Patent Description:
Vehicles (e.g., automobiles) are equipped with an infrared sensor as an example of an electromagnetic wave sensor. The infrared sensor transmits and receives infrared rays (electromagnetic waves). The infrared sensor transmits infrared rays toward the outside of a vehicle and receives the infrared rays that have struck and have been reflected by an object outside of the vehicle. By transmitting and receiving the infrared rays in such a manner, the infrared sensor detects the object outside of the vehicle.

An infrared sensor cover (electromagnetic wave sensor cover) is arranged on the front side (the outside of the vehicle) of the infrared sensor in a transmission direction of infrared rays. The infrared sensor cover is used to prevent the infrared sensor from being directly seen from the outside of the vehicle. <CIT>, <CIT>, and <CIT> disclose electromagnetic wave sensor covers.

Known examples of such an infrared sensor cover include an infrared sensor cover disclosed in <CIT>. Such an infrared sensor cover includes a cover base, a heater wire, and an anti-reflection (AR) coating layer. The cover base is located on a path of infrared rays that are transmitted and received by the infrared sensor. The heater wire is disposed on the front surface of the cover base. The heater wire generates heat when energized. The AR coating layer limits the reflection of infrared rays.

The above-described infrared sensor cover is configured to melt ice and snow that adhere to the infrared sensor cover using the heat generated by the heater wire. This limits situations in which the adhesion of ice and snow to the infrared sensor cover prevents the passage of infrared rays. In addition, the AR coating layer of the above-described infrared sensor cover limits the reflection of infrared rays. Thus, the infrared sensor cover limits a decrease in the detection accuracy of the infrared sensor.

In the infrared sensor cover, while the AR coating layer limits the reflection of infrared rays, the heater wire is arranged so as to extend in a serpentine manner at a predetermined pitch. Thus, causing the heater wire to generate heat produces a temperature difference between a portion of the infrared sensor cover that includes the heater wire and a portion of the cover that does not include the heater wire. Thus, the infrared sensor cover is not evenly heated. Accordingly, the ice and snow that adhere to the infrared sensor cover are not able to be efficiently melted.

Accordingly, an electromagnetic wave sensor cover capable of being evenly heated while limiting the reflection of electromagnetic waves is desired.

An electromagnetic wave sensor cover that solves the above-described problem is specified in claim <NUM>.

A first embodiment of an electromagnetic wave sensor cover employed in an infrared sensor cover <NUM> for a vehicle <NUM> will now be described with reference to the drawings. The direction in which the vehicle <NUM> travels forward is hereinafter referred to as the front. The reverse direction is hereinafter referred to as the rear.

As shown in <FIG>, the front end of the vehicle <NUM> includes a front-monitoring infrared sensor <NUM>. The infrared sensor <NUM> is an example of an electromagnetic wave sensor. The infrared sensor <NUM> transmits infrared rays IR (electromagnetic waves), each having a wavelength of <NUM> or the like, toward the front of the vehicle <NUM> and receives the infrared rays IR that have struck and have been reflected by an object outside of the vehicle. Such an object includes, for example, a vehicle leading the vehicle <NUM> and pedestrians.

Since the infrared sensor <NUM> transmits the infrared rays IR toward the front of the vehicle <NUM> as described above, the infrared sensor <NUM> transmits the infrared rays IR in a direction from the rear to the front of the vehicle <NUM>. The front in the transmission direction of the infrared rays IR substantially matches the front of the vehicle <NUM>. The rear in the transmission direction of the infrared rays IR substantially matches the rear of the vehicle <NUM>. Thus, the front in the transmission direction of the infrared rays IR is hereinafter simply referred to as "frontward," "front," or the like, and the rear in the transmission direction of the infrared rays IR is hereinafter simply referred to as "rearward," "rear," or the like.

As shown in <FIG>, the rear half of the outer portion of the infrared sensor <NUM> corresponds to a box-shaped case <NUM> having a closed end. The front half of the outer portion of the infrared sensor <NUM> is a box-shaped cover <NUM> having a closed end. The case <NUM> includes a tubular peripheral wall <NUM> and a bottom wall <NUM>. The bottom wall <NUM> is located at the rear end of the peripheral wall <NUM>. The entire case <NUM> is made of a synthetic resin material, such as polybutylene terephthalate (PBT). The case <NUM> includes a transmitting portion <NUM> and a receiving portion <NUM> on the front surface of the bottom wall <NUM>. The transmitting portion <NUM> transmits the infrared rays IR. The receiving portion <NUM> receives the infrared rays IR.

