Coloured PVB

Abstract

A method is directed to modifying the color in transmission of an optical system that incorporates an electrochromic device. Such a modified optical system makes it possible to combine the notions of effectiveness in the chromatic variation on the one hand, and of limitation of losses in light transmission on the other hand.

Claims

1. A process for modifying the color in transmission of an optical system from an initial state having an initial color (L*i; a*i; b*i) to a final state having a final color (L*f; a*f; b*f), said process comprising at least one step of modifying the absorbance properties of an optical element of said optical system so that at least 50% of the total absorption (A) of said optical element occurs in an effective absorption range ([λ.sub.min; λ.sub.max]) defined so that a variation in absorption by the optical system of any amount of light at a wavelength λ comprised in said effective absorption range ([λ.sub.min; λ.sub.max]) generates a variation ΔC(λ) in chromaticity distance (C.sub.i, C(λ)) smaller than 0, where:
ΔC(λ)=C(λ)−C.sub.i=√{square root over ((a*(λ)−a*ref).sup.2+(b*(λ)−b*ref).sup.2)}−√{square root over ((a*i−a*ref).sup.2+(b*i−b*ref).sup.2)} (C.sub.i, C(λ)) is the chromaticity distance between a given value ((a*i; b*i), (a*(λ); b*(λ))) of chromaticity in transmission and a reference value (a*ref; b*ref) of chromaticity in transmission, (a*i; b*i) is the initial value of chromaticity in transmission, (a*(λ); b*(λ)) is the value of chromaticity in transmission obtained following the variation in absorption achieved at the wavelength λϵ[λ.sub.min; λ.sub.max].

2. The process for modifying the color in transmission of an optical system as claimed in claim 1, wherein said optical system comprises at least one electrochemical functional system with electrically controllable optical and/or energy properties.

3. The process for modifying the color in transmission of an optical system as claimed in claim 2, wherein said at least one electrochemical functional system comprises at least one ionically conductive layer arranged between a first electrode coating and a second electrode coating, the at least one electrochemically active layer being configured to pass reversibly between a first state and a second state having optical and/or energy transmission properties different from the first state.

4. The process for modifying the color in transmission of an optical system as claimed in claim 1, wherein said optical system comprises at least the following optical elements: a functional assembly comprising a substrate of glazing function, on which substrate said electrochemical functional system with electrically controllable optical and/or energy properties is formed, and a lamination interlayer, which is interposed between the functional assembly and a counter-substrate of glazing function.

5. The process for modifying the color in transmission of an optical system as claimed in claim 4, wherein the lamination interlayer is made of PVB.

6. The process for modifying the color in transmission of an optical system as claimed in claim 1, further comprising a step of determining said effective absorption range ([λ.sub.min; λ.sub.max]), using a representation of the chromatic variations (Δa*(λ); Δb*(λ)) in transmission and of the variations ΔTL(λ) in losses in light transmission generated as a function of the wavelength λ at which said optical system undergoes a variation in the amount of light absorbed.

7. The process for modifying the color in transmission of an optical system as claimed in claim 1, wherein said reference chromaticity value (a*ref; b*ref) has a saturation value of zero.

8. The process for modifying the color in transmission of an optical system as claimed in claim 1, wherein said optical element is modified so that its absorption spectrum is substantially centered on a wavelength at which the ratio ΔC(λ)/ΔTL(λ) is maximal, where: TL(λ) is the light transmission of said optical system, as determined according to standard NF EN 410, and ΔTL(λ) is the variation in light transmission TL(λ) generated by the variation in the amount of light absorbed by the optical system at the wavelength λ.

9. The process for modifying the color in transmission of an optical system as claimed in claim 1, wherein said optical element is modified so that at least 55% of its total absorption (A) is comprised in said effective absorption range ([λ.sub.min; λ.sub.max]).

10. The process for modifying the color in transmission of an optical system as claimed in claim 9, wherein said optical element is modified so that at least 70% of its total absorption (A) is comprised in said effective absorption range ([λ.sub.min; λ.sub.max]).

11. The process for modifying the color in transmission of an optical system as claimed in claim 1, wherein the modifying step comprises coloring the bulk of said optical element to be modified and/or depositing a colored coating on at least one of the faces of said optical element to be modified.

12. The process for modifying the color in transmission of an optical system as claimed in claim 1, further comprising at least one preliminary step of measuring the spectrum in transmission of said optical system.

