Lens with slits

Abstract

A lens (1) comprising a centrally extending axis (A) being perpendicular to a lens axis (L) of the lens, the lens being adapted for retracting light rays from a light source positioned off-axis with respect to the centrally extending axis, the lens comprising a first plurality of slits (21, 22, 23, 24) extending entirely in an interior of the lens and comprising a length (l) extending perpendicular to the centrally extending axis, a width (w) extending parallel to the centrally extending axis and a thickness (t) extending perpendicular to both the length and the width, the first plurality of slits being mutually parallel and covering between 40% and 60% of a plane in which both the centrally extending axis (A) and the length (l) and width (w) of the first plurality of slits (21, 22, 23, 24) extend.

Claims

1. A lens comprising a centrally extending axis (A) being perpendicular to a lens axis (L) of the lens, the lens being adapted for refracting light rays from a light source positioned off-axis with respect to the centrally extending axis, the lens comprising a first plurality of slits extending entirely in an interior of the lens and comprising a length (l) extending perpendicular to the centrally extending axis, a width (w) extending parallel to the centrally extending axis and a thickness (t) extending perpendicular to both the length and the width, the first plurality of slits being mutually parallel and covering between 40% and 60% of a plane in which both the centrally extending axis (A) and the length (l) and width (w) of the first plurality of slits extend.

2. A lens according to claim 1, wherein the first plurality of slits cover 50% of the plane in which both the centrally extending axis and the length (l) and width (w) of the first plurality of slits extend.

3. A lens according to claim 1, and further comprising a second plurality of slits extending entirely in the interior of the lens and comprising a length extending perpendicular to the centrally extending axis, a width extending parallel to the centrally extending axis and a thickness extending perpendicular to both the length and the width, the second plurality of slits being mutually parallel and covering between 40 and 60% of a plane in which both the centrally extending axis and the length and width of the second plurality of slits extend, the second plurality of slits extending in an angle different from zero with respect to the first plurality of slits.

4. A lens according to claim 3, wherein the second plurality of slits cover 50% of the plane in which both the centrally extending axis and the length and width of the second plurality of slits extend.

5. A lens according to claim 3, and further comprising a third plurality of slits extending entirely in the interior of the lens and comprising a length extending perpendicular to the centrally extending axis, a width extending parallel to the centrally extending axis and a thickness extending perpendicular to both the length and the width, the third plurality of slits being mutually parallel and covering between 40 and 60% of a plane in which both the centrally extending axis and the length and width of the third plurality of slits extend, the third plurality of slits extending in an angle different from zero with respect to both the first plurality of slits and the second plurality of slits.

6. A lens according to claim 5, wherein the third plurality of slits cover 50% of the plane in which both the centrally extending axis and the length and width of the third plurality of slits extend.

7. A lens according claim 1, wherein any one or more of the first plurality of slits, the second plurality of slits and the third plurality of slits terminate in a distance from an outer surface of the lens.

8. A lens according to claim 1, wherein the width of each slit of any one or more of the first plurality of slits, the second plurality of slits and the third plurality of slits is between 0.1 mm and of a height of the lens measured along the centrally extending axis (A).

9. A lens according claim 1, wherein the distance between adjacent slits of any one or more of the first plurality of slits, the second plurality Of slits and the third plurality of slits is between 0.1 mm and of a height of the lens measured along the centrally extending axis (A).

10. A lens according to claim 1, wherein the thickness of each slit of any one or more of the first plurality of slits, the second plurality of slits and the third plurality of slits is larger than 1000 nm.

11. A lens according to claim 1, wherein end surfaces of the slits of any one or more of the first plurality of slits, the second plurality of slits and the third plurality of slits facing towards adjacent slits are lambertian scattering.

12. A collimator comprising a lens according to claim 1.

13. A lighting device comprising a lens according to claim 1 and at least two light sources adapted for, in operation, emitting light the at least two light sources being positioned spaced apart and off-axis with respect to the centrally extending axis (A).

14. A lighting device according to claim 13, wherein the width of each slit of any one or more of the first plurality of slits, the second plurality of slits and the third plurality of slits of the lens is chosen such as to be larger than a wavelength of the light emitted by the at least two light sources and smaller than a distance between the at least two light sources.

