Reflector for Uniformly Illuminating an Area, Particularly a Film Window of a Film or Slide Projector, and Reflector Lamp

U.S. Ser. No. 275,150, filed June 19, 1981, Rakitsch and Bodmer, assigned to the assignee of the present application.

Reference to related U.S. Pat. Nos.: 4,021,659, 4,035,631, and to; German Patent Disclosure Document Nos.: DE-OS 21 48 478, to which U.S. Pat. No. 3,758,770 corresponds, DE-OS 23 63 378, to which U.S. Pat. No. 3,825,742 corresponds.

The present invention relates to a reflector structure which is used for illuminating an area, and more particularly for illuminating a film window for a film or slide projection system in which the film window is illuminated as uniformly as possible with the aid of the reflector.

With an areal light source, for instance a coiled filament and a smooth reflector surface, local brightness variations occur in the film window due to imaging of the filament. It has been proposed to roughen the reflector surface by knurling or facetting. A reflector having a surfce of numerous small reflective areas is described for instance in the U.S. Pat. Nos. 4,021,659 and 4,035,631. Other surface structures are described, among others, in German Disclosure Document Nos. DE-OS 21 48 478 and DE-OS 23 63 378 to which U.S. Pat. No. 3,758,770, Morasz and 3,825,742, Levin, correspond. The structures of the reflector surface described therein cause the beams to fan out or spread more than with a smooth reflector surface. Thus, the image of the filament structure in the film window is blurred, but a certain non-uniformity in the film window illumination remains nevertheless. As a result of the increased stray light, the utilized light flux falls. This has to be compensated by increasing the power input of the lamp; this, in turn, causes a higher temperature in the film window than with smooth reflector surfaces. The common feature of all these solutions is that the reflectors have one contour only.

It is an object to provide a reflector with a surface by which a good uniformity of the film window illumination may be obtained and stray light is avoided as far as possible.

Briefly, in accordance with the invention, the reflector for illuminating an area is characterized in that the reflector has at least two contours which are rotation-symmetrical with respect to the optical axis of the total system, their generatrices follow elliptical curves, i.e. sectional cuts through a cone, and the reflector is divided into zonal regions with successive regions lying on different contours. The focal points of the generatrices may coincide or may be displaced relatively to each other in the optical axis or on a parallel to the optical axis. Depending on the purpose the reflector is to serve, the contours may follow identical or non-identical elliptical curves. The parts forming the different contours must be present with a certain relative surface area. For example, when two contours A and B are used, the area proportions F.sub.A and F.sub.B of the total reflective area F should satisfy the equation F.sub.A =xF and F.sub.B =(1-x) F, with 0.2.ltoreq.x.ltoreq.0.8. Depending on the desired light distribution, the ratios of the areas of the different contours may be from between 1:4 and 1:1. The succession of the zones of different contours also depends on the application purpose. The reflector shall have at least two zonal regions which are rotation symmetrical with respect to the optical axis. The reflector area of a contour is formed by the rotation of a generatrix which follows the curve of a elliptical curve. The geometrical axis of at least one of the elliptical curves may be inclined relative to the optical axis, that is, include an angle with the optical axis of the total system.

By matching the different contours of the elliptical curves to one another, for instance such that the contour of one conical section yields a spot-type illumination and the contour of the other conical section yields a saddle-type illumination, a highly uniform illumination of the film window may be achieved.

With the reflector shapes of the invention which reflect the image of the luminous element by means of several reflector contours into the film window, the uniformity may be improved both in projection and in illumination. The decisive factor is that, in projection, the uniformity of the 1st order is improved over the entire projection screen and not only the uniformity of the 2nd order, i.e. small local variations in film window illumination as with the initially described reflectors which have numerous small reflective areas on one contour. To explain the above in more detail: the uniformity of the 1st order describes the shape of the illumination intensity across the projection screen, i.e. the basic shape of the curve; the uniformity of the 2nd order describes the slight variations in illumination intensity caused by images of lamp structures or other attendant phenomena in the film window arising within the basic illumination curve shape. An explanation for this improvement may be that a reflector whose contour corresponds to a single elliptical curve relationship can always primarily project only one object point into one image point. When, however, two contours are used, two object points may be projected into one image point or, conversely, on object point may be projected into two image points. When several contours are used, a corresponding number of associations of object point/image point may be determined.

