DE2406890A1 - DICHROITIC MIRROR - Google Patents

Description

Applicant: Canon Kabushiki Kaisha, No. 30-2, 3-chome, Shimomaruko, Ohta-ku, Tokyo, Japan

Dichroic mirror

The invention relates to a dichroic mirror according to the preamble of the main claim.

For example, dichroic mirrors have three color component beam splitter systems Application found in the Vieg of a single incoming light beam are arranged between the lens and the pick-up tube of television cameras. The incoming bundle of white rays Light falls on the dichroic mirror and is divided into two Components divided, one of which is reflected, the other is transmitted. The reflected and the transmitted Components have a complementary relationship to one another. The Reflexio.nsr and complementary transmission characteristics the dichroic mirrors are used in the above-mentioned beam splitter systems to generate color separation images of a given object to be recorded by the pickup tubes. In most cases the necessary color division is achieved by using two dichroic mirrors, in particular one reflective blue dichroic mirror I and a red reflective dichroic

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shear mirror II in a spatial arrangement to each other be attached, as shown in Figure 1. Both mirrors transmit a green frequency band while they have a reflect blue or red frequency band, which leads to the fact that the white bundle of rays reflected from the object, which is to be recorded, divided into red, green and blue light as it passes through the beam splitter system will. It is desirable that the transmission characteristics of the blue reflective and red reflective dichroic Mirrors have a uniform spectral distribution, otherwise the distribution of the light energy in the green spectral range does not become uniform, so that a high quality image cannot be obtained without causing a color shift occurs.

One type of dichroic mirror is shown in FIG. The dichroic mirror is constructed from alternately arranged layers of a material with a high refractive index H and a material with a low refractive index L, which are formed by vacuum coating on a transparent substrate S in the order that the first layer is adjacent to Substrate has a high refractive index and that the outermost layer is exposed to the ambient air. Vacuum deposition is a method of creating the layers because it allows precise control of the thicknesses of the layers. To maintain maximum reflectance at a construction wavelength, the optical thickness of a layer with a high refractive index and the subsequent layer with a low refractive index should be adjusted in the ratio of either 3 λ ^ ϊ fyk or λ / 4: X / h, where λ means the construction wavelength as far as this relates to a blue reflecting and a red reflecting dichroic mirror. With the arrangements according to the state of

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Technology almost - all layers had an optical thickness, which are essentially an odd multiple of a quarter wavelength corresponded to the design wavelength. Because of this, every conventional one produces blue, reflective dichroic mirrors have a pass band that has a large ripple from 500 mu to βθθ mu in a larger area Wavelength than the design wavelength, as indicated by curve a of Figure 5 for a multilayer structure with 3 λ / 4: λ / 4 and with curve c in FIG. 6 for a multilayer structure with λ / 4: λ / 4. The conventional red reflecting dichroic mirrors each produce a transmission range with a large one Waviness from 500 mu to 56Ο mu in a smaller area Wavelengths as the design wavelength as shown by curves b and d of Figure 5 and Figure 6, respectively.

As in the case of the dichroic mirror of the type shown in FIG. 2, another type of dichroic mirror can be formed according to FIG . The thickness of the layers in Figure 3 is of the same order of magnitude as the thickness of the layers of Figure 2. In Figure 3, however, the layers are arranged in reverse order, i.e. low refractive index, high refractive index, low refractive index, etc., and not high refractive index, low refractive index and high., refractive index as in the case of Figure To maximize the reflectance at a construction wavelength, the optical thickness of a layer with low refractive index and the subsequent layer with high refractive index should be controlled in a ratio that is either> / 4: 3 "K / \ or λ / 4: λ / 4 in relation to the construction wavelength, as far as the blue-reflecting and red-reflecting dichroic mirrors according to the state

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■ ⁴ "2406830

According to the case described above, in this arrangement, the blue reflecting dichroic mirror produces a transmission region having a large ripple of 500 mu to 600 mu in a range longer wavelengths than the design wavelength, while the red reflecting dichroic mirror has one Provides a pass band that has a large ripple of 500 τψ to 5βθ mu in a range of shorter wavelengths than the design wave

has length.

