Projection objective

Description

FIELD OF THE INVENTION

The invention relates to a projection objective having a variable focal length, preferably serving to image tilting mirror matrices or reflecting and/or transmitting LCDs, more specifically the invention relates to a projection objective comprising three groups of lenses arranged on a common optical axis, wherein, starting from the side facing the projection screen, the first lens group, serving the purpose of focussing, and the second lens group, serving the purpose of varying the focal length, are arranged on the optical axis in a movable manner, and the third lens group is stationary.

BACKGROUND OF THE INVENTION

Generally known projection objectives having variable focal lengths and used especially for images of the aforementioned type, as described, for example, in EP 058 651 B1 and U.S. Pat. No. 5,644,435, differ from each other essentially in the number of lenses, their arrangement and the technical construction data in connection with the conditions applicable to the optical system.

Since it is desirable for applications, in most cases, to vary the size and location of the image, thus adjusting the projection to the most diverse spatial dimensions, use is made mainly of zoom projection systems.

Therefore, projection objectives consist mainly of a zoom device, a compensating device and a focusing device of the type known from photographic lenses.

As a result in order to achieve a high image quality, such systems comprise many lens elements, are structurally very complex and, thus, very expensive. There is a need in the art for a simple, inexpensive projection objective including relatively few lens elements.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention, to provide a projection objective of the above-mentioned type, which reduces the technical complexity of its production due to a small number of optical elements having minimized dimensions, while simultaneously ensuring a good imaging performance.

According to the invention, this object is achieved in that the following conditions are met:
1.0 h max<dG2-G3<1.5 h max
and
s≦10 mm,
wherein h max is the maximum object height, dG2-G3 is the distance between lens group G2 and lens group G3 in a first position, and s is the object-side intercept distance. That is, s is the distance from the object side vertex of lens group G3 to the object along the optical axis.

The projection objective according to the invention comprises relatively few optical elements and is, thus, very inexpensive to manufacture.

In order to improve its imaging performance, the projection objective is conveniently designed such that the following conditions are met:
1.2 fl<fG1<1.7 fl,
0.7 fl<fG2<1.1 fl,
1.5 fl<fG3<2.0 fl,
wherein

• • fl=the total focal length of the objective in a first position,
  • fG1=the absolute value of the focal length of the first lens group G1,
  • fG2=the absolute value of the focal length of the second lens group G2, and
  • fG3=the absolute value of the focal length of the third lens group G3.

For example, one embodiment of the invention consists in that, starting from the side facing the projection screen,

• • the first lens group consists of a negative meniscus, a positive meniscus and a negative lens,
  • the second lens group consists of a negative meniscus or of a negative assembly which consists of a positive meniscus and of a negative meniscus, a positive lens and a positive assembly which consists of a positive lens and of a negative lens, and
  • the third lens group consists of at least one positive lens.

Advantageous further embodiments of the projection objective according to the invention consist in that the following conditions are met:
2.0<L/fl<30
1.68<navg<1.74
40.0<vavg<44.0,
wherein

• • L=the entire length of the objective,
  • navg=the average refractive index of the objective, and
  • vavg=the average value of the Abbe dispersion number of the objective.

The projection objective according to the invention will be explained in more detail with reference to embodiment examples. In doing so, identical reference numerals designate identical elements in the individual Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of the projection objective;

FIG. 2 shows a schematic representation of the projection objective with the assembly being arranged as the first element in the second lens group G2;

FIG. 3 shows a schematic representation of the radiuses Ri and of the air spaces or lens thicknesses di according to FIG. 2;

FIG. 4 shows a schematic representation of the projection objective with the meniscus lens being arranged as the first element in the second lens group G2;

FIG. 5 shows a schematic representation of the radiuses Ri and of the air spaces or lens thicknesses di according to FIG. 4;

FIG. 6 shows a graph of the imaging errors for the final positions of the focal length range (f=19.2 and f=22.5) of a first embodiment example according to FIGS. 2 and 3;

FIG. 7 shows a graph of the imaging errors for the final positions of the focal length range (f=25.8 and f=30.0) of a second embodiment example according to FIGS. 2 and 3;

FIG. 8 shows a graph of the imaging errors for the final positions of the focal length range (f=19.2 and f=22.5) of a first embodiment example according to FIGS. 4 and 5;

FIG. 9 shows a graph of the imaging errors for the final positions of the focal length range (f=25.8 and f=30.0) of a second embodiment example according to FIGS. 4 and 5.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the general structure of the projection objective comprising, starting from the projection screen 1 and arranged on the common optical axis 2, the first lens group G1,the second lens group G2 and the third lens group G3, wherein, as is evident from the second partial image, the second lens group G2, which is used for correction of the focal length (<-fok.->), occupies a changed (corrected) position on the optical axis 2. The third lens group G3 facing the object remains unchanged in its position on the optical axis 2, while the first lens group G1 is displaceably arranged for the purpose of focusing the objective.

FIG. 2 shows the schematic representation of the projection objective comprising a negative meniscus lens 3, a positive meniscus lens 4 as well as a negative lens 5, arranged in the lens group G1.

In this example, the lens group G2 consists of a positive meniscus lens 6 and a negative meniscus lens 7 (assembly), of a positive lens 8 and of a positive assembly consisting of a positive lens 9 and of a negative lens 10.

The lens group G3 is characterized by a positive lens 11.

FIG. 3 shows the denoting of the radiuses Ri (i=1 to 16) and the air spaces or lens thicknesses di (i=1 to 16) of the variant of the projection objective shown in FIG. 2. On this basis, said variant is represented by two embodiment examples differing in their constructional data. The constructional data of the embodiment examples thereof are shown in Tables 1 and 2 in conjunction with Table 5, which follow the embodiment examples.

In variation of the example according to FIGS. 2 and 3, FIG. 4 shows the representation of the projection objective with a negative meniscus lens 12 being arranged as the first element in the second lens group G2, instead of the assembly consisting of the positive meniscus lens 6 and a negative meniscus lens 7. In addition, FIG. 5 shows the denoting of the radii Ri (i=1 to 15) and the air spaces or lens thicknesses di (i=1 to [[14]]16). Based thereon, this alternative embodiment is characterized by two embodiment examples differing in their constructional data, shown in Tables 3 and 4 in conjunction with Table 5. FIG. 5 also shows the air space d16, the object-side vertex to object distance or object-side intercept distance (designated “s”.)

FIGS. 6 and 7 are graphs showing the imaging errors for the end positions of the focal length range f=19.2 or f=22.5 (first embodiment example) and f=25.8 and f=30.0 (second embodiment example) according to the variant of the projection objective of FIGS. 2 and 3.

Analogous to this type of representation, FIGS. 8 and 9 show the imaging errors of both embodiment examples of the projection objective according to the arrangements of groups of lenses of FIGS. 4 and 5 for the same end positions of the focal length ranges.

The present invention may be embodied in other specific forms without departing from the spirit of the essential attributes thereof; therefore, the illustrated embodiments should be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.