Patent Publication Number: US-9835318-B2

Title: Luminaire

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority of Japanese Patent Application Number 2014-181664, filed Sep. 5, 2014, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a luminaire capable of wireless communication. 
     2. Description of the Related Art 
     Luminaires capable of wireless communication have been conventionally known. A luminaire capable of wireless communication includes an antenna for wireless communication, and performs processes according to wireless signals received by the antenna (for example, see Japanese Unexamined Patent Application Publication No. 2013-145634). 
     SUMMARY OF THE INVENTION 
     It is aesthetically desirable that the antenna included in the luminaire is unnoticeable in the state where the luminaire is installed. The antenna is therefore placed in the luminaire body of the luminaire or in a housing in which a power circuit is contained. 
     However, the luminaire body and the housing containing the power circuit are typically made of metal, for safety measures and the like. Such a metal housing blocks electric waves, which causes a problem in ensuring the communication function of the antenna. 
     The present disclosure accordingly has an object of providing a luminaire that ensures the communication function of wireless communication and also improves the communication quality. 
     To achieve the object stated above, a luminaire according to one aspect of the present disclosure includes: a power circuit that supplies power to a light source; an antenna that at least one of transmits and receives a wireless signal; and a metal housing that contains the antenna and the power circuit. The metal housing has an opening, and the antenna and the opening are arranged to cause a polarization plane of an electric wave most strongly radiated from the antenna and a polarization plane of an electric wave most strongly radiated from the opening to substantially coincide with each other. 
     This structure ensures the communication function of wireless communication and also improves the communication quality. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The figures depict one or more implementations in accordance with the present teaching, by way of examples only, not by way of limitations. In the figures, like reference numerals refer to the same or similar elements. 
         FIG. 1  is an exploded perspective view illustrating a luminaire according to Embodiment 1 of the present disclosure; 
         FIG. 2  is a side view illustrating the luminaire according to Embodiment 1 of the present disclosure; 
         FIG. 3  is a diagram illustrating a wireless module including an antenna according to Embodiment 1 of the present disclosure; 
         FIG. 4  is a diagram illustrating the positional relationship between an opening of the luminaire body and the antenna according to Embodiment 1 of the present disclosure; 
         FIG. 5A  is a diagram illustrating the strength of vertical polarization of the antenna according to Embodiment 1 of the present disclosure; 
         FIG. 5B  is a diagram illustrating the strength of horizontal polarization of the antenna according to Embodiment 1 of the present disclosure; 
         FIG. 6  is a diagram illustrating the strength of electric wave with respect to the positional relationship between the opening of the metal housing and the antenna according to Embodiment 1 of the present disclosure; 
         FIG. 7  is a diagram illustrating the positional relationship between the opening of the metal housing and the antenna according to Embodiment 1 of the present disclosure; 
         FIG. 8A  is a diagram illustrating a radiation pattern in the case where the angle between the opening of the metal housing and the antenna is 90° according to Embodiment 1 of the present disclosure; 
         FIG. 8B  is a diagram illustrating a radiation pattern in the case where the angle between the opening of the metal housing and the antenna is 0° according to Embodiment 1 of the present disclosure; 
         FIG. 9  is a diagram illustrating the positional relationship between the opening of the metal housing and the antenna according to Embodiment 2 of the present disclosure; 
         FIG. 10A  is a plan view illustrating the shape of the opening of the metal housing according to Embodiment 2 of the present disclosure; 
         FIG. 10B  is a plan view illustrating the shape of an opening group of the metal housing according to Embodiment 2 of the present disclosure; 
         FIG. 10C  is a plan view illustrating another shape of the opening group of the metal housing according to Embodiment 2 of the present disclosure; 
         FIG. 10D  is a plan view illustrating another shape of the opening group of the metal housing according to Embodiment 2 of the present disclosure; 
         FIG. 10E  is a plan view illustrating another shape of the opening group of the metal housing according to Embodiment 2 of the present disclosure; 
         FIG. 10F  is a plan view illustrating another shape of the opening group of the metal housing according to Embodiment 2 of the present disclosure; 
         FIG. 10G  is a plan view illustrating another shape of the opening group of the metal housing according to Embodiment 2 of the present disclosure; 
         FIG. 11  is a diagram illustrating the relationship between the shape of the opening group of the metal housing and the communication performance according to Embodiment 2 of the present disclosure; 
         FIG. 12  is a diagram illustrating the positional relationship between a plurality of opening groups and the antenna according to a variation of Embodiment 2 of the present disclosure; 
         FIG. 13  is a sectional view illustrating a luminaire according to Embodiment 3 of the present disclosure; 
         FIG. 14A  is a perspective view illustrating a circuit case according to Embodiment 3 of the present disclosure; 
         FIG. 14B  is a bottom view illustrating the circuit case according to Embodiment 3 of the present disclosure; 
         FIG. 15  is an exploded perspective view illustrating the circuit case according to Embodiment 3 of the present disclosure; 
         FIG. 16A  is a perspective view illustrating the arrangement of openings of the metal housing according to Embodiment 3 of the present disclosure; 
         FIG. 16B  is a perspective view illustrating the arrangement of openings of the metal housing according to a comparative example of Embodiment 3 of the present disclosure; 
         FIG. 17A  is a diagram illustrating a radiation pattern in the XZ plane (plane parallel to openings) of the metal housing according to Embodiment 3 of the present disclosure; 
         FIG. 17B  is a diagram illustrating a radiation pattern in the YZ plane (plane perpendicular to openings) of the metal housing according to Embodiment 3 of the present disclosure; 
         FIG. 18A  is a diagram illustrating a radiation pattern in the XZ plane of the metal housing according to a comparative example of Embodiment 3 of the present disclosure; 
         FIG. 18B  is a diagram illustrating a radiation pattern in the YZ plane of the metal housing according to a comparative example of Embodiment 3 of the present disclosure; 
         FIG. 19  is a diagram illustrating the relationship between the opening width of openings in the metal housing and the radiation pattern according to Embodiment 3 of the present disclosure; 
         FIG. 20A  is a perspective view illustrating the arrangement of a plurality of openings of the metal housing according to Embodiment 3 of the present disclosure; 
         FIG. 20B  is a perspective view illustrating the arrangement of a plurality of openings of the metal housing according to a comparative example of Embodiment 3 of the present disclosure; 
         FIG. 21A  is a diagram illustrating a radiation pattern in the YZ plane of the metal housing according to Embodiment 3 of the present disclosure; 
         FIG. 21B  is a diagram illustrating a radiation pattern in the YZ plane of the metal housing according to a comparative example of Embodiment 3 of the present disclosure; 
         FIG. 22  is an exploded perspective view illustrating another example of the luminaire according to Embodiment 3 of the present disclosure; 
         FIG. 23A  is an exploded perspective view illustrating a luminaire according to Variation 1 of Embodiment 3 of the present disclosure; 
         FIG. 23B  is an exploded perspective view illustrating a luminaire according to Variation 2 of Embodiment 3 of the present disclosure; 
         FIG. 23C  is an exploded perspective view illustrating a luminaire according to Variation 3 of Embodiment 3 of the present disclosure; 
         FIG. 24A  is an exploded perspective view illustrating a luminaire according to Variation 4 of Embodiment 3 of the present disclosure; and 
         FIG. 24B  is an exploded perspective view illustrating a luminaire according to Variation 5 of Embodiment 3 of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following describes a luminaire according to each of the embodiments of the present disclosure in detail, with reference to drawings. Each of the embodiments described below shows a general or specific example. The numerical values, shapes, materials, structural elements, the arrangement and connection of the structural elements, etc. shown in the following embodiments are mere examples, and therefore do not limit the scope of the present disclosure. Of the structural elements in the following embodiments, structural elements not recited in any one of the independent claims representing superordinate concepts are described as optional structural elements. 
     Each of the drawings is a schematic diagram, and does not necessarily provide exact illustration. The same structural members are given the same reference signs in the drawings. 
     Embodiment 1 
     Luminaire 
     An overview of a luminaire according to Embodiment 1 is described first, with reference to  FIGS. 1 and 2 .  FIG. 1  is an exploded perspective view illustrating luminaire  1  according to this embodiment.  FIG. 2  is a side view illustrating luminaire  1  according to this embodiment. 