The contours may be distributed over the reflector in an arbitrary arrangement. It is important, however, that the overall length of the boundaries of the respective reflector contours should be as small as possible and the mold slants, e.g. for the blank manufacture, require only a small angle in space, with respect to the axis of the reflector.

FIG. 1 shows a perspective top view of the reflector;

FIG. 2 shows a section through the reflector;

FIGS. 3a to 3c illustrate relative illumination intensity distributions;

FIG. 4 shows a diagram which illustrates schematically the position of the coordinate systems used for the elliptical curves.

The reflector 1 in FIG. 1 is preferably made of borosilicate glass and is divided into five zones. The three zonal areas 2a to 2c have a contour K.sub.1 (FIG. 2). The alternately arranged zonal areas 3a and 3b have a different contour K.sub.2. The path of rays is illustrated in FIG. 2. FIG. 3a shows the illumination intensity distribution of contour 1 which yields a spot-type illumination. Here and in the following FIGS. 3b and 3c, W denotes the widest dimension of the area to be illuminated. FIG. 3b shows the illumination intensity distribution of contour 2 which yields a saddle-type illumination characteristic in the film window. In FIG. 3c is plotted the composite, improved illumination curve generated by the reflector which is composed of the two contour parts. In this example, the parts forming the different contours have a surface area ratio of 5:3 (area K.sub.1 to area K.sub.2). The widest dimension of the area to be illuminated, for example of a film window, is shown by the arrow W in FIG. 3c, from which the essential uniform illumination obtained throughout its dimension will be apparent.

In the diagram of FIG. 4, the two elliptical zones or contours are shown in their position to the optical axis of the total system. The coordinates X.sub.01, Y.sub.01 belong to the center of the inclined coordinate system X', Y' which is associated with the contour 1. Accordingly, the center of the coordinate system associated with the contour 2 is determined by X.sub.02, Y.sub.02. The coordinate system X', Y' is inclined relative to the coordinate system X, Y by the angle .phi., the X-axis being given by the optical axis of the total system. F.sub.1 and F.sub.2, respectively, designate the focal points of the two elliptical contours or curves.

The elliptical zones are placed in the form of bands, or strips (see FIG. 1) in the reflector, which is essentially cup-shaped.

Reflectors of this type having several contours are particularly suited for lamps with small luminous elements, large film windows and the use of only slightly opened ojectives. Good results are obtained, for instance, with the following exemplary embodiment: the reflector comprises two zonally arranged contours which follow identical elliptical curves. The reflector diameter is 50 mm. The equations for the contors 1 and 2 are: X'.sup.2 /41.25.sup.2 +Y'.sup.2 /24.65.sup.2 =1, with X.sub.01 =41.24 mm and Y.sub.01 =-0.37 mm (for contour K.sub.1) and X.sub.02 =43.54 mm and Y.sub.02 =-0.37 mm (for contour K.sub.2) and .phi.=1.42.degree. (FIG. 4). The zone adjacent the apex is associated with the contour K.sub.1. Its area is about 4% of the total reflector surface. A zone associated with the contour K.sub.2 then follows, and has 10% of the reflector surface. This is followed alternately by zones of the contours K.sub.1 of 16%, K.sub.2 of 25% and again K.sub.1 of 45% (see also FIGS. 1 and 2). The optical mounting distance is 35 mm, the film window has the dimensions 9.60 mm.times.7.0 mm (16 mm-projection), the image forming objective is 1:1.3/35. The lamp bulb used with, and for example sealed to, this reflector is a halogen cycle incandescent lamp of 24 V/250 W with a filament diameter of 2.6 mm and a filament length of 4.6 mm. The lamp bulb L is shown only schematically in FIG. 2, positioned with its filament F on the optical axis including the focal planes of the two contours.