Ultra-thin thin-film optical interference filter

Abstract

To provide a thin-film interference filter.SOLUTION: A thin-film interference filter has a thin-film interference multi-layer stack composed of individual thin-film filters arranged in groups to form a plurality of repeat unit blocks. The thin-film interference filter is flexible enough to be bendable to a radius of curvature of 250 mm or even less without permanently damaging the thin-film interference filter. The thin-film interference filter may have a second thin-film interference multi-layer stack composed of individual thin-film layers arranged in repeat unit blocks, and such thin-film layers may have different optical transmission spectra from each other. At least one interlayer between the first thin-film interference multi-layer stack and the second thin-film interference multi-layer stack may block a range of wavelengths of infrared, visible, or ultraviolet light. A defect layer of a different optical thickness from neighboring thin-film layers forms the repeat unit blocks, thereby creating a Fabry Perot cavity resonator.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present application relates to an optical interference filter, particularly a multilayer thin film filter.

Background Art

[0002] Optical interference filters are inherently complex and costly to manufacture because of the complex structures required to obtain high optical density (O.D.) while maintaining high spectral selectivity. The ability to finely adjust the spectral shape of the filtered light is one of the advantages of interference filters.

[0003] Traditionally, such interference filters are made by vacuum deposition of transparent thin film optical layers onto a plastic or glass substrate. The substrate with the thin film layers deposited thereon typically has a thickness in the range of 0.5 mm to 10 mm. Coating for each layer and the resulting filter cut generate tension in the thin film stack, and this tension often causes bending and crack generation on the thin film filter, especially when the substrate is too thin. This problem is more prominent for filters with a large number of layers to achieve high optical performance. A large number of layers are required to obtain a high optical density. A wide spectral cutoff range requires a large number of layers. A sharp transition edge between a high transmission level and a low transmission level often requires a complex layer structure with a large number of layers having different refractive indices. Similarly, suppressing side reflection bands to produce a flat transmission curve often requires a complex layer structure and a large number of layers having different refractive indices. Conventionally, filter membranes or films mainly composed of thin plastics have been made mainly by three methods.

[0004] 1. Coextrusion: In this process, two or more materials typically flow through a feed block and form a multi-layer stack of materials. This multi-layer flow of materials is then pushed into a series of layer multipliers where the original layer stack is split in one direction in various ways and recombined vertically, with the aim of doubling the number of layers while changing the width or height of the stack. Next, the final multi-layer flow is pushed into a die to spread the multi-layer stack into a multi-layer film state. This process has certain drawbacks. A) The multi-layer film or sheet is limited to unit blocks with periodically repeated layers. In other words, the same multi-layer stack emerging from the feed block is periodically doubled. The multipliers are only capable of providing various splitting and recombination ratios, and as a result, the thickness ratio will vary from one multiplier to the next. Customization of individual layers is not feasible. B) This process is only useful for all plastic filters that can be processed by coextrusion equipment. For example, a glass filter cannot be introduced.

[0005] 2. Coating on a Flexible Substrate: The web coating process is widely used in the window film industry, where a roll of plastic film is supplied into a vacuum chamber to deposit a thin film layer. In many cases, there are a number of deposition sources in series, and as a result, each deposition source deposits one layer at a time. This method is often used for a single layer structure with only a few antireflection layers, scratch protection, or heat dissipation layers, which often include as many as 20 coating layers. Given the brutality of the coating source material for the thin film layer, a large number of layers causes cracking of the thin film stack when the flexible substrate is bent. For this reason, complex thin film filters cannot be made on a flexible substrate by this method. In a similar manner, a sheet of the flexible substrate is attached to a drum or fixture provided within a closed vacuum chamber. This method produces a very small amount of product, but allows for somewhat more complex layers with a larger number of layers made. Despite the fragile nature of the layers and the limitations on bending ability, the uses of these products are limited. These two methods of growing multilayer filters on a flexible substrate are also used to produce small-sized particles of the multilayer filter by intentionally cracking the multilayer filter layer once it has been grown on the flexible substrate. The flexible substrate makes it easier to produce particles from such filters.