     Luminaire  1  according to this embodiment is capable of wireless communication. In detail, luminaire  1  performs wireless communication with another apparatus such as a remote control or a mobile information terminal such as a smartphone or a tablet terminal. In more detail, luminaire  1  transmits a wireless signal to another apparatus, or receives a wireless signal from another apparatus. For example, in the case where luminaire  1  receives a wireless signal from another apparatus, luminaire  1  performs a process such as turning on the light, turning off the light, dimming, or toning based on the received wireless signal. 
     Luminaire  1  includes light source  2 , luminaire body  10 , feeding socket  20 , non-feeding socket  30 , reflector  40 , circuit case  50 , power circuit  60 , and wireless module  70  including antenna  71 , as illustrated in  FIGS. 1 and 2 . 
     Luminaire  1  has one surface (i.e. the surface opposite to reflector  40 ) of luminaire body  10  fixed to an installation surface such as a ceiling. Luminaire  1  is long, as illustrated in  FIGS. 1 and 2 . The X axis direction is the transverse direction of reflector  40  of luminaire  1 , the Y axis direction is the longitudinal direction of reflector  40 , and the Z axis direction is the direction perpendicular to reflector  40 . Hereafter, “up” means the Z axis positive direction (toward the installation surface such as the ceiling) and “down” means the Z axis negative direction (the direction of light irradiation) in  FIGS. 1 and 2 . 
     Each structural member in luminaire  1  is described in detail below. 
     [Light Source] 
     Light source  2  is a light source for illumination including a light emitting module having a light emitting element, and emits predetermined light. Light source  2  is removably placed in luminaire body  10 . 
     In this embodiment, light source  2  is a straight-tube light emitting diode (LED) lamp. The straight-tube LED lamp includes, for example, a long housing such as a glass bulb, an LED module placed in the housing, and a feeding base and non-feeding base provided at the ends of the housing. 
     The LED module is actually a chip-on-board (COB) light emitting module in which an LED chip is mounted directly on a board, though the LED module is not limited to this. For example, the LED module may be a light emitting module in which a surface-mount-device (SMD) LED element is used as a light emitting element. The SMD LED element is a packaged LED element obtained by mounting an LED chip in a cavity of a case made of resin and enclosing phosphor-containing resin in the cavity. The light emitting element included in light source  2  may be any other solid light emitting element, for example, a semiconductor light emitting element such as a semiconductor laser or an electroluminescence (EL) element such as an organic EL or inorganic EL element. 
     [Luminaire Body] 
     Luminaire body  10  is an example of a metal structure. Luminaire body  10  is formed by bending a plate member made of metal such as aluminum to have an opening on the lower side. 
     Luminaire body  10  is a container that contains circuit case  50  (power circuit  60 ) and wireless module  70  (antenna  71 ). In detail, luminaire body  10  is substantially a long rectangular parallelepiped, and has its open surface covered by reflector  40 . Feeding socket  20  and non-feeding socket  30  are provided at the respective ends of luminaire body  10  so as to protrude downward below reflector  40 . 
     In this embodiment, luminaire body  10  and reflector  40  constitute a metal housing that contains power circuit  60  and antenna  71 . 
     Luminaire body  10  has opening  11 . In this embodiment, only one opening  11  is formed in a side surface of luminaire body  10 , as illustrated in  FIG. 2 . 
     Opening  11  is a slit opening. In detail, opening  11  is substantially rectangular in shape. Opening  11  is formed, for example, with its long side direction (Y axis direction) being parallel to the long side direction of luminaire body  10 . The length (L in  FIG. 2 ) of the long side of opening  11  is greater than or equal to substantially half (λ/2) the wavelength λ corresponding to the wireless signal frequency. 
     The term “substantially half the wavelength” means that the length is essentially or approximately equal to the half wavelength. The term “substantially half the wavelength” may include the range of plus and minus several % (e.g. 5%) with respect to the half wavelength. For example, the length of the long side of opening  11  may be several % shorter than half (λ/2) the wavelength λ corresponding to the wireless signal frequency. The same applies to other similar descriptions. 
     [Feeding Socket and Non-Feeding Socket] 
     Feeding socket  20  is a socket for feeding power to light source  2 . Feeding socket  20  is formed, for example, by insert molding using a resin material and a pin receiving terminal made of metal. 
     A base pin (e.g. a pair of L-shaped pins) of a feeding base of light source  2  is inserted in feeding socket  20 . The inserted base pin is in contact with the metal pin receiving terminal in feeding socket  20 . For example, the pin receiving terminal is connected to power circuit  60  in circuit case  50 . Power for lighting light source  2  can thus be fed to light source  2  via feeding socket  20 . 
     Non-feeding socket  30  is a non-feeding socket for holding light source  2 . Non-feeding socket  30  is formed, for example, by injection molding using a resin material. A base pin (e.g. a T-shaped pin) of a non-feeding base of light source  2  is inserted in non-feeding socket  30 . 
     [Reflector] 
     Reflector  40  is an example of a metal structure. Reflector  40  is formed by processing a plate member made of metal, like luminaire body  10 . For example, reflector  40  is painted white or milk white so that its lower surface has a reflection function. Although this embodiment describes an example where reflector  40  is separate from luminaire body  10 , reflector  40  may be formed integrally with luminaire body  10 . 
     [Circuit Case] 
     Circuit case  50  is a case that contains power circuit  60 . Circuit case  50  is formed, for example, by bending a plate member made of metal such as aluminum. 
     For example, circuit case  50  has a through hole through which a lead wire (not illustrated) is inserted. The lead wire electrically connects power circuit  60  and antenna  71 . 
     Circuit case  50  may contain not only power circuit  60  but also antenna  71 , as in Embodiment 3 described later. Detailed description will be given in Embodiment 3. 
     [Power Circuit] 
     Power circuit  60  is a circuit (lighting circuit) for supplying power to light source  2 . In detail, power circuit  60  converts AC power supplied from system power or the like via a cable (not illustrated) into DC power and supplies the power to light source  2 . 
     Power circuit  60  is formed, for example, on a printed wiring board. In detail, power circuit  60  includes a diode bridge rectifier circuit for converting AC power into DC power and a DC-DC converter. Power circuit  60  may be realized as one integrated circuit (IC) having the same functions as a rectifier circuit and a DC-DC converter. 
     [Wireless Module and Antenna] 
       FIG. 3  is a diagram illustrating wireless module  70  including antenna  71  according to this embodiment. 
     Wireless module  70  includes antenna  71 , wireless control circuit  72 , printed wiring board  73 , and connector  74 , as illustrated in  FIG. 3 . 
     Antenna  71  is a pattern antenna for transmitting or receiving a wireless signal. In other words, antenna  71  is a conductive pattern formed on printed wiring board  73 , as illustrated in  FIG. 3 . Providing antenna  71  on printed wiring board  73  as a pattern antenna enables a reduction in size of antenna  71 . 
     Note that antenna  71  is any antenna that performs at least one of transmission and reception of a wireless signal, and is not limited to a pattern antenna. For example, antenna  71  may be a chip antenna. 
     In this embodiment, antenna  71  is a substantially L-shaped pattern antenna. As illustrated in  FIG. 3 , antenna  71  has tip portion  71   a  and root portion  71   b . Tip portion  71   a  is a part on the tip side of antenna  71 . Root portion  71   b  is a part on the wireless control circuit  72  side of antenna  71 . Tip portion  71   a  and root portion  71   b  are substantially orthogonal to each other. 
     Although antenna  71  is not limited to be substantially L-shaped but may be linear or the like in shape, substantially L-shaped antenna  71  enables a reduction in size of wireless module  70 . 
     In this embodiment, the frequency band of the wireless signal transmitted or received by antenna  71  is the ultra high frequency (UHF) band or the super high frequency (SHF) band. 
     Wireless control circuit  72  is an integrated circuit that controls the transmission or reception of the wireless signal by antenna  71 . In this embodiment, wireless control circuit  72  obtains a predetermined command included in the wireless signal which antenna  71  has received from a remote control or the like. Wireless control circuit  72  controls power circuit  60  according to the obtained command. In detail, wireless control circuit  72  controls power circuit  60  to turn on or off light source  2 . 
