EP4647575A1

EP4647575A1 - Smart window switchable between transmission mode and reflection mode

Info

Publication number: EP4647575A1
Application number: EP23914940.4A
Authority: EP
Inventors: Mikyung LIM; Jae-Hyun Kim; Kwangseop Kim; Hyeon-Don Kim; Seong-Jae Jeon; Bongkyun Jang; Suwan JEON; Hak-Joo Lee
Original assignee: Korea Institute of Machinery and Materials KIMM; Center for Advanced Meta Materials
Current assignee: Korea Institute of Machinery and Materials KIMM; Center for Advanced Meta Materials
Priority date: 2023-01-03
Filing date: 2023-11-16
Publication date: 2025-11-12
Also published as: WO2024147476A1

Abstract

A smart window is switchable between a transmissive mode and a reflective mode, and includes first, second, third and fourth layers sequentially stacked. The air layer is disposed between the second and third layers. The first, second, third and fourth layers and the air layer are transparent in visible and near-infrared wavelengths of incident sunlight. A thickness of the air layer is variable.

Description

BACKGROUND

1. Field of Disclosure

The present disclosure of invention relates to a smart window switchable between a transmissive mode and a reflective mode, and more specifically the present disclosure of invention relates to a smart window switchable between a transmissive mode and a reflective mode capable of controlling a solar radiation actively. The smart window may switch between the reflective mode and the transmissive mode in a near-infrared rays and may selectively block a solar radiation energy, and then heating and cooling loads may be decreased with maintaining constant visibility by maintaining transmissive performance in a visible light.

2. Description of Related Technology

These days, for purposes such as urban aesthetics and lighting, the exterior walls of buildings are mainly constructed with glass windows. However, these glass windows have a significant impact on building energy consumption, causing energy loss inside the building in the winter, and also causing unnecessary waste of cooling energy as excessive solar energy flows in through the windows in the summer.
Furthermore, in addition to the glass windows used as the exterior walls of the buildings, the technical need to minimize internal energy consumption by adjusting the transmittance of solar radiant energy according to the season or external environment is increasing in the case of vehicle windows as well.
However, in the case of the glass windows applied to such buildings or vehicles, problems may occur if visibility varies as the transmittance of the solar radiant energy is adjusted. Then, there is a need for the development of active smart windows to minimize visibility problems by controlling the transmittance of the solar radiant energy while maintaining visible light transmittance.
Even in the past, as in Korea Laid-open Patent No. 10-2002-0002420 , technology for structures that selectively scatter or transmit radiation has been developed. However, this is implemented based on the operation of liquid crystals, and has limitations in that it is effective only for certain polarizations.
Further, European Laid-open Patent No. 2269100 discloses a structure that controls the reflectance or transmittance of the surface of a building or vehicle through light scattering as a technology for managing glare. However, this technology is a passive technology that implements switching between the transmissive mode and the reflective mode depending on temperature, and has the limitation of being unable to switch the modes using an electrical signal at the desired time as an active technology.
Related prior arts include Korea Laid-open Patent No. 10-2002-0002420 and European Laid-open Patent No. 2269100 .