15. A lighting device according to claim 13, wherein the thickness of each slit of any one or more of the first plurality of slits, the second plurality of slits and the third plurality of slits is chosen such as to be larger than a wavelength of the light emitted by the at least two light sources.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the invention.

(2) FIG. 1 shows a lens according to a first embodiment of the invention and comprising a first plurality of slits in a cross sectional view in a plane perpendicular to a plane in which both a centrally extending axis being perpendicular to a lens axis of the lens and the length and the width of a first plurality of slits extend.

(3) FIG. 2 shows the lens according to FIG. 1 in a cross sectional views in a plane in which both a centrally extending axis being perpendicular to a lens axis of the lens and the length and the width of a first plurality of slits extend.

(4) FIG. 3 shows a perspective view of a lighting device according to the invention comprising a plurality of light sources and a lens according to a second embodiment of the invention and comprising a first and a second plurality of slits.

(5) FIG. 4 shows a schematic view of a collimator comprising a lens according to the invention.

(6) FIGS. 5A and 5B show plots of the intensity distribution of a lighting device according to FIG. 3 in which the lens comprises no slits and in which a part of the surface of the lens adapted for emitting light received from the light sources is 1 degree Gaussian scattering and 10 degrees Gaussian scattering, respectively.

(7) FIGS. 5C and 5D show plots of the intensity distribution of a lighting device according to FIG. 3 in which the lens comprises a first and a second plurality of slits and in which a part of the surface of the lens adapted for emitting light received from the light sources is 1 degree Gaussian scattering and 10 degrees Gaussian scattering, respectively.

(8) FIG. 6 shows a perspective view of a lighting device according to the invention comprising a plurality of light sources and a lens according to a third embodiment of the invention and comprising a first, a second and a third plurality of slits.

(9) FIGS. 7A and 7B show plots of the intensity distribution of a lighting device according to FIG. 6 in which the lens comprises no slits and in which a part of the surface of the lens adapted for emitting light received from the light sources is 1 degree Gaussian scattering and 10 degrees Gaussian scattering, respectively.

(10) FIGS. 7C and 7D show plots of the intensity distribution of a lighting device according to FIG. 6 in which the lens comprises a first, a second and a third plurality of slits and in which a part of the surface of the lens adapted for emitting light received from the light sources is 1 degree Gaussian scattering and 10 degrees Gaussian scattering, respectively.

(11) As illustrated in the figures, the sizes of layers and regions are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of embodiments of the present invention. Like reference numerals refer to like elements throughout.

DETAILED DESCRIPTION

(12) The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

(13) FIGS. 1 and 2 show a lens 1 according to a first embodiment of the invention in mutually perpendicular cross sectional views. The lens 1 comprises a lens axis L, an outer surface 13, a centrally extending axis A being perpendicular to the lens axis L and a first plurality of slits 21, 22, 23, 24.

(14) The first plurality of slits 21, 22, 23, 24 extend entirely in an interior of the lens. The first plurality of slits 21, 22, 23, 24 comprise a length, l, extending perpendicular to the centrally extending axis A, a width, w, extending parallel to the centrally extending axis A and a thickness, t, extending perpendicular to both the length, l, and the width, w. The slits of the first plurality of slits 21, 22, 23, 24 are mutually parallel. The first plurality of slits 21, 22, 23, 24 cover in the embodiment shown a total of about 50% of a plane in which both the centrally extending axis A and the length 1 and width w of the first plurality of slits 21, 22, 23, 24 extendcf. FIG. 2.

(15) Thus, as shown in the figures and irrespective of the embodiment the lens axis L may be seen as a horizontally extending axis, while the centrally extending axis A may be seen as a vertical axis. Likewise, and also irrespective of the embodiment, the plane in which both the centrally extending axis A and the length and width of a given plurality of slits extend may be seen as a vertically extending plane or simply vertical plane.

