle of incidence, which allows it to be totally internally reflected. Moreover, this geometry ensures that none of the light emanating from the lamp 11' is reflected back into the lamp and wasted. Though the optical system has been described with reference to exemplary embodiments, it should be noted that the scope of the invention is not restricted to these embodiments and parts thereof. The skilled person will realise that numerous variations to both the first reflector 3, 3' and the second reflector 4 are possible without deviating from the scope of the invention, in particular in respect of the number of sectors 12, 13, 14; 12', 13'; 19, 29, 21 used in the individual reflectors.