EP0250191A2

EP0250191A2 - Improved reflector for pre-focussed reflector lamps

Info

Publication number: EP0250191A2
Application number: EP19870305285
Authority: EP
Inventors: Anthony Leonard Isaacs
Original assignee: Thorn EMI PLC
Current assignee: Thorn EMI PLC
Priority date: 1986-06-19
Filing date: 1987-06-15
Publication date: 1987-12-23
Also published as: GB8614988D0

Abstract

A surface of a lamp reflector is partially covered with facets which have substantially elliptical perimeters, there being regions of unfacetted surface between some or all of the facets. The perimeter of each facet may touch that of its immediate neighbours.

Description

The present invention relates to reflectors of the kind commonly used with pre-focussed reflector lamps in particular, but not solely, low voltage tungsten halogen lamps.
Reflectors of this kind are usually parabolic or elliptical in shape and, as is well known in the art, are often faceted. One purpose of this faceting is to prevent imaging of the lamp filament from distorting what would otherwise be a reasonably uniform illumination.
Conventionally, these facets have been rectangular in shape, but we have also advantageously used hexagonal/diamond shaped facets. The hexagonal/diamond pattern is derived from a pattern of intersecting curved shapes which are, to a first mathematical approximation, ellipses. The pattern is first created on a mould from which the reflectors are then made. Each grinding into the surface of the mould produces, to a first mathematical approximation, an ellipse. These ellipses form circular and radial patterns of ellipses and it is the intersection of the ellipses which are not on the same radial or circular bands which give the final hexagonal/diamond pattern. Thus at least part of the surface of the reflector is completely covered with hexagonal/diamond shapes facts, each facet being in contact with a different neighbouring facet on each of its sides. In addition, as a consequence of the way the facets are created, each hexagonal/diamond shaped facet forms a dimple in the surface of the reflector; the depth of the dimple being defined as the normal deviation of the centre of the facet from the original surface. Thus, while facets of this type have the desired effect of defocussing the filament, they also distort the surface of the reflector by their very presence; no part of the original surface is left undistorted. This causes excessive scattering of the light from the lamp.
It is an object of the invention to produce a facet which will have the desired effect of defocussing the filament and at the same time, minimise the surface distortion and the scattering of light which it may cause.
According to the present invention there is provided a lamp reflector suitable for a prefocussed lamp, the lamp reflector having a surface which is partially covered with facets there being a region of unfacetted surface between a facet and at least one of its immediate neighbours.
The said facets may be derived from shapes which are, to a first mathematical approximation, ellipsoidal. The ellipsoidal shapes either touch, do not touch at all or intersect such that the resulting facets respectively touch at a point, do not touch at all or touch at a line to leave some of the original surface between some or all of the facets; the original surface being defined as the surface of the reflector as it was before being faceted. Thus the resulting surface conforms more closely to the original surface than do those surfaces obtained using the prior art techniques; it is therefore less distorted and so gives less scattering of the light from the filament, while still having the desired effect of defocussing the filament.
In a preferred embodiment of the invention the facets come into contact with each other, but do not intersect, resulting in a pattern of facets each having a perimeter which, to a first mathematical approximation, is an ellipse, with the spaces between the facets comprising the original surface. The facets may be offset or alternatively, they can be in line. In either case each band or row of facets is, in general, progressively more shallow than the preceding band or row as the pole of the reflector is approached, the depth of a facet being defined as the normal deviation of the centre of the facet from the original surface. However, nearer to the pole, the depth of the facets can become so shallow as to be insignificant and this tends to make manufacture of these facets difficult.
In an alternative embodiment of the invention faceting may cease at a point where manufacture has become difficult or, preferably, an increased facet depth can be used by reducing the number of facets in the row or rows closest to the pole of the reflector. A preferred reduction in the number of facets is in the ratio of 2:1, which leaves the overall facet pattern uniform.
In another embodiment of the invention the aforementioned near ellipses do not come into contact with each other at all, giving the reflector a surface which is covered with non-touching near elliptically shaped facets; where the areas between the facets are parts of the original, undistorted surface.
In yet another embodiment of the invention the near ellipses do intersect, but not sufficiently to remove completely the original undistorted surface between some or all of them.
For each of the embodiments of the invention mentioned above existing manufacturing techniques, of the kind used to produce the hexagonal/diamond faceted reflectors, may be used to create the desired effect.
In order that the invention may be clearly understood and readily carried into effect, specific embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, of which

Figure 1 is a schematic front view of a hexagonal/diamond faceted reflector.
Figure 2 is a schematic front view of a hexagonal/diamond facted reflector, which shows how the facets are created.
Figure 3 is a schematic front view of a preferred embodiment of the present invention.
Figure 4 is a schematic front view of another embodiment of the present invention.