[0006] 3. Nano-Lamination: In this method, nanoscale layers of materials with various refractive indices are directly laminated onto a flexible substrate in a roll-to-roll manner. A major drawback of this method and the resulting product is the lack of uniformity and controllability of the individual sublayers at the submicron scale. Therefore, the resulting filter product does not function well as a highly selective filter.

[0007] U.S. Patent Application Publication No. 2014 / 0242329 (A1) describes a method for manufacturing a thin film optical filter using compression molding of structured preform blocks. This method enables the production of thin film optical interference filters in the form of all-plastic flexible ultra-thin films and sheets. This method addresses two major drawbacks of traditional vacuum-coated thin film filters by providing extremely high scalability and demonstrating high performance while being able to conform to curved and bent surfaces with ultra-thin filters that can be bent to a radius of curvature of 250 mm or even less without permanently damaging, deforming, or cracking the thin film optical filter or the thin film layers. Regarding the thermal compression forming method of thin film filters, U.S. Patent Application Publication No. 2014 / 0242329 (A1) is incorporated by reference in its entirety and made a part of this specification.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Summary of the Invention

[0009] The present invention provides a thin film interference filter having a first thin film interference multilayer stack composed of individual thin film layers arranged in groups to form a plurality of first repeating unit blocks, the thin film interference filter being flexible enough to be bent to a radius of curvature of 250 mm or less, or even less, without permanently damaging, deforming, or cracking the thin film interference filter as a whole or the thin film layers in at least one of the multilayer stacks.

[0010] The second thin film interference multilayer stack, composed of individual thin film layers arranged in groups to form a plurality of second repeating unit blocks, preferably has a different light transmission spectrum from the first thin film interference multilayer stack.

[0011] At least one intermediate layer positioned between the first thin-film interference multilayer stack and the second thin-film interference multilayer stack has a thickness that is 10 to 1000 times the thickness of each individual thin-film layer in the first thin-film interference multilayer stack. The intermediate layer may be an absorption layer for blocking the wavelength range of infrared light, visible light, or ultraviolet light for effective absorption of selected wavelengths.

[0012] A jacket layer sandwiching one or more thin-film interference multilayer stacks may be provided for physical protection of the one or more thin-film interference multilayer stacks.

[0013] In addition, 1 to 15 layers of the antireflection thin-film layers deposited on the outer surface of at least one of the first jacket layer or the second jacket layer improve the optical properties of the filter. The antireflection thin-film layer may mainly consist of a polymer or glass and may be formed by co-thermoforming with the first jacket layer and the first thin-film interference multilayer stack during thermoforming. As a variant, at least some of the antireflection thin-film layers may be formed by coating the first jacket layer after thermoforming.

[0014] For effective protection, each of the first jacket layer and the second jacket layer has a thickness that is 10 to 1000 times the thickness of each individual thin-film layer in the first thin-film interference multilayer stack. At least one of the first jacket layer or the second jacket layer may have a dual function by also being an absorption layer that blocks the wavelength range of infrared light, visible light, or ultraviolet light.

[0015] Each of the individual thin-film layers in the first multilayer stack has a thickness in the range from 5 nm to 5000 nm, and the thin-film interference filter has an overall thickness in the range from 0.01 mm to 1 mm.

[0016] The thin film interference filter has a transmission spectrum that varies between a low transmittance of at most 20% of the incident light of the first wavelength and a high transmittance of at least 80% of the incident light of the second wavelength. At least one transition edge between the low transmittance and the high transmittance has a width of at most 5% of the third wavelength between the first wavelength and the second wavelength, and at this width, the thin film interference filter transmits 50% of the incident light.