More specifically, the infrared sensor <NUM> includes the case <NUM> to which the transmitting portion <NUM> and the receiving portion <NUM> are coupled, and the cover <NUM> that is located frontward from the case <NUM> and covers the transmitting portion <NUM> and the receiving portion <NUM> from the front. The cover <NUM> is made of, for example, polycarbonate (PC), polymethyl methacrylate (PMMA), cyclo olefin polymer (COP), or resin glass. The cover <NUM> permits the passage of infrared rays.

The infrared sensor cover <NUM> is disposed in front of the infrared sensor <NUM> and is separate from the infrared sensor <NUM>. The infrared sensor cover <NUM> is an example of the electromagnetic wave sensor cover. The infrared sensor cover <NUM> includes a plate-shaped cover body <NUM>. The cover body <NUM> is located frontward from the cover <NUM>. The cover body <NUM> indirectly covers the transmitting portion <NUM> and the receiving portion <NUM> from the front, with the cover <NUM> located between the cover body <NUM> and the transmitting and receiving portions <NUM>, <NUM>. The cover body <NUM> is fixed to the vehicle <NUM> by a fixing member (not shown).

The cover body <NUM> has a structure including four layers laid out in the front-rear direction. More specifically, the cover body <NUM> includes a hard coating layer <NUM>, a base layer <NUM>, a metal oxide layer <NUM>, and a low refractive index layer <NUM> that are laminated in this order from the front side. The cover body <NUM> includes two electrodes <NUM> that sandwich the metal oxide layer <NUM> in the up-down direction.

The base layer <NUM> is a part of the skeleton frame of the cover body <NUM>. The base layer <NUM> includes a front surface <NUM> and a rear surface <NUM> in the transmission direction of the infrared rays IR. The base layer <NUM> is made of a transparent synthetic resin material that permits passage of the infrared rays IR. The base layer <NUM> of the present embodiment is made of polycarbonate (PC). The base layer <NUM> may be made of material other than PC; that is, polymethyl methacrylate (PMMA), cyclo olefin polymer (COP), or the like.

The hard coating layer <NUM> permits passage of the infrared rays IR and has a higher hardness than the base layer <NUM>. The hard coating layer <NUM> is formed by applying a known finishing agent to the front surface <NUM> of the base layer <NUM>. Examples of the finishing agent include an organic hard coating agent (e.g., acrylate agent, oxetane agent, and silicone agent), an inorganic hard coating agent, and an organic-inorganic hybrid hard coating agent. The hard coating agent may be an agent that is hardened when irradiated with ultraviolet rays, or may be an agent that is hardened when receiving heat.

The hard coating layer <NUM> makes the cover body <NUM> more resistant to impact. Thus, the hard coating layer <NUM> limits situations in which the front surface of the cover body <NUM> is damaged by pebbles or the like. Further, the hard coating layer <NUM> makes the cover body <NUM> more resistant to weather. Thus, the hard coating layer <NUM> limits situations in which sunlight, wind, rain, temperature change, or the like varies the properties of the cover body <NUM> and degrades the cover body <NUM>.

The metal oxide layer <NUM> is laminated adjacent to the rear surface <NUM> of the base layer <NUM>. The metal oxide layer <NUM> permits passage of the infrared rays IR and is conductive. Resistance heating is performed for the metal oxide layer <NUM> through energization. That is, the entire metal oxide layer <NUM> generates heat through Joule heating that occurs when current is supplied to the metal oxide layer <NUM>. Thus, the metal oxide layer <NUM> functions as a heater.

The metal oxide layer <NUM> of the present embodiment is made of indium zinc oxide (IZO, registered trademark), which is transparent and conductive. The metal oxide layer <NUM> may be made of material that is transparent and conductive other than IZO (registered trademark). Examples of such material include indium tin oxide (ITO), aluminum zinc oxide (AZO), gallium zinc oxide (GZO), antimony tin oxide (ATO), or fluorine tin oxide (FTO).