SUMMARY

The present invention is developed to solve the above-mentioned problems of the related arts. The present invention provides a smart window switchable between a transmissive mode and a reflective mode capable of controlling a solar radiation actively. The smart window may switch between the reflective mode and the transmissive mode in a near-infrared rays and may selectively block a solar radiation energy, and then heating and cooling loads may be decreased with maintaining constant visibility by maintaining transmissive performance in a visible light.
According to an example embodiment, a smart window includes first, second, third and fourth layers sequentially stacked. The air layer is disposed between the second and third layers. The first, second, third and fourth layers and the air layer are transparent in visible and near-infrared wavelengths of incident sunlight. A thickness of the air layer is variable.
In an example, as the thickness of the air layer may be changed, transmittance of a visible light of the incident sunlight may not be changed and the near-infrared wavelength of the incident sunlight may be transmissive or reflective.
In an example, the thickness of the air layer may be changed in a range between 0 and λ_a/4na, λ_a may be an arbitrary wavelength selected such that λ_a/2 falls within a range of the visible wavelength of the sunlight, and na may be refractive index of the air layer.
In an example, even though the thickness of the air layer is changed, the transmittance through which the sunlight may pass through the smart window and the reflectance through which the sunlight is reflected by the smart window may remain constant at λ_a/2.
In an example, when refractive index of the first layer is n1, refractive index of the second layer is n2, refractive index of the third layer is n3, refractive index of the fourth layer is n4, and refractive index of the air layer is na, n3 ≥ n2 > n1 > na, or n3 ≥ n2 > n4 > na.
In an example, the thickness of the second layer may be λ₂/4n2 and the thickness of the third layer may be λ₃/4n3. λ₂ and λ₃ may be arbitrary wavelengths selected such that λ₂/2 and λ₃/2 respectively fall within the range of the visible wavelength of the sunlight.
In an example, the first and fourth layers may have the same refractive index, and each of the first and fourth layers may have the thickness larger than that of each of the second layer, the third layer and the air layer.
In an example, the fifth layer may be disposed between the third and fourth layers, and the fifth layer may be transparent in the visible and near-infrared wavelengths of the sunlight.
In an example, refractive index of the fifth layer may be smaller than that of the third layer and may be larger than that of the fourth layer.
In an example, the thickness of the fifth layer may be λ₅/4n5. n5 may be refractive index of the fifth layer, and λ₅ may be an arbitrary wavelength selected such that λ₅/2 falls within a range of the visible wavelength of the sunlight.
In an example, the smart window may further include an additional layer disposed between the first and second layers, the fourth and fifth layers, or the third and fifth layers. The additional layer may be transparent in the visible and near-infrared wavelengths of the sunlight. Refractive index of the additional layer may be a value between refractive index of the layer disposed on the additional layer and refractive index of the layer disposed beneath the additional layer.
In an example, the smart window may further include a through hole passing through the third and fourth layers to be connected to the air layer. The air of the air layer may be removed to outside through the through hole, or the air may be provided to the air layer through the through hole.
In an example, the smart window may further include a base layer disposed under the fourth layer; and an air bag disposed between the fourth layer and the base layer, and changed according to an injection of an air.
In an example, the smart window may further include a variable structure disposed in the fourth layer. A height of the variable structure may be variable.
In an example, an upper portion of the variable structure may be fixed at the second layer, and a lower portion of the variable structure may be fixed at the fourth layer. The third layer and the fourth layer may be partially disposed inside of the air layer.
In an example, the smart window may further include a base layer disposed under the fourth layer; and a variable structure disposed between the fourth layer and the base layer, wherein a height of the variable structure is variable.
In an example, the smart window may further include an electrode part comprising first and second electrodes. The first electrode may be formed on the third layer and formed inside of the air layer, and the second electrode may be formed on the second layer and formed inside of the air layer to face the first electrode.
In an example, the smart window may further include an electrode part comprising first and second electrodes. The first electrode may be formed on the third layer and formed inside of the air layer, and the second electrode may be formed on the first layer and formed toward an outside.
In an example, the smart window may further include a filler formed at a side of the air layer.
In an example, the smart window may further include a variable structure formed at a side of the air layer. The variable structure may be a shape memory alloy or a piezoelectric element.
According to the present example embodiments, in the smart window with a structure in which multiple layers including an air layer are stacked, since each layer is transparent in the visible wavelengths of incident sunlight, each layer does not limit the user's field of view, and as the thickness of the air layer changes, near-infrared wavelengths of sunlight are transmitted or reflected. Thus, an inflow of radiant energy into the room may be controlled. Then, in the summer, the loss of cooling energy may be minimized by minimizing the amount of radiant heat transfer, and in the winter, the heating effect may be improved by maximizing the amount of radiant heat transfer. Thus, the energy efficiency of buildings and vehicles may be improved.
By defining the correlation between the thickness and refractive index of the stacked layers and varying the thickness of the intervening air layer, it is possible to easily control the inflow of solar radiation energy that transmits or reflects incident near-infrared rays. In addition, even if the transmission or reflection of the near-infrared rays is controlled, the transmittance in the visible light region is maintained constant, so the user's field of view is not limited. Thus, it may be effectively applied to various transparent glass windows that require good visibility.
In particular, the thickness of the air layer may be easily controlled by a structure that directly injects or removes air from the outside, or alternatively, by adding a structure for applying pressure to the structure of the smart window, it may be easily controlled through pressure control. Thus, easy control of the transmittance of the near-infrared rays may be possible.
That is, the thickness of the air layer may be varied through various structures. For example, air may be directly injected or removed from the outside, or the expansion or contraction of the air bag may be utilized by interposing the air bag.
Alternatively, a pair of electrodes acting on attractive and repulsive forces may be placed on both sides of the air layer to control the thickness of the air layer through selective application of current, or selectively, electrodes may be arranged in various ways within the range of action of attractive and repulsive forces.
Furthermore, a variable structure having a variable length may be interposed in an arbitrary layer or on an air layer, and the length of the variable structure may be varied. Here, the variable structure may be controlled to have a variable length in certain situations, such as a shape memory alloy or a piezoelectric element.
Thus, easy control of the transmittance of the near-infrared rays may be possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a smart window according to an example embodiment of the present invention;
FIG. 2A is an example structure for explaining reflective and transmissive characteristics of the smart window of FIG. 1, and FIG. 2B and FIG. 2C are graphs showing transmittance and reflectance of the example structure of FIG. 2A;
FIG. 3A is another example structure for explaining reflective and transmissive characteristics of the smart window of FIG. 1, and FIG. 3B and FIG. 3C are graphs showing transmittance and reflectance of the example structure of FIG. 3A;
FIG. 4A is still another example structure for explaining reflective and transmissive characteristics of the smart window of FIG. 1, and FIG. 4B and FIG. 4C are graphs showing transmittance and reflectance of the example structure of FIG. 4A;
FIG. 5A and FIG. 5B are cross-sectional views illustrating a changing state of a thickness of an air layer in the smart window of FIG. 1, and FIG. 5C and FIG. 5D are graphs showing transmittance and reflectance according to the thickness of the air layer is changed;
FIG. 6A is a cross-sectional view illustrating a smart window according to another example embodiment of the present invention, and FIG. 6B and FIG. 6C are graphs showing transmittance and reflectance for the smart window of FIG. 6A;
FIG. 7A to FIG. 7D are graphs showing transmittance and reflectance according to the thickness of the air layer is changed in the smart window of FIG. 6A;
FIG. 8A to FIG. 8C are cross-sectional views illustrating a smart window according to still another example embodiment of the present invention;
FIG. 9 is a cross-sectional view illustrating a smart window according to still another example embodiment of the present invention;
FIG. 10 is a cross-sectional view illustrating a smart window according to still another example embodiment of the present invention;
FIG. 11 is a cross-sectional view illustrating a smart window according to still another example embodiment of the present invention;
FIG. 12 is a cross-sectional view illustrating a smart window according to still another example embodiment of the present invention;
FIG. 13 is a cross-sectional view illustrating a smart window according to still another example embodiment of the present invention; and
FIG. 14 is a cross-sectional view illustrating a smart window according to still another example embodiment of the present invention.

3, 4, 5, 6, 7, 8, 9, 10, 11 :	smart window
10 :	first layer	20 :	second layer
30 :	third layer	40 :	fourth layer
31, 41:	through hole	50 :	fifth layer
55 :	additional layer	60 :	air layer
71, 81,91 :	substrate	72,82, 92, 93 :	coating layer
160 :	filler	170:	air bag
180 :	base layer	100101 :	electrode part
200,201 :	variable structure

