Antireflective coating and method of manufacturing same

Optical: systems and elements – Light interference – Produced by coating or lamina

Reexamination Certificate

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C359S581000, C428S156000, C428S410000, C428S426000, C065S031000

Reexamination Certificate

active

06359735

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to an antireflective coating comprising a carrier layer consisting of an optically transparent material, which presents, at least on one surface side, antireflective characteristics in view of the wavelengths of radiation incident on the surface. Moreover, inventive methods of manufacturing the antireflective coating are disclosed.
PRIOR ART
At the interfaces of transparent media such as glass or synthetic panes, which are preferably used for windows, screens or instrument display surfaces, one part of the light incident on the interfaces is always reflected, i.e. reflected back into the space. As a result of the reflection phenomena occurring on the interface of the transparent media the transparency characteristics as well as the reading suitability of screens or displays are substantially impaired. For an improvement of the transparency properties or the reading suitability of screens of any kind in general antireflection measures are known which take each a different influence on the reflective characteristics at the interfaces.
For instance, reflecting surfaces may be subjected to an antireflection treatment by providing their surface with a suitable roughness. Even though the roughening of the interface surface causes a reflection of the same share of the incident light back into space any light beams incident in parallel onto the surface are reflected in various directions as a result of surface roughness. In this manner, clear mirror images are avoided, which means that light sources which would be normally reflected with sharp edges imaged on the interface, merely result in a fairly homogeneous brightening on the roughened surface. In this manner, strong differences in luminosity are avoided and the reflections are felt to be by far less troublesome.
This kind of antireflection treatment has been successfully employed, for instance, on displays characterised by the term “antiglare coating”. An essential advantage of this antireflection technique is the suitability for copy moulding of the structures by low-cost stamping processes. The disadvantage of this kind of antireflection treatment resides, however, in the aspect that the hemispheric reflection, i.e. the sum of reflecting and diffuse reflection into the total rear space region is not increased in the most expedient case, so that the background brightness of screen glass surfaces so prepared is comparatively high. This results not least in a substantial reduction of the contrast of an eye or display present behind such an antiglare coating.
Another possibility of antireflection treatment of optical surfaces consists in the application of appropriate interference layers. In that technique the surface to be subjected to antireflection treatment is coated with one or several thin layers having a suitable refractive index and an appropriate thickness. The structure of the interference layer is so configured that destructive interference phenomena occur in the reflected radiation field in suitable wavelength ranges, so that reflexes from light sources are strongly reduced in terms of their brightness. However, their image remains sharp in the reflected optical path, in distinction from the aforementioned antiglare layer. Even with a visual residual reflection of <0.4% the sharp mirror images produce sometimes a more troublesome effect than the comparatively high brightness of antiglare surfaces. The contrast ratio is good. For the majority of screens and further applications, however, interference layers are too expensive from a manufacturing point of view.
A third alternative of antireflection treatment of optical surfaces consists in the introduction of so-called sub-wavelength grids, which leads to a gradient of the refractive index at the interface of an optically transparent medium, so that an optical effect is produced which resembles that of interference layers. Such a gradient of refractive indices is realised by surface structures if the structures are smaller than the wavelengths of the incident light. Die production of periodic structures by means of holographic exposure in a photo resist layer applied on the surface of a transparent medium is expediently suitable to this end.
Examples of such sub-wavelength grids may be taken from the prior art documents DE 38 31 503 C2 and DE 2 422 298 A1.
Such sub-wavelength surface grids presenting periods of 200 to 300 nm are suitable for wide-band reflection reduction. Such surfaces, which are also known by the term “moth-eye antireflection surfaces”, are described in details in an article by M. C. Hutley, S. J. Willson, “The Optical Properties of Moth-Eye Antireflection Surfaces”, in: OPTICA ACTA, 1982, vol. 29, No. 7, pages 993-1009. Even though the substantial advantage of such “moth-eye layers” resides in the manufacturing technique which can be reproduced by means of stamping processes at reasonable costs and which resemble those of antiglare structures, the large-area production of such structures is very difficult in view of the very narrow tolerance ranges in optics in view of the variance of structure depths and a very high aspect ratio, i.e. a very high ratio between the structure depth and the period of the structures, which may give rise to adulterating colour effects. Moreover, the images of light sources are imaged on surfaces so processed, in the reflected image as sharply as in the case of interference layers.
BRIEF DESCRIPTION OF THE INVENTION
The present invention is based on the problem of improving an antireflective coating comprising a carrier layer consisting of an optically transparent material, which presents, at least on one side, antireflective properties in view of the wavelengths of the radiation incident on the surface, in such a way that particularly in application on screen surfaces the contrast ratio will be substantially not impaired by the reflective properties at the optical interface. Discrete reflection images such as those occurring in interference layers and reflections on sub-wavelength grids should be avoided. The inventive antireflective coating should particularly present hemispherical reflective characteristics whose degree of reflection is by far below that of normal antiglare layers. Moreover, a method should be provided for manufacturing the inventive antireflective coating, which is also suitable for producing large-area antireflective coatings, despite low manufacturing costs.
The solution to the problem underlying the invention is described in claim
1
. Claim
7
relates to an inventive manufacturing method. The dependent claims contain each the features expediently improving the respective inventive ideas.
In accordance with the invention an antireflective coating comprising a carrier layer consisting of an optically transparent material, which presents, at least on one surface side, antireflective properties with respect to the wavelengths of the radiation incident on the surface, is so configured that the antireflective surface side has a surface roughness with stochastically distributed structures—so-called macro structures—and that the macro structures are additionally modulated with periodically sequenced surface structures—the so-called micro structures—which have cycle or period lengths which are smaller than the wavelengths of the radiation incident on the antireflective surface.
The present invention is based on the idea of combining the advantages of the reflective properties of the aforedescribed known antiglare layers with the characteristics of sub-wavelength grids. On account of the superimposition of macro structures and micro structures on one and the same optical surface firstly discrete reflection images are prevented on account of the macro structures, and secondly the share is dramatically reduced by hemispheric reflection on the surface on account of the micro structures. In particular in the application on monitor display surfaces the inventive antireflective coating results in a substantial increase of the contrast conditions, a large destruction

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