(16) Adjacent slits of the first plurality of slits 21, 22, 23, 24 are arranged spaced apart with a distance d. Furthermore, the slits of the first plurality of slits 21, 22, 23, 24 are arranged such as to terminate in a distance from the outer surface 13 of the lens 1. Alternatively, the slits of the first plurality of slits 21, 22, 23, 24 may be arranged such as to terminate at the outer surface 13 of the lens 1.

(17) It is noted that irrespective of the embodiment the length, width and thickness of the slits of the respective pluralities of silts as well as the distance between adjacent slits of a plurality of slits may vary. Thus, all slits need not have the same length, width and/or thickness, but may have different lengths, widths and/or thicknesses. Likewise the distance between adjacent slits of a plurality of slits may vary within a plurality of slits and/or between different pluralities of slits.

(18) In FIGS. 1 and 2 two light sources 5, 6 emitting light rays 51, 52, 61, 62 are shown such as to illustrate the path of the light rays 51, 52, 61, 62 through the lens 1. In FIG. 2 only the light source 5 is visible, the other light source 6 being hidden behind the light source 5. The light sources 5 and 6 emit light of two different colors, for example red and blue. The light sources 5 and 6 may be two differently colored LEDs.

(19) The two light sources 5, 6 are arranged off axis with respect to the centrally extending axis A, i.e. on mutually opposite sides of the plane in which both the centrally extending axis A and the length, l, and width, w, of the first plurality of slits 21, 22, 23, 24 extend and in the same distance from the said plane.

(20) The lens 1 is provided with a first surface part or segment 12 adapted for receiving and coupling light into the lens 1 as well as a second surface part or segment 11 adapted for coupling mixed light out of the lens 1. The second surface part or segment 11 of the lens 1 may be diffusing or scattering.

(21) Light emitted by the two light sources 5, 6 is thus coupled into the lens 1 at the first surface segment 12. When propagating through the lens 1, about half of the rays 51, 52 coming from the light source 5 and about half of the rays 61, 62 coming from the light source 6 will hit a slit of the first plurality of slits 21, 22, 23, 24 and be reflected at the slits 21, 22, 23, 24 along the plane in which both the centrally extending axis A and the length, l, and width, w, of the first plurality of slits 21, 22, 23, 24 extendcf. FIG. 1. Similarly, about half of the rays 51, 52 coming from the light source 5 and about half of the rays 61, 62 coming from the light source 6 will not hit a slit of the first plurality of slits 21, 22, 23, 24 and will thus cross the plane in which both the centrally extending axis A and the length, l, and width, w, of the first plurality of slits 21, 22, 23, 24 extendcf. likewise FIG. 1. The light is thus mixed. Finally the thus mixed light is emitted through the second surface part or segment 11 of the lens 1. In case the second surface part or segment 11 of the lens 1 is diffusive or scattering a further mixing of the light will be obtained.

(22) FIG. 4 illustrates schematically a collimator 9 comprising a lens 1 according to the invention. Any lens according to the invention may be employed in a collimator. The lens 1 is arranged centrally in the collimator 9. Also the collimator comprises a TIR surface 91. Thereby light emitted by a light source 5 and propagating though the collimator 9 comprising the lens 1 is eventually emitted as both collimated and mixed light 51, 52.

(23) Turning now to FIG. 3, a perspective view of a lighting device 200 according to the invention is shown. The lighting device 200 comprises four light sources 5, 6, 7, 8 arranged on a base 210 such as a printed circuit board. Two of the light sources, namely light sources 5 and 7, emit light of one specific color, e.g. green, while the other two of the light sources, namely light sources 6 and 8 emit light of another specific color, e.g. red. The lighting device 200 further comprises a second embodiment of a lens 100 according to the invention.

(24) The lens 100 of the lighting device 200 differs from that shown in FIGS. 1 and 2 and described above in that it further comprises a second plurality of slits 31, 32, 33, 34.

(25) The second plurality of slits 31, 32, 33, 34 extend entirely in an interior of the lens. Similarly to the first plurality of slits 21, 22, 23, 24, the second plurality of slits 31, 32, 33, 34 comprise a length extending perpendicular to the centrally extending axis A, a width extending parallel to the centrally extending axis A and a thickness extending perpendicular to both the length and the width. The slits of the second plurality of slits 31, 32, 33, 34 are mutually parallel. The second plurality of slits 31, 32, 33, 34 cover in the embodiment shown in FIG. 3 a total of about 50% of a plane in which both the centrally extending axis A and the length and width of the second plurality of slits 31, 32, 33, 34 extend.