[0017] Other details and other advantages of the present application will become apparent from the following description of the accompanying drawings. The drawings are provided attached to this specification for illustrative purposes only and are not intended to limit the scope of the present invention.

Brief Description of the Drawings

[0018]

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Mode for Carrying Out the Invention

[0019] Throughout the following description, the term "or" is used as an inclusive term and, unless otherwise specified, indicates one or the other or both options. Thus, the individual features described, for example, an absorption layer, a defect, a unit block, a jacket layer having various refractive indices, various layer thicknesses, etc., can be combined with a single multilayer thin film optical interference filter. Some of the layers are simultaneously stretched through a furnace, and some of the layers are deposited by coating within the same filter. The present invention provides various forms of filters and layer structures having physical characteristics and specifications related to flexibility and optical performance.

[0020] In the first embodiment shown in FIG. 1, the multilayer thin film optical interference filter 10 has two jacket layers 12 and a multilayer stack 16 sandwiched between the jacket layers 12. The multilayer stack 16 is composed of dozens of thin film layers 18 shown in partial detail in FIG. 2. The jacket layers may be transparent over the entire wavelength range of infrared (IR), visible, and ultraviolet (UV) light, or at least over all wavelengths transmitted by the multilayer stack 16. As a result, the jacket layers 12 do not substantially affect the optical properties of the thin film filter. The term "substantially" as used in this application means within the range of 10%. As a modification, one or both of the jacket layers 12 can constitute an absorption layer that blocks one or more wavelength ranges that would otherwise be transmitted by the multilayer stack 16 if not so configured. In this application, unless otherwise specified, the term "block" means transmission of less than 50% of the incident light energy, while the other means absorbing more than 50% of the incident light energy. Each jacket layer 12 has a thickness in the range from 10 times to 1000 times the thickness of each of the individual thin film layers 18 in the multilayer stack 16.

[0021] In a general embodiment, the thin film interference filter 10 includes a combination of a thin film interference multilayer 18 and an absorptive or transparent intermediate layer 20. Such a multilayer and intermediate layer have a thickness in the range from 10 times to 1000 times the thickness of the individual thin film layers in the multilayer stack 16. This combination is surrounded on both sides by layers 12 of jacket material, each having a thickness in the range from 10 times to 1000 times the thickness of each of the individual thin film layers in the multilayer stack 16. The multilayer interference film or film of FIG. 4 has, for example, two multilayer stacks 16 sandwiched between two jacket layers. Another layer 20 is provided between the two multilayer stacks 16, and this layer 20 is thicker than each of the individual thin film layers 18 of the multilayer stack 16. This thick layer 20 may be an absorption layer or a transparent layer.

[0022] The layer 18 in the multilayer stack 16 is in a thickness range of 5 nm to 5,000 nm, depending on the target wavelength for filtering, the refractive index of the material, and the optical performance of the filter determined by the layer structure and the thickness distribution between the layers to meet the conditions for destructive or constructive interference. The total thickness of the filter film or film 10, including the protective jacket layers 12 on both sides and any intermediate layer 20 if present, is in the range of 0.05 mm to 1 mm.

[0023] The filter film 10 is flexible such that it can be bent to a radius of curvature in the range of 3 mm to 250 mm depending on the filter thickness and the constituent materials, without permanently damaging, deforming, or cracking the interference filter 10 or the thin film layers 18 in the multilayer stack 16 as a whole.

[0024] The filter structure may further include up to 15 layers of antireflection thin films 22 deposited on the outside of any jacket layer 12 that serves to reduce reflectivity, as shown in FIG. 5. The multilayer stack 16 is sandwiched between two jacket layers 12. On the outside of one of the jacket layers 12, several antireflection layers 22 are present to promote light transmission. These antireflection layers 22 may be mainly composed of polymer or glass, and such antireflection layers 22 may be manufactured by a thermal compression forming method or coated on the filter film after all other layers 18, and optionally the filter subassembly including layers 12 or 22 are made.