DETAILED DESCRIPTION

The invention is described more fully hereinafter with Reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity.
It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
Hereinafter, the invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown.
FIG. 1 is a cross-sectional view illustrating a smart window according to an example embodiment of the present invention.
Referring to FIG. 1, the smart window 1 according to the example embodiment includes a first layer 10, a second layer 20, and air layer 60 (Air), a third layer 30 and a fourth layer 40 sequentially stacked from an outdoor where a sunlight is incident.
The first layer 10 may be positioned to face the outdoor where the sunlight is incident, and the fourth layer 40 may be positioned to face an indoor.
Here, the following explanation, an example will be given where the first layer 10 is positioned to face the outdoor where the sunlight is incident, but this is for convenience of explanation. Thus, the fourth layer 40 may be positioned to face the outdoor where the sunlight is incident, and the first layer 10 may be positioned to face an indoor. In the smart window 1 according to the present example embodiment, any of the first and fourth layers 10 and 40 may be positioned to face the outdoor.
The first to fourth layers 10, 20, 30, 40, and the air layer 60 are transparent in visible and near-infrared wavelengths of incident sunlight. Since the layers above are all transparent in the visible wavelength, the user in an indoor does not experience problems such as limited visibility when observing an outdoor through the smart window 1. Here, for example, the wavelength of the visible light of the sunlight may be about 400~700nm, and the wavelength of the near-infrared light of the sunlight may be about 700~2,500nm.
To be transparent in the visible and near-infrared wavelengths of the sunlight, the first to fourth layers 10, 20, 30 and 40 may include a material selected from a glass, titanium dioxide (TiO₂), silicon nitride (SiN), tantalum pentoxide (Ta₂O₅), silicon dioxide (SiO₂), polyethyleneterephthalate (PET) and polydimethylsiloxane (PDMS).
Here, the first layer 10 and the fourth layer 40 are the layers respectively forming an outermost layer and an inner most layer of the smart window 1, and thus each of the first and fourth layers 10 and 40 may be a film or a substrate having a thickness larger than that of the other layers. For example, a thickness d1 of the first layer 10 and a thickness d4 of the fourth layer 40 may be larger than about 50µm.
In addition, when refractive index of the first layer 10 is n1, refractive index of the second layer 20 is n2, refractive index of the third layer 30 is n3, refractive index of the fourth layer 40 is n4, and refractive index of the air layer 60 is na, the refractive index n1 of the first layer 10 may be the same as the refractive index n4 of the fourth layer 40.
Further, the above refractive indexes satisfy the following equations. $n 3 \geq n 2 > n 1 > na$ $n 3 \geq n 2 > n 4 > na$
The refractive index of the air layer 60 disposed in the middle is the smallest, and the refractive indexes of the second layer 20 and the third layer 30 respectively disposed on upper and lower surfaces of the air layer 60 are relatively large. The refractive index of the third layer 30 is larger than that of the second layer 20.
In addition, when a thickness of the second layer 20 is d2 and a thickness of the third layer 30 is d3, the thicknesses satisfy the following equations. $λ_{2} / 4 n 2$ $λ_{3} / 4 n 3$
Here, λ₂ and λ₃ are arbitrary wavelengths selected such that λ₂/2 and λ₃/2 respectively fall within the range of the visible wavelength of the sunlight. Each of λ₂/2 and λ₃/2 is within the range of 400~700nm which is an example visible wavelength, and then each of λ₂ and λ₃ is arbitrary wavelength included in the range of 800~1,400nm. Here, the range of 800~1,400nm is within the range of 700~2,500nm which is an example near-infrared wavelength explained above. Further, it is sufficient for each of the λ₂ and λ₃ to be selected as any wavelength in the above range, and they do not need to be the same.
The thickness of the air layer 60 (da) may be variable within a range between 0 and λ_a/4na. Here, λ_a is defined as the same for each of λ₂ and λ₃ defined above, and λ_a is an arbitrary wavelength included within the range of each of λ₂ and λ₃.
In the present example embodiment, the thickness of the air layer 60 (da) is merely changed within the above range. As the thickness of the air layer 60 changes, the near-infrared wavelength of the incident sunlight passes through or is reflected from the smart window 1. The conversion of the near-infrared wavelength into a transmissive mode and a reflective mode will be explained in detail with reference to the following drawings.
As a structure changing the thickness of the air layer 60 (da) within the range between 0 and λ_a/4na, although not shown in the figure, an additional air injection line connected from an outside to the air layer 60 may be formed.
The air injection line may be connected from the outdoor to the air layer 60 with passing through the first and second layers 10 and 20, or may be connected with passing through the third and fourth layers 30 and 40, and the connected direction of the air injection line may not be limited.
As the air injection line injects the air to the air layer 60 directly, the thickness of the air layer 60 increases, or as the air injection line removes the air from the air layer 60 directly, the thickness of the air layer 60 decreases.
Alternatively, the air of the air layer 60 may be controlled by a pressure applied from outside. For example, although not shown in the figure, an additional pressure supply for applying the pressure may be configured on an outer surface of the fourth layer 40.
Then, as the air is applied to the pressure supply and the pressure supply is expanded, the pressure due to the expansion of the pressure supply may be provided to the air layer 60 and the thickness of the air layer 60 may decrease. Alternatively, as the air applied to the pressure supply is removed and the pressure supply contracts, an external force applied to the air layer 60 is removed by the contraction of the pressure supply and the thickness of the air layer 60 may increase.
Accordingly, the thickness of the air layer 60 may be controlled by various methods mentioned above, and the method may not be limited thereto.
Even though the thickness of the air layer 60 changes, the transmittance and the reflectance of the visible light of the sunlight passing through the smart window 1 are not changed and is uniformly maintained. Thus, when the transmittance of the visible light is maintained high at an initial state, the transmittance of the visible light may be maintained relatively high regardless of the thickness of the air layer 60.
Accordingly, the state in which the transmittance and the reflectance of the visible light are maintained regardless of the change of the thickness of the air layer 60 and the transmittance and the reflectance changes, are explained in detail referring to the following drawings.
FIG. 2A is an example structure for explaining reflective and transmissive characteristics of the smart window of FIG. 1, and FIG. 2B and FIG. 2C are graphs showing transmittance and reflectance of the example structure of FIG. 2A.