(26) The second plurality of slits 31, 32, 33, 34 furthermore generally extend in an angle different from zero with respect to the first plurality of slits 21, 22, 23, 24. In an embodiment the second plurality of slits 31, 32, 33, 34 and the first plurality of slits 21, 22, 23, 24 extend in such an angle with respect to each other that the second plurality of slits 31, 32, 33, 34 and the first plurality of slits 21, 22, 23, 24 are radially evenly distributed when seen in a plane perpendicular to the centrally extending axis A. In the embodiment shown in FIG. 3, the second plurality of slits 31, 32, 33, 34 extend in an angle of 90 degrees with respect to the first plurality of slits 21, 22, 23, 24.

(27) Adjacent slits of the second plurality of slits 31, 32, 33, 34 are arranged spaced apart with a distance corresponding to the distance d between adjacent slits of the first plurality of slits 21, 22, 23, 24. Alternatively, adjacent slits of the second plurality of slits 31, 32, 33, 34 may be arranged spaced apart with a distance differing from the distance d between adjacent slits of the first plurality of slits 21, 22, 23, 24.

(28) Furthermore, the slits of the second plurality of slits 31, 32, 33, 34 are arranged such as to terminate in a distance from the outer surface 13 of the lens 100. Alternatively, the slits of the second plurality of slits 31, 32, 33, 34 may be arranged such as to terminate at the outer surface 13 of the lens 100.

(29) The four light sources 5, 6, 7, 8 are arranged off axis with respect to the centrally extending axis A. More particularly, the four light sources 5, 6, 7, 8 are arranged pair-wise on mutually opposite sides of the plane in which both the centrally extending axis A and the length, l, and width, w, of the first plurality of slits 21, 22, 23, 24 extend and in the same distance from the said plane, and on mutually opposite sides of the plane in which both the centrally extending axis A and the length and width of the second plurality of slits 31, 32, 33, 34 extend and in the same distance from the said plane, respectively.

(30) Light emitted by the four light sources 5, 6, 7, 8 is thus coupled into the lens 100 at the first surface segment 12. When the light emitted by the four light sources 5, 6, 7, 8 propagates through the lens 100, about half of the rays coming from each of the four light sources 5, 6, 7, 8 will hit a slit of the first plurality of slits 21, 22, 23, 24 or the second plurality of slits 31, 32, 33, 34 and be reflected at the said slit along the relevant planecf. FIG. 3. Furthermore, about a quarter of the rays coming from each of the four light sources 5, 6, 7, 8 will not hit a slit of the first plurality of slits 21, 22, 23, 24 and will thus cross the plane in which both the centrally extending axis A and the length, l, and width, w, of the first plurality of slits 21, 22, 23, 24 extendcf. likewise FIG. 3. Similarly, about a quarter of the rays coming from each of the four light sources 5, 6, 7, 8 will not hit a slit of the second plurality of slits 31, 32, 33, 34 and will thus cross the plane in which both the centrally extending axis A and the length and width of the second plurality of slits 31, 32, 33, 34 extendcf. likewise FIG. 3. The light is thus mixed. Finally the thus mixed light is emitted through the second surface part or segment 11 of the lens 100. In case the second surface part or segment 11 of the lens 100 is diffusive or scattering a further mixing of the light will be obtained.

(31) The light mixing effect of a lens 100 in a lighting device 200 as described above is illustrated by means of FIGS. 5A to 5D. The lighting device 200 comprises two light sources emitting red light and two light sources emitting green light. In FIGS. 5A to 5D the light emitted by the lighting device 200 is represented by grey scale tones such that grey corresponds to red light and white corresponds to green light, while black corresponds to mixed light.

