FR2543696A1 - INTERFERENCE FILTER WITH A CONVEYOR BELT - Google Patents

Abstract

THE INTERFERENCE FILTER EQUIPPED WITH A TRANSPARENT BAND, CONSTITUTED BY A SYSTEM OF INTERFERENCE LAYERS APPLIED ON A SUPPORT TRANSPARENT TO LIGHT AND WHICH CAN BE CLOSED BY A COVER GLASS CONNECTED BY A BINDER AND WHICH CONSISTS OF SEVERAL GROUPS OF SUPERIMPOSED AND SYMMETRICAL FILTER LAYERS, WHICH ARE EACH TIME CONNECTED BY AN INTERMEDIATE LAYER N, IS CHARACTERIZED IN THAT THE NUMBER OF INDIVIDUAL LAYERS, IN ONE OR MORE EXTERNAL GROUPS OF FILTER LAYERS OF THE INTERINFERENCE LAYER SYSTEM, IS DIMINISHED, NUMBER OF EXTERNAL INDIVIDUAL LAYERS H OR N IN RELATION TO THE NUMBER OF INDIVIDUAL LAYERS IN ONE OR MORE INTERNAL GROUPS OF FILTERING LAYERS, AND OR BY THE FACT THAT, IN ONE OR MORE EXTERNAL GROUPS OF FILTERING LAYERS OF THE INTERFERENCE LAYER SYSTEM, THE OPTICAL THICKNESS OF THE CENTRAL INDIVIDUAL LAYER CONCERNED G2H OR G2N IS LESS THAN THE OPTICAL THICKNESS OF THE CENTRAL INDIVIDUAL LAYER CONCERNED IN ONE OR SEVERAL INTERNAL GROUPS OF FILTERING LAYERS.

Description

The present invention relates to a filter

  interference provided with a transparent band of a

  bandwidth from 3 to 30% of the central wavelength of

the transparent strip.

  Such filters are used to select limited spectral domains; they are therefore used in optical devices or apparatuses of the field

UV, VIS and near IR.

  Interference filters united with a transparent band generally consist of a system of metallic and non-metallic or only non-metallic layers which are applied to each other on a support transparent to light and protected against any external influence by a cover glass connected by a binder and possibly having a function

  filtering by forming an additional block.

  Interference filters provided with metallic and non-metallic layers transparent to light are known from German Patent 716,153 The disadvantage of these filters lies in the fact that the transmission is greatly reduced in the area of transparency due

  high light absorption by metallic layers.

  In document DE-PS 913 005, interference filters

  are described comprising transparent layer systems

  light annuities non-metallic, high refraction but low alternation Such filters have a

  very high maximum transmission in the trans-

  appearance due to the low absorption of non-metallic layers The fact that the high transmission range remains limited to only a narrow wavelength band

  is a drawback for many uses.

  Constructive broadening of the transparency range of this type of filter by reducing the number of layers results in high transparency to light, inadmissible for use in the stopping range, and deterioration

  the steepness of the slope of the spectral borders deli-

mitting the transparent strip.

  By having identical filters of this type one after the other provided with a reduced number of layers and therefore having an enlarged range of transparency, filters with multiple cavities are obtained having a low transparency to light in the field stop and an extended transparency domain with steep spectral limit (Knittel 1 'Optics of

  Thin Films "John Wile & Sons, London-New York 1976).

  However, a disadvantage arises for such filters, when the necessary arrangement, one after the other of more than 3 filter elements, by the fact that a structure of absorption bands appears in the transparent band of the filter, structure which results in a decrease in transmission In addition, this band structure reacts very significantly to thickness errors

layer due to manufacturing.

  Furthermore, it is known to manufacture an interference filter with a transparent band in

  combining a filter with a transparent domain

  Enlarged short wave rence with a second pre-filter

  sensing a widened long-wave transparency domain so that the transparency domains of the overlapping filters define a wavelength band (HA Macleod "Thin Fims Optical Filters" Adam Hilger LTD, London, 1969, p 154 ff) ' Such a provision is also presented

in document DD-PS 6369.

  Interference filters of this type to filter a spectral transparency range have

  the disadvantage that for their realization, there is

  sity of two different filters having different layer systems which requires more technical means In addition, the respect of spectral dimensions is not satisfactory due to the superposition of errors

  due to the manufacture of the two filters.

  The object of the invention is to improve the quality of optical interference filters fitted with a transparent band The drawbacks of the solutions

  known techniques should be discarded.

  The object according to the invention consists in creating

  optical interference filters fitted with a trans-

  parent which allows high transmission by eliminating

  maximum band structure in the trans band

  parent of the filter, without deteriorating the advantageous filtering qualities, such as the corresponding bandwidth of the transparent band and the

  good stopping effect in the area of lateral stopping.