The low refractive index layer <NUM> is laminated adjacent to the rear surface of the metal oxide layer <NUM>. That is, the metal oxide layer <NUM> and the low refractive index layer <NUM> are laminated adjacent to each other in the transmission direction of the infrared rays IR, and a laminate of the metal oxide layer <NUM> and the low refractive index layer <NUM> is laminated on the rear surface <NUM> of the base layer <NUM>. The low refractive index layer <NUM> permits passage of the infrared rays IR and is made of material that has a lower refractive index than the material of the metal oxide layer <NUM>. The low refractive index layer <NUM> of the present embodiment is made of silicon dioxide (SiO<NUM>). Silicon dioxide is insulative. Thus, the low refractive index layer <NUM> covers the rear surface of the metal oxide layer <NUM> so as to insulate the metal oxide layer <NUM> that is energized. The low refractive index layer <NUM> accordingly stops the flow of current between the metal oxide layer <NUM> and another member.

The two electrodes <NUM> are made of metal (e.g., copper). The two electrodes <NUM> are in contact with the metal oxide layer <NUM> so as to be electrically connected to the upper and lower ends of the metal oxide layer <NUM>, respectively. That is, the two electrodes <NUM> sandwich the metal oxide layer <NUM> in the up-down direction to energize the metal oxide layer <NUM> so that the metal oxide layer <NUM> generates heat.

In this case, the two electrodes <NUM> are in contact with the base layer <NUM>. Further, in this case, the two electrodes <NUM> are located outside of a region of the cover body <NUM> through which the infrared rays IR pass. The two electrodes <NUM> are thinner than the metal oxide layer <NUM>. The two electrodes <NUM> are connected to a power supply (not shown). Thus, the metal oxide layer <NUM> is energized by the power supply (not shown) via the two electrodes <NUM>.

The operation of the infrared sensor cover <NUM> will now be described.

In the vehicle <NUM> including the infrared sensor <NUM>, when the infrared rays IR are transmitted from the transmitting portion <NUM>, the infrared rays IR are applied to the rear part of the cover body <NUM>. The reflection of the applied infrared rays IR on the rear part of the cover body <NUM> is limited by an interference structure between the low refractive index layer <NUM> and the metal oxide layer <NUM>, which are located at the rear part of the cover body <NUM>.

More specifically, the low refractive index layer <NUM> and the metal oxide layer <NUM> are configured such that reflected waves of the infrared rays IR reflected on the boundary of an air layer and the low refractive index layer <NUM> and reflected waves of the infrared rays IR reflected on the boundary of the low refractive index layer <NUM> and the metal oxide layer <NUM> interfere with each other and weaken each other.

After sequentially passing through the low refractive index layer <NUM> and the metal oxide layer <NUM>, the infrared rays IR sequentially pass through the base layer <NUM> and the hard coating layer <NUM>. The infrared rays IR that have passed through the cover body <NUM> in the above-described manner strike and are reflected by an object outside of the vehicle. Such an object includes, for example, a vehicle leading the vehicle 11and the pedestrians. The reflected infrared rays IR again pass through the hard coating layer <NUM>, the base layer <NUM>, the metal oxide layer <NUM>, and the low refractive index layer <NUM> of the cover body <NUM> in this order.

The infrared rays IR that have passed through the cover body <NUM> are received by the receiving portion <NUM>. In the infrared sensor <NUM>, the infrared rays IR transmitted from the transmitting portion <NUM> and the infrared rays IR received by the receiving portion <NUM> are used to recognize the object and detect the distance between the vehicle <NUM> and the object, the relative speed, and the like.

Since the interference structure between the low refractive index layer <NUM> and the metal oxide layer <NUM> limits the reflection of the infrared rays IR, a larger amount of the infrared rays IR pass through the cover body <NUM>. Thus, the cover body <NUM> does not notably hinder passage of the infrared rays IR. Thus, the amount in which the cover body <NUM> attenuates the infrared rays IR transmitted from the transmitting portion <NUM> easily falls within an allowable range. As a result, the infrared sensor <NUM> easily recognizes the object and detects the distance between the object and the vehicle <NUM>, the relative speed, and the like in an effective manner.

The entire metal oxide layer <NUM> generates heat when energized. This heat is evenly conveyed to the entire cover body <NUM> so that the entire cover body <NUM> is heated. Thus, the front surface of the cover body <NUM>, which is exposed to the outside of the vehicle, is evenly heated as well. Thus, even if ice and snow adhere to any portion of the front surface of the cover body <NUM>, the ice and snow are quickly melted by the heat conveyed from the metal oxide layer <NUM>.