The invention therefore aims to achieve the above To avoid waviness in the transmission areas of the dichroic mirrors, and it is therefore an object of the present invention the creation of dichroic mirrors which have improved transmission characteristics. This task is carried out by solved the subject matter of the main claim.

The dichroic mirrors according to the invention are able to generate a uniform distribution of the light energy, those in the transmitted light components free of any Ripple is when the incident light has a continuous spectral distribution.

Essential features of the invention are thus to be seen in the fact that a dichroic mirror is created from a plurality of layers of a material with a high refractive index and of a material with a low Refractive index exists, 'which alternate successively in Arranged in the form of a stack on a surface of a substrate are. The dichroic mirror separates the light falling on it in the area of the visible Spectrum in two components, one of which is transmitted and the other is reflected. The first layer, the second Layer and the layer with the highest number in the stack, calculated from the substrate, each have an optical thickness

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set that of -an odd number of a quarter wavelength deviates from the design wavelength in order to obtain a uniform distribution of the permeability coefficients about the let through. Components.

According to the invention, a dichroic thin-film or thin-layer coating is thus created in which successive layers alternate from a material with a low refractive index such as MgFp and SiO ₂ and a material with a high refractive index such as ZrO ₂ , CeO ₂ , TiO ₂ , ZnS and mixtures thereof by vacuum deposition are applied to a transparent substrate, the first layer, the second layer and the layer with the largest number counting from the substrate are adjusted with regard to their optical thickness ^ that this deviates from an odd multiple of a quarter wavelength of the construction wavelength, while the other layers can be deposited in optical thicknesses with a ratio of either. Λ / 4: 3 λ / 4 or / 1/4: A / 4 follow one another, as is generally known.

Figure 4 is a vector diagram illustrating the relationship between amplitude and phase of reflectivity between the reflected and incident rays at a particular wavelength in the middle of the band at which the ripple occurs caused by the coating and that shown in Figure 3 The arrangement shown, in which the coating consists of eleven layers, should be prevented. The vector diagram shows the process by which the coating and arrangement are developed and how it works. In. FIG. 4 shows a curve which consists of twelve vectors and which starts from a point 0 with a vector r 1. The length of the vector r ^ is the amplitude of the reflectivity for the

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11th layer proportional. A vector r, _Q , the length of which is in the same ratio as above to the amplitude of the reflectivity for the 10th layer, starts at the end of the vector r i, making a phase angle Ο ... to the direction of the vector r . ₁ has.
the following formula determines:

of the vector r.- has. The phase angle 0 ,. is through

θ π ⁼ '■ * (nd) ,, * cos # _n ..

11 A - "- I 11

Therein, (nd) ₁₁ denotes the optical thickness of the 11th layer and Oi ₁₁ denotes the angle of refraction at the outer surface of the 11th layer. When other vectors r _q to r _{Q are} successively appended in the manner described above and when the end point of the vector r _Q coincides with point 0 or when a vector r is zero, no ripple occurs at the desired wavelength. It can be seen from the vector diagram that one can reduce the waviness to zero by changing the phase angle for some of the intermediate layers between the second and the outermost layer or their optical thicknesses. In vacuum deposition, however, as the number of layers of modified thickness increases, so does the difficulty of avoiding a cumbersome and time-consuming process. In addition, the complete prevention of waviness cannot be achieved by merely changing the thickness of the first layer or only the thickness of the outermost layer. This is only possible if at least three layers in the coating are adjusted to an optical layer thickness which deviates from an odd multiple of a quarter wavelength, so that the condition that the vector r.zero is fulfilled.