     In this embodiment, wireless control circuit  72  performs communication using ZigBee® which is one of the standards for wireless personal area networks (WPANs). The communication method of wireless control circuit  72  is, however, not limited to this, and may be Bluetooth® or a wireless local area network (LAN). 
     Printed wiring board  73  is a board on which antenna  71  and wireless control circuit  72  are mounted. 
     Connector  74  is a connector for connecting power circuit  60  and each of wireless control circuit  72  and antenna  71 . For example, a receiving-side connector (not illustrated) to which connector  74  is connectable is provided in luminaire body  10 . The receiving-side connector is electrically connected to power circuit  60  via a lead wire or the like. By connecting connector  74  to the receiving-side connector, wireless control circuit  72  can be electrically connected to power circuit  60 . 
     Connecting connector  74  and the receiving-side connector allows the position of wireless module  70  in luminaire body  10  to be fixed. The positional relationship between antenna  71  and opening  11  can thus be determined, as described later. 
     Power circuit  60  and wireless control circuit  72  may be connected directly to each other via a lead wire. 
     Although power circuit  60  and the structure including antenna  71  and wireless control circuit  72  are provided on different boards in this embodiment, these components may be provided on one board. In this case, a shorter wire length contributes to more stable operation, and a smaller number of components contributes to lower cost. 
     [Positional Relationship Between Antenna and Opening] 
     The following describes the positional relationship between antenna  71  and opening  11  formed in luminaire body  10  according to this embodiment, with reference to  FIG. 4 . 
       FIG. 4  is a diagram illustrating the positional relationship between opening  11  of luminaire body  10  and antenna  71  according to this embodiment. 
     In this embodiment, luminaire body  10  is combined with reflector  40  to constitute a metal housing which is substantially a rectangular parallelepiped. The metal housing has opening  11  and contains antenna  71  inside, as illustrated in  FIG. 4 . 
     In this case, since antenna  71  in the metal housing and opening  11  are close to each other, antenna  71  and opening  11  are coupled to each other. As a result, an electric wave generated from antenna  71  causes a current to flow on the surface of the metal housing. In other words, opening  11  of the metal housing functions as a slot antenna. In the slot antenna, an electric field is generated in the short side direction of opening  11 . Hence, in the example illustrated in  FIG. 4 , the polarization plane of the electric wave most strongly radiated from opening  11  is parallel to the short side direction of opening  11 . 
     In this embodiment, antenna  71  and opening  11  are arranged so that the polarization plane of the electric wave most strongly radiated from antenna  71  and the polarization plane of the electric wave most strongly radiated from opening  11  substantially coincide with each other. In detail, antenna  71  is positioned so that the polarization plane of the electric wave most strongly radiated from antenna  71  is at an angle of at least ±15° with respect to the long side direction of opening  11 . 
     The strength of the electric wave radiated from antenna  71  is described below, with reference to  FIGS. 5A and 5B . 
       FIGS. 5A and 5B  are diagrams respectively illustrating the strengths of vertical polarization and horizontal polarization of antenna  71  according to this embodiment. The vertical polarization is the direction parallel to root portion  71   b  of antenna  71 , and the horizontal polarization is the direction parallel to tip portion  71   a  of antenna  71 . 
     As can be understood from the comparison of  FIGS. 5A and 5B , the vertical polarization is stronger than the horizontal polarization. Though other polarizations are not illustrated, the polarization plane of the electric wave most strongly radiated from antenna  71  in this embodiment is the vertical polarization. In other words, the polarization plane of the electric wave most strongly radiated from antenna  71  is parallel to root portion  71   b.    
     Accordingly, antenna  71  is positioned so that root portion  71   b  is perpendicular to the long side direction of opening  11 , as illustrated in  FIG. 4 . As a result, the polarization plane (the vertical polarization illustrated in  FIG. 5A ) of the electric wave most strongly radiated from antenna  71  and the polarization plane (the short side direction of opening  11 ) of the electric wave most strongly radiated from opening  11  coincide with each other. 
     The positional relationship between opening  11  of the metal housing and antenna  71  and the maximum gain of the electric wave radiated from opening  11  are described below, with reference to  FIGS. 6 to 8B . 
       FIG. 6  is a diagram illustrating the strength of the electric wave with respect to the positional relationship between opening  11  of the metal housing and antenna  71  according to this embodiment. In detail,  FIG. 6  illustrates the result of simulating the strength of the electric wave depending on the angle between opening  11  and antenna  71 , while changing the angle. 
     The horizontal axis in  FIG. 6  represents the angle between the long side direction of opening  11  and root portion  71   b  of antenna  71 . Since the polarization plane of the electric wave most strongly radiated from antenna  71  is parallel to root portion  71   b  of antenna  71 , the horizontal axis in  FIG. 6  corresponds to the angle between the long side direction of opening  11  and the polarization plane of the electric wave most strongly radiated from antenna  71 . The vertical axis in  FIG. 6  represents the maximum gain of the electric wave radiated from opening  11  (main lobe strength) and the total efficiency. 
       FIG. 7  is a diagram illustrating the positional relationship between opening  11  of the metal housing and antenna  71  according to this embodiment. The strength of the electric wave in the case where the angle between the long side direction of opening  11  and root portion  71   b  of antenna  71  is 0°, 5°, 15°, 45°, 75°, and 90° is simulated as illustrated in  FIG. 7 . 
     As illustrated in  FIG. 6 , the maximum gain and the total efficiency are saturated and stable in the range greater than or equal to 15° and less than or equal to 90°. This indicates that antenna  71  is to be installed in luminaire body  10  so that the angle between the long side direction of opening  11  and root portion  71   b  of antenna  71  is in the range greater than or equal to 15° and less than or equal to 90°. 
       FIGS. 8A and 8B  are diagrams respectively illustrating radiation patterns in the case where the angle between opening  11  of the metal housing and antenna  71  is 90° and in the case where the angle between opening  11  of the metal housing and antenna  71  is 0° according to this embodiment. 
     The maximum strength of the main lobe is about 3 dB in the case where the angle is 90°, and about −23 dB in the case where the angle is 0°, as illustrated in  FIGS. 8A and 8B . Thus, the strength of the electric wave is greater in the case where the angle is 90° than in the case where the angle is 0°. 
     The above indicates that antenna  71  is to be positioned so that the polarization plane of the electric wave most strongly radiated from antenna  71  is at an angle of at least ±15° with respect to the long side direction of opening  11 . In other words, antenna  71  is to be positioned so that the polarization plane of the electric wave most strongly radiated from antenna  71  is in the range greater than or equal to 15° and less than or equal to 165°, and greater than or equal to 195° and less than or equal to 345°, with respect to the long side direction of opening  11 . 
     [Conclusion] 
     As described above, luminaire  1  according to this embodiment includes: power circuit  60  that supplies power to light source  2 ; antenna  71  that at least one of transmits and receives a wireless signal; and luminaire body  10  that contains antenna  71  and power circuit  60 . Luminaire body  10  has opening  11 , and antenna  71  and opening  11  are arranged to cause a polarization plane of an electric wave most strongly radiated from antenna  71  and a polarization plane of an electric wave most strongly radiated from opening  11  to substantially coincide with each other. 
     Thus, in this embodiment, the metal housing has opening  11  that functions as a slot antenna to radiate the electric wave from antenna  71  placed in the metal housing to the outside. This ensures the communication function of wireless communication. 
     Moreover, in this embodiment, antenna  71  and opening  11  are arranged so that the polarization plane of the electric wave most strongly radiated from antenna  71  and the polarization plane of the electric wave most strongly radiated from opening  11  substantially coincide with each other. The strength of the electric wave from opening  11  can be increased in such a way, as illustrated in  FIG. 6  and the like. This improves the communication quality of wireless communication. 
     For example, the metal housing is substantially a rectangular parallelepiped, opening  11  is substantially rectangular in shape, a long side direction of opening  11  is parallel to a long side direction of the metal housing, and antenna  71  is positioned with the polarization plane of the electric wave most strongly radiated from antenna  71  being at an angle of at least ±15° with respect to the long side direction of opening  11 . 
     The strength of the electric wave from opening  11  can be increased in such a way, thus improving the communication quality of wireless communication. 
     For example, opening  11  is substantially rectangular in shape, and a length of a long side of opening  11  is greater than or equal to substantially half a wavelength corresponding to a frequency of the wireless signal. 
