Linear light source

The present invention relates to a linear light source, and more particularly to a highly water-resistant, environmentally friendly, low power-consumption linear light source adapted to be driven with small power.

Conventionally, a neon tube, fluorescent tube, etc. are used as a light emitter which can provide a linear light source extending over a distance of several meters.

However, a neon tube or fluorescent tube needs a high voltage, and it may possibly cause an electric shock or current to leak from it. Therefore, it cannot be used in water, rain or snow. Further, since it is formed from a glass tube, it may not be used in a place where it will possibly be broken by collision with a person, car or the like. The glass tube is very fragile.

For a circular shape, the glass tube has to be elaborately bent or curved in conformity to the curvature of the circle. The glass tube bending or curving cannot be easily done but needs high skill and much experience. This will increase the manufacturing costs. Furthermore, the neon tube or fluorescent tube consumes a power as large as several tens W per unit length of 1m. For a long use, a commercial power supply has to be available in a place not far from the neon tube or fluorescent tube.

To solve all the above problems, there have been proposed a light transmission tube comprising a flexible tube and a transparent core solution or soft transparent polymer filled in the tube, and a braided plastic optical fiber product.

Briefly speaking, a light generated from a source is admitted into a light transmission tube at one end thereof and emitted from the lateral side of the tube along a length of several tens of meters. Since the light source and glowing portion can thus be separated from each other, the light transmission tube can be used in water, outdoors or in a place where an explosion may possibly take place. Further, the tube will not possibly be easily broken. Different from the glass tubes such as neon tube, fluorescent tube, etc., it needs no troublesome, elaborate working but can be easily worked.

However, when such a conventional light transmission tube is used to emit light from the lateral side thereof along a length of several tens of meters, its luminous efficiency is low. For an increased intensity of light, a light source capable of providing an output ranging from 50 to 250 W is required for use with the light transmission tube. Also, when the light source and light transmission tube are used in water, outdoors or in a place where an explosion may possibly take place, the light source should be protected, which will cause the light source itself to have such a large volume as to require a wide space for storage.

Attention is also drawn to US-A-4 733 332 which discloses a linear light source (illuminating device) (see Figs. 1-3) comprising a light transmission tube including a transparent core in the form of a light-transmitting rod 1 and a clad (transparent protective tube 4 together with air filled clearance 5 between core and tube), and a light source a,b disposed at an axial end of the light transmission tube, wherein light emitted from the light source a,b and incident upon an end of the light transmission tube is emitted from a lateral side of the tube.

Accordingly, the present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing a linear light source usable without any problem in water, rain, snow or in a place where an explosion may possibly take place, adapted to provide sufficient illumination from the lateral side thereof even with small power, and designed to be very compact for storage in a limited space.

The present invention provides a linear light source comprising: <ul id="ul0001" list-style="none" compact="compact"><li>a light transmission tube including a transparent core and a clad having a smaller refractive index than the core, wherein the light transmission tube further includes a band-shaped light reflective layer disposed between the clad and core along the length of the clad;</li><li>a light-reflecting protective layer provided on the outer surface of the clad positioned to cover the light reflective layer;</li><li>a water-proof light source disposed at at least one axial end of the light transmission tube; and</li><li>a drive unit to turn on and off the light source, the light emitted from the light source and incident upon an end of the light transmission tube being emitted from a lateral side of the light transmission tube.</li></ul>

The invention will be further described, by way of example only, with reference to the accompanying drawings, wherein: <ul id="ul0002" list-style="none" compact="compact"><li>FIG. 1 schematically illustrates the construction of a linear light source according to the present invention;</li><li>FIG. 2 is an axial-sectional view of a light transmission tube;</li><li>FIG. 3 is a cross-sectional view of a light transmission tube;</li><li>FIG. 4 is a cross-sectional view of a variant of a light transmission tube;</li><li>FIG. 5 is an explanatory drawing of another variant of a light transmission tube;</li><li>FIG. 6 is an explanatory drawing of still another variant of a light transmission tube;</li><li>FIG. 7 is a cross-sectional view of a light transmission tube to which a light-reflecting protective layer is attached;</li><li>FIG. 8 is a cross-sectional view of a light transmission tube secured to a channel;</li><li>FIG. 9 is a cross-sectional view of a light transmission tube secured to another channel;</li><li>FIG. 10 is an explanatory drawing of a light transmission tube used on a traffic control sign;</li><li>FIG. 11 is an explanatory drawing of a light transmission tube used in a guide way;</li><li>FIG. 12 is an explanatory drawing of a light transmission tube used on a staircase;</li><li>FIG. 13 is an explanatory drawing of a light transmission tube used on a signboard;</li><li>FIG. 14 is an explanatory drawing of an indicator panel in which a light transmission tube is used in a segmented form;</li><li>FIG. 15 is an explanatory drawing of a light transmission tube used as an indirect illumination inside a car;</li><li>FIG. 16 is an explanatory drawing of a light transmission tube used as a foot light on the lower portion of a car door;</li><li>FIG. 17 is an explanatory drawing of a light transmission tube used as a side marker lamp on the lateral side of a car; and</li><li>FIG. 18 is an explanatory drawing of a light transmission tube used as a stop sign on the rear side of a car trunk lid.</li></ul>