13. An optical system the color of which has been modified by a process as claimed in claim 1.

14. The optical system as claimed in claim 13, further comprising the following optical elements: a functional assembly comprising a substrate of glazing function, on which substrate an electrochemical functional system with electrically controllable optical and/or energy properties is formed, and a lamination interlayer, which is interposed between the functional assembly and a counter-substrate of glazing function.

15. The optical system as claimed in claim 13, further comprising a reflection-controlling coating to modify the color of the optical system in reflection toward one of the sides of said optical system.

16. The optical system as claimed in claim 15, wherein said reflection-controlling coating is arranged between said functional assembly and said counter substrate.

17. The optical system as claimed in claim 15, wherein the reflection-controlling coating is made of silicon nitride (SiNx) in order to attenuate the color of the optical system in reflection toward one of the sides of said optical system.

18. A process for manufacturing an optical system as claimed in claim 13.

19. A method comprising utilizing an optical system as claimed in claim 13 as an architectural glazing, as an exterior glazing of an internal partition or glazed door, as a glazing in windows or internal partitions of transportation means including trains, airplanes, automobiles, boats and ships, as glazings for display screens including computer screens or television screens, for spectacles or objectives of cameras or to protect solar panels.

20. An optical system comprising: a first optical element that has an initial color (L*i; a*i; b*i) in transmission, a second optical element that is such that the optical system has a final color (L*f; a*f; b*f) in transmission, wherein at least 50% of the total absorption (A) of said second optical element is comprised in an effective absorption range ([λ.sub.min; λ.sub.max]) defined so that a variation in absorption by the optical system of any amount of light at a wavelength λ comprised in said effective absorption range ([λ.sub.min; λ.sub.max]) generates a variation ΔC(λ) in chromaticity distance (C.sub.i, C(λ)) smaller than 0, where:
ΔC(λ)=C(λ)−C.sub.i=√{square root over ((a*(λ)−a*ref).sup.2+(b*(λ)−b*ref).sup.2)}−√{square root over ((a*i−a*ref).sup.2+(b*i−b*ref).sup.2)} (C.sub.i, C(λ)) is the chromaticity distance between a given value ((a*i; b*i), (a*(λ); b*(λ))) of chromaticity in transmission and a reference value (a*ref; b*ref) of chromaticity in transmission, (a*i; b*i) is the initial value of chromaticity in transmission, (a*(λ); b*(λ)) is the value of chromaticity in transmission obtained following the variation in absorption achieved at the wavelength λϵ[λ.sub.min; λ.sub.max].

Description

(1) Other features and advantages of the invention will become apparent on reading the following description of particular embodiments, which are given by way of simple nonlimiting illustrative examples, and the appended figures, in which:

(2) FIG. 1 is a graph illustrating the spectral sensitivity of the photosensitive rods and cones present in a human eye,

(3) FIG. 2 is a graph showing two absorption spectra allowing a color of given chromaticity in transmission to be obtained,

(4) FIG. 3 is a graphical representation of the sensitivity of the human eye as a function of the wavelength of an incident ray,

(5) FIG. 4 is a schematic view in cross section of an optical system according to one particular embodiment of the invention,

(6) FIG. 5 is a flowchart illustrating the successive steps of a process for modifying the color in transmission of an optical system according to one particular embodiment of the invention,

(7) FIG. 6 is a graph illustrating, in a CIE L*a*b* color space, the modification of the color in transmission of an optical system according to one particular embodiment of the invention,

(8) FIG. 7 is a graphical representation of the chromatic variations (Δa*(λ); Δb*(λ)) in transmission and variations ΔTL(λ) in losses in light transmission as a function of the wavelength λ at which an optical system according to one particular embodiment of the invention undergoes a variation in the amount of absorbed light,

(9) FIG. 8 is a graphical representation of the absorption spectrum of a modified optical device according to one particular embodiment of the invention.

(10) In the various figures, unless otherwise indicated, the same reference numbers have been used to reference elements that are similar or identical.

(11) The various elements illustrated by the figures are not necessarily shown to actual scale, the accent more being placed on the representation of the general operation of the invention.

(12) A plurality of particular embodiments of the invention are described below. It will be understood that the present invention is in no way limited by these particular embodiments and that other embodiments may perfectly well be implemented.