     This enables efficient transmission or reception of the wireless signal. 
     For example, antenna  71  is mounted on printed wiring board  73 . 
     This enables a reduction in size of the antenna and a reduction in the number of components. 
     For example, opening  11  is a slit opening. 
     Such opening  11  can efficiently function as a slot antenna. 
     Embodiment 2 
     Luminaire 
     The following describes a luminaire according to Embodiment 2. The following description mainly focuses on the differences from Embodiment 1, and may omit the same parts. 
       FIG. 9  is a diagram illustrating the positional relationship between first opening  120  of the metal housing and antenna  71  according to this embodiment. 
     In this embodiment, first opening  120  is formed in side surface  110  of the metal housing instead of opening  11  in Embodiment 1, as can be seen from the comparison with  FIG. 4 . First opening  120  differs in shape from opening  11 . 
     First opening  120  is described in detail below. 
     [Opening] 
       FIG. 10A  is a plan view illustrating the shape of first opening  120  of the metal housing according to this embodiment. 
     In this embodiment, first opening  120  is formed in side surface  110  of the metal housing, as illustrated in  FIG. 9 . In detail, first opening  120  is formed in side surface  110  of luminaire body  10 . 
     First opening  120  is a slit opening. As illustrated in  FIG. 10A , first opening  120  has long side portion  120   a  and two short side portions  120   b . Two short side portions  120   b  are positioned orthogonally at the respective ends of long side portion  120   a.    
     Width d of first opening  120  is substantially constant. Let L 1  be the length of long side portion  120   a , and L 2  be the length of each short side portion  120   b . The length of first opening  120  along the slit direction is L 1 +2×L 2 . Thus, the length of first opening  120  along the slit direction is the total length of long side portion  120   a  and two short side portions  120   b.    
     In this embodiment, the length of first opening  120  along the slit direction is greater than or equal to substantially half the wavelength corresponding to the wireless signal frequency. For example, when the wireless signal uses the frequency band of 2.4 GHz, half the wavelength corresponding to this frequency is about 62 mm. Hence, for example, first opening  120  illustrated in  FIG. 10A  is formed so that length L 1  of long side portion  120   a  is 62 mm, length L 2  of each short side portion  120   b  is 4 mm, and width d is 0.5 mm. 
     With first opening  120  illustrated in  FIG. 10A , the standing wave ratio (SWR) is 1.3, and the main lobe strength is 2.6 dB. Thus, the power efficiency is improved, and the electric wave is radiated efficiently. The detailed antenna characteristics of first opening  120  are illustrated in (a) in  FIG. 11 . 
     [Opening Group] 
     Although  FIG. 10A  illustrates an example where only one first opening  120  is formed in side surface  110  of the metal housing, this is not a limitation. A plurality of openings including first opening  120  may be formed in side surface  110  of the metal housing. In other words, an opening group including a plurality of openings may be formed in side surface  110  of the metal housing. 
     The following describes examples of such an opening group with reference to  FIGS. 10B to 10G .  FIGS. 10B to 10D  are plan views respectively illustrating the shapes of opening groups  101  to  103  of the metal housing according to this embodiment.  FIGS. 10E to 10G  are plan views respectively illustrating the shapes of opening groups  104  to  106  of the metal housing according to comparative examples. 
     As illustrated in  FIGS. 10B to 10G , opening groups  101  to  106  are each formed in side surface  110  of the metal housing. Side surface  110  of the metal housing includes first plate portion  111 , second plate portion  112 , and connecting portion  113 . 
     First plate portion  111  is a part surrounded by the plurality of openings (i.e. closing plate). Second plate portion  112  is a part outside the plurality of openings. Connecting portion  113  is a part between an end of one of the plurality of openings and an end of another one of the plurality of openings, and connects first plate portion  111  and second plate portion  112 . In other words, the plurality of openings and the plurality of connecting portions  113  are provided between first plate portion  111  and second plate portion  112 . 
     First plate portion  111 , second plate portion  112 , and connecting portions  113  are formed, for example, by processing one plate member made of metal. Thus, first plate portion  111 , second plate portion  112 , and connecting portions  113  are integrally formed from the same member. First plate portion  111 , second plate portion  112 , and connecting portions  113  therefore have substantially the same surface color. 
     As illustrated in  FIGS. 10B to 10G , opening groups  101  to  106  each include a plurality of openings, first plate portion  111 , and a plurality of connecting portions  113 . Each opening group thus has a knockout structure. 
     The plurality of openings include first opening  120 , second openings  121  to  123 , etc. 
     Example 1 
     Opening group  101  illustrated in  FIG. 10B  is made up of two first openings  120 , first plate portion  111 , and two connecting portions  113 . 
     Two first openings  120  have parts substantially parallel to each other. In detail, two first openings  120  are arranged so that respective long side portions  120   a  are parallel to each other. The ends of two first openings  120  face each other. 
     First plate portion  111  is a part surrounded by two first openings  120 . First plate portion  111  is substantially rectangular, as illustrated in  FIG. 10B . First plate portion  111  is connected to second plate portion  112  by two connecting portions  113 . 
     Example 2 
     Opening group  102  illustrated in  FIG. 10C  is made up of one first opening  120 , two second openings  121 , first plate portion  111 , and three connecting portions  113 . First plate portion  111  is substantially rectangular. One first opening  120 , two second openings  121 , and three connecting portions  113  are provided along the periphery of first plate portion  111 , i.e. along the periphery of the substantially rectangular shape. 
     Two second openings  121  each have long side portion  121   a  and short side portion  121   b . Short side portion  121   b  is formed at the end of long side portion  121   a , and is orthogonal to long side portion  121   a . Thus, each second opening  121  is a substantially L-shaped slit opening. The length of second opening  121  along the slit direction is less than or equal to half the length of first opening  120  along the slit direction. 
     Two second openings  121  are arranged so that two long side portions  121   a  and long side portion  120   a  of first opening  120  are parallel to each other. Respective long side portions  121   a  of two second openings  121  lie on a straight line, and connecting portion  113  is provided between long side portions  121   a . Each of two short side portions  120   b  of first opening  120  and short side portion  121   b  of a corresponding one of two second openings  121  lie on a straight line, and connecting portion  113  is provided between short side portion  120   b  and short side portion  121   b.    
     Example 3 
     Opening group  103  illustrated in  FIG. 10D  is made up of one first opening  120 , one second opening  122 , two second openings  123 , first plate portion  111 , and four connecting portions  113 . One first opening  120 , one second opening  122 , two second openings  123 , and four connecting portions  113  are provided along the periphery of first plate portion  111 , i.e. along the periphery of the substantially rectangular shape. 
     Second opening  122  is a substantially rectangular opening. Second opening  122  is positioned in parallel with long side portion  120   a  of first opening  120 . The length of second opening  122  along the slit direction is less than or equal to half the length of first opening  120  along the slit direction. 
     Two second openings  123  each have long side portion  123   a  and short side portion  123   b . Short side portion  123   b  is formed at the end of long side portion  123   a , and is orthogonal to long side portion  123   a . Thus, each second opening  123  is a substantially L-shaped slit opening. The length of second opening  123  along the slit direction is less than or equal to half the length of first opening  120  along the slit direction. 
     Second opening  122  and respective long side portions  123   a  of two second openings  123  lie on a straight line, and connecting portion  113  is provided between each end of second opening  122  and corresponding long side portion  123   a . Each of two short side portions  120   b  of first opening  120  and short side portion  123   b  of a corresponding one of two second openings  123  lie on a straight line, and connecting portion  113  is provided between short side portion  120   b  and short side portion  123   b.    
     Comparative Example 1 
     Opening group  104  illustrated in  FIG. 10E  is made up of two second openings  122 , four second openings  123 , first plate portion  111 , and six connecting portions  113 . Two second openings  122 , four second openings  123 , and six connecting portions  113  are provided along the periphery of first plate portion  111 , i.e. along the periphery of the substantially rectangular shape. 
     One second opening  122  and respective long side portions  123   a  of two second openings  123  lie on a straight line. The other second opening  122  and respective long side portions  123   a  of the other two second openings  123  also lie on a straight line. The directions in which these elements are arranged are parallel to each other. In other words, two second openings  122  are arranged in parallel with each other. Connecting portion  113  is provided between second opening  122  and long side portion  123   a  of each of two second openings  123 . 