Referring to FIG. 2A, in case that a coating layer 72 is coated between the air layer 60 (Air) and the substrate 71, the change of state of the transmittance and the reflectance is explained. This may be called as an anti-reflection coating method.
The coating layer 72 has refractive index n72 lower than refractive index n71 of the substrate 71, and the thickness d72 of the coating layer 72 may be λ/4n72. Here, λ has the same definition of each of λ₂ or λ₃ explained above.
At this structure, the change of state of the transmittance and the reflectance is explained referring to FIG. 2B and FIG. 2C.
Here, the refractive index of the substrate 71 is 2.43 and the substrate 71 has titanium dioxide (TiO₂). The refractive index of the coating layer 72 is 2, and the coating layer 72 has silicon nitride (SiN). λ is 1,100nm.
Referring to FIG. 2B and FIG. 2C, in contrast to the state in which the substrate 71 is merely formed, when the coating layer 72 is formed on the substrate 71, destructive interference occurs at the corresponding wavelength λ and the reflectance decreases and the transmittance increases.
Further, at the half wavelength λ/2 of the corresponding wavelength λ, the transmittance and the reflectance are maintained uniformly.
FIG. 3A is another example structure for explaining reflective and transmissive characteristics of the smart window of FIG. 1, and FIG. 3B and FIG. 3C are graphs showing transmittance and reflectance of the example structure of FIG. 3A.
Contrary to the so-called anti-flection coating explained above, referring to FIG. 3A, when a coating layer 82 is coated between the air layer 60 (Air) and the substrate 81 and the refractive index n82 of the coating layer 82 is larger than the refractive index n81 of the substrate 81, the change of the state of the transmittance and the reflectance is explained below.
Here, the thickness d82 of the coating layer 82 is λ/4n82, and λ has the same definition of each of λ₂ or λ₃ explained above.
The refractive index of the substrate 81 is 1.5 and the substrate 81 has the glass. The refractive index of the coating layer 82 is 2.43, and the coating layer 82 has titanium dioxide (TiO₂). λ is 1,100nm.
Referring to FIG. 3B and FIG. 3C, in contrast to the state in which the substrate 81 is merely formed, when the coating layer 82 is formed on the substrate 81, constructive interference occurs at the corresponding wavelength λ and the reflectance increases and the transmittance decreases.
Further, at the half wavelength λ/2 of the corresponding wavelength λ, the transmittance and the reflectance are maintained uniformly.
FIG. 4A is still another example structure for explaining reflective and transmissive characteristics of the smart window of FIG. 1, and FIG. 4B and FIG. 4C are graphs showing transmittance and reflectance of the example structure of FIG. 4A.
Alternatively, referring to FIG. 4A, when coating layers 92 and 93 different from each other is coated between the air layer 60 (Air) and the substrate 91, the change of the state of the transmittance and the reflectance is explained below.
Here, when the refractive index of the lower coating layer 92 is n92 and the refractive index of the upper coating layer 92 is n93, the thickness d92 of the lower coating layer 92 is λ/4n92 and the thickness d93 of the upper coating layer 93 is λ/4n99, and λ has the same definition of each of λ₂ or λ₃ explained above. The refractive index n92 of the lower coating layer 92 is larger than the refractive index n91 of the substrate 91, and the refractive index n93 of the upper coating layer 93 is smaller than the refractive index n92 of the lower coating layer 92.
In addition, the refractive index of the substrate 91 is 1.5 and the substrate 91 has the glass. The refractive index of the lower coating layer 92 is 2.43, and the lower coating layer 92 has titanium dioxide (TiO₂). The refractive index of the upper coating layer 93 is 2, and the lower coating layer 93 has silicon nitride (SiN). λ is 1,100nm.
In this structure, the upper coating layer is additionally formed between the air layer 60 and the coating layer 82, with the coating layer 82 formed between the air layer 60 and the substrate 81. Here, the refractive index of the upper coating layer is between that of the air layer 60 and that n82 of the coating layer 82.
In this structure, referring to FIG. 4B and FIG. 4C, in contrast to the state in which the lower coating layer 92 is formed on the substrate 91, when the lower and upper coating layers 92 and 93 are formed on the substrate 91, at the corresponding wavelength λ, the reflectance decreases again and the transmittance increases again due to the anti-reflection effect.
Further, at the half wavelength λ/2 of the corresponding wavelength λ, the transmittance and the reflectance are maintained uniformly.
Accordingly, since the wavelength is selected such that λ/2 is the wavelength included within the visible wavelength, the transmittance and the reflectance are uniformly maintained in the visible light region at any structures explained above, and the transmittance and the reflectance are changed and controlled only in the near-infrared region.
Hereinafter, the stacked structure of the smart window 10 according to the present example embodiment is explained below.
FIG. 5A and FIG. 5B are cross-sectional views illustrating a changing state of a thickness of an air layer in the smart window of FIG. 1, and FIG. 5C and FIG. 5D are graphs showing transmittance and reflectance according to the thickness of the air layer is changed.
Here, the thickness d2 of the second layer 20 is λ/4n2 and the thickness d3 of the third layer 30 is λ/4n3.
Through the Equation (1) and Equation (2), the thickness d2 of the second layer 20 is defined as λ₂/4n2 and the thickness d3 of the third layer 30 is defined as λ₃/4n3, and λ₂ and λ₃ are arbitrary wavelengths. However, hereinafter, for the convenience of explanation, the case in which both of λ₂ and λ₃ are selected as the same wavelength λ. Here, λ is included within the range of each of λ₂ and λ₃ defined above.
In addition, each of the first and fourth layers 10 and 40 has the refractive index of 1.5 and includes the glass. The second layer 20 has the refractive index of 2 and includes silicon nitride (SiN). The third layer 30 has the refractive index of 2.43 and includes titanium dioxide (TiO₂). λ is 1,100nm.
Here, since λ is 1,100nm, the thickness of the second layer 20 is 137.5nm from Equation (3) and the thickness of the third layer 30 is 113.2nm from Equation (4).
First, when the thickness of the air layer 60 is 0 (zero) as in FIG. 5A, referring to FIG. 5C and FIG. 5D, the third layer 30 having the relatively larger refractive index is formed on the fourth layer 40 and thus the reflectance at the corresponding wavelength λ increases, but the reflectance at the corresponding wavelength λ decreases since the second layer 20 is a material with the medium refractive index and satisfies the anti-reflective coating conditions.
However, when the thickness of the air layer 60 is λ/4na (for example, 275nm) as in FIG. 5B, referring to FIG. 5C and FIG. 5D, the stacked structure of the first and second layers 10 and 20 stacked on the air layer 60 and the stacked structure of the fourth and third layers 40 and 30 stacked under the air layer 60 has the larger reflectance at the corresponding wavelength λ, and thus the reflectance is amplified more by the air layer 60 and finally the reflectance at the corresponding wavelength λ increases relatively significantly.