(32) FIGS. 5A and 5B show plots of the intensity distribution of a lighting device of the type shown in FIG. 3 but comprising a lens having no slits and in which a part of the surface of the lens adapted for emitting light received from the light sources is 1 degree Gaussian scattering (FIG. 5A) and 10 degrees Gaussian scattering (FIG. 5B), respectively. As may be seen from FIG. 5A, the color intensity distribution plot made using a lens with no slits clearly shows four areas of virtually unmixed colors, namely two grey and two white areas. FIG. 5B illustrates that even when the part of the surface of the lens adapted for emitting light received from the light sources is scattering, the areas with unmixed colors do not disappear.

(33) FIGS. 5C and 5D show plots of the intensity distribution of a lighting device according to FIG. 3 in which the lens 100 comprises a first plurality of slits 21, 22, 23, 24 and a second plurality of slits 31, 32, 33, 34 and in which a part 11 of the surface 13 of the lens 100 adapted for emitting light received from the light sources 5, 6, 7, 8 is 1 degree Gaussian scattering (FIG. 5C) and 10 degrees Gaussian scattering (FIG. 5D), respectively. FIG. 5C illustrates that when using a lens 100 with slits the colors are well mixed, although some artifacts, shown as a grey dotted shadow in FIG. 5C, appear along the rim of the distribution plot. FIG. 5D illustrates that the said artifacts may easily be smoothened out and the color mixing improved further, when the part 11 of the surface 13 of the lens 100 adapted for emitting light received from the light sources 5, 6, 7, 8 is scattering.

(34) Turning now to FIG. 6, a perspective view of another lighting device 201 according to the invention is shown. The lighting device 201 comprises three light sources 5, 6, 7 arranged on a base 210 such as a printed circuit board. The three light sources emit light of mutually different colors. For instance, a first one of the light sources emits green light, a second one emits red light and a third one emits blue light. The lighting device 200 further comprises a third embodiment of a lens 101 according to the invention.

(35) The lens 101 of the lighting device 201 differs from that shown in FIG. 3 and described above in that it further comprises a third plurality of slits 41, 42, 43, 44.

(36) The third plurality of slits 41, 42, 43, 44 extend entirely in an interior of the lens. Similarly to the first plurality of slits 21, 22, 23, 24, the third plurality of slits 41, 42, 43, 44 comprise a length extending perpendicular to the centrally extending axis A, a width extending parallel to the centrally extending axis A and a thickness extending perpendicular to both the length and the width. The slits of the third plurality of slits 41, 42, 43, 44 are mutually parallel. The third plurality of slits 41, 42, 43, 44 cover in the embodiment shown in FIG. 6 a total of about 50% of a plane in which both the centrally extending axis A and the length and width of the third plurality of slits 41, 42, 43, 44 extend.

(37) The third plurality of slits 41, 42, 43, 44 furthermore generally extend in an angle different from zero with respect to both the first plurality of slits 21, 22, 23, 24 and the second plurality of slits 31, 32, 33, 34. In an embodiment the third plurality of slits 41, 42, 43, 44, the second plurality of slits 31, 32, 33, 34 and the first plurality of slits 21, 22, 23, 24 extend in such an angle with respect to each other that the third plurality of slits 41, 42, 43, 44, the second plurality of slits 31, 32, 33, 34 and the first plurality of slits 21, 22, 23, 24 are radially evenly distributed when seen in a plane perpendicular to the centrally extending axis A. In the embodiment shown in FIG. 6, the third plurality of slits 41, 42, 43, 44 extend in an angle of 60 degrees with respect to both the second plurality of slits 31, 32, 33, 34 and the first plurality of slits 21, 22, 23, 24.

(38) Adjacent slits of the third plurality of slits 41, 42, 43, 44 are arranged spaced apart with a distance corresponding to the distance d between adjacent slits of the first plurality of slits 21, 22, 23, 24. Alternatively, adjacent slits of the third plurality of slits 41, 42, 43, 44 may be arranged spaced apart with a distance differing from the distance d between adjacent slits of the first plurality of slits 21, 22, 23, 24 and/or differing from the distance between adjacent slits of the second plurality of slits 31, 32, 33, 34.

(39) Furthermore, the slits of the third plurality of slits 41, 42, 43, 44 are arranged such as to terminate in a distance from the outer surface 13 of the lens 101. Alternatively, the slits of the third plurality of slits 41, 42, 43, 44 may be arranged such as to terminate at the outer surface 13 of the lens 101.