As schematically shown in FIG. 1, a linear light source according to the present invention comprises a light transmission tube 1 having a light reflective layer 13 disposed therein, light source 2 and drive unit 3 as main components.

The light transmission tube 1 includes a transparent tubular clad 11, a transparent core 12 having a larger refractive index than the clad 11, and a light reflective layer 13 disposed between the core 12 and the inner wall of the clad 11 (this side of the clad will be referred to as "rear side" hereafter) opposite to a side from which the light is to be emitted (this side of the clad will be referred to as "front side" hereafter). The light transmission tube 1 further comprises a light-reflecting protective layer 14 formed on the rear side of the clad 11 to cover the light reflective layer 13 and complement the light reflective layer 13.

The diameter and length of the core 12 in the light transmission tube 1 are not limited to any special values, but when one LED is used as the light source 2, for example, the core diameter should be about 2 to 30 mm, preferably be 4 to 15 mm,and the core length should be about 0.1 to 5 m, preferably 0.2 to 2 m.

The light reflective layer 13 may be formed like a band along the length of the clad in a position somewhat inwardly off the core surface.

The tubular clad 11 should preferably made of a flexible material moldable into a tube form and having a small refractive index such as plastic, elastomer or the like. More particularly, it should be made of a material selected from polyethylene, polypropylene, polyamide, polystyrene, ABS resin, polymethyl methacrylate, polycarbonate, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, polyethylene-vinyl acetate copolymer, polyvinyl alcohol, polyethylene-polyvinyl alcohol copolymer, fluorocarbon resin, silicone resin, natural rubber, polyisoprene rubber, polybutadiene rubber, styrene-butadiene copolymer, butyl rubber, butyl rubber halide, chloroprene rubber, acryl rubber, EPDM, acrylonitrile-butadiene copolymer, fluorocarbon rubber, silicone rubber, etc.

Among these materials, a silicone polymer or fluorocarbon polymer having a smaller refractive index should preferably be selected for making the clad 11. More particularly, the more preferable materials for the clad 11 include silicone polymers such as polydimethyl siloxane polymer, polymethyl phenyl siloxane polymer, fluorosilicone polymer and the like, polytetrafluoroethylene (PTFE), fluorinated ethylene propylene copolymer (FEP), perfluoroalkoxyethylene copolymer (PFE), polychlorotrifluoroethylene (PCTFE), ethylene tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride, polyvinyl fluoride, vinylidene fluoride-ethylene chloride trifluoride copolymer, vinylidene fluoride-propylene hexafluoride copolymer, vinylidene fluoride-propylene hexafluoride-ethylene tetrafluoride copolymer, ethylene tetrafluoride propylene rubber, fluorinated thermoplastic elastomer, etc. Among others, a fluorinated polymer should preferably be used to make the clad 11. One of these materials may be used or two or more may also be used as blended.

On the other hand, the transparent core 12 should be made of a solid material such as methacrylated polymer, polycarbonate, polystyrene, silicone rubber, ethylidene norbornene polymer, SBS, SIS, SEBS (styrene-ethylene-butadiene-styrene block polymer) or the like. Among others, the methacyrylated polymer should preferably be used to make the core 12.

The methacryl polymers for use to make the core 12 includes homopolymers derived from polymerization of one monomer selected from acryl, methacryl and esters of them with monatomic alcohols, or copolymers derived from copolymerization of two or more monomers. The monatomic alcohols include those with 1 to 22 carbons. Among others, a monomer selected from among acryl, methacryl and esters of them with lower alcohols (with 1 to 5 carbons, preferably 1 to 3 carbons, most preferably 1 carbon) or a copolymer of them with a monomer represented by the following usual formula(1) should preferably be used since they have an excellent flexibility and light permeability. <chemistry id="chem0001" num="0001"><img file="EP0899503B1_D0001.tif" /></chemistry> where R¹ is a hydrogen atom or methyl group, R² is an alkyl group with a number of carbons from 8 to 20, preferably 10 to 16, more particularly 12 to 14. The higher alkyl groups may be either homoalkyl group or mixed alkyl group, and should more preferably be an alkyl group with 12 carbons and a mixed alkyl group with 13 carbons. The ratio between the alkyl group with 12 carbons and that with 13 carbons should usually be 20 : 80 to 80 : 20, more particularly 40 : 60 to 60 : 40. The ratio of copolymerization between a monomer selected from the acryl, methacryl and lower alcohol esters of them and the monomer represented by the above formula (1) should appropriately be selected to be 5 : 95 to 79 : 21, more preferably 30 : 70 to 65 : 35.