[0025] The overall optical performance of the filter device

[0026] The optical filter 10 described herein blocks portions of the spectral wavelength range from 300 nm to 25 microns for optical applications across the UV to visible light spectrum and up to the IR. Throughout this specification, the terms "substantially" and "about" indicate a deviation of up to ±15%, preferably ±5%.

[0027] Filter 10 has a transmission spectrum with at least one transition edge between a low transmittance and a high transmittance. For the purposes of this particular embodiment, the high transmittance is defined as a transmittance exceeding 80% of the incident light. The low transmittance is defined as a transmittance of at most 20% of the incident light. An example of a transition edge is shown in FIG. 6. FIG. 6 shows a transmission spectrum with a transition edge 24 from a low transmittance to a high transmittance having an increasing wavelength λ. The edge gradient of at least one such transition edge 24 between the low transmittance and the high transmittance is in the range from 0.02% to 5%. This means that the difference Δλ between the wavelength λ80 at which the transmittance reaches 80% closest to the high transmittance range and the wavelength λ20 at which the transmittance reaches 20% closest to the low transmittance range is in the range from 0.02% to 5% of the wavelength λ50 at which the transmittance is equal to 50% on the upper edge between the two points. The wavelength difference Δλ for the transition edge is, for example, less than 0.05% of the 50% transmittance wavelength λ50 where the transmission band itself has a width Δλ of only 0.1 nm (as shown in FIGS. 12 and 13), while on the other hand, the transition edge may extend over several percent of the 50% transmittance wavelength λ50 for a wider band (as shown in FIGS. 7, 10, and 11).

[0028] The transition edge may be defined, for example, between a 20% transmittance and a 50% transmittance between transmittance levels different from those shown in this example, in which case the transmittance in the band does not reach, for example, a higher transmittance level. In that case, the reference wavelength λ50 is the wavelength at which the transmittance is equal to 50% of the highest transmittance level of the transition edge.

[0029] Although the transmittance level may vary, a transmittance of up to 94% can be achieved for high transmittance wavelengths with no antireflection layer deposited on the filter surface and the surrounding air having a refractive index of approximately 1. With an additional antireflection layer provided, the transmittance for high transmittance wavelengths can reach up to 99% under the same ambient conditions.

[0030] The filter spectrum may have up to 20 transition edges from high transmittance to low transmittance and from low transmittance to high transmittance to provide a number of transmittance and cutoff ranges between adjacent transition edges. FIGS. 7 and 12 show, for example, all four transition edges 24 having two cutoff bands, and FIG. 13 shows all eight transition edges having four cutoff bands, half of which are low-to-high transmittance edges and the other half of which are high-to-low transmittance edges, respectively. The band full width at half maximum λ50 (FWHM) of each transmittance band or cutoff band should preferably be in the range from 0.1% to 75% of the central wavelength of the same band. The lower limit of 0.1% corresponds to a very narrow notch filter or band filter as will be described below for a Fabry-Perot cavity resonator having the spectra shown in FIGS. 12 and 13, while the upper level corresponds to a wide notch or band filter. Further details regarding the layer structure providing such a transmittance curve are as follows.

[0031] The transmittance within the low transmittance range can reach low values of 0.1%, 0.01%, 0.001%, 0.0001%, or even 0.00001% by using a sufficient number of interference layers 18 or by adding an absorption layer 20 or 12 that blocks a range of wavelengths.

[0032] Various layer structures of the filter device

[0033] As schematically shown in FIG. 3, the thin film multilayer stack 16 in the filter membrane may be composed of unit blocks 14 that repeat many times within the stack 16. Each unit block may be composed of up to 12 sub-layers of up to 5 different materials. The sub-layers of each repeating unit block may be in a thickness range from 1% to 75% of the total physical thickness of the repeating unit block 14. FIG. 3 shows three layers 18 of the same thickness δ1 and a unit block 14 with different materials having different refractive indices from each other, resulting in equal or different optical path lengths or optical thicknesses. As a variant, FIG. 8 shows a unit block of 5 different materials in 5 layers with different thicknesses δ1, δ2, δ3, δ4, δ5.