Such a structure limits situations in which the adhesion of ice and snow to the front surface of the cover body <NUM> prevents passage of the infrared rays IR. As a result, even in the case of snowy weather, the infrared sensor <NUM> sufficiently recognizes the object and detects the distance between the object and the vehicle <NUM>, the relative speed, and the like.

As described above, the metal oxide layer <NUM> of the infrared sensor cover <NUM> in the present embodiment has two functions: namely, the metal oxide layer <NUM> functions as a heater and functions to limit reflection of the infrared rays IR with the low refractive index layer <NUM>.

The first embodiment described above in detail has the following advantages.

(<NUM>-<NUM>) The infrared sensor cover <NUM> is configured to be employed in the infrared sensor <NUM> including the transmitting portion <NUM>, which transmits the infrared rays IR, and the receiving portion <NUM>, which receives the infrared rays IR. The infrared sensor cover <NUM> includes the cover body <NUM>. The cover body <NUM> is configured to cover the transmitting portion <NUM> and the receiving portion <NUM> from the front in the transmission direction of the infrared rays IR from the transmitting portion <NUM>. The cover body <NUM> includes the base layer <NUM>, one or more metal oxide layers <NUM>, one or more low refractive index layers <NUM>, and the two electrodes <NUM>. The base layer <NUM> is made of synthetic resin and permits passage of the infrared rays IR. The metal oxide layer <NUM> permits passage of the infrared rays IR and is conductive. The low refractive index layer <NUM> permits passage of the infrared rays IR and is made of material that has a lower refractive index than the material of the metal oxide layer <NUM>. The two electrodes <NUM> are disposed in contact with the metal oxide layer <NUM> to energize the metal oxide layer <NUM> so that the metal oxide layer <NUM> generates heat. The base layer <NUM> includes the front surface <NUM> and the rear surface <NUM> in the transmission direction of the infrared rays IR. The metal oxide layer <NUM> and the low refractive index layer <NUM> are laminated adjacent to each other in the transmission direction of the infrared rays IR. The laminate of the metal oxide layer <NUM> and the low refractive index layer <NUM> is laminated on the rear surface <NUM> of the base layer <NUM>.

In this structure, energizing the metal oxide layer <NUM> causes the entire metal oxide layer <NUM> to generate heat. This allows the cover body <NUM> to be evenly heated. In addition, the low refractive index layer <NUM>, which has a lower refractive index than the metal oxide layer <NUM>, is laminated adjacent to the metal oxide layer <NUM>. Thus, reflection of the infrared rays IR is limited. This allows the entire cover body <NUM> to be evenly heated while limiting reflection of the infrared rays IR on the cover body <NUM>.

(<NUM>-<NUM>) In the infrared sensor cover <NUM>, the laminate of the metal oxide layer <NUM> and the low refractive index layer <NUM> is laminated on the rear surface <NUM> of the base layer <NUM> in the transmission direction of the infrared rays IR.

This structure allows the base layer <NUM> to protect the metal oxide layer <NUM> and the low refractive index layer <NUM>.

(<NUM>-<NUM>) In the infrared sensor cover <NUM>, the metal oxide layer <NUM> is made of indium zinc oxide.

Since indium zinc oxide is transparent, this structure allows the infrared rays IR to pass through the metal oxide layer <NUM> more easily. Additionally, since the metal oxide layer <NUM> is made of indium zinc oxide, which is transparent, the metal oxide layer <NUM> is hard to see from the outside. This improves the design of the infrared sensor cover <NUM>.

(<NUM>-<NUM>) In the infrared sensor cover <NUM>, the low refractive index layer <NUM> is made of silicon dioxide.

Silicon dioxide is readily available. Thus, this structure facilitates manufacturing of the low refractive index layer <NUM>.

(<NUM>-<NUM>) In the infrared sensor cover <NUM>, the electrodes <NUM> are made of metal.

Metal is readily available. Thus, this structure facilitates manufacturing of the electrodes <NUM>.

