Optical element

An optical element comprises zones of different refraction power or curvature in order to achieve a desired pattern of brightness distribution. In Particular, a darker spot in a central illumination region of an illuminating arrangement including a cold light reflector lamp the light of which is supplied over a fiber optic light guide should be avoided. The optical element has a rotational symmetric annular zone of low or no curvature the outer ring diameter of which being larger and the inner ring diameter being smaller than the diameter of the light exit surface of the fiber optic light guide. A second rotational symmetric zone has a stronger curvature than the first zone. This second zone is situated within the inner ring diameter of the first zone. Both zones face the light exit surface of the light guide. A third zone has a curvature different to the curvatures of the first and second zones, and is situated at the light exit side of the optical element. This third zone is opposite both to the second zone in a rotational symmetric manner and, at least in part, to the first zone in radial relationship.

FIELD OF THE INVENTION 
The present invention relates to an optical element comprising zones of 
different curvature in order to avoid reduced brightness in a central 
illumination region of an illuminating arrangement including a cold light 
reflector lamp or metal oxide vaporized mirror lamp, the light of which 
being supplied to a fiber optical light guide. 
BACKGROUND OF THE INVENTION 
Lamps of the above-mentioned type comprise a reflector through the central 
region of which the lamp's base is passing. The glass bulb of the lamp 
itself has a seal stud at its end remote from the reflector. This 
constructive arrangement affects, however, the emission characteristic of 
the lamp in a disadvantageous manner in that the light intensity is 
significantly smaller in the central region than in regions situated more 
off-side the optical axis. If light emitted from such a reflector lamp is 
guided to an object to be illuminated, for example, through a fiber 
optical light guide, the same distribution characteristic of light 
intensity will, in principle, appear at the exit surface of this light 
guide. The maximum solid angle of light emission at light's exit from the 
light guide will be limited precisely by the numerical aperture of the 
glass fiber. 
The solid or special angle of light emission of reflector lamps described 
above amounts about to 70.degree., and glass fiber light guides of 3 mm, 5 
mm or 8 mm are used, for example, to guide the light to the object to be 
illuminated. The dark central spot occurring due to the constructive 
design of the reflector lamp described above will appear at the exit of 
the light guide the more significantly, the smaller the active diameter of 
the bundle of the fiber optic light guide is. This disadvantageous effect 
is utterly spoiling, for instance, when illuminating an object to be 
examined by microscope. Such a brightness distribution is especially 
disadvantageous with light guides comprising fiber bundles which run in 
common on the light entrance side, but are separated at the light exit 
side, i.e. which are then divided into a plurality of light guides. 
In order to avoid a dark central spot, it has already been suggested to 
arrange the reflector lamp inclined with respect to the axis of the light 
entrance surface of the light guide. This, however, results merely in a 
size reduction of the dark central spot. Even with an inclined lamp, the 
above-mentioned dark spot appears, some times due to tolerances in 
manufacture of the reflector lamp. Moreover, a special drawback will occur 
with small active diameters of the light guide, even with an inclined 
lamp, in that the improvement in brightness distribution in the central 
region of an illuminated surface is rather insignificant. 
Furthermore, it has been suggested to arrange a wedge-like optical element 
in the region between the reflector lamp and the entrance surface of the 
light guide in order to avoid a dark central spot. Although this known 
suggestions results in an improvement in brightness distribution in the 
central region, the average light intensity over the surface to be 
illuminated will be reduced significantly by such a measure. The reason 
for this disadvantageous effect is that, although some light intensity is 
transferred into the central angular region by wedge-like elements, light 
is also directed concurrently to the light guide in an angular region 
which lies outside its numerical aperture and, therefore, cannot be 
transferred any further. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to avoid the drawbacks of the 
prior art and, more particularly, to achieve an optimal distribution of 
brightness at an illuminated surface, especially by avoiding a dark spot 
in a central region of the illuminated surface. For in many applications, 
such as in microscopy, it is required to have light of at least the same 
brightness available within the central region of an illuminated surface 
as in more external regions of the illuminated surface. 