The accompanying drawing serves to further explain the invention. Show in it:

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Figure 1 is a schematic representation of an optical system according to the prior art for separating the red, green and blue spectral bands;

Figures 2 and 5 are cross-sectional views of dichroic Mirrors constructed in accordance with the invention, wherein the Layers for the purpose of a clearer representation with considerably exaggerated thickness are drawn;

Figure 4 is a vector diagram to explain the principle, with which the ripple is avoided;

Figures 5 and β permeability diagrams, which one for dichroic mirrors according to the prior art; and

Figures 7 »8, 9 and 10 permeability diagrams which are obtained by the dichroic mirrors in accordance with preferred embodiments of the invention are constructed.

The details of the invention will become even more apparent on the basis of the following description of preferred exemplary embodiments. A dichroic mirror contains a coating made up of a plurality of layers. The layers consist of a material with a high refractive index H and a material with a low refractive index L, these materials being applied by vacuum coating to a transparent substrate in the order shown. The first layer, adjacent to the substrate, consists of a material with a high refractive index. It has an optical thickness of J Jl / 4 of the design wavelength λ. The outermost layer is made of a material with a low refractive index. It has an optical thickness of Ti / K of design wavelength λ. The detailed description of this arrangement should be given below using the designation customary for thin layers *

G (3HL) ^m A

Herein, G means a glass substrate

H a layer of a high refractive index material with an optical thickness of Ά /% 0 9 8 3 4/0 8 5 6

L a layer of a material with low

Refractive index with an optical thickness of λ, / 4 m an integer other than zero] and A the environment (air).

To avoid ripple in a range of wavelengths greater than the design wavelength, should:

(1) a dichroic multilayer structure with the construction formula G (3HL) ^m A modified such that the optical thickness of the first layer is less than 3 λ / 4, that the optical thickness of the second layer is less than λ / 4 and that the optical thickness of the outermost layer is greater than λ / 4.

(2) A structure of the construction formula G (3HL) ^m 3HA should

Be modified so that the optical thickness of the first layer applied to the substrate is less than 3 λ / 4 that the optical thickness of the second layer is less than λ / 4 and that the optical thickness of the outermost layer is less than 3 A / 4.

(3) A structure with the construction formula G (HL) ^m A should be modified so that the thickness of the first and second layers is less than A / 4 and that the thickness of the outermost layer is greater than A / 4.

(4) A structure with the construction formula G (HL) ^m HA should be modified in such a way that the layer thickness of the first, second and outermost layers is each less than λ / 4.

To avoid waviness in a range of smaller wavelengths than the construction wavelength

(5) a structure with the construction formula G (3HL) ^m A be modified so that the layer thickness of the first layer is greater than 3 A / ^ j that the layer thickness of the second layer is greater than λ / Κ and that the layer thickness of the outermost Layer is smaller than X / k .

(6) A structure with the construction formula G (3HL) ^m 3HA should be modified in such a way that it contains either a combination (a) of a first layer that is thinner than 3λ / \, a second layer that is thicker than ä / K and an outermost layer,

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which is thinner than 3 V ^, contains or a combination (b) _; a first layer thicker than 3 -V ^, a second layer thicker than A / 4 ^ and an outermost layer thicker than 3 Λ / 4.

(7) A structure with the construction formula G (HL) ^m A should be modified so that it contains a first and a second layer, each thicker than Λ / 4, and an outermost layer, which is thinner than Λ / 4 .

(8) A structure with the construction formula G (HL) ¹¹¹ HA should be modified so that it contains a first, a second and an outermost layer, each of which is thicker than

In Figure 2 is a dichroic mirror according to a Embodiment of the invention shown. This contains a transparent substrate S with a normal light reflective Surface made of a suitable material such as glass, which has an index of refraction which can be varied as desired, in the present case an index of refraction of 1.52. A dichroic coating is applied to one surface of the substrate S. This cover consists of transparent, substantially colorless layers 1 with 10, which consist of at least two different materials. One of these Materials has a high refractive index H, in the present case 2.20, the other a low refractive index L-, im present case 1.38. It should be mentioned that the high refractive index material is used as the first layer is used.