A second embodiment of an electromagnetic wave sensor cover employed in an infrared sensor cover <NUM> for a vehicle <NUM> will now be described with reference to the drawings. As shown in <FIG>, the second embodiment is the same as the first embodiment except for the arrangement of the infrared sensor cover <NUM> on the infrared sensor <NUM>. Thus, only the difference of the second embodiment from the first embodiment will be described, and redundant descriptions will be omitted. The same reference numerals are given to the components of the second embodiment that are the same as the corresponding components of the first embodiment.

As shown in <FIG>, the rear half of the outer portion of the infrared sensor <NUM> corresponds to the case <NUM> and the front half corresponds to a cover <NUM>. The cover <NUM> of the infrared sensor <NUM> is used as the infrared sensor cover <NUM>, which is an example of the electromagnetic wave sensor cover. The infrared sensor cover <NUM> includes a tubular peripheral wall <NUM> and a plate-shaped cover body <NUM>. The cover body <NUM> is located at the front end of the peripheral wall <NUM>.

The cover body <NUM> is sized so as to close the front end of the case <NUM>. That is, the cover body <NUM> has the same structure as the cover body <NUM> (see <FIG>) of the first embodiment and is smaller than the cover body <NUM>. The cover body <NUM> covers the transmitting portion <NUM> and the receiving portion <NUM> from the front.

The second embodiment described above in detail has the following advantage in addition to advantages (<NUM>-<NUM>) to (<NUM>-<NUM>).

(<NUM>-<NUM>) The cover <NUM> of the infrared sensor <NUM> is used as the infrared sensor cover <NUM>. That is, the infrared sensor cover <NUM> is a part of the infrared sensor <NUM>. This reduces the amount of space the infrared sensor <NUM> occupies in the vehicle <NUM> as compared with when the infrared sensor cover <NUM> is separate from the infrared sensor <NUM> in the first embodiment.

The above-described embodiments may be modified as follows. The above-described embodiments and the following modifications can be combined as long as the combined modifications remain technically consistent with each other.

The cover body <NUM> or <NUM> may be changed to a cover body <NUM> shown in <FIG>. The cover body <NUM> includes the hard coating layer <NUM>, base layer <NUM>, low refractive index layer <NUM>, metal oxide layer <NUM>, low refractive index layer <NUM>, metal oxide layer <NUM>, and low refractive index layer <NUM> that are laminated in this order from the front side. The cover body <NUM> further includes two electrodes <NUM> that sandwich three low refractive index layers <NUM> and two metal oxide layers <NUM> in the up-down direction. That is, the cover body <NUM> includes multiple (three in this case) metal oxide layers <NUM> and multiple (two in this case) low refractive index layers <NUM>, and the two metal oxide layers <NUM> and the three low refractive index layers <NUM> are alternately laminated. Additionally, the cover body <NUM> includes the two electrodes <NUM> that extend in the front-rear direction so as to sandwich all the metal oxide layers <NUM> and all the low refractive index layers <NUM> in the up-down direction.

In this structure, multiple metal oxide layers <NUM> with refractive indices different from those of multiple low refractive index layers <NUM> are alternately laminated. This further limits reflection of the infrared rays IR (electromagnetic waves) on the cover body <NUM> while evenly heating the cover body <NUM>. Additionally, the two electrodes <NUM> extend in the front-rear direction so as to sandwich all the metal oxide layers <NUM> and all the low refractive index layers <NUM> in the up-down direction. This allows the two electrodes <NUM> to protect all the metal oxide layers <NUM> and all the low refractive index layers <NUM> while energizing all the metal oxide layers <NUM>.

In the cover body <NUM> shown in <FIG>, as long as the metal oxide layers <NUM> and the low refractive index layers <NUM> are alternately laminated, the number of the metal oxide layers <NUM> and the number of the low refractive index layers <NUM> may be changed. In this case, the rearmost layer of the metal oxide layers <NUM> and the low refractive index layers <NUM> is preferably a low refractive index layer <NUM>. Further, in this case, the frontmost layer of the metal oxide layers <NUM> and the low refractive index layers <NUM> may be a metal oxide layer <NUM>.

In the cover body <NUM> shown in <FIG>, the two electrodes <NUM> may sandwich one or more metal oxide layers <NUM> in the up-down direction. That is, for example, the two electrodes <NUM> may sandwich only one metal oxide layer <NUM>, or may sandwich only one metal oxide layer <NUM> and one low refractive index layer <NUM>. Alternatively, the two electrodes <NUM> may extend so as not to sandwich the low refractive index layer <NUM> located at the rearmost end.