In addition, as shown in Table 1 below for the case in which the thickness of the air layer 60 is 0 and 275 nm, the transmittance (%) and the reflectance (%) in the entire wavelength range, visible light range, and near-infrared range are is exemplified.

[Table 1] Transmittance and reflectance in each wavelength region according to change of thickness of air layer

	d=0nm	d=275nm
Transmittance in entire wavelength range (300~2,500nm) (%)	91.6	70.4
Reflectance in entire wavelength range (300~2,500nm) (%)	8.4	29.7
Transmittance in visible light range (400~700nm) (%)	92.7	94.1
Reflectance in near-infrared range (700~2,500nm) (%)	9.6	47.3

In Table 1, when calculating the average, the distribution of solar radiation energy was considered. Referring to Table 1, overall, it may be confirmed that 21.2% of the total solar energy may be controlled, the visible light transmittance is maintained at over 90%, and the reflectance in the near-infrared region may be controlled at 37.7%.
Accordingly, in the smart window 10 according to the present example embodiment, the thickness of the air layer 60 is controlled to be 0 (zero), the reflectance in the near-infrared decreases and the transmittance in the near-infrared increases, so that the solar radiation energy may be effectively transmitted. Alternatively, the thickness of the air layer 60 is controlled to be λ/4na, the reflectance in the near-infrared increases and the transmittance in the near-infrared decrease, so that the solar radiation energy may be blocked. Thus, switching between the transmissive mode and the reflective mode for wavelengths in the near-infrared region may be performed.
As illustrated in FIG. 5C and FIG. 5D, at the λ/2 half of the corresponding wavelength λ, the reflectance and the transmittance are maintained uniformly and thus the transmittance in the visible light region are maintained uniformly. Thus, there is no problem with limited visibility.
FIG. 6A is a cross-sectional view illustrating a smart window according to another example embodiment of the present invention, and FIG. 6B and FIG. 6C are graphs showing transmittance and reflectance for the smart window of FIG. 6A.
The smart window 2 according to the present example embodiment is substantially same as the smart window 1 of FIG. 1, except for a fifth layer 50 formed additionally, and thus the same reference numerals are used for the same element and any repetitive explanation will be omitted.
Referring to FIG. 6A, in the smart window 2 according to the present example embodiment, the fifth layer 50 is disposed between the third and fourth layers 30 and 40.
Here, the fifth layer 50 is transparent in the visible and near-infrared wavelengths of the sunlight, and may include a material selected from a glass, titanium dioxide (TiO₂), silicon nitride (SiN), tantalum pentoxide (Ta₂O₅), silicon dioxide (SiO₂), polyethyleneterephthalate (PET) and polydimethylsiloxane (PDMS).
Here, when refractive index of the fifth layer 50 is n5, Equation (5) is satisfied. In Equation (5), n3 is the refractive index of the third layer 30 and n4 is the refractive index of the fourth layer 40,
$n 3 > n 5 > n 4$
In addition, when a thickness of the fifth layer 50 is d5, d5 satisfies Equation (6) below. $d 5 = λ_{5} / 4 n 5$
Here, λ₅ is an arbitrary wavelength selected such that λ₅/2 falls within a range of the visible wavelength of the sunlight.
Accordingly, in case that the fifth layer 50 is additionally formed, the transmittance and the reflectance of the smart window 2 are explained below.
First, each of the first and fourth layers 10 and 40 has the refractive index of 1.5 and includes a glass, each of the second and fifth layers 20 and 50 has the refractive index of 2 and includes silicon nitride (SiN), and the third layer 30 has the refractive index of 2.43 and includes titanium dioxide (TiO₂). λ₅ is 1,100nm.
Here, since λ₅ is 1,100nm, the thickness of each of the second and fifth layers 20 and 50 is 137.5nm from Equation (3) and Equation (6), and the thickness of the third layer 30 is 113.2nm from Equation (4).
Thus, as illustrated in FIG. 6B and FIG. 6C, as the thickness of the air layer 60 changes from 0 (zero) to λ₅/4na (for example, 275nm), the reflectance at the corresponding wavelength λ₅ increases relatively significantly.

As shown in Table 2 below for the case in which the thickness of the air layer 60 is 0 and 275 nm, the transmittance (%) and the reflectance (%) in the entire wavelength range, visible light range, and near-infrared range are is exemplified.

[Table 2] Transmittance and reflectance in each wavelength region according to change of thickness of air layer

	d=0nm	d=275nm
Transmittance in entire wavelength range (300~2,500nm) (%)	94.7	70.2
Reflectance in entire wavelength range (300~2,500nm) (%)	5.3	29.8
Transmittance in visible light range (400~700nm) (%)	92.4	90.4
Reflectance in near-infrared range (700~2,500nm) (%)	3.5	45.8