In the following, reference should be made to FIGS. 7 and 8 for an understanding of the effect exerted by the invention. FIGS. 7 and 8 show the transmission characteristics versus the wavelength of a red reflecting and a blue reflecting dichroic mirror for different numbers of layers. Curve 1 of FIG. 7 shows a transmission characteristic of the blue reflecting dichroic mirror with a ten-layer coating according to the construction formula G (3HL) 5a with the modification (1) in FIG

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optical thickness of the first, second and tenth layers, the condition being fulfilled that the vector r of FIG. 4 is zero. Curve 4 shows a transmission characteristic of another, blue-reflecting dichroic mirror with an eleven-layer coating of the construction formula G (3HL) JHA with the modification (2). Curve 2 shows a transmission characteristic of a red reflecting dichroic mirror with a ten-layer coating of the construction formula G (JHL) ^ A with the modification (5) , while curve 3 shows a transmission characteristic of another red reflecting dichroic mirror with an eleven-layer coating - of the construction formula G (3HL ) Represents OHA with modification (6) - (a). Curve 11 of Figure 8 shows a transmission characteristic of a blue reflecting dichroic mirror with a fourteen

T- layered coating of construction formula G (HL) A with modification (3). Curve 12 of Figure 8 shows a transmission characteristic of another blue reflecting dichroic mirror with a 15-layer coating of the construction formula G (HL) 'HA with the modification (4). Curve 13 shows a transmission characteristic of a red reflecting dichroic mirror with a 13-layer coating of the construction formula G (HL) HA with the modification (8).

By comparing these transmission characteristics with those shown in Figures 5 and 6, of which curves a, b, c and "d correspond to curves 1 and 2 of Figure 7 and curves 11 and of Figure 8, it can be seen that the distribution the transmittance coefficient in a green spectral region is uniform and with respect to the dichroic mirror according to the The prior art is vastly improved, which requires that the optical thicknesses of all layers in the Plating to significantly odd multiples of Λ / 4 der Design wavelength are set.

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The invention is also concerned with dichroic Multi-layer structures that differ from the cases described above with regard to the order of the layers. To completely eliminate ripple in a range of wavelengths greater than the design wavelength ^ ,, the following measures are taken:

(9) A dichroic ^ multilayer structure of Könstruktionsformel G (Lj5H) ^m A should be modified such that the optical Dicke.einer first applied to the substrate layer is smaller than the ^4/2 I-, that the optical thickness of a second layer is smaller than 3λ / k and that the optical thickness of the outermost layer is less than 3> V4. -

(10) A structure with a construction formula G (L3H) ^m LA should be modified to include a first layer thinner than -V4, a second layer thinner than 3 "V4, and an outermost layer that is is thicker than A / 4.

(11) A structure having a construction formula G (LH) ^m A should be modified to include first, second and outermost layers each thinner than A / 4.

(12) A structure with a construction shape G (LH) ¹¹¹ LA should be modified in such a way that it contains a first and a second layer, each thinner than 3/4, as well as an outermost layer. Layer that is thicker than

To avoid waviness in a range of wavelengths, which are smaller than the design wavelength, the following measures are taken:

(13) A structure of the structural formula G (L3H) ^m A should be modified in such a way that it contains either a combination (a) of a first layer that is thicker than tyk, a second and an outermost layer, each thinner than 3 Contains λ / 4 or a combination (b) of a first layer, which is thicker than λ / 4, and a second and an outermost layer, which. are each thicker than

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(14) A structure with a construction formula G (L3H) ^m LA should be modified to include a first layer thicker than Λ / 4, a second layer thicker than 3 A / 4, and an outermost layer Layer thinner than X / k.