     Respective short side portions  123   b  of two second openings  123  lie on a straight line. Respective short side portions  123   b  of the other two second openings  123  also lie on a straight line. The directions in which these elements are arranged are parallel to each other. Connecting portion  113  is provided between two short side portions  123   b.    
     Opening group  104  does not include first opening  120 , as illustrated in  FIG. 10E . Accordingly, the length of every opening in opening group  104  along the slit direction is less than substantially half the wavelength corresponding to the wireless signal frequency. 
     Comparative Example 2 
     Opening group  105  illustrated in  FIG. 10F  is made up of two second openings  121 , one second opening  122 , two second openings  123 , first plate portion  111 , and five connecting portions  113 . 
     Opening group  105  has the shape that combines opening group  102  illustrated in  FIG. 10C  and opening group  104  illustrated in  FIG. 10E , as illustrated in  FIG. 10F . Two second openings  121 , one second opening  122 , two second openings  123 , and five connecting portions  113  are provided along the periphery of first plate portion  111 , i.e. along the periphery of the substantially rectangular shape. 
     Opening group  105  does not include first opening  120 , as illustrated in  FIG. 10F . Accordingly, the length of every opening in opening group  105  along the slit direction is less than substantially half the wavelength corresponding to the wireless signal frequency. 
     Comparative Example 3 
     Opening group  106  illustrated in  FIG. 10G  is made up of two second openings  121 , one second opening  122 , two second openings  123 , first plate portion  111 , and five connecting portions  113 . 
     Opening group  106  has opening group  105  illustrated in  FIG. 10F  turned upside down, as illustrated in  FIG. 10G . 
     [Characteristics of Opening Group] 
     The antenna characteristics of opening groups  101  to  106  mentioned above are described below, with reference to  FIG. 11 . 
       FIG. 11  is a diagram illustrating the relationship between the shape of the opening group of the metal housing and the communication performance according to this embodiment. In  FIG. 11 , (a) to (g) respectively correspond to first opening  120  and opening groups  101  to  106  illustrated in  FIGS. 10A to 10G .  FIG. 11  illustrates the standing wave ratio (SWR), the main lobe characteristics (strength, direction, angle width), and the side lobe strength in each case. 
     As illustrated in (a) to (d) in  FIG. 11 , in first opening  120  and opening groups  101  to  103 , the SWR is less than or equal to 1.5, and the electric wave is radiated efficiently. In opening groups  101  to  103 , the main lobe strength is 5.1 dB to 5.7 dB, which is sufficiently high as compared with the case where only first opening  120  is provided (2.6 dB) as illustrated in  FIG. 10A . Thus, by forming any of opening groups  101  to  103  in the metal housing, the electric wave can be radiated more efficiently, with it being possible to improve the communication quality of wireless communication. 
     In opening groups  104  to  106 , on the other hand, the SWR is high, and the main lobe strength is low, as illustrated in (e) to (g) in  FIG. 11 . This indicates that opening groups  104  to  106  each fail to function as an antenna. 
     Opening groups  104  to  106  each do not include first opening  120 , as illustrated in  FIGS. 10E to 10G . In other words, opening groups  104  to  106  each include only openings less than substantially half the wavelength corresponding to the wireless signal frequency. Conversely, the inclusion of an opening (e.g. first opening  120 ) greater than or equal to substantially half the wavelength corresponding to the wireless signal frequency enables the opening group to function as an antenna. 
     [Conclusion] 
     As described above, in the luminaire according to this embodiment, the metal housing has one or more openings in side surface  110 , the one or more openings are each a slit opening, and a length of first opening  120  along a slit direction is greater than or equal to substantially half a wavelength corresponding to a frequency of the wireless signal, first opening  120  being at least one of the one or more openings. 
     Thus, in this embodiment, the metal housing has first opening  120  that functions as a slot antenna to radiate the electric wave from antenna  71  placed in the metal housing to the outside. This ensures the communication function of wireless communication. 
     Moreover, in this embodiment, the length of first opening  120  along the slit direction is greater than or equal to substantially half the wavelength corresponding to the wireless signal frequency. The electric wave can therefore be efficiently radiated, as can be understood from the comparison between (a) and (e) to (g) in  FIG. 11 . This improves the communication quality of wireless communication. 
     For example, the metal housing has a plurality of openings in side surface  110 , and side surface  110  of the metal housing includes: first plate portion  111  surrounded by the plurality of openings; second plate portion  112  that is a part outside the plurality of openings; and connecting portion  113  that is a part between an end of one of the plurality of openings and an end of another one of the plurality of openings and connects first plate portion  111  and second plate portion  112 . 
     By forming the plurality of openings according to the knockout structure in this way, the electric wave can be radiated more efficiently than in the case where only one first opening  120  is formed, as can be understood from the comparison between (a) and (b) to (d) in  FIG. 11 . In addition, the knockout structure can keep external dust or foreign matter from entering the metal housing. This prevents the situation where heat is generated as a result of dust and the like adhering to power circuit  60  and consequently fire or smoking occurs. 
     For example, the plurality of openings may include a plurality of first openings  120 , and the plurality of first openings  120  may have parts substantially parallel to each other. 
     In this way, the electric wave can be radiated more efficiently than in the case where only one first opening  120  is formed, as can be understood from the comparison between (a) and (b) in  FIG. 11 . 
     For example, the plurality of openings may include second opening  121  different from first opening  120 , and a length of second opening  121  along a slit direction may be less than or equal to half the length of first opening  120  along the slit direction. 
     In this way, the electric wave can be radiated more efficiently than in the case where only one first opening  120  is formed, as can be understood from the comparison between (a) and (c) to (d) in  FIG. 11 . 
     For example, first plate portion  111 , second plate portion  112 , and connecting portion  113  may have substantially a same surface color. 
     This makes the openings unnoticeable, which is desirable in aesthetic terms. Besides, in the case where the openings are formed in reflector  40 , a reduction in reflection function of reflector  40  can be suppressed. 
     Variation of Embodiment 2 
     The following describes a variation of the luminaire according to Embodiment 2, with reference to drawings. 
     In the luminaire in this variation, a plurality of opening groups are formed in side surfaces of the metal housing. 
       FIG. 12  is a diagram illustrating the positional relationship between a plurality of opening groups  101  and antenna  71  according to this variation. 
     In the luminaire according to this variation, luminaire body  10  and reflector  40  constitute the metal housing, and two opening groups  101  are formed in side surfaces of the metal housing. In detail, opening group  101  is formed in each of opposite side surfaces  110  and  110   a  of the metal housing. Two opening groups  101  face each other with antenna  71  in between. 
     Thus, for example, a plurality of opening groups  101  each including first plate portion  111 , connecting portion  113 , and a plurality of openings are formed in side surfaces  110  of the metal housing in this variation. 
     This enables the radiation pattern to be changed. 
     Although two opening groups  101  are provided in the example illustrated in  FIG. 12 , different opening groups may be provided instead. For example, opening group  101  and opening group  102  may be formed respectively in side surface  110  and side surface  110   a . Moreover, two opening groups  101  may be formed in one side surface  110 . 
     For example, the luminaire may further include an insulator in one or more openings. 
     In the knockout structure, for instance, there is a possibility of erroneous piercing during construction. Such erroneous piercing can be suppressed by providing the insulator. 
     Embodiment 3 
     [Luminaire] 
     The following describes a luminaire according to Embodiment 3. While a straight-tube LED lamp is used as an example of the luminaire in Embodiments 1 and 2, a downlight is used as an example in this embodiment. 
       FIG. 13  is a sectional view illustrating luminaire  201  according to this embodiment. In detail,  FIG. 13  illustrates the state in which luminaire  201  is installed in embedding hole  81  formed in ceiling  80 . 
     Luminaire  201  according to this embodiment is, for example, a recessed luminaire such as a downlight that is embedded in ceiling  80  of a house or the like to emit light downward (toward the floor or wall). Luminaire  201  is capable of wireless communication. 
     As illustrated in  FIG. 13 , luminaire  201  includes luminaire body  210  and circuit case  220 . Light source  202  is contained in luminaire body  210 . Power circuit  60  and wireless module  70  that includes antenna  71  are contained in circuit case  220 . Luminaire  201  further includes heat radiation fin  230 , attachment spring  240 , and attachment portion  250 . 