[0034] The optical thickness of the internal sub-layers can vary in various cases up to 90% lower or higher than the average optical thickness of all the layers 18 in the unit block 14, due to either the difference in refractive index or thickness between the sub-layers. The optical thickness is defined as the product of the physical thickness, e.g., δ1, δ2, δ3, δ4, δ5 and the optical refractive index of the material which may vary with wavelength.

[0035] For example, the optical thickness of an individual unit block 14 may vary in small increments such that the optical thickness or refractive index as a function of thickness (position) across the entire multilayer stack can approximate a sine wave or follow a periodic curve as a whole. This results in a quasi-rugate structure without the need to provide a continuously varying refractive index of a rugate structure across the entire thickness of the multilayer stack 16. In the simplest form, a periodic refractive index function similar to a sawtooth function can be formed as a discrete approximation to a sine wave function with only three different refractive indices.

[0036] The filter membrane 10 can have as few as five repeating unit blocks 14 or as many as a thousand unit blocks 14, and not all of them need to be identical. The unit blocks 14 can be arranged in various ways within the multilayer stack 16 of the thin film filter 10. In one embodiment, in a single case, all the unit blocks can have the same total thickness. In other embodiments shown by the plots 101 - 106 of FIG. 9, the unit blocks 14 in the filter stack 16 can be the same in terms of the material and order of the layers 18, except for the magnification regarding their total thickness. This variation can be in the linear pattern shown by plot 101 or in a non-linear pattern that increases from one end to the other end of the multilayer stack 16 as shown by plots 102, 103. In another embodiment, the magnification can decrease from the highest value at one end of the stack 16 to the lowest value and then increase and return to a high value as schematically shown by plot 104, or it can have the reverse relationship as schematically shown by plot 105. There may be a number of cycles of linear or non-linear variation of the magnification of the unit block thickness across the multilayer stack 16, as schematically shown by plot 106.

[0037] Another embodiment can include a combination of at least two unit block forms of plots 101 - 106 (or other plots). The thicknesses of the unit blocks 14 plotted in FIG. 9 are not an exhaustive list of thickness variations, and the number of unit blocks schematically shown is kept small for simplicity. A typical thin film filter 10 will have from dozens to hundreds of unit blocks 14.

[0038] For example, FIG. 10 depicts the transmittance spectrum of a filter including a quarter-wavelength layer stack 16 that forms two sets for each of different wavelengths. In a representative embodiment, each quarter-wavelength layer stack 16 has 126 double layers, and each double layer is a unit block of two layers, or several double layers can form a single unit block corresponding to 252 layers per multilayer stack 16 composed of polymethyl methacrylate (PMMA) and a second thermoplastic polymer having a refractive index different from that of PMMA. The layers 18 in one stack 16 each have a thickness of approximately 81 nm, and the layers 18 in the second stack 16 each have a thickness of approximately 108 nm. This configuration results in a transmittance curve that includes two notches.

[0039] The filter 10 used for the transmittance spectrum of FIG. 10 has a 0.025 mm thick intermediate layer 22 of PMMA between the two stacks 16 and one 0.025 mm thick layer of PMMA provided on each side of the device as a protective jacket layer 12. The total thickness of this double-notch filter 10 is approximately 0.122 mm. The transmittance curve of this device provides blocking of more than 99.9% in two wavelength ranges of approximately 488 nm and 647 nm over a blocking bandwidth of about 30 nm to 40 nm and a transition gradient of less than 3% as described above.

[0040] When the number of double layers in each stack 16 is reduced to 36, the resulting filter can still block up to 99% in the same wavelength range. However, by simultaneously stretching the filter layers, many double layers can be created without the need for an expensive coating process.