(13) According to one particular embodiment and such as illustrated in FIG. 4, the invention relates to an optical system 1 comprising at least the following optical elements:

(14) a functional assembly 2 comprising a substrate 3 of glazing function, on which substrate a functional electrochemical system 4 with electrically controllable optical and/or energy properties is formed, said electrochemical functional system 4 comprising at least one ionically conductive layer arranged between a first electrode coating and a second electrode coating, the least one ionically conductive layer being suitable for reversibly passing between a first state and a second state having different optical and/or energy transmission properties from the first state, and

(15) a lamination interlayer 5 made of PVB, interposed between the functional assembly 2 and a counter-substrate 6 of glazing function.

(16) In particular, this optical system 1 comprises:

(17) a first optical element that has what is called an “initial” color (L*i; a*i; b*i) in transmission,

(18) a second optical element that is such that the optical system 1 has what is called a “final” color (L*f; a*f; b*f) in transmission, and that is characterized in that at least 50% of the total absorption of this second optical element is comprised in an effective absorption range (λ.sub.min; λ.sub.max) defined so that a variation in absorption by the optical system 1 of any amount of light at a wavelength λ comprised in the effective absorption range [λ.sub.min; ×.sub.max] generates a variation ΔC(λ) in chromaticity distance (C.sub.i, C(λ)) smaller than 0.

(19) According to this particular embodiment, the electrochemical functional system 4 with electrically controllable optical and/or energy properties is formed on that face of the substrate 3 which is opposite the lamination interlayer 5. According to one alternative embodiment, this functional system may be interposed between the substrate 3 and the lamination interlayer 5.

(20) According to one particular embodiment and such as illustrated in FIG. 4, the optical system 1 comprises a reflection-controlling coating 7 made of silicon nitride (SiNx) and arranged on the internal face of the counter-substrate 6. This coating 7 in particular allows the color in reflection of the optical system 1 to be attenuated from the point of view of an observer positioned on the exterior of a building equipped with such an optical system 1. Such an optical coating furthermore allows the amount of exterior light transmitted by the optical system to be increased or, in other words, the overall losses in transmission to be decreased, such as already described in the present text.

(21) According to alternative embodiments, the optical system 1 may adopt various glazing configurations in which the various optical elements are organized along the optical axis X:

(22) single glazing in which two sheets of glazing function, the substrate 3 and the counter-substrate 6, are laminated into a single glazing: (exterior of a building) counter-substrate 6/lamination interlayer 5/functional assembly 2 comprising the substrate 3 (building interior);

(23) double glazing, shown in FIG. 4, in which two among three sheets of glazing function, the substrate 3 and the counter-substrate 6, are laminated into a laminated glazing: (exterior of a building) counter-substrate 6/lamination interlayer 5/functional assembly 2 comprising the substrate 3/cavity 8 filled with an inert gas/low-E layer/glass sheet 9 (building interior);

(24) triple glazing in which two among four glass sheets, the substrate 3 and the counter-substrate 6, are laminated together: (exterior of a building) counter-substrate 6/lamination interlayer 5/functional assembly 2 comprising the substrate 3/cavity filled with an inert gas/glass sheet/cavity filled with an inert gas/low-E layer/glass sheet (building interior).

(25) Generally, the glazing comprises a laminated glazing. This laminated glazing is intended to be placed on the exterior side of the building. By double glazing what is meant is an assembly of two glazings spaced apart and separated by a gas-filled or vacuum-filled cavity, and by triple glazing what is meant is an assembly of three glazings spaced apart and separated by two respective gas- or vacuum-filled cavities. As described above, at least one of the spaced-apart glazings of a multiple glazing may be laminated. In all these configurations, the counter-substrate 6 is intended to be positioned toward the exterior.

(26) It will be noted that the electrochromic device included in the optical system 1 according to the invention may include one or more transparent conductive coatings such as SnO.sub.2:F or ITO and/or one or more networks of conductors such as metal wires. The electrochemically active layer and the one or more layers of electrolyte may take the form of a gel solution and/or of an ionically conductive polymer and/or of one or more mineral layers deposited by magnetron cathode sputtering, CVD or the sol-gel process, without departing from the spirit of the invention.

(27) An optical system 1 according to the invention makes it possible to combine the notions of effectiveness of the chromatic variation on the one hand, and of limitation of losses in light transmission on the other hand. This advantageous optical system 1 may in particular be obtained by implementing a process for modifying its color in transmission, from an initial color (L*i; a*i; b*i) to a final color (L*f; a*f; b*f), such as described in more detail in the rest of the text and illustrated in FIG. 5.