(40) The three light sources 5, 6, 7 are arranged off axis with respect to the centrally extending axis A. More particularly, the three light sources 5, 6, 7 are arranged evenly distributed around the vertically extending central lens axis A. More particularly, and in the embodiment shown, the light source 5 is arranged directly under the first plurality of slits 21, 22, 23, 24, the light source 6 is arranged directly under the second plurality of slits 31, 32, 33, 34 and the light source 7 is arranged directly under the third plurality of slits 41, 42, 43, 44.

(41) Light emitted by the three light sources 5, 6, 7 is thus coupled into the lens 101 at the first surface segment 12. When light propagating through the lens 101, about half of the rays coming from each of the three light sources 5, 6, 7 will hit the slits of the first plurality of slits 21, 22, 23, 24, the second plurality of slits 31, 32, 33, 34 and the third plurality of slits 41, 42, 43, 44 in such a way that each plurality of slits causes a virtual overlap of light emitted by two neighboring light sourcescf. FIG. 6. The light is thus mixed. Finally the thus mixed light is emitted through the second surface part or segment 11 of the lens 101. In case the second surface part or segment 11 of the lens 101 is diffusive or scattering a further mixing of the light will be obtained.

(42) The light mixing effect of a lens 101 in a lighting device 201 as described above is illustrated by means of FIGS. 7A to 7D. The lighting device 201 comprises three light sources one emitting red light, one emitting green light and one emitting blue light. In FIGS. 7A to 7D the light emitted by the lighting device 201 is represented by grey scale tones such that white corresponds to red light, light grey to blue light and dark grey corresponds to green light, while black corresponds to mixed light.

(43) FIGS. 7A and 7B show plots of the intensity distribution of a lighting device according to FIG. 6 but comprising a lens with no slits and in which a part of the surface of the lens adapted for emitting light received from the light sources is 1 degree Gaussian scattering (FIG. 7A) and 10 degrees Gaussian scattering (FIG. 7B), respectively. As may be seen from FIG. 7A, the color intensity distribution plot made using a lens with no slits clearly shows areas of virtually unmixed colors, namely light grey, dark grey and white areas. FIG. 7B illustrates that even when the part of the surface of the lens adapted for emitting light received from the light sources is scattering, the areas with unmixed colors do not disappear.

(44) FIGS. 7C and 7D show plots of the intensity distribution of a lighting device 201 according to FIG. 6 in which the lens 101 comprises a first plurality of slits 21, 22, 23, 24, a second plurality of slits 31, 32, 33, 34 and a third plurality of slits 41, 42, 43, 44 and in which a part 11 of the surface 13 of the lens 101 adapted for emitting light received from the light sources is 1 degree Gaussian scattering (FIG. 7C) and 10 degrees Gaussian scattering (FIG. 7D), respectively. FIG. 7C illustrates that when using a lens 101 with slits the colors are well mixed, although some distinct unmixed areas remain. FIG. 7D illustrates that the remaining distinct unmixed areas may be smoothened out and the color mixing improved further, when the part 11 of the surface 13 of the lens 100 adapted for emitting light received from the light sources 5, 6, 7, 8 is scattering.

(45) Finally, it is noted that lenses according to the invention are simple to manufacture. For example, a lens 1 according to the first embodiment and as shown in FIGS. 1 and 2 may be made by combining two half lenses. A first example of such two half lenses are one half with protrusions and one part with a flat surface, where the protrusions and the flat surface is joined, e.g. glued, together. A second example of such two half lenses are two parts both having a flat surface, where the flat surfaces are joined, e.g. glued, together with a multilayer or perforated foil in between. A third example of such two half lenses are two parts both having a flat surface, where the two flat surfaces are glued together with thick glue dots. Lenses 100 and 101 according to the second and third embodiments, respectively may be manufactured in an analogous manner by joining four lens parts or six lens parts, respectively.

(46) Also, it is feasible to provide a lens according to the invention with more than three pluralities of slits such as to enable mixing of light from an even larger number of light sources, possibly emitting light of even more different colors.

(47) The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

(48) Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.