[0041] FIG. 11 discloses another embodiment of a filter that constitutes a band filter. The illustrated embodiment is the transmittance spectrum of a filter with 580 double layers of the same polymer material as mentioned in the above embodiment, and the individual layer thicknesses vary within the range from 138 nm to 243 nm and are 0.025 mm thick outside the protective jacket layer. This filter has a total thickness of approximately 0.27 mm.

[0042] To create the selective bands of high and low transmittance including the bands disclosed in the above embodiments, a sheet having a thickness much thicker than the final layer 18 and optionally layers 12, 22 but the same relative thickness ratio as the final layer is stacked within the preform, and this preform is then optionally repeatedly compressively stretched and longitudinally stretched from the furnace, and finally, the layer thickness is very much reduced so that the layer thickness reaches the desired dimensions while maintaining these thickness ratios.

[0043] In another embodiment, the periodicity of the unit block 14 with variable magnification as described above is interrupted by at least one defect layer made of at least one of the constituent materials or different materials in such a way that the thickness of at least one defect layer does not follow the periodic pattern of the unit block 14 of the rest of the multilayer stack 16. This configuration creates a Fabry - Perot cavity resonator that produces a very narrow band of high transmittance.

[0044] FIG. 12 shows a representative example of the transmittance spectrum of a filter 10 composed of 1800 layers in total of unit blocks 14 where the average layer thickness of each unit block 14 varies in the range from 64 nm to 114 nm. A defect layer 22 with a thickness of 178 nm that interrupts the layer thickness periodicity results in an optical transmittance curve representing a narrow - band filter. The filter used in the example of FIG. 12 has a protective jacket layer with a thickness of 0.025 mm on each side of the filter having a total filter thickness of approximately 0.21 mm.

[0045] Finally, FIG. 13 shows the transmittance spectrum of a filter with three defect layers 22 of different thicknesses that interrupt the layer periodicity three times. The filter used in FIG. 13 has similar protective jacket layers 12 with a thickness of 0.025 mm on both sides of the filter having a total filter thickness of about 0.211 mm.

[0046] As a variant of PMMA, polycarbonate can be used as the main matrix polymer in association with other thermoplastic polymers having a refractive index different from that of polycarbonate. Chalcogenide glass materials containing various ratios of arsenide, sulfur, selenide or germanium have demonstrated thermal and mechanical properties equivalent to those of certain thermoplastic resins such as polycarbonate, polyetherimide and polyethersulfone. The ultra-thin flexible filter is preferably made of alternating layers of at least one polymer and at least one such glassy material.

[0047] The above description relates to preferred embodiments of the present invention, but as will be understood, the present invention is capable of modification, variation and change without departing from the proper scope and fair meaning of the present invention as set forth in the appended claims.

Claims

**Claim 1** A thin-film interference filter comprising: a first thin-film interference multilayer stack composed of individual thin-film layers arranged in a group so as to form a plurality of first repeating unit blocks; the thin-film interference filter is flexible enough to be bent to a radius of curvature of 250 mm or less without permanently damaging, deforming, or cracking the thin-film interference filter or the thin-film layers in the at least one multilayer stack. **Claim 2** The thin-film interference filter according to claim 1, further comprising a second thin-film interference multilayer stack composed of individual thin-film layers arranged in a group so as to form a plurality of second repeating unit blocks, the second thin-film interference multilayer stack having a light transmission spectrum different from that of the first thin-film interference multilayer stack. **Claim 3** having at least one intermediate layer located between the first thin-film interference multilayer stack and the second thin-film interference multilayer stack; The thin-film interference filter according to claim 2, wherein the at least one intermediate layer has a thickness 10 to 1000 times the thickness of each of the individual thin-film layers in the first thin-film interference multilayer stack. **Claim 4** The thin-film interference filter according to claim 3, wherein at least one of the at least one intermediate layer is an absorption intermediate layer that blocks the wavelength range of infrared light, visible light, or ultraviolet light. **Claim 5** The thin-film interference filter according to claim 1, further comprising a first jacket layer and a second jacket layer, wherein the first thin-film interference multilayer stack is disposed between the first jacket layer and the second jacket layer. **Claim 6** The thin-film interference filter according to claim 6, further comprising 1 to 15 layers of an antireflection thin-film layer deposited on at least one outer surface of the first or the second jacket layer. **Claim 7** The thin-film interference filter according to claim 6, wherein 1 to 15 layers of the antireflection thin-film layer are mainly composed of a polymer or glass. **Claim 8** The thin-film interference filter according to claim 6, wherein 1 to 15 layers of the antireflection thin-film layer are formed by co-extrusion with the first jacket layer and the first thin-film interference multilayer stack during a thermoforming process. **Claim 9** The thin film interference filter according to claim 6, wherein 1 to 15 layers of the antireflection thin film layer are formed by coating the first jacket layer after thermocompression forming.