Figure 1 is a schematic front view of the hexagonal/diamond faceted reflector already known in the art. Figure 2 shows the pattern of intersecting curved shapes which are, to a first mathematical approximation ellipses and which, although themselves not visible on the finished article, create the hexagonal/diamond faceted pattern shown in Figure 1. Comparison of Figures 1 and 2 shows quite clearly how the desired effect is achieved.
The facet pattern depicted in Figure 3 represents a preferred embodiment of the invention. Here the curved shapes, which are ellipses to a first mathematical approximation, come into contact with each other, but substantially do not intersect and the spaces between them are the original surface of the reflector. Because the ellipses used to form this facet pattern do not intersect, they can be made smaller than the necessarily intersecting ellipses which are used to create the known hexagonal/diamond facet pattern. Since the ellipses, which form the elliptically faceted pattern of this embodiment of the invention, are smaller than those used to create a hexagonal/diamond faceted pattern, on a reflector of the same size, the dimples in the original surface formed by these elliptically shaped facets are not as deep as those formed by the hexagonal/diamond facets; that is, the normal deviation of the centre of the facet from the original surface is not as great. Consequently, as a result of this and the fact that parts of the original surface are left undistorted in the areas between the ellipses, the elliptically faceted surface of the reflector, as shown in Figure 3, more closely resembles the original surface than does the hexagonal/diamond faceted surface of the prior art and so cause less scattering of the light from the lamp; while at the same time still having the desired effect of defocussing the lamp filament. In figure 3 the facets are shown offset, but the desired effect can equally well be obtained if the facets are in line.
The facets shown in Figure 3 become progressively shallower closer to the pole of the reflector until their depth becomes so shallow as to be insignificant; this makes manufacture of these facets difficult towards that point. Figure 4 shows an alternative embodiment of the invention which allows the facet depth to be increased by reducing the number of facets from that row onwards. In Figure 4 the reduction of the number of facets in each row is in the ration of 2:1, which leaves the overall facet pattern uniform. However, a different ratio may be desired.
Other embodiments of the invention using curved shapes other than near ellipses will be apparent to those skilled in the art.
Thus the invention described herein provides a faceted reflector preferably, though not necessarily, for use with a pre-focussed reflector lamp; which achieves the desired effect of defocussing the filament of the lamp but, as a result of reduced surface distortion, gives less scattering of the light from the lamp than is normally associated with faceted lamp reflectors of the kind known in the art. The invention has the additional advantage that existing manufacturing techniques may be used for its production.

Claims

1. A lamp reflector suitable for a prefocussed lamp, the lamp reflector having a surface which is partially covered with facets there being a region of unfacetted surface between a facet and at least one of its immediate neighbours.

2. A lamp reflector according to Claim 1 wherein each said facet has a substantially elliptical perimeter.

3. A lamp reflector according to Claim 1 or Claim 2, there being a continuous region of unfacetted surface around some or all of the facets.

4. A lamp reflector according to Claim 1 or Claim 2 wherein the perimeter of a facet touches that of its immediate neighbours.

5. A lamp reflector according to Claim 5 wherein the perimeter of each facet touches that of its immediate neighbours.

6. A lamp reflector according to Claim 1 wherein some of the facets occupy bands which extend radially from the pole of the reflector.

7. A lamp reflector according to Claim 1 wherein the facets occupy circular bands, each containing the same number of facets.

8. A lamp reflector according to Claim 1 wherein the facets occupy circular bands, and the band or bands closest to the pole of the reflector contain fewer facets than do bands further from the pole.

9. A lamp reflector according to Claim 8 wherein the band or bands closest to the pole contain half the number of facets contained by the bands further from the pole.