The cover body <NUM> or <NUM> may be changed to a cover body <NUM> shown in <FIG>. The cover body <NUM> includes the hard coating layer <NUM>, low refractive index layer <NUM>, metal oxide layer <NUM>, base layer <NUM> that are laminated in this order from the front side. The cover body <NUM> includes two electrodes <NUM> that sandwich the metal oxide layer <NUM> in the up-down direction. That is, the cover body <NUM> has a laminate of the metal oxide layer <NUM> and the low refractive index layer <NUM> laminated on the front surface <NUM> of the base layer <NUM>. In this case, the position of the metal oxide layer <NUM> and the position of the low refractive index layer <NUM> may be replaced with each other.

The two electrodes <NUM> may be made of material other than metal.

The low refractive index layer <NUM> may be made of material other than silicon dioxide.

The hard coating layer <NUM> may be omitted.

The infrared sensor <NUM> in which the transmitting portion <NUM> and the receiving portion <NUM> are covered by the infrared sensor cover <NUM> or <NUM> may transmit and receive infrared rays IR each having a wavelength of <NUM>, instead of a wavelength of <NUM>.

The infrared sensor covers <NUM> and <NUM> are applicable to a structure in which the infrared sensor <NUM> is located at a part that differs from the front part of the vehicle <NUM>, for example, at the rear part of the vehicle <NUM>. In this case, the infrared sensor <NUM> transmits the infrared rays IR toward the rear of the vehicle <NUM>. Further, in this case, the infrared sensor covers <NUM> and <NUM> are located frontward from the transmitting portion <NUM> and the receiving portion <NUM> in the transmission direction of the infrared rays IR; that is, located rearward from the transmitting portion <NUM> and the receiving portion <NUM> in the vehicle <NUM>. Likewise, the infrared sensor covers <NUM> and <NUM> are applicable to a structure in which the infrared sensor <NUM> is located at a diagonally front side or a diagonally rear side of the vehicle <NUM>.

The infrared sensor covers <NUM> and <NUM> are applicable to a structure in which the infrared sensor <NUM> is mounted in a conveyance that differs from the vehicle <NUM> (e.g., a train, an airplane, or a ship).

The electromagnetic wave sensor cover may be applied to an electromagnetic wave sensor other than the infrared sensor <NUM>, such as a millimeter wave sensor.

Claim 1:
An electromagnetic wave sensor cover configured to be employed in an electromagnetic wave sensor including a transmitting portion that transmits an electromagnetic wave in form of infrared rays
and a receiving portion that receives the electromagnetic wave the electromagnetic wave sensor cover comprising a cover body (<NUM>; <NUM>; <NUM>; <NUM>) configured to cover the transmitting portion and the receiving portion from a front in a transmission direction of the electromagnetic wave from the transmitting portion, wherein
the cover body (<NUM>; <NUM>; <NUM>; <NUM>) includes:
a base layer (<NUM>) made of synthetic resin and permitting passage of the electromagnetic wave;
a metal oxide layer (<NUM>) permitting passage of the electromagnetic wave and being conductive;
a low refractive index layer (<NUM>) permitting passage of the electromagnetic wave; and
two electrodes (<NUM>) disposed in contact with the metal oxide layer (<NUM>) to energize the metal oxide layer (<NUM>) so that the metal oxide layer (<NUM>) generates heat,
the base layer (<NUM>) includes a front surface (<NUM>) and a rear surface (<NUM>) in the transmission direction,
the metal oxide layer (<NUM>) and the low refractive index layer (<NUM>) are laminated adjacent to each other in the transmission direction,
a laminate of the metal oxide layer (<NUM>) and the low refractive index layer (<NUM>) is laminated on the rear surface (<NUM>) of the base layer (<NUM>), and
the low refractive index layer (<NUM>) is laminated adjacent to a rear surface of the metal oxide layer (<NUM>) in the transmission direction and made of material that has a lower refractive index than material of the metal oxide layer (<NUM>) so that a reflected wave of the electromagnetic wave reflected on a boundary of an air layer and the low refractive index layer (<NUM>) and a reflected wave of the electromagnetic wave reflected on a boundary of the low refractive index layer (<NUM>) and the metal oxide layer (<NUM>) interfere with each other and weaken each other.