     As illustrated in  FIG. 13 , the X axis direction is the direction of connection between luminaire body  210  and circuit case  220  in the plane parallel to ceiling  80 , the Y axis direction is the direction orthogonal to the X axis direction in the plane parallel to ceiling  80 , and the Z axis direction is the direction perpendicular to ceiling  80 . 
     Each structural member in luminaire  201  is described in detail below. 
     Light source  202  is a light source for illumination including a light emitting module having a light emitting element, and emits predetermined light. In this embodiment, light source  202  includes a COB light emitting module, or a light emitting module using an SMD LED element. 
     Luminaire body  210  is a housing which is substantially a truncated cone. A plurality of heat radiation fins  230  protruding outward are provided on the outer peripheral surface of luminaire body  210 . Attachment spring  240  is also attached to the outer peripheral surface of luminaire body  210 . 
     Circuit case  220  is a metal housing that contains power circuit  60  and antenna  71 . Circuit case  220  is formed, for example, by bending a plate member made of metal such as aluminum. 
     The detailed structure of circuit case  220  will be described later. 
     Heat radiation fins  230  are fins for radiating heat generated when light source  202  emits light, to the outside. For example, heat radiation fins  230  are formed integrally with luminaire body  210 . 
     Attachment spring  240  is fixed to the outer peripheral surface of luminaire body  210 , and is biased outward. Attachment spring  240  is used to install luminaire  201  (luminaire body  210 ) into embedding hole  81 . 
     Attachment portion  250  is connected to a cable (not illustrated) that is connected to system power (utility power) which is the supplier of AC power. Attachment portion  250  feeds AC power obtained via the cable, to power circuit  60  in circuit case  220  via cable  260 . Attachment portion  250  is provided at one end of circuit case  220  in the longitudinal direction. 
     Cable  260  is a cable for supplying the AC power received by attachment portion  250  to power circuit  60  in circuit case  220 . Cable  261  is a cable for supplying the power from power circuit  60  in circuit case  220  to light source  202  in luminaire body  210 . 
     [Circuit Case] 
     Circuit case  220  according to this embodiment is described in detail below. 
       FIGS. 14A and 14B  are respectively a perspective view and bottom view illustrating circuit case  220  according to this embodiment. 
     Circuit case  220  according to this embodiment is a polyhedral metal housing, and has upper cover  221  and lower cover  222  that are combined so as to create a gap. By combining upper cover  221  and lower cover  222 , openings  223  and  224  are formed in circuit case  220 , as illustrated in  FIGS. 14A and 14B . Thus, openings  223  and  224  are gaps created when combining upper cover  221  and lower cover  222 . 
       FIG. 15  is an exploded perspective view illustrating circuit case  220  according to this embodiment. 
     Upper cover  221  and lower cover  222  are an example of a metal structure. Upper cover  221  and lower cover  222  are each formed by processing a plate member made of metal. Upper cover  221  and lower cover  222  each have a plurality of substantially rectangular surfaces. 
     Upper cover  221  has opening  225 . Opening  225  is a gap formed in upper cover  221 . 
     Opening  225  is formed along a side of a substantially rectangular surface. For example, upper cover  221  has surfaces  221   a  to  221   c , as illustrated in  FIG. 15 . Surfaces  221   a  and  221   b  are perpendicular to surface  221   c . Opening  225  is formed along two sides of surface  221   c . In detail, opening  225  is formed between surface  221   c  and each of surfaces  221   a  and  221   b.    
     As illustrated in  FIG. 14A , opening  225  is positioned on sides of circuit case  220 . In detail, opening  225  extends over two or more continuous sides of circuit case  220 . 
     Moreover, opening  225  connects to opening  224 . In other words, opening  225  forms one continuous opening together with opening  224 , and this opening extends over three continuous sides of the polyhedron (i.e. three sides of surface  221   c ). The length of the opening is greater than or equal to substantially half the wavelength corresponding to the wireless signal frequency. 
     The radiation pattern of the electric wave can be changed by providing the plurality of openings  223  to  225  in circuit case  220  in the above-mentioned manner. 
     [Arrangement of Openings and Radiation Pattern] 
     The following describes how the radiation pattern changes according to the arrangement of openings in the metal housing, with reference to  FIGS. 16A to 18B . 
       FIGS. 16A and 16B  are each a diagram illustrating the arrangement of openings in the metal housing according to this embodiment. 
     Metal housing  300  illustrated in  16 A is a metal housing which models circuit case  220  according to this embodiment. Metal housing  300  is a polyhedron, and has surfaces  301  to  305 . Surfaces  304  and  305  are parallel to each other, and perpendicular to surfaces  301  to  303 , as illustrated in  FIG. 16A . 
     Metal housing  300  has two openings  310  and  311 . Two openings  310  and  311  are parallel to each other. 
     Opening  310  is formed on sides of surface  304 . In detail, opening  310  is formed between surface  304  and each of surfaces  301  to  303 . Thus, opening  310  extends over three continuous sides of surface  304 . 
     Opening  311  is formed on sides of surface  305 . In detail, opening  311  is formed between surface  305  and each of surfaces  301  to  303 . Thus, opening  311  extends over three continuous sides of surface  305 . 
       FIG. 16B  illustrates metal housing  320  that differs only in the positions of openings from metal housing  300  illustrated in  FIG. 16A . Metal housing  320  illustrated in  FIG. 16B  has two openings  330  and  331 . Two openings  330  and  331  are parallel to each other. 
     Opening  330  is formed on a side of surface  304 . In detail, opening  330  is formed between surface  303  and surface  304 . 
     Opening  331  is formed on a side of surface  305 . In detail, opening  331  is formed between surface  303  and surface  305 . 
     In  FIGS. 16A and 16B , the YZ plane is the plane parallel to surface  302 , the XY plane is the plane parallel to surface  303 , and the XZ plane is the plane parallel to surfaces  304  and  305 . Thus, the X axis, the Y axis, and the Z axis are orthogonal to each other. Surface  303  corresponds to the bottom surface, e.g. the surface on the side installed on ceiling  80 . In other words, the living space exists on the surface  303  side (i.e. the Z axis negative direction). 
       FIGS. 17A and 17B  are diagrams illustrating radiation patterns respectively in the XZ plane (plane parallel to the openings) and the YZ plane (plane perpendicular to the openings) of metal housing  300  illustrated in  FIG. 16A .  FIGS. 18A and 18B  are diagrams illustrating radiation patterns respectively in the XZ plane and the YZ plane of metal housing  320  illustrated in  FIG. 16B .  FIGS. 17A to 18B  illustrate the results of simulating the radiation pattern of the electric wave using metal housings  300  and  320 . 
     The radiation pattern of metal housing  300  has wide directivity on the bottom side in both of the XZ plane and the YZ plane, as illustrated in  FIGS. 17A and 17B . Metal housing  300  can radiate the electric wave in a wide area on the bottom side, and receive the electric wave from a wide area on the bottom side. 
     On the other hand, the radiation pattern of metal housing  320  has null point on the bottom side in both of the XZ plane and the YZ plane, as illustrated in  FIGS. 18A and 18B . Thus, in the case where metal housing  320  is used as a slot antenna, the transmission or reception of electric wave is interfered in an area on the bottom side. 
     As described above, in this embodiment, the radiation pattern of the electric wave radiated from the metal housing can be changed according to the arrangement of openings in the metal housing (i.e. circuit case  220 ). Here, more excellent radiation pattern characteristics are attained in the case where the opening extends over two or more continuous sides of the polyhedron as in metal housing  300  illustrated in  FIG. 16A , than in the case where the opening is formed only on one side of the polyhedron as in metal housing  320  illustrated in  FIG. 16B . 
     [Opening Width of Openings and Radiation Pattern] 
     The following describes the relationship between the opening width of openings and the radiation pattern, with reference to  FIG. 19 .  FIG. 19  is a diagram illustrating the relationship between the opening width of openings in the metal housing and the radiation pattern according to this embodiment. 
     The opening width of an opening is the width of a slit opening in the direction orthogonal to the slit direction. For example, in the case where the opening is substantially rectangular in shape, the opening width is the width in the transverse direction. 