In Table 2, when calculating the average, the distribution of solar radiation energy was considered. Referring to Table 2, overall, it may be confirmed that 24.5% of the total solar energy may be controlled, the visible light transmittance is maintained at over 90%, and the reflectance in the near-infrared region may be controlled at 42.3%.
Accordingly, in the smart window 2 according to the present example embodiment, the fifth layer 50 is inserted as the layer having the medium refractive index, and thus the reflectance may be decreased more in the near-infrared region. Thus, the control range of the reflectance may be increased more compared to the smart window 1 of FIG. 1.
FIG. 7A to FIG. 7D are graphs showing transmittance and reflectance according to the thickness of the air layer is changed in the smart window of FIG. 6A.
Referring to FIG. 7A to FIG. 7D, the transmittance and the reflectance are illustrated for the cases in which the thickness of the air layer 60 is changed variously.
Referring to FIG. 7A and FIG. 7B, even when the air layer 60 has a fine thickness, for example, 10nm or 20nm, it may be confirmed that there is no significant difference in the reflectance and the transmittance compared to the case in which the thickness of the air layer 60 is 0 (zero).
Accordingly, when the thickness of the air layer 60 is 10nm or 20nm, a fine gap is formed between the second and third layers 20 and 30. Even though the fine gap is formed, the reflectance and the transmittance of the smart window 1 remain almost the same.
Alternatively, referring to FIG. 7C and FIG. 7D, when the air layer 60 has the thickness of 200nm or 300nm similar to 250nm, the transmittance and the reflectance are similar to those in case that the thickness of the air layer 60 is 250nm.
Accordingly, in controlling the thickness of the air layer 60, even though it is not precisely controlled to have a specific thickness, the characteristics of the reflectance and the transmittance may be maintained to be almost similar within a certain range, thus reducing the need for very precise control of the air layer 60.
FIG. 8A to FIG. 8C are cross-sectional views illustrating a smart window according to still another example embodiment of the present invention.
The smart window 2, 4 and 5 according to the present example embodiment is substantially same as the smart window 2 of FIG. 6A, except that an additional layer 55 is additionally formed in various positions, and thus the same reference numerals are used for the same element and any repetitive explanation will be omitted.
Hereinafter, the additional layer 55 is transparent in the visible and near-infrared wavelengths of the sunlight, and may include a material selected from a glass, titanium dioxide (TiO₂), silicon nitride (SiN), tantalum pentoxide (Ta₂O₅), silicon dioxide (SiO₂), polyethyleneterephthalate (PET) and polydimethylsiloxane (PDMS).
Referring to FIG. 8A, in the smart window 3 according to the present example embodiment, the additional layer 55 is formed between the first and second layers 10 and 20, and the refractive index of the additional layer 55 is between the refractive index of the first layer 10 disposed on the additional layer 55 and the refractive index of the second layer 20 disposed under the additional layer 55.
In addition, referring to FIG. 8B, in the smart window 4 according to the present example embodiment, the additional layer 55 is formed between the fifth and fourth layers 50 and 40, and the refractive index of the additional layer 55 is between the refractive index of the fifth layer 50 disposed on the additional layer 55 and the refractive index of the fourth layer 40 disposed under the additional layer 55.
Further, referring to FIG. 8C, in the smart window 5 according to the present example embodiment, the additional layer 55 is formed between the third and fifth layers 30 and 50, and the refractive index of the additional layer 55 is between the refractive index of the third layer 30 disposed on the additional layer 55 and the refractive index of the fifth layer 50 disposed under the additional layer 55.
Accordingly, even though the additional layer 55 having the above refractive index is disposed between the layers, as in the case in which the fifth layer 50 having the above refractive index is disposed between the layers, switching between the reflective mode and the transmissive mode is possible as the thickness of the air layer 30 changes and the transmittance in the visible light region is uniformly maintained.
Hereinafter, various example embodiments of the smart window in which the thickness of the air layer 60 changes are explained.
FIG. 9 is a cross-sectional view illustrating a smart window according to still another example embodiment of the present invention.
The smart window 6 according to the present example embodiment is substantially same as the smart window 1 of FIG. 1, except that through holes 31 and 41 and a filler 160 are formed, and thus the same reference numerals are used for the same element and any repetitive explanation will be omitted.
Referring to FIG. 9, in the smart window 9, as the structure changing the thickness of the air layer 60, the through holes 31 and 41 are formed through the third layer 30 and the fourth layer 40.
A first through hole 31 extends from the air layer 60 and extends through the third layer 30, and a second through hole 41 extends aligned with the first through hole 31 and extends through the fourth layer 40.
Thus, the first and second through holes 31 and 41 extend to open the air layer 60 and the outside to each other, and although not shown in the figure, the air may flow through the first and second through holes 31 and 41.
An air supplier (not shown) may be connected to the through holes 31 and 41 to supply the air to the through holes 31 and 41. Thus, the air may be provided to the air layer 60 or may be removed from the air layer 60 by an operation of the air supplier.
As explained above, the thickness of the air layer 60 (da) may be changed between 0 and λ_a/4na, and thus the reflectance and the transmittance in the near-infrared rays may be changed.
Thus, the thickness of the air layer 60 decreases to 0 (zero) by removing the air from the air layer 60, and alternatively, the thickness of the air layer 60 increased to λ_a/4na by supplying the air to the air layer 60.
In FIG. 9, it is illustrated that the through holes 31 and 41 extend through the third and fourth layers 30 and 40, but the through holes 31 and 40 may extend through the first and second layers 10 and 20. Further, the through holes 31 and 41 may extend to a side of the third layer 30 with passing through the third layer 30, or may extend to a side of the fourth layer 40 with passing through the third and fourth layers 30 and 40. Accordingly, the connecting line of the through holes may be variously changed.
In addition, in the present example embodiment, to change the thickness of the air layer 60, the filler 160 is configured at a side of the air layer 60.
The filler 160 is formed along the side of the air layer 60 between the second and third layers 20 and 30.
The filler 160 maintains the structure of the air layer 60 as a whole. Since the filler 160 is positioned along the side of the air layer 60, the thickness of the air layer 60 may decrease at a central portion of the air layer 60 and the filler 160 is not interfered or affected by the decrease of the thickness of the air layer 60. That is, the shape or the structure of the filler 160 is maintained uniformly.
Alternatively, the filler 160 may include a flexible material with variable height, and thus as the thickness of the air layer 60 decreases, the filler 160 may be contracted together.
However, the filler 160 should prevent the injected air from leaking to outside and maintain the thickness of the air layer 60 constant by the injected air, and thus the filler 160 should have a certain level of strength and airtightness.
FIG. 10 is a cross-sectional view illustrating a smart window according to still another example embodiment of the present invention.
The smart window 7 according to the present example embodiment is substantially same as the smart window 6 of FIG. 9, except that an air bag 170 are formed, and thus the same reference numerals are used for the same element and any repetitive explanation will be omitted.
Referring to FIG. 10, in the smart window 7, the air bag 170 is disposed between the fourth layer 40 and the base layer 180.
Here, the base layer 180 is illustrated different from the fourth layer 40 in the figure, but the base layer 180 may include the material same as the fourth layer 40.
Thus, the air bag 170 may be disposed on the fourth layer 40, or the air bag 170 is disposed in the fourth layer 40 formed with a relatively larger thickness.
The air bag 170 may be a polymer bag. The air bag 170 is expanded by an air supplied from outside and then the height of the air bag 170 increases, and the air bag 170 contracts or shrinks when the air is removed and then the height of the air bag 170 decreases. Thus, although not shown in the figure, an additional air supply may be configured to be connected to the air bag 170.
Accordingly, when the air bag 170 expands by the air supplied from outside, the thickness of the air layer 60 decreases due to the pressure due to the expansion of the air bag 170. Alternatively, when the air bag 170 contracts as the air is removed, the thickness of the air layer 60 increases due to volume reduction of the air bag 170.
Thus, the thickness of the air layer 60 is changed between 0 and λ_a/4na, and the reflectance and the transmittance in the near-infrared rays may be changed.
In the figure, the air bag 170 is illustrated to be disposed as a single pack shape, but at least two pack shape air bags may be disposed in the fourth layer 40 or may be disposed between the fourth layer 40 and the base layer 180. Here, the plurality of the air bags are controlled to expand or contract at the same time, and the thickness of the air layer 60 may be uniformly changed.
FIG. 11 is a cross-sectional view illustrating a smart window according to still another example embodiment of the present invention.
The smart window 8 according to the present example embodiment is substantially same as the smart window 6 of FIG. 9, except that an electrode part 100 is formed in the air layer 60, and thus the same reference numerals are used for the same element and any repetitive explanation will be omitted.
Referring to FIG. 11, in the smart window 8 according to the present example embodiment, the electrode part 100 is formed on the air layer 60.
In the air layer 60, the electrode part 100 includes a first electrode 110 formed on an upper surface of the third layer 30 and a second electrode 120 formed on a lower surface of the second layer 20.
Here, the first and second electrodes 110 and 120 are disposed inside of the air layer 60, and are aligned to face each other along an up and down direction which is a stacked direction of the layers.
In addition, in FIG. 11, each of the first and second electrodes 110 and 120 includes two lines spaced apart from each other, but a plurality of lines more than two may be disposed to be spaced apart from each other by a predetermined distance.
The arrangement and the shape of the first and second electrodes 110 and 120 may be variously changed, and it is sufficient if the first and second electrodes 110 and 120 are arranged so that attractive and repulsive forces act on each other.
The first and second electrodes 110 and 120 of the electrode part 100 may include a transparent electrode, and thus the transmissive state of the visible light through the smart window 8 may be maintained.