(15) A structure with a construction formula G (LH) ^m A should be modified so that it contains a first, a second and an outermost layer, each of which is thicker than .A / 4.

(ΐβ) Its structure with a construction formula G (LH) ^m LA should be modified so that it contains a first and a second layer, each thicker than λ / k, and an outermost layer, which is thinner than'λ / 4th

In another embodiment, as shown in Figure 3, a dichroic mirror includes a transparent substrate with a refractive index of 1.52, on one surface of which a first layer of a material with a low refractive index of 1.33, a second layer with a material with a high refractive index of 2.20, as well as additional alternating layers are applied. Such a dichroic mirror has transmission characteristics as shown in FIGS. Curve 21 of Figure 9 shows the transmission characteristic of a blue reflecting dichroic mirror with an eleven-layer coating of construction formula G (LJH) - ⁵ LA with modification (10), in the optical thicknesses of the first, second and outermost layers, which condition is met becomes that the vector r of Figure 4 becomes zero. The curve 22 shows a transmission characteristic of a red reflecting dichroic mirror with a 12-layer coating of the construction formula Q (L3H) A with the modification (13) - (a). Curve 23 of FIG. 9 shows a transmission characteristic of a red reflecting dichroic mirror with an eleven-layer coating of construction formula G (L3H) ^ LA with modification (14).

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Curve 31 in Fig. 10 shows a transmission characteristic of a blue reflective dichroic mirror with a 13-layer Covering the construction formula G (LH) LA with the modification (l2). Curve 32 shows a transmission characteristic of a blue reflective dichroic mirror with a 14-layer Covering the construction formula G (LH) Ά with the modification (11). Curve 13 shows a transmission characteristic, one red reflective dichroic mirror with a l4 layer Covering the construction formula G (LH) 'LA with the modification (16).

From the above description it can be seen that the described embodiments of the invention have the initially solve the mentioned problem by creating a dichroic mirror that generates a pass band that is free of Waviness is independent of the number of layers in the coating. Regarding the construction of the Brings multilayer structure from the dichroic mirrors. an increase in the number of layers increases the steepness of the transmission curve in the border area of the short and long wavelengths, so that the number of layers corresponds to the required power of the dichroic mirror can be changed. In addition, according to the present Invention for the multilayer structure seldom a dependence on the refractive index of the substrate, so that it is not necessary is to change the structure according to the refractive index of the substrate as with anti-reflective coatings.

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Claims (1)