       FIG. 19  illustrates the radiation pattern with the opening width of 0.5 mm and 1.0 mm in the case where the frequency used is 2.4 GHz. The radiation pattern in each of the XZ plane and the YZ plane is illustrated in  FIG. 19 . 
     For example, in the XZ plane, the main lobe strength is −23.4 dB when the opening width is 0.5 mm, and −11.6 dB when the opening width is 1.0 mm. In the YZ plane, on the other hand, the main lobe strength is −16.7 dB when the opening width is 0.5 mm, and −30.5 dB when the opening width is 1.0 mm. 
     Thus, the radiation pattern differs significantly when the opening width is different. This indicates that constant opening width of openings contributes to more stable wireless communication. For example, the opening width of the openings can be maintained by providing, between the openings, an insulator for maintaining the opening width of the openings. 
     As an example, opening  223  illustrated in  FIG. 14A  is formed by combining upper cover  221  and lower cover  222  illustrated in  FIG. 15 . This raises the possibility that the opening width of opening  223  at the time of or after the combination is not a desired width. 
     Providing an insulator in opening  223 , however, allows the opening width of opening  223  to be maintained. The insulator is made of an insulating resin material as an example. The insulator can be provided in opening  223 , for instance, by applying the insulating resin material and curing it by light irradiation or the like. 
     The insulator provided in opening  223  may be any insulator. For example, an insulating plate member may be pinched in opening  223 . 
     [Distance Between a Plurality of Openings and Radiation Pattern] 
     The following describes the relationship between the distance between a plurality of openings and the radiation pattern, with reference to  FIGS. 20A to 21B . 
       FIGS. 20A and 20B  are each a diagram illustrating the arrangement of a plurality of openings in the metal housing according to this embodiment. In detail,  FIGS. 20A and 20B  respectively illustrate metal housings  400  and  420  that differ only in the positions of openings from metal housing  300  illustrated in  FIG. 16A . 
     Metal housing  400  illustrated in  FIG. 20A  has two openings  410  and  411  parallel to each other. Metal housing  420  illustrated in  FIG. 20B  has two openings  430  and  431  parallel to each other. 
     Openings  410  and  411  and openings  430  and  431  have the same length and width. In detail, openings  410  and  411  and openings  430  and  431  have a length greater than or equal to substantially half the wavelength corresponding to the wireless signal frequency. Openings  410  and  411  and openings  430  and  431  are formed in surface  303  which corresponds to the bottom surface of a corresponding one of metal housings  400  and  420 . 
     Openings  410  and  411  are at distance D 1  from each other, as illustrated in  FIG. 20A . Openings  430  and  431  are at distance D 2  from each other, as illustrated in  FIG. 20B . 
     Distance D 1  is a length greater than or equal to substantially half the wavelength corresponding to the wireless signal frequency. Distance D 2  is a length less than substantially half the wavelength corresponding to the wireless signal frequency. Thus, distance D 1  is longer than distance D 2 . 
       FIG. 21A  is a diagram illustrating the radiation pattern in the YZ plane of metal housing  400  illustrated in  FIG. 20A .  FIG. 21B  is a diagram illustrating the radiation pattern in the YZ plane of metal housing  420  illustrated in  FIG. 20B .  FIGS. 21A and 21B  respectively illustrate the results of simulation using metal housings  400  and  420 . 
     The strength of the electric wave from metal housing  400  is greater than the strength of the electric wave from metal housing  420 , as illustrated in  FIGS. 21A and 21B . Thus, the electric wave can be efficiently radiated in the case where the distance between two openings arranged in parallel is greater than or equal to substantially half the wavelength corresponding to the wireless signal frequency. 
     [Conclusion] 
     As described above, in luminaire  201  according to this embodiment, circuit case  220  (the metal housing) includes two or more metal structures that are combined with each other to create a gap, openings  224  and  225  are each the gap, and a length of openings  224  and  225  is greater than or equal to substantially half a wavelength corresponding to a frequency of the wireless signal. 
     Thus, the opening of the length greater than or equal to substantially half the wavelength corresponding to the wireless signal frequency is provided. This enables efficient radiation of the electric wave, and improves the communication quality of wireless communication. Moreover, since the gap created by combining the two or more metal structures is used as the opening, there is no need to form the opening in the metal structure. A reduction in the number of manufacturing steps contributes to lower cost. 
     For example, circuit case  220  is a polyhedron, and openings  224  and  225  are each positioned on a side of the polyhedron. 
     In this way, the radiation pattern can be changed according to the position of the opening. In other words, the radiation pattern can be changed according to on which side of the polyhedron the opening is provided. Hence, an appropriate radiation pattern can be formed depending on, for example, the installation location of luminaire  201 . 
     For example, opening  225  extends over two or more continuous sides of the polyhedron. 
     It is desirable to reduce the size of circuit case  220  (metal housing) in which power circuit  60  is contained, in order to reduce the size of luminaire  201 . Reducing the size of circuit case  220 , however, makes it difficult to form an opening with a necessary length. 
     In this embodiment, on the other hand, the opening extends over two or more sides of the polyhedron, and so is guaranteed to have a length greater than or equal to substantially half the wavelength corresponding to the wireless signal frequency. This enables efficient radiation of the electric wave, and improves the communication quality of wireless communication. 
     For example, circuit case  220  has a plurality of openings  225  that are parallel to each other. 
     This enables efficient radiation of the electric wave, and improves the communication quality of wireless communication. 
     For example, a distance between the plurality of openings  225  parallel to each other is greater than or equal to substantially half the wavelength corresponding to the frequency of the wireless signal. 
     Setting the distance between the plurality of openings to be greater than or equal to substantially half the wavelength corresponding to the wireless signal frequency enables efficient radiation of the electric wave, and improves the communication quality of wireless communication. 
     For example, the luminaire may further include an insulator provided in opening  225  to maintain an opening width of opening  225 . 
     The opening width of the opening can be maintained in this way, as a result of which stable wireless communication can be performed. 
     Although this embodiment describes the type of downlight in which luminaire body  210  and circuit case  220  are separated physically and connected by cable  261  as illustrated in  FIG. 13 , this is not a limitation. For example, the type of downlight in which luminaire body  210 A and circuit case  220 A are connected directly to each other as in luminaire  201 A illustrated in  FIG. 22  may be used. 
     Circuit case  220 A is a metal housing which is substantially a rectangular parallelepiped, and contains power circuit  60  and wireless module  70  (not illustrated). Circuit case  220 A has openings  221 A and  222 A. The openings are shown by thick lines (thick solid line or thick dotted line) in  FIG. 22 . 
     Openings  221 A and  222 A are provided at an end of substantially rectangular parallelepiped circuit case  220 A in the longitudinal direction. Openings  221 A and  222 A are each a slit opening extending over two sides of circuit case  220 A. Openings  221 A and  222 A are parallel to each other. 
     With this structure, too, circuit case  220 A functions as a slot antenna to enable wireless communication. 
     Variations of Embodiment 3 
     Although Embodiment 3 describes an example where the luminaire is a downlight, the luminaire is not limited to such. For example, the luminaire may be a straight-tube LED lamp as described in Embodiment 1. 
     The following describes variations of Embodiment 3, with reference to  FIGS. 23A to 24B . 
     [Variation 1] 
       FIG. 23A  is an exploded perspective view illustrating luminaire  500  according to Variation 1 of Embodiment 3. 
     Luminaire  500  illustrated in  FIG. 23A  differs from luminaire  1  according to Embodiment 1 in that luminaire body  510  and circuit case  550  are included instead of luminaire body  10  and circuit case  50 . 
     Luminaire body  510  has openings  511  to  514 , instead of opening  11 . In this variation, wireless module  70  is, for example, contained in circuit case  550 . Circuit case  550  accordingly has openings  551  to  554 . The openings are shown by thick lines (thick solid line or thick dotted line) in  FIG. 23A . 
     Openings  511  and  512  are formed at one end of long luminaire body  510  in the longitudinal direction. Openings  511  and  512  are each an L-shaped slit opening extending over two sides of luminaire body  510 . Openings  511  and  512  are parallel to each other. 
     Openings  513  and  514  are formed at the other end of long luminaire body  510  in the longitudinal direction. Openings  513  and  514  are each an L-shaped slit opening extending over two sides of luminaire body  510 . Openings  513  and  514  are parallel to each other. 