Although not shown in the figure, the electrode part 100 is connected to an outer power supplier and a current is provided to each of the first and second electrodes 110 and 120. Then, when the repulsive force is applied between the first and second electrodes 110 and 120, the thickness of the air layer 60 increases due to the repulsive force. Alternatively, when the attractive force is applied between the first and second electrodes 110 and 120, the thickness of the air layer 60 decreases due to the attractive force.
Accordingly, the current applied to each of the first and second electrodes 110 and 120 is controlled and the attractive and repulsive forces are selectively induced, and then the thickness of the air layer 60 (da) may be controlled within a range between 0 and λ_a/4na, and the reflectance and the transmittance in the near-infrared rays may be variously changed.
FIG. 12 is a cross-sectional view illustrating a smart window according to still another example embodiment of the present invention.
The smart window 9 according to the present example embodiment is substantially same as the smart window 8 of FIG. 11, except for an arrangement position of an electrode part 101, and thus the same reference numerals are used for the same element and any repetitive explanation will be omitted.
Referring to FIG. 12, in the smart window 9 according to the present example embodiment, the first electrode 110 of the electrode part 101 is formed on the upper surface of the third layer 30 inside of the air layer 60. However, the second electrode 130 of the electrode part 101 is formed on an outer surface (an upper surface) of the first layer 10 outside of the air layer 60.
In the present example embodiment, a space for forming the electrode in the air layer 60 is insufficient, the second electrode 130 is formed on the outer surface of the first layer 10 and space utilization may be improved.
Here, in the figure, the second electrode 130 is illustrated to be formed on the outer surface of the first layer 10, but the first electrode 110 may be formed on the lower surface of the second layer 20 and the second layer 130 may be formed on an outer surface (a lower surface) of the fourth layer 40.
In the present example embodiment, the electrode part 101 is connected to the outer power supplier and the current is provided to each of the first and second electrodes 110 and 130.
Then, when the repulsive force is applied between the first and second electrodes 110 and 130, the thickness of the air layer 60 increases due to the repulsive force. Alternatively, when the attractive force is applied between the first and second electrodes 110 and 130, the thickness of the air layer 60 decreases due to the attractive force.
Accordingly, the current applied to each of the first and second electrodes 110 and 130 is controlled and the attractive and repulsive forces are selectively induced, and then the thickness of the air layer 60 (da) may be controlled within a range between 0 and λ_a/4na, and the reflectance and the transmittance in the near-infrared rays may be variously changed.
FIG. 13 is a cross-sectional view illustrating a smart window according to still another example embodiment of the present invention.
The smart window 10 according to the present example embodiment is substantially same as the smart window 6 of FIG. 9, except for a variable structure 200, and thus the same reference numerals are used for the same element and any repetitive explanation will be omitted.
Referring to FIG. 13, in the smart window 10 according to the present example embodiment, the variable structure 200 is formed at a side of the air layer 60.
An upper side of the variable structure 200 makes contact with the second layer 20 and is fixed, and a lower side of the variable structure 200 makes contact with the fourth layer 41 and is fixed. Here, the fourth layer 41 may have a stepped portion at a side, so that the lower side of the variable structure 200 is fixed at the stepped portion of the fourth layer 41.
In addition, the third layer 31 is formed on the fourth layer 41, and as illustrated in the figure, the third layer 31 is formed on the fourth layer 41 which has the stepped portion and disposed inside of the air layer 60. Thus, the third layer 31 is disposed inside of the air layer 60.
The variable structure 200 is formed to cover the side of the air layer 60 as a whole. The variable structure 200 extends from the second layer 20 to a portion of the fourth layer 41 and is formed along the side of the air layer 60, and thus a predetermined space 61 may be formed between the variable structure 200 and the third and fourth layers 31 and 41.
A length of the variable structure 200 is variable, and thus the thickness of the air layer 60 is changed as the length of the variable structure 200 is changed along an up and down direction.
The variable structure 200 may be a shape memory alloy or a piezoelectric element.
When the variable structure 200 is a shape memory alloy, the variable structure 200 returns to its original shape when heat is applied even though the variable structure 200 is in a compressed or contracted state. When the variable structure 200 is compressed or contracted, the thickness of the air layer 60 may be maintained at 0 (zero). Depending on the degree of application of heat, etc., the variable structure 200 may return to its original shape while the air layer 60 may change to a predetermined thickness (λ_a/4na or less).
To this end, the shape of the variable structure 200 may be designed so that when the variable structure 200 finally returns to its original shape, the thickness of the air layer 60 becomes λ_a/4na.
In addition, in the case of the variable structure 200, the structure that has returned to its original shape through cooling may be induced to be compressed or contracted again, and through this, the thickness of the air layer 60 may be changed.
Further, although not shown in the figure, a heat supplier may be additionally configured to apply the heat to the variable structure 200.
Alternatively, when the variable structure 200 is a piezoelectric element, the voltage applied to the piezoelectric element is controlled to change the shape of the variable structure 200. The variable structure 200 is controlled to be compressed or contracted, and then the thickness of the air layer 60 is controlled to be 0 (zero), and alternatively, the variable structure 200 is controlled to be expanded and then the thickness of the air layer 60 is changed to be a predetermine thickness (λ_a/4na or less).
Thus, the voltage applied to the variable structure 200 is controlled, to control a size, that is a height of the variable structure 200, and then the thickness of the air layer 60 may be controlled in a range between 0 and λ_a/4na.
Further, although not shown in the figure, a voltage controller may be additionally configured to apply the voltage to the variable structure 200.
FIG. 14 is a cross-sectional view illustrating a smart window according to still another example embodiment of the present invention.
The smart window 11 according to the present example embodiment is substantially same as the smart window 10 of FIG. 13, except for a positon of a variable structure 200, and thus the same reference numerals are used for the same element and any repetitive explanation will be omitted.
Referring to FIG. 14, in the smart window 11 according to the present example embodiment, the variable structure 201 is disposed between the fourth layer 40 and the base layer 180.
Here, the base layer 180 is an additional layer disposed under the fourth layer 40, but the material and the characteristics of the base layer 180 may be the same as those of the fourth layer 40.
In addition, as explained above, the base layer 180 is not configured to an additional structure, and the fourth layer 40 is formed to have relatively larger thickness and to be as the base layer 180. That is, the variable structure 201 may be disposed in the fourth layer 40.
Here, a plurality of the variable structures 201 may be arranged to be spaced apart from each other between the fourth layer 40 and the base layer 180, and the arrangement and the number of the variable structures 201 may not be limited thereto. When the plurality of the variable structures 201 is formed, the variable structures 201 should be controlled so that the heights of the variable structures 201 change substantially equally.
Further, as explained above, the variable structure 201 may be a shape memory alloy or a piezoelectric element, and the control of the variable structure 201 may be the same as explained above.
According to the present example embodiments, in the smart window with a structure in which multiple layers including an air layer are stacked, since each layer is transparent in the visible wavelengths of incident sunlight, each layer does not limit the user's field of view, and as the thickness of the air layer changes, near-infrared wavelengths of sunlight are transmitted or reflected. Thus, an inflow of radiant energy into the room may be controlled. Then, in the summer, the loss of cooling energy may be minimized by minimizing the amount of radiant heat transfer, and in the winter, the heating effect may be improved by maximizing the amount of radiant heat transfer. Thus, the energy efficiency of buildings and vehicles may be improved.
By defining the correlation between the thickness and refractive index of the stacked layers and varying the thickness of the intervening air layer, it is possible to easily control the inflow of solar radiation energy that transmits or reflects incident near-infrared rays. In addition, even if the transmission or reflection of the near-infrared rays is controlled, the transmittance in the visible light region is maintained constant, so the user's field of view is not limited. Thus, it may be effectively applied to various transparent glass windows that require good visibility.
In particular, the thickness of the air layer may be easily controlled by a structure that directly injects or removes air from the outside, or alternatively, by adding a structure for applying pressure to the structure of the smart window, it may be easily controlled through pressure control. Thus, easy control of the transmittance of the near-infrared rays may be possible.
That is, the thickness of the air layer may be varied through various structures. For example, air may be directly injected or removed from the outside, or the expansion or contraction of the air bag may be utilized by interposing the air bag.
Alternatively, a pair of electrodes acting on attractive and repulsive forces may be placed on both sides of the air layer to control the thickness of the air layer through selective application of current, or selectively, electrodes may be arranged in various ways within the range of action of attractive and repulsive forces.
Furthermore, a variable structure having a variable length may be interposed in an arbitrary layer or on an air layer, and the length of the variable structure may be varied. Here, the variable structure may be controlled to have a variable length in certain situations, such as a shape memory alloy or a piezoelectric element.
Thus, easy control of the transmittance of the near-infrared rays may be possible.
Although the exemplary embodiments of the present invention have been described, it is understood that the present invention should not be limited to these exemplary embodiments but various changes and modifications can be made by one ordinary skilled in the art within the spirit and scope of the present invention as hereinafter claimed.