Therefore, in accordance with the invention, an optical element of the kind 
described is suggested wherein this optical element comprises a first 
annular, rotational symmetric zone having a small or no curvature, the 
outer ring diameter of this zone being larger and its inner ring diameter 
being smaller than the diameter of the light exit surface of the fiber 
optic light guide; a second rotational symmetric zone of a stronger 
curvature than the curvature of the first zone, this second zone being 
situated within the inner ring diameter of the first zone and the two 
zones facing the exit surface of the light guide; and a third zone of a 
curvature different with respect to the curvatures of the first and second 
zones which is situated at the light exit side of the optical element, 
this third zone being arranged opposite both to the second zone in 
rotational symmetric relationship and, at least partially, to the first 
zone in radial relationship. By suitably selecting the dimensions of the 
zones and their curvatures, thickness' and diameters, it is achieved to 
transfer light from the angular regions of higher intensity to solid 
angular regions close to the axis at the light exit side after the optical 
element, on the one hand, and to reduce significantly the maximum solid 
angle of the light cone exiting the optical element, on the other hand. 
Thus, the radiation of the optical element is influenced in a favorable 
manner in such a way that a predetermined brightness distribution of the 
surface to be illuminated is achieved. 
According to a preferred embodiment of the invention, it is suggested that 
the second zone forms a concave lens surface, and the third zone forms a 
convex lens surface. By such a combination of optically effective zones at 
both sides of the optical element, darkness in the central region, the 
so-called dark spot, will already be reduced by such a simple design of 
the optical element. 
In order to be able to combine mechanically the optical element with an 
illuminating arrangement or the light guide, particularly in a 
predetermined position, it is suggested, according to another preferred 
embodiment, that a, preferably cylindrical, wall is provided at the outer 
edge of the optical element, outside the first zone, which extends in the 
direction of the rotational axis of the optical element and which 
tensionally and releasably embraces the light guide as a holder.

DETAILED DESCRIPTION OF THE DRAWINGS 
In FIG. 1, 1 designates a light guide which forms part of an illuminating 
arrangement not shown being, for example, equipped with a halogen cold 
light reflector lamp. Such a reflector lamp has a radiation characteristic 
which has a significantly lower light intensity in a central region as 
compared with the remaining regions. This is due to a seal stud of the 
lamp bulb and to the lamp's base passing through the reflecting mirror. 
Light emitted by the reflector lamp passes through the light guide 1 and 
is used, for example, for illuminating an object (not shown) to be viewed 
by a microscope. In principle, the same characteristic showing a 
non-uniform brightness distribution will result at the light guide's exit 
2. The maximum solid angle of light emitted will be limited precisely by 
the numerical aperture of the glass fiber of the light guide. 
A dark central spot within a field to be illuminated, due to the 
significantly lower light intensity in the central region, is the more 
clearly visible the smaller the active bundle diameter of the glass fibers 
of the light guide is. For illuminating the object field of a microscope, 
glass fiber light guides are used which have an active diameter of 3 to 8 
millimeters only. Such a dark central spot will also appear if a fiber 
bundle of a light guide is subdivided into a plurality of fiber bundles, 
e.g. into two or three fiber bundles, for illuminating an object. 
In order to avoid or to eliminate the central dark spot, an optical element 
3, that faces the light exit side 2, is arranged within the path of rays 
of the fiber optic light guide 1. 
The optical element 3 is formed as a rotational symmetric light dome of 
glass or light transmissive plastic material. The arrangement of the 
optical element 3 before the exit side 2 of the light guide 1 is such that 
the rotational axis 4 of the optical element 3 is perpendicular to the 
exit surface 2 of the light guide 1 and is aligned with the axis 5 of the 
light guide 1. 
At the light entrance side of the optical element 3, there are two 
optically differently effective zones 6, 7, whereas at the light exit side 
of the optical element 3 a third optically effective zone 8 is provided. 
Thus, the zones 6, 7 and 8 constitute optically effective surfaces. At the 
light entrance side of the optical element 3, the zone 6 of the embodiment 
shown by way of example is an annular plane surface for marginal rays 9, 
9' emitted from the light guide 1 of the illuminating arrangement. The 
marginal rays 9 leave the optical element 3 as marginal rays 9' over the 
curved and optically effective surface of the zone 8 at the light exit 
side of the optical element 3. 