(28) In a first step (S1), the spectrum in transmission of the optical system 1 is measured using a spectrophotometer or any other known device of equivalent function.

(29) On the basis of this measurement, the initial chromaticity value (a*i; b*i) in transmission of the optical system is determined (step S2), for example using a known color space such as the CIE XYZ (1931) system.

(30) According to one particular embodiment, and such as illustrated in FIG. 6, this initial chromaticity value (a*i; b*i) in transmission is positioned in the known CIE L*a*b* color space, which is commonly called the CIE LAB space, in which: The L* component is lightness, and is measured from 0 (black) to 100 (white), The component a* represents a range of 600 levels on an axis extending from the color green (−300) to the color red (+299), The component b* represents a range of 600 levels on an axis extending from the color blue (−300) to the color yellow (+299).

(31) Thus, in the example illustrated in FIG. 6, the value (a*i; b*i), measured in the clear state, of the initial chromaticity in transmission of an optical device (1) is (−10,4805; 10,9183), this corresponding to a yellow-green color. This value is represented by a square. It will be noted that in other embodiments, this initial chromaticity value (a*i; b*i) in transmission may vary over all of the visible spectrum, without departing from the spirit of the invention.

(32) Subsequently, the initial chromaticity distance C.sub.i, between this initial chromaticity value (a*i; b*i) in transmission and a given reference chromaticity value (a*ref; b*ref) is determined (step S3). This given reference value relates to a reference color that it is sought to obtain. It differs from the “final color” that is obtained at the end of the modifying process. The initial chromaticity distance C.sub.i, corresponds to the norm of the vector extending from the initial chromaticity point (a*i; b*i) to the chosen reference chromaticity point (a*ref; b*ref).

(33) According to the particular embodiment illustrated in FIG. 6, the chosen reference chromaticity value (a*ref; b*ref) is of zero saturation (0; 0). Such a process, commonly referred to as neutralization, aims to attenuate a color in order that it have no hue in transmission other than that of the light rays transmitted from the exterior. With regard to the CIE L*a*b* coordinate system, such a neutralization implies a colorimetric variation toward a color of higher a* component and of lower b* component. According to this embodiment, the initial chromaticity distance C.sub.i satisfies the following equation:
C.sub.i=√{square root over ((a*i).sup.2+(b*i).sup.2)}

(34) According to one alternative embodiment, the chosen reference chromaticity value (a*ref; b*ref) has a non-zero saturation and therefore corresponds to a particular color to be obtained in transmission, whether this be for reasons of technical and/or esthetic nature. According to this alternative embodiment, the initial chromaticity distance C.sub.i satisfies the following equation:
C.sub.i=√{square root over ((a*i−a*ref).sup.2+(b*i−b*ref).sup.2)}

(35) The variation ΔC(λ) in this chromaticity distance between the initial chromaticity value (a*i; b*i) and a chromaticity value (a*(λ); b*(λ)) in transmission obtained following a variation in absorption at the wavelength λ, which value is represented by a triangle, expresses the capacity of the optical system 1 to get closer to the desired chromaticity in transmission, i.e. to the reference chromaticity. Thus, this variation ΔC satisfies the following equation:
ΔC(λ)=C(λ)−C.sub.i=√{square root over ((a*(λ)−a*ref).sup.2+(b*(λ)−b*ref).sup.2)}−√{square root over ((a*i−a*ref).sup.2+(b*i−b*ref).sup.2)}

(36) A negative ΔC(λ) value expresses a variation in chromaticity, after absorption of light at a wavelength λ, allowing the initial chromaticity to be brought closer to the one desired, the obtained chromaticity being “closer” to the desired chromaticity than the initial chromaticity. In contrast, the obtainment of a positive ΔC(λ) value, after absorption of light at a given wavelength λ, expresses an increase in the distance between the desired chromaticity, i.e. the reference chromaticity, and the initial chromaticity.

(37) FIG. 7 shows three estimated curves of chromatic variations (Δa*(λ); Δb*(λ)) in transmission and of variations ΔTL(λ) in losses in light transmission related to a substantially monochromatic absorption (absorption spectrum full width at half maximum smaller than 5 nm) implemented, for various wavelengths λ in all of the optical spectrum, for a known SAGE™ electrochromic double glazing having the following characteristics: A counter-substrate made of glass of a thickness of 4 mm, An interlayer made of polymer of a thickness of 0.89 mm, A substrate made of glass of 2.2 mm thickness, coated with an electrochromic layer, A cavity filled with argon of 12.2 mm thickness, A counter-glass of 4 mm thickness.