Claims

A smart window comprising:
first, second, third and fourth layers sequentially stacked,

wherein an air layer is disposed between the second and third layers,

wherein the first, second, third and fourth layers and the air layer are transparent in visible and near-infrared wavelengths of incident sunlight,

wherein a thickness of the air layer is variable.
The smart window of claim 1, wherein as the thickness of the air layer is changed, transmittance of a visible light of the incident sunlight is not changed and the near-infrared wavelength of the incident sunlight is transmissive or reflective.
The smart window of claim 2, wherein the thickness of the air layer is changed in a range between 0 and λ_a/4na,
wherein λ_a is an arbitrary wavelength selected such that λ_a/2 falls within a range of the visible wavelength of the sunlight, and na is refractive index of the air layer.
The smart window of claim 3, wherein even though the thickness of the air layer is changed, the transmittance through which the sunlight passes through the smart window and the reflectance through which the sunlight is reflected by the smart window remain constant at λ_a/2.
The smart window of claim 1, wherein when refractive index of the first layer is n1, refractive index of the second layer is n2, refractive index of the third layer is n3, refractive index of the fourth layer is n4, and refractive index of the air layer is na,
n3 ≥ n2 > n1 > na, or n3 ≥ n2 > n4 > na.
The smart window of claim 5, wherein the thickness of the second layer is λ₂/4n2 and the thickness of the third layer is λ₃/4n3,
wherein λ₂ and λ₃ are arbitrary wavelengths selected such that λ₂/2 and λ₃/2 respectively fall within the range of the visible wavelength of the sunlight.
The smart window of claim 1, wherein the first and fourth layers have the same refractive index, and each of the first and fourth layers has the thickness larger than that of each of the second layer, the third layer and the air layer.
The smart window of claim 1, wherein the fifth layer is disposed between the third and fourth layers, and the fifth layer is transparent in the visible and near-infrared wavelengths of the sunlight.
The smart window of claim 8, wherein refractive index of the fifth layer is smaller than that of the third layer and is larger than that of the fourth layer.
The smart window of claim 8, wherein the thickness of the fifth layer is λ₅/4n5, wherein n5 is refractive index of the fifth layer, and λ₅ is an arbitrary wavelength selected such that λ₅/2 falls within a range of the visible wavelength of the sunlight.
The smart window of claim 8, further comprising:
an additional layer disposed between the first and second layers, the fourth and fifth layers, or the third and fifth layers,

wherein the additional layer are transparent in the visible and near-infrared wavelengths of the sunlight,

wherein refractive index of the additional layer is a value between refractive index of the layer disposed on the additional layer and refractive index of the layer disposed beneath the additional layer.
The smart window of claim 1, further comprising:
a through hole passing through the third and fourth layers to be connected to the air layer,

wherein an air of the air layer is removed to outside through the through hole, or the air is provided to the air layer through the through hole.
The smart window of claim 1, further comprising:
a base layer disposed under the fourth layer; and

an air bag disposed between the fourth layer and the base layer, and changed according to an injection of an air.
The smart window of claim 1, further comprising:
a variable structure disposed in the fourth layer,

wherein a height of the variable structure is variable.
The smart window of claim 14, wherein an upper portion of the variable structure is fixed at the second layer, and a lower portion of the variable structure is fixed at the fourth layer,
wherein the third layer and the fourth layer are partially disposed inside of the air layer.
The smart window of claim 1, further comprising:
a base layer disposed under the fourth layer; and

a variable structure disposed between the fourth layer and the base layer, wherein a height of the variable structure is variable.
The smart window of claim 1, further comprising:
an electrode part comprising first and second electrodes,

wherein the first electrode is formed on the third layer and formed inside of the air layer, and the second electrode is formed on the second layer and formed inside of the air layer to face the first electrode.
The smart window of claim 1, further comprising:
an electrode part comprising first and second electrodes,

wherein the first electrode is formed on the third layer and formed inside of the air layer, and the second electrode is formed on the first layer and formed toward an outside.
The smart window of claim 1, further comprising:
a filler formed at a side of the air layer.
The smart window of claim 1, further comprising:
a variable structure formed at a side of the air layer,

wherein the variable structure is a shape memory alloy or a piezoelectric element.