Thus, it will be recognized by those skilled in the art that by choosing an 
appropriate refractive power, i.e. curvature thickness, transmissive 
material etc., of the three zones, which, in general, will have different 
curvatures, a desired brightness distribution can be achieved. For by 
mathematically determining and dimensioning the curvatures, thickness' or 
diameters of the optical element 3, light is transferred from the angular 
regions of higher intensity towards those angular regions which are close 
to the axes 4 and 5 behind the optical element 3, as may be seen from the 
path of marginal rays 9 and 9'. In addition, by cooperation of the curved 
surface of zone 7 with that of zone 8, which is also curved, but generally 
with a different curvature, the maximum solid angle of the light cone 
emitted from the optical element will be significantly reduced, as may be 
seen from the path of inner rays 10 and 10'. From the path of exiting rays 
9' and 10', it may also be clear that, just for the purpose of avoiding a 
dark central spot, it is convenient if the refractive power from the outer 
zone 6 towards the region of the rotational axes 4, 5 is decreasing. 
FIG. 2 shows a plot graphically illustrating the effect of the optical 
element 3 in comparison with light emitted by the light guide 1 without 
using an optical element 3 after it. In this diagram, the relative 
illumination measuring values E are shown as a curve in relation to the 
radii R of the illuminated object field. Therein, curve a illustrates the 
relative illumination measuring values without any optical element, curve 
b are the evaluated or weighted relative average value of illumination 
over 40 mm without any optical element, curve c the relative illumination 
measuring values when using the optical element 3, and curve d the 
evaluated or weighted relative average value of illumination over 40 mm 
when using the optical element 3. 
These curves a to d are defined by the measuring values of illumination 
intensity listed in the table of FIG. 3. For carrying out these 
measurements, a light guide of an active diameter of 5 mm was used. The 
relative measurement of the illumination intensities was made in a 
distance of 60 mm. The light sensitive measuring surface of the photometer 
employed had a diameter of 2 mm. Measurements were carried out within a 
measuring region of a radius R=30 mm, the evaluation being made over a 
radius r=20 mm. 
From the diagram according to FIG. 2 and its curve a, the central dark 
region, appearing when no optical element 3 is used, is clearly visible. 
If the optical element 3 is arranged in the manner shown in FIG. 1, curve 
c shows clearly that the central dark region does no longer exist. In 
addition, the weighted average value of illumination intensity is, in 
accordance with curve d, substantially higher than without having arranged 
an optical element 3 (curve b). 
The invention is not limited to the above described embodiment. For 
example, it may be suitable in dependence on the light emission 
characteristic of a respective lamp or at the exit side of the light guide 
1 to modify the optically effective zones of the element 3. Thus, it may 
be convenient to provide more than two optically differently effective 
zones either at the light entrance side of the optical element 3 and/or at 
the light exit side of the optical element 3. Instead of an annular plane 
surface of zone 6, a curved surface may be provided also in this region in 
order to change an emission characteristic to comply with a special object 
or task. 
Furthermore, a continuous or a discontinuous transition of adjacent 
optically effective zones may be chosen in accordance with the brightness 
distribution requested for an illuminated object field. In this respect, 
the upper part of FIG. 1 shows a discontinuous, i.e. sudden, transition 
from zone 7 to zone 6, while the lower part shows a continuous or gradual 
transition 7'. It is clear that such continuous or discontinuous 
transitions can also be provided at the light exit side of the optical 
element 3 provided there are more than one zone 8 on this side. 
In order to connect the optical element 3 to the light guide 1, cylindrical 
wall portions 11 may, for example, be provided at the outer edge of the 
optical element 3, preferably being integral with the material of the 
optical element 3, which extends in the direction of the rotational axes 
4, 5, and which enables putting the optical element 3 onto the light guide 
1 or a mount 12 thereof in a socket-like manner. At the inner surface of 
wall portions 11, one or more notches or the like for engagement with 
respective grooves in the mount 12 may be provided which enable fixed, but 
releasable fastening of the optical element 3 to the light guide 1 or its 
mount 12 in at least one predetermined distance. It is clear that instead 
of these crown-like individual wall portions strips 11, preferably 
angularly uniformly distributed over the periphery of the optical element 
3, a substantially cylindrical wall, may extend parallel to the axes 4, 5. 
The advantage of individual strips, however, is that they provide a 
springy, resilient engagement with the mount 12. 
The optical element 3 may also be made of colored or stained glass or other 
transmissive material if the use of colored light is requested for some 
application in microscopy material investigation or the like. Instead of 
shaping the optical element 3 as a dome, any other suitable structure and 
shape may be chosen. The optical element according to the invention may be 
used to achieve any distribution of light intensity desired and is not 
restricted to avoiding a dark central spot. 
Optical elements, mainly when made of plastic material, may be produced by 
injection molding in a simple way in which case aspherically curved zones 
may easily be provided.