  The solution to this problem is obtained in accordance with

  mement to the invention thanks to an interference filter provided with a transparent band constituted by a system

  interference layers applied as a trans-

  parent to the light and closed by a cover glass fixed by a binder, system which can consist of several groups of superimposed filter layers and symmetrically constructed, for example by presenting the structure of layers g 2 H, Ig 2 NH, -H Ng 2 HNH or HN Hg 2 NHNH, etc., layers which, in each case, are connected by an intermediate layer N, the individual non-metallic layers which are transparent to light, H, N or g 2 H, g 2 N of the groups of filter layers having different refractive indices n H and n and an optical layer thickness equal to 1/4 of the wavelength LO of the filter or a g multiple to an integer with g = 1,2,3 double of the value 1/4 AO, by the fact that the number of individual layers, in one or more external groups of filter layers of the interference layer system> is reduced by the number of individual external layers H or N relative to the number individual layers in one or more groups internal layers, and / or the fact that in one or more external groups of layers

  filters of the interference layer system, the thick-

  optical thickness of the central individual layer concerned g 2 H or g 2 N is less than the optical thickness of the

  individual central layer concerned in one or more

  internal groups of filter layers.

  It is therefore advantageous for the layers indi-

  N or g 2 N have a lower refractive index compared to the individual layers H or g 2 H. But it is also possible that the individual layers N or g 2 H have compared to the individual layers H or g 2 H refractive index

or higher.

  Furthermore, it is advantageous that the refractive index N N of the intermediate layer N coincides with the refractive index n T of the support and; in the case of a cover glass fixed by a binder, with the index

  of refraction n K of the fixing binder of the cover glass

ture.

  Otherwise in the presence of diffuse refractive indices

  for the intermediate layer N of the support and the

  cover glass, an additional intermediate layer

  may be provided between the support and the interference layer system and / or between the interference layer system and the attached cover glass

by a binder, if it exists.

  The essence of the invention consists in that in the interference layer system according to the interference filter according to the invention, at least one external group of filter layers is distinguished from one or more internal groups of filter layers of the same structure by the number of individual layers

  H or N or by the optical thickness of the individual layers

  dual central or by the two characteristics Indeed, the number of individual layers H or N or the optical thickness of the individual central layers g 2 H or g 2 N must be less in at least one external group of filter layers to one or more groups

internal layers.

  The group of applied lower layers is designated by external groups of filter layers.

  on the support and the group of upper layers applied

  against the outside cover, for example a cover glass, but also the second group of

  lower or upper layers and so on.

  It is advantageous that the groups of outer filter layers are only reduced by the two individual outer layers 0/4 H or N of these groups of filter layers compared to the groups of inner filter layers with identical structure, or that only the central individual layer g 2 H or g 2 N of the group of external filter layers is reduced by 2 H or 2 N layers, therefore by O / 2 in the optical thickness

  compared to the inner group of filter layers.

  The advantage of the interference layer system according to the invention resides in the fact that the annoying structures of the bands in the area of transparency of the filter have been eliminated or greatly limited and a higher transmission is therefore obtained. filtering qualities remain largely maintained such as bandwidth

  and the stop effect in the lateral stop domain.

  Usual glass plates or filters

  known glass with which ob obtains wire effects-

  additional known trage can be used in

  as support for the layers or cover glass.

  Various other features of the invention

  emerge - & 'elsewhere from the detailed description which follows.

  Embodiments of the subject matter of the in-

  are shown, by way of non-limiting examples

tatives, to the accompanying drawings.

  Figs 1 and 3 represent the spectral transmission T for an interference filter having

  a state-of-the-art layer system

than.

  Figs 2, 4, 5 and 6 represent the trans-

  spectral mission T for embodiments 1,

  2, 3 and 4 described according to the invention.

  The spectral representations were con-

  stunned in the wave number 3 A 0 / _ and each relate to the wavelength of the central filter o of the transparency range of the filter, -length which can be deliberately chosen and which serves at the same time as a measure of length for the individual layers because H and N are layers with an optical thickness of A / 4 A By determining for each use case o the wavelength _o for the central wavelength of the filter, the wavelength spectrum as well as the optical thickness of each individual layer are generally fixed In fig 2, 4 and 6, the wavelength scale is entered in the case oj o = 550 nm In this particular case, all individual layers H and N have an optical thickness

  of 1/4 o = 137.5 nm and the individual layers center

  trales g 2 H and g 2 N an optical thickness magnified by the factor 2 g, the value 2 g (g = whole number) being

  given in the corresponding examples.

Example 1

  Example 1 is based on a system

  teme of layers of an interference filter having these

  faults, system which is modified according to the invention.

  The system of layers is cropped by 7 oups The known system of layers is caroose of 7 identical groups of filter layers of structure GH 2 NH, arranged on a glass support one on the other, they are connected by layers N and protected by a cover glass connected by a binder The layer system can be symbolically characterized as follows: glass NG 2 NG 2 NG 2 NG 2 NG 2 NG 2 NG 2 N / glass connected by a binder.

  N denotes individual or inter-

  cryolite mediaries with a refractive index n N = 1.30 and H denotes individual layers of zinc sulphide having a refractive index n H = 2.36 The optical thickness of the individual layers H and N is for each of 1 / 4; , that of the individual central layers o 2 N (g = 1) is 1/2 O in the groups G 2 for the groups of filtering layers consisting of 3 individual layers, this results in an optical thickness

of; For each of the groups.