10. The thin film interference filter according to claim 5, wherein each of the first jacket layer and the second jacket layer has a thickness that is 10 to 1000 times the thickness of each individual thin film layer in the first thin film interference multilayer stack.

11. The thin film interference filter according to claim 5, wherein at least one of the first jacket layer or the second jacket layer is an absorption layer that blocks the wavelength range of infrared light, visible light, or ultraviolet light.

12. The thin film interference filter according to claim 1, wherein each of the individual thin film layers in the first multilayer stack has a thickness in the range from 5 nm to 5000 nm.

13. The thin film interference filter according to claim 1, wherein the thin film interference filter has an overall thickness in the range from 0.01 mm to 1 mm.

14. The thin film interference filter according to claim 1, wherein the thin film interference filter has a transmission spectrum that varies between a low transmittance of at most 20% of the incident light of the first wavelength and a high transmittance of at least 80% of the incident light of the second wavelength.

15. The transmission spectrum has at least one transition edge between the low transmittance and the high transmittance, and the transition edge has a width of at most 5% of a third wavelength between the first wavelength and the second wavelength. At the width, the thin film interference filter transmits 50% of the incident light. The thin film interference filter according to claim 13.

16. The thin film interference filter according to claim 1, having a transmission spectrum with at least one transition edge between a high transmittance of at least 50% of the incident light of the first wavelength and a low transmittance of at most 20% of the incident light of the second wavelength. The transition edge has a width of at most 5% of the first wavelength. At the width, the thin film interference filter transmits 50% of the maximum transmittance adjacent to the transition edge.

17. The thin film interference filter according to claim 1, further having at least one defect layer with an optical thickness different from that of the immediately adjacent individual thin film layer forming the repeating unit block.

18. The thin-film interference filter according to claim 1, wherein at least two of the individual thin-film layers within each of the first repeating unit blocks differ from each other in at least one of thickness and refractive index with respect to a specific wavelength.

19. The thin-film interference filter according to claim 18, wherein at least two of the individual thin-film layers differ from each other in the optical thickness with respect to the specific wavelength defined as a product obtained by multiplying the refractive index by the thickness.

20. The thin-film interference filter according to claim 18, wherein the next individual thin-film layer differs from the next thin-film layer in optical thickness or refractive index by only a small step such that the refractive index of the next thin-film layer follows an individual sinusoidal pattern to form a quasi-rugate filter.

21. The thin-film interference filter according to claim 1, wherein at least two of the individual thin-film layers within the repeating unit blocks are arranged in the same refractive index order.

22. The thin-film interference filter according to claim 18, wherein at least two of the individual thin-film layers within the repeating unit blocks are arranged in the same thickness order, and each of the individual thin-film layers of one unit block has a thickness different from that of each of the individual thin-film layers of another unit block by a certain magnification, and as a result, the one unit block is different from the another unit block by the certain magnification.

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