Abstract:
An interference filter having 17 coatings of alternating high and low index materials is located on the illumination path of an optical device and provides correction for green color distortion on the observation path and correction for blue color distortion on the photo path in order that both the observed image and photographed image are color neutral.

Description:
BACKGROUND OF THE INVENTION 
     The invention pertains to a color compensation filter for optical devices comprising observation and photo ray paths, in particular for microscopes. Complex optical devices that, apart from the usual photo ray path, are equipped with an observation ray path may require the correction of color distortions that are caused by the effects of the optical components on the spectrum in connection with the weighting of the color sensitivity of the film material. This can result in color distortions in that part of the ray path directed at the observer (observer ray path) that are different from those of the part of the ray path being used for recording purposes (photo ray path). Such a case involves the correction, or rather compensation, of a combination of color distortions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph of the light tranmission characteristics of a filter according to the present invention; 
     FIG. 2 is a diagram of light paths in a microscope; and 
     FIG. 3 shows a section through a color compensating filter according to the present invention. 
    
    
     It is the task of the invention discussed here to insert a compensation filter in the optical ray path of complex devices of this type, which, in turn, allows for selective color compensation in various wave-length areas. 
     The problem is solved through the use of a color compensation filter which consists of a layered interference filter. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates the effect on the spectrum of the layered interference filter system of this invention. A wide, basin-shaped saddle between 470 nm and 580 nm can be seen in the green area. This area of a selected transmission reduction of the color green is responsible for the correction of the color distortion of the color green. 
     The sharp upswing in FIG. 1 at between 390 and 430 nm--the 50% factor of the transmission is 423±5--is responsible for the reduction of the shorter wave lengths, so that the additional color distortion of the color blue is corrected. 
     The structure of the component layers as shown in FIG. 3 is outlined in the following chart: 
     
         ______________________________________Layer Refractive Index               Optical, Thickness t (λ = 500______________________________________               nm)1     n.sub.H = 2.05               0.6630 * λ/42     n.sub.L = 1.38               0.4700 * λ/43     n.sub.H = 2.05               2.0773 * λ/44     n.sub.L = 1.38               0.9616 * λ/45     n.sub.H = 2.05               0.9722 * λ/46     n.sub.L = 1.38               0.4910 * λ/47     n.sub.H = 2.05               0.9722 * λ/48     n.sub.L = 1.38               0.4910 * λ/49     n.sub.H = 2.05               0.9722 * λ/410    n.sub.L = 1.38               0.4910 * λ/411    n.sub.H = 2.05               0.9722 * λ/412    n.sub.L = 1.38               0.4910 * λ/413    n.sub.H = 2.05               0.9722 * λ/414    n.sub.L = 1.38               0.4910 * λ/415    n.sub.H = 2.05               0.7495 * λ/416    n.sub.L = 1.38               0.6179 * λ/417    n.sub.H = 2.05               0.3019 * λ/4______________________________________ 
    
     Layer 1 borders on the medium air (refractive index n=1); layer 17 is on the glass substrate, which has a refractive index n G  between 1.57 and 1.9. The angle of incidence can be between 0° and ±10°. H refers to a high refractive substance with a refractive index n H  =2.05, that can vary by a factor of ±0.05. The material referred to here belongs to the group zirconium dioxide (ZrO 2 ), tantalum pentoxide (Ta 2  O 5 ), hafnium (IV) oxide (HfO 2 ), or a mixture of metal oxides and rare earth oxides. The material with the refractive index n L  =1.38 is magnesium fluoride (MgF 2 ). 
     As is indicated in the chart, the component layers 7/8, 9/10, 11/12 and 13/14 represent identical layers of layers 5/6. 
     The interference filter discussed here selectively alters the light intensity of the various wave-length areas, so that, for instance, the photomicrographs provide a color-neutral reproduction of the microscopic object. As shown in FIG. 2, the visual observation of an object through the microscope also shows a color-neutral image. In addition, the interference filter is applied to a glass surface on which no other layer of any sort has been placed before. This is the case with a diffusing screen, for instance, which is located inside the illumination ray path of the optical device. The interference filter being introduced here can basically be applied to either side of the diffusing screen. It is, however, advisable to apply it to the smooth, flat side. The diffusing screen is positioned perpendicular to the illumination ray path. The suggested layer sequence simplifies this rather common positioning. Another advantage of a diffusing screen, is that it is an optical component that is easily retrieved, so that the replacement of this particular part is a relatively uncomplicated procedure. This allows for the correction of color distortion defects without having to dismantle all of the parts of the optical device entirely. By no longer requiring an additional substrate support for the interference filter, yet another function of the diffusing screen (in addition to its intended function) is demonstrated. 
     The diffusing screen is located inside the illumination ray path in front of the aperture stop (on the lamp side). The position is, for the most part, a minor concern for the interference filter. Care must, however, be taken to insure that the interference filter is located inside the illumination ray path, since the light that is not transmitted through the interference filter is essentially reflected. 
     With the invention discussed here, a variation of ±5% must be taken into account for the optical thicknesses indicated in the diagram as well as the claim. The manufacturing of the interference filter occurs in a high vacuum by a vaporization process.