(38) It will be noted that the choice of a SAGE™ electrochromic double glazing is purely arbitrary. Thus, a graphical representation similar to that of FIG. 7 may be obtained for any given optical system.

(39) To return to the subject of FIG. 7, the estimated curves of the chromatic variations Δa*(λ) and Δb*(λ) are represented by squares and rhombuses, respectively. The estimated curve of losses ΔTL(λ) in light transmission is represented by triangles and this curve corresponds to the transposition of the curve illustrated in FIG. 3 to the graph of FIG. 7.

(40) With regard to the CIE L*a*b* color space illustrated in FIG. 6, a negative Δa* value bears witness to a chromatic variation toward the green whereas a positive Δa* value refers to a chromatic variation toward the red. Following the same reasoning, a negative Δb* value bears witness to a chromatic variation toward the blue whereas a positive Δa* value refers to a chromatic variation toward the yellow. Following the transposition of this reasoning to the graph of FIG. 7, it may be seen that the chromaticity in transmission tends: toward the color red in the absorption wavelength range comprised between 480 and 575 nm (positive Δa*), toward the color green in the absorption wavelength ranges respectively comprised between 400 and 480 nm, and between 575 and 800 nm (negative Δa*), toward the color yellow in the absorption wavelength range comprised between 400 and 500 nm (positive Δb*(λ)), toward the color blue in the absorption wavelength range comprised between 500 and 800 nm (negative Δb*(λ)).

(41) The chromatic variations in transmission (Δa*(λ); Δb*(λ)) may respectively be defined using the following equations:
Δa*(λ)=a*(λ)−a*i
Δb*(λ)=b*(λ)−b*i

(42) With regard to these two equations, to the aforementioned one defining ΔC(λ), and to the known values of a*i, a*ref, b*i and b*ref, it is easy to calculate the variation ΔC(λ) in chromaticity distance affecting the optical system 1.

(43) In a following step (S4), using FIG. 7 an effective absorption range [λ.sub.min; λ.sub.max] is determined in which a variation in absorption by the optical system (1) of any amount of light at a wavelength λ comprised in said effective absorption range [λ.sub.min; λ.sub.max] generates a modification of the initial chromaticity toward the reference chromaticity, or in other words a variation ΔC(λ) in chromaticity distance (C.sub.i, C(λ)), smaller than 0.

(44) With regard to the chromatic variations Δa*(λ); Δb*(λ)) in transmission of an optical device 1 as a function of absorption wavelengths, such as illustrated in FIG. 7, it is determined that the effective absorption range [λ.sub.min; λ.sub.max] meeting this criterion lies approximately between 490 and 588 nm.

(45) According to one particular embodiment, the effective absorption range [λ.sub.min; λ.sub.max] is determined so that the value of the ratio ΔC(λ)/ΔTL(λ) is maximal. The spectral transmittance of the optical system varies with the ratio ΔC(λ)/ΔTL(λ). Thus, assuming that the variation ΔTL(λ) is smaller than 0, the higher the value of ΔC(λ)/ΔTL(λ), the better the transmission of the incident light by the system at an absorption wavelength λ. In other words, the increase in ΔC(λ)/ΔTL(λ) allows the losses in transmission generated by the optical system to be decreased.

(46) Surprisingly, it has been determined in the case of the example illustrated in FIG. 6, for which example the measured value (a*i; b*i) of initial chromaticity in transmission is (−10,4805; 10,9183), that the value of ΔC(λ)/ΔTL(λ) is maximal at an absorption wavelength of 550 nm. However, this value is located in the wavelength range in which the eye is the most sensitive. This result therefore goes against an established technical prejudice, namely that it is recommended to avoid absorbing transmitted light in a spectral region in which the human eye is the most sensitive, in order to limit losses ΔTL(λ) in light transmission. Contrary to received ideas, it has therefore been established by the inventors that, with a view to modifying the color in transmission of an optical system while preserving a satisfactory clearness, it is more important to take into account the ratio ΔC(λ)/ΔTL(λ) than to consider in isolation the loss ΔTL(λ) in light transmission of the system.