     Openings  551  and  552  are formed at one end of substantially rectangular parallelepiped circuit case  550  in the longitudinal direction. Openings  551  and  552  are each a slit opening extending over two sides of circuit case  550 . Openings  551  and  552  are parallel to each other. 
     Openings  553  and  554  are formed at the other end of substantially rectangular parallelepiped circuit case  550  in the longitudinal direction. Openings  553  and  554  are each a slit opening extending over two sides of circuit case  550 . Openings  553  and  554  are parallel to each other. 
     With this structure, too, circuit case  550  and luminaire body  510  each function as a slot antenna to enable wireless communication. 
     [Variation 2] 
       FIG. 23B  is an exploded perspective view illustrating luminaire  500   a  according to Variation 2 of Embodiment 3. 
     Luminaire  500   a  illustrated in  FIG. 23B  differs from luminaire  1  according to Embodiment 1 in that luminaire body  510   a  and circuit case  550  are included instead of luminaire body  10  and circuit case  50 . 
     Luminaire body  510   a  has openings  511   a  to  514   a , instead of opening  11 . In this variation, wireless module  70  is, for example, contained in circuit case  550 . The openings are shown by thick lines (thick solid line or thick dotted line) in  FIG. 23B . 
     Openings  511   a  and  512   a  are formed in side surfaces near one end of long luminaire body  510   a  in the longitudinal direction. Openings  511   a  and  512   a  are each a substantially rectangular slit opening formed in luminaire body  510   a . Openings  511   a  and  512   a  are parallel to each other. 
     Openings  513   a  and  514   a  are formed in side surfaces near the other end of long luminaire body  510   a  in the longitudinal direction. Openings  513   a  and  514   a  are each a substantially rectangular slit opening formed in luminaire body  510   a . Openings  513   a  and  514   a  are parallel to each other. 
     With this structure, too, circuit case  550  and luminaire body  510   a  each function as a slot antenna to enable wireless communication. 
     [Variation 3] 
       FIG. 23C  is an exploded perspective view illustrating luminaire  500   b  according to Variation 3 of Embodiment 3. 
     Luminaire  500   b  illustrated in  FIG. 23C  differs from luminaire  1  according to Embodiment 1 in that luminaire body  510   b  and circuit case  550  are included instead of luminaire body  10  and circuit case  50 . 
     Luminaire body  510   b  has openings  511   b  and  512   b , instead of opening  11 . In this variation, wireless module  70  is, for example, contained in circuit case  550 . The openings are shown by thick lines (thick solid line or thick dotted line) in  FIG. 23C . 
     Openings  511   b  and  512   b  are formed in side surfaces at the center of long luminaire body  510   b  in the longitudinal direction. Openings  511   b  and  512   b  are each a substantially rectangular slit opening formed on a side of the corresponding side surface of luminaire body  510   b . Openings  511   b  and  512   b  are parallel to each other. 
     With this structure, too, circuit case  550  and luminaire body  510   b  each function as a slot antenna to enable wireless communication. 
     [Variation 4] 
     Although foregoing Variations 1 to 3 describe an example where the luminaire includes a cylindrical straight-tube LED lamp having a feeding base and a non-feeding base as light source  2 , this is not a limitation. This variation describes a luminaire that includes a line light source having a half-cylindrical cover. 
       FIG. 24A  is an exploded perspective view illustrating luminaire  600  according to Variation 4 of Embodiment 3. 
     Luminaire  600  includes light source  602 , luminaire body  610 , connectors  620  and  630 , power circuit  60 , and wireless module  70 . 
     Light source  602  includes a long LED module and a long translucent cover for covering the LED module. The LED module is, for example, a COB light emitting module in which an LED chip is directly mounted on a board. 
     Luminaire body  610  is an example of a metal structure. Luminaire body  610  is formed, for example, by bending a plate member made of metal such as aluminum to have opening  611  on the lower side. While luminaire body  510  in Variation 1 and the like is substantially a rectangular parallelepiped, luminaire body  610  in this variation is a flat and long polyhedron. In detail, luminaire body  610  is convex, and has opening  611  in the convex portion. 
     Luminaire body  610  is connected to connector  620 . Connector  620  is a connector for feeding power to light source  602 , and is connected to connector  630  to feed power from an external power source (e.g. utility power) to light source  602 . In detail, connector  630  is connected to power circuit  60 . Power circuit  60  converts power supplied via connectors  620  and  630  and supplies the converted power to light source  602 . 
     Power circuit  60  and wireless module  70  are positioned on the opposite side to the cover of the LED module. Thus, power circuit  60  and wireless module  70  are contained in luminaire body  610  through opening  611  of luminaire body  610  when luminaire body  610  and light source  602  are combined. 
     Luminaire body  610  has openings  612  and  613 . 
     Openings  612  and  613  are arranged along opening  611  of long luminaire body  610 . Openings  612  and  613  are each a substantially rectangular slit opening formed on a side of a side surface of luminaire body  610 . Openings  612  and  613  are parallel to each other. Openings  612  and  613  remain open even when luminaire body  610  and light source  602  are combined, and so can radiate the electric wave from wireless module  70 . 
     With this structure, too, luminaire body  610  functions as a slot antenna to enable wireless communication. 
     [Variation 5] 
     Although Variation 4 describes an example where luminaire body  610  is convex and light source  602  is outside luminaire body  610 , this is not a limitation. This variation describes a luminaire in which light source  602  is contained in luminaire body  610 . 
       FIG. 24B  is an exploded perspective view illustrating luminaire  700  according to Variation 5 of Embodiment 3. 
     Luminaire  700  includes light source  602 , luminaire body  710 , connectors  620  and  630 , power circuit  60 , and wireless module  70 . 
     Luminaire body  710  is an example of a metal structure. Luminaire body  710  is formed, for example, by bending a plate member made of metal such as aluminum to have opening  711  on the lower side. Luminaire body  710  is a flat and long polyhedron. In detail, luminaire body  710  is concave, and has opening  711  in the concave portion. Surfaces  712  and  713  forming the concave portion are, for example, reflection surfaces. 
     Power circuit  60  and wireless module  70  are contained in luminaire body  710  through opening  711  of luminaire body  710  when luminaire body  710  and light source  602  are combined, as in Variation 4. 
     Luminaire body  710  has openings  714  and  715 . 
     Openings  714  and  715  are arranged along opening  711  of long luminaire body  710 . Openings  714  and  715  are each a substantially rectangular slit opening formed on a side of a corresponding one of surfaces  712  and  713  of luminaire body  710 . Openings  714  and  715  are parallel to each other. Openings  714  and  715  remain open even when luminaire body  710  and light source  602  are combined, and so can radiate the electric wave from wireless module  70 . 
     With this structure, too, luminaire body  710  functions as a slot antenna to enable wireless communication. 
     (Other Variations) 
     Although the luminaire according to the present disclosure has been described by way of the foregoing embodiments and their variations, the present disclosure is not limited to the foregoing embodiments. 
     Although the foregoing embodiments describe an example where the antenna is mounted on the printed wiring board, this is not a limitation. The antenna may be a single antenna or the like. 
     Although the foregoing embodiments describe an example where the metal housing having one or more openings is a polyhedron such as substantially a rectangular parallelepiped, this is not a limitation. The metal housing may have a curved surface. For example, the metal housing may be substantially a cylinder or substantially a truncated cone. 
     Embodiments 2 and 3 and their variations may be realized independently of Embodiment 1. In detail, Embodiments 2 and 3 and their variations may be realized each as a structure that does not include the matter described in the independent claim representing a superordinate concept. For example, in Embodiments 2 and 3, etc., the antenna and the opening may be arranged without substantial coincidence between the polarization plane of the electric wave most strongly radiated from the antenna and the polarization plane of the electric wave most strongly radiated from the opening. 
     Other variations obtained by applying various changes conceivable by a person skilled in the art to the embodiments and any combinations of the structural elements and functions in the embodiments without departing from the scope of the present disclosure are also included in the present disclosure. 
     While the foregoing has described what are considered to be the best mode and/or other examples, it is understood that various modifications may be made therein and that the subject matter disclosed herein may be implemented in various forms and examples, and that they may be applied in numerous applications, only some of which have been described herein. It is intended by the following claims to claim any and all modifications and variations that fall within the true scope of the present teachings.