Claims /l.yDichroic mirror with a substrate on which as Coating alternating layers of a material with a high refractive index and a material with a low refractive index are applied, characterized in that at least the first layer, the second layer and the layer with the largest number from the substrate is set to an optical thickness which deviates from an odd multiple of a quarter wavelength of the design wavelength in order to create a pass-through area free of undulation, 2. dichroic shear mirror according to claim 1, characterized in that it comprises a substrate, are applied to the as coatings alter- • nating layers of a material having a high refractive index and a material having a low refractive index, wherein the odd-numbered layers of the substrate of counted consist of the material with high refractive index, while the even layers consist of the material with low refractive index, and that the optical thickness of a layer with high refractive index and the subsequent layer with low refractive index with respect to the construction wavelength Λ in the ratio of 3λ / 4: λ / 4 and that at least the first layer, the second layer and the layer with the largest number, calculated from the substrate, are each set to an optical thickness that is an odd multiple of λ / 4 of the construction wavelength X deviates. 3 · dichroic mirror according to claim 2, characterized in that in the event that the material with the low refractive index is used for the layer with the highest number 4 09834/0856 the optical thickness of the first layer is set smaller than 3 λ / ² * · * ^the ^ er second layer smaller than A / 4 and that of the layer with the highest number larger than % / h to reduce the waviness on the page Avoid longer wavelengths of the construction wavelength. Dichroic mirror according to Claim 2, characterized in that when the layer with the largest number is off is made of a material with a low refractive index, the first layer is adjusted to an optical thickness, which is greater than 3 Λ ^ * ^ ie second layer on one optical thickness greater than -V ^, and the layer with the highest number to an optical thickness that is less than A / 4 to avoid the occurrence of ripples on the short side to avoid the design wavelength. 5. Diehroitischer Spiegel according to claim 2, characterized in that, if the layer with the highest number is made of a material with a high refractive index, the first layer is adjusted to an optical layer thickness that is less than 3 Ί / 4, the second layer to an optical thickness which is smaller than V ^> and the layer with the largest number to an optical thickness which is smaller than 3λ / ^ f in order to avoid ripples on the long-wave side of the design wavelength. ^: 6. dichroic mirror according to claim 2, characterized in that when the layer with. the largest number consists of a material with a high refractive index, the first layer is controlled to an optical thickness that is less than 3 X / h, the second layer to an optical layer thickness that is greater than ^ / 4, and the layer with the largest number to a layer thickness that is less than 3 -V4, in order to avoid ripples on the short-wave side of the construction wavelength. '4 09834/0856 7. dichroic mirror according to claim 2, characterized in that that if the layer with the largest number is made of the material with the high refractive index, then the first Layer is set to an optical thickness which is greater than J λ / 4, the second layer to an optical thickness, which is larger than λ / 4, and the layer with the largest 'Number to an optical thickness greater than 3 λ / 4, by squares on the short-wave side of the design wavelength to avoid. 8. Dichroic mirror according to claim I, characterized in that it contains a substrate on which a coating in alternating layers of a material with a high refractive index and a material with a low refractive index are applied, the odd-numbered layers counted from the substrate of the material with the high refractive index, the even layers consist of the material with the low refractive index, and whereby'die optical thickness of a layer with high refractive index and the subsequent layer with low refractive index, based on the construction wavelength λ, in the ratio λ / 4: λ / 4 are related to each other, and that. at least the first layer, the second layer and the layer with the largest number, calculated from the substrate, are each set to an optical thickness which deviates from an odd multiple of λ / 4 of the design wavelength X. 9. dichroic mirror according to claim 8, characterized in that when the layer with the highest number consists of the material with a low refractive index, the first and second layers are each set to an optical thickness which is smaller than fyK, and that the layer with the largest number is set to an optical thickness 409834/0856 which is greater than λ / k in order to avoid ripples on the longer wavelength side of the design wavelength. 10. Dichroic mirror according to claim 8, characterized in that when the layer with the highest number is made of a material having a low refractive index, the first and second layers are each set to an optical thickness,. which is larger than tyk, and that the layer with the highest number is set to an optical thickness which is smaller than V ^ * in order to avoid ripples on the shorter-wave side of the design wavelength. 11. Dichroic mirror according to claim 8, characterized in that when the layer with the highest number consists of the material with the high refractive index, the first layer, the second layer and the layer with the largest number are each set to an optical thickness which is smaller than λ / K in order to avoid ripples on the long wavelength side of the design wavelength. 