(47) Following the determination of the effective absorption range [λ.sub.min; λ.sub.max] (step S4), the absorbance properties of an optical element of said optical system 1 are modified (step S5) so that at least 50% of the total absorption (A) of this element is achieved in an effective absorption range [λ.sub.min; λ.sub.max] defined so that a variation in absorption by the optical system 1 of any amount of light at a wavelength λ comprised in said effective absorption range [λ.sub.min; λ.sub.max] generates a variation ΔC(λ) in chromaticity distance (C.sub.i, C(λ)) smaller than 0.

(48) It will be noted that this modification of the absorbance properties may be achieved by replacing the targeted optical element, by modifying its intrinsic properties and/or by adding a colored coating. Alternatively, the subject matter of the invention may take the form of a process for manufacturing an optical system in which the absorbance properties of one of its optical elements have already been modified, compared to a known prior-art optical system. Thus, according to this particular embodiment, the invention relates to a process for manufacturing an optical system 1 that comprises:

(49) a first optical element that has what is called an “initial” color (L*i; a*i; b*i) in transmission,

(50) a second optical element that is such that the optical system 1 has what is called a “final” color (L*f; a*f; b*f) in transmission, and that is characterized in that at least 50% of the total absorption of this second optical element is comprised in an effective absorption range (λ.sub.min; λ.sub.max) defined so that a variation in absorption by the optical system 1 of any amount of light at a wavelength λ comprised in the effective absorption range [λ.sub.min; λ.sub.max] generates a variation ΔC(λ) in chromaticity distance (C.sub.i, C(λ)) smaller than 0.

(51) FIG. 8 is a graphical representation of the absorption spectrum (rhombuses) of a SAGE™ optical device 1, such as already described in the present text, in which the bulk of the PVB lamination interlayer 5 has been colored. This graph in addition illustrates the variation in the ratio ΔC(λ)/ΔTL(λ) (curve through the squares) as a function of the absorption wavelength. By tinting the PVB, the absorption spectrum of the optical device 1 has been modified so that at least 58% of the total absorption (A) of this optical element is comprised in the effective absorption range [λ.sub.min; λ.sub.max]. Such as indicated in the present text and illustrated in FIG. 8, this effective absorption range [λ.sub.min; λ.sub.max] lies approximately between 290 and 588 nm.

(52) It will be noted that tinting a PVB so that at least one preset portion of its total absorption (A) is comprised in a preset wavelength range presents no particular technical difficulty, providing of course that these two parameters are judiciously determined/chosen beforehand, this being, inter alia, the crux of the invention.

(53) Alternatively or in combination, the modification (S5) of the absorbance properties of an optical element of the optical system 1 is achieved via the addition, to the material, of semiconductor or metal nanoparticles and/or the deposition of a colored coating on at least one of the faces of the optical element 1 to be modified.

(54) It will be noted that according to particular embodiments, the absorbance properties of one or more elements of the optical system 1 may alternatively or in combination be modified.

(55) Thus, and according to one particular embodiment, the ionically conductive layer integrated into the electrochromic device is or forms part of the optical elements the absorbance properties of which are modified.

(56) In addition, the choice of the desired portion of the total absorption (A) comprised in the effective absorption range [λ.sub.min; λ.sub.max] may differ from one embodiment to the next without however departing from the spirit of the invention.

(57) In order to observe the reproduction of the present invention by an optical system 1 comprising at least one optical element the absorbance properties of which have been modified, it is possible to proceed as follows: Measure the spectrum in transmission of said modified optical system, Compare this spectrum to that of said optical device before modification, in order to deduce therefrom the modification parameters of said optical element, namely: the effective absorption range [λ.sub.min; λ.sub.max] used, the chosen portion of the total absorption (A) comprised in this effective absorption range [λ.sub.min; λ.sub.max].

(58) It will be noted that the spectrum in transmission of said optical system before modification may already be known, since it is that of a standard optical system, and/or be determinable by replacement of said modified optical element with a standard optical element and subsequent measurement of the spectrum in transmission of the optical system incorporating this standard replacement element. By “standard optical element” what is meant is an optical element having a negligible influence on the obtained spectrum in transmission of the optical system.

(59) The values described in the present text must not be understood as being strictly limited to the mentioned numerical values. Rather, unless otherwise indicated, each value designates both the exactly mentioned value and a range of functionally equivalent values encompassing this value.

(60) Although particular embodiments of the present invention have been illustrated and described, it is obvious that various other changes and modifications may be made within the spirit and scope of the invention. The present text is therefore intended to cover in the appended claims any modification falling within the scope of the present invention.