  The spectral transmission of these known interference layer systems is shown in Fig 1 and shows the defect in the band structure in the

  transparent band which falls down to below T = 0.5.

  This known device is modified according to the invention.

  tion by the fact that the two outer groups of the seven groups of filter layers G 2 of the layer system are each exchanged by the group of layers G 1 = 2 H, the simplest group which in principle represents a layer

  individual with an optical thickness of 1/2 to 0.

  Therefore, the inter-

  ference is symbolically characterized by glass / NG 1 NG 2 NG 2 NG 2 NG 2 NG 2 NG 1 N / glass connected by a binder La

  spectral transmission of this system with layers of inter-

  rence is shown in fig 2.

  It appears that for the inter- layered system

  ference according to the invention, the band structure

  annoying has been largely eliminated while the other qua-

  filtering units are maintained such that the width

  tape and stop effect as well as others.

  Example 2 Based on the conditions set in Example 1, Example 2 is based on the known interference layer system

  fitted with a striped structure, characterized symbolically-

  ment per glass / NG 3 NG 3 NG 3 NG 3 NG 3 N / glass connected by a binder consisting of 5 identical groups of filter layers G =

  HN 2 HNH with each 5 individual layers.

  The same materials were used as for

the layers of example 1.

  FIG. 3 shows the spectral transmission and the disadvantageous band structure in the field of the transparent band of the filter. The two external groups are each replaced by the group G 2 = H 2 NH of 3 individual layers according to the invention of this system of layers, group in which the number of individual layers is decreased by deleting

  of the two individual layers (H) with respect to G 3.

  The new interference layer system according to the invention can be symbolically characterized

  per glass / NG 2 NG 3 NG 3 NG 3 NG 2 N / glass connected by a binder.

  The higher spectral transmission in the range of transparency of the filter is due to the absence of the annoying absorption bands as it appears

of the filtering curve (fig 5).

  Example of realization 3 In a way analogous to the examples of realization

  1 and 2, another interference filter in accordance with the

  vention is considered according to the symbolic characteristic:

  glass / G 3 NG 4 NG 4 G 3 / glass connected by a binder, whose system

  teme of interference layers is formed by two inner groups of filter layers Ga = HN 4 HNH and by the two outer groups of filter layers G 3 = HN 2 HN 1 EI and by an individual intermediate layer N

  in a matter mentioned in Example 1.

  Here, the central individual layer 2 H (g = 1) in the two outer groups of filter layers G 3, according to the invention, has a reduced optical thickness

  1/2 o compared to the individual layer 4 H inter-

  medial (g = 2) of the two interior groups G 4.

  The filter curve shown in Fig 5 does not show an unfavorable band structure in the

transparency area of the filter.

  Exemp_lle of realization 4 By using the same material of layer as in

  the previous examplesg we use an inter-

  ferences with the characteristic

  glass / G 3 NG 3 NG 5 NG 3 NG 3 / glass connected by a binder.

  The four outer groups of wire layers

  trantes G 3 with G 3 = HN 2 HNH are each composed of 5

  individual layers while the group of fil layers

  inner trantes G 5 with G 5 HNH 2 NHNH is composed of 7

individual layers.

  The filter curve shown in fig 6 of

  this interference filter does not show a defective structure

  vorable stripes in the filter transparency area.

Claims (4)

EVENDICATIONS   1 Interference filter fitted with a trans-   parent, made up of a system of inter-   applied on a support transparent to light and which can be closed by a cover glass connected by a binder and which is composed of several groups of superimposed and symmetrical filter layers, for example of a structure of layers g 2 H, Hg 2 N Hp H Ng 2 HNH or HN Hg 2 NHNH, etc., which are each time connected by an intermediate layer N, the individual non-metallic layers which are transparent to light, H, N or g 2 H, 92 N of the groups of filter layers with different refractive indices n E and N as well as an optical layer thickness of 1/4 of the wavelength   of the central filter or a g multiple of a number   tier with g = 1, 2, 3 twice the value 1/4, characterized in that the number of individual layers, in one or more external groups of filter layers of the interference layer system is reduced by the number of layers individual external H or N in relation to the number of individual layers in one or more internal groups of filter layers, and / or by the fact that in one or more external groups of filter layers of the layer system   interference, the optical thickness of the indi-   central vacuum of the concerned g 2 H or g 2 N is less than the optical thickness of the central individual layer concerned in one or more internal groups of filter layers. 2 interference filter according to claim 1, characterized in that the individual layers N or g 2 N have a lower refractive index compared to the individual layers H or g 2 H. 3 interference filter according to claim 1, characterized in that the individual layers N or g 2 N have relative to the individual layers H or   g 2 H a higher refractive index.   4 Interference filter according to claim 1, characterized in that the refractive index N of the intermediate layer N coincides with the refractive index n T of the support and, in the case of a fixed cover glass by a binder, with the refractive index n K of the binder for fixing the cover glass.   Interference filter according to claim   1, characterized in that in the presence of refractive indices   For the intermediate layer N of the support and the cover glass, an additional intermediate layer may be provided between the support and the interference layer system and / or between the interference layer system and the attached cover glass. by a binder, if it exists.