12. Dichroic mirror according to claim 8, characterized in that, when the layer with the highest number is made of the material with the high refractive index, the first layer, the second layer and the layer with the highest number each to one optical thickness are adjusted · greater is.t _(as A / 4 in order to avoid ripples on the short wavelength side of the design wavelength. 15. Dichroic mirror according to claim 1, characterized in that that it contains a substrate on which a coating in alternating layers of a material with a low Refractive index and a material with a high "409834/0856 Refractive index is attached, with the odd-numbered layers calculated from the substrate_. consist of the low refractive index material, the even layers of the high refractive index material, and whereL the optical thickness of a low refractive index layer and the next subsequent high refractive index layer with respect to a design wavelength λ in a ratio of / 1/4 · 3, λ / 4 are to each other, and that at least the first layer, the second layer and the layer with the highest number, calculated from the substrate, are each set to an optical thickness that is an odd multiple of λ / Κ of the construction wavelength λ deviates. 14 * dichroic mirror according to claim 13, characterized in that that if the layer with the highest number consists of the material with the low refractive index, then the the first layer is set to an optical thickness which is smaller than Λ / 4, the second layer to an optical thickness Thickness is set that is less than 3 - ^ / 4, and the Layer with the highest number is set to an optical thickness that is greater than Λ / 4 in order to avoid ripples aμf to avoid the long wavelength side of the design wavelength. 15 dichroic mirror according to claim 13, characterized in that that if the layer with the highest number of the material with the. low refractive index, the the first layer is set to an optical thickness which is greater than A / 4, the second layer to an optical thickness, which is greater than 3 λ / 4, and the layer with the highest Number to an optical thickness that is smaller than λ / 4 in order to avoid ripples on the short-wave side of the design wavelength 409834/0856 l6. Dichroic mirror according to Claim 13, characterized in that, when the layer with the highest number consists of the material with the high refractive index, the first layer is set to an optical thickness which is smaller than λ / k second layer to an optical thickness that is less than 3 λ / Κ, Λΐηά. the highest numbered layer to an optical thickness less than 3λ / k in order to avoid ripples on the long wavelength side of the design wavelength. IT. Dichroic mirror according to claim 13, characterized in that when the layer with the highest number consists of the material with the high refractive index, the first layer is set to an optical thickness which is smaller than λ / 4, the second layer to an optical thickness that is smaller than 3 Λ / ^, 'νΰΩ, ά the layer with the highest number to an optical thickness that is smaller. than 3 λ / ^ t to avoid ripples on the short-wave side of the design wavelength. 18. Dichroic mirror according to claim I3, characterized in that when the layer with the highest number consists of material with the high refractive index, the first layer is set to an optical thickness which is greater than λ / ^ * the second layer to an optical thickness that is greater than 3 λ / ^, and the layer with the `` highest number to an optical thickness that is greater than 3 fyh, in order to avoid inferences on the short-wave side of the design wavelength. 19 dichroic mirror according to claim 1, characterized in that that it contains a substrate which is a coating of alternating layers of a material with a low refractive index and a material with a 4,983 / 0856 has a high refractive index, the odd layers from the substrate from the material with low refractive index, the even layers from the material with high refractive index and wherein the optical thickness of a layer with low refractive index and the next subsequent layer with high refractive index with respect a construction wavelength \ in a ratio of λ / k : ^ / 4 to each other, and that at least the first layer, the second layer and ■ the layer with the highest number counted from the substrate are each set to an optical thickness, many deviate from an odd multiple of - ^ A of the construction wavelength λ . 20 »Dichroic mirror according to claim 19, characterized in that that if the layer with the highest number is made of the low refractive index material consists, the first layer and the second layer are each set to an optical thickness which is smaller is as λ / 4, and that the highest-numbered layer is set to an optical thickness which is larger than Λ / 4 to get ripples on the longwave side of the Avoid construction wavelength. 21. A dichroic mirror according to claim 19, characterized in that when the layer having the highest number out of the material of low refractive index, the first and zweit.e _{i (.Schicht} are each set to an optical thickness greater is as ^ / 4, and that the layer with the highest number is set to an optical thickness which is smaller than λ / 4 in order to avoid ripples on the short-wave side of the design wavelength. 409834/0856 22. Dichroic mirror according to claim 19 »characterized in that that if the layer with the highest number is made of the high refractive index material, the first layer, the second layer, and the highest numbered layer are each set to an optical thickness are, . which is smaller than λ / 4 to eliminate ripples on the long wavelength side of the design wavelength to avoid. 23 · Dichroic mirror according to claim IS, characterized in that when the layer with the highest number consists of the material with a high refractive index, the first layer, the second layer and the layer with the highest number are each set to an optical thickness , which is greater than V4 to avoid ripples on the short-wave side of the design wavelength. : 409834/0856 - Blank page