The diameter of the core 12 is not limited to any special value, but it should be 2 to 30 mm, preferably 5 to 15 mm.

The light reflective layer 13 should preferably be formed from light-scattering particles. The light-scattering particles for use to form the light reflective layer 13 include organic polymer particles such as silicone resin particles, polystyrene resin particles, etc., metal oxide particles such as Al₂O₃, TiO₂, SiO₂, etc., sulfate particles such as BaSO₄, etc., carbonate particles such as CaCO₃, etc. One type of the above-mentioned particles may be used, or two or more of them may be used in conjunction, to form the light reflective layer 13.

The mean particle size should be 0.1 to 30 m, preferably 1 to 15 m. If the mean particle size exceeds 30 m, the particles will readily precipitate in a core solution being injected into the clad tube 11 in the process of forming the light transmission tube 1 as will be described later. The ready precipitation of the particles will be inconvenient for the formation of the light transmission tube 1.

The thickness of the light reflective layer 13 is not limited to any special value, but should be 10 to 200 m, preferably 50 to 100 m. If the layer thickness is too small, the light will be less reflected and thus the intensity of light will be lower. On the contrary, if the light reflective layer 13 is too thick, the light will be much reflected and the intensity of light be higher, which however is true for a portion of the layer 13 near to the light source while the light intensity will be lower at a portion of the layer 13 farther from the light source.

The light-reflecting protective layer 14 may be a layer which will prevent a light, leaked from the light reflective layer 13, from emitting through the layer. The layer should preferably be one which will not absorb the leakage light but reflect it. More particularly, the layer 14 may be a foil of a metal such as silver, aluminium or the like, a metal sheet, reflective sheet, metal-evaporated sheet or the like, or a sheet on which light-scattering particles like the above-mentioned ones are applied.

The light-reflecting protective layer 14 may be formed on the outer surface of the clad 11 to cover just the light reflective layer 13 as shown in FIG. 3. Alternatively, it may be formed on the outer surface of the clad 11 and extended sufficiently beyond the light reflective layer 13 toward the front side of the clad 11 as to have a light emission window 15 as shown in FIG. 4. The light emission window 15 is a portion of the clad 11 which remains not covered with the light-reflecting protective layer 14.

In forming the light transmission tube 1, the above-mentioned light-scattering particles are dispersed in the core solution containing the above-described monomer, the solution is put into a clad tube which becomes the clad 11, the clad tube is closed at opposite ends thereof and placed horizontally for about 30 to 48 minutes to allow the dispersed light-scattering particles to settle onto the bottom of the clad tube. To settle the light-scattering particles, centrifugation may be applied as the case may be. Thereafter, the monomer contained the core solution is polymerized and hardened. Thus, the light reflective layer 13 formed from the light-scattering particles is formed between the clad 11 and core 12, or in a position somewhat inwardly of the outer surface of the core 12.

The monomer polymerization is not limited to any special method. The monomer may be polymerized by a conventional method in which an organic peroxide such as t-butyl hydroperoxide, di-t-butyl peroxide, rauloyl peroxide, benzoyl peroxide, dimilistyle peroxydicarbonate, t-butyl peroxyacetate, t-butyl peroxy(2-ethylhexanoate) or the like, and a polymerization initiator such as an azo-compound such as cumilperoxioctoate, azobisisobutyronitrile, azobiscyclohexanenitrile or the like, is incorporated into the monomer and polymerized at 50 to 120 °C for 1 to 20 hours. For this polymerization, it is recommendable to polymerize the monomer while pressurizing the core solution from one end or both ends of the clad tube since the pressurization will not prevent air bubbles from taking place in the core.

The light source 2 is disposed at at least one axial end of the light transmission tube 1 (at the left end in this embodiment). It uses an LED (light emitting diode). There are available LEDs which provide red, blue, green, yellow, orange, or white rays of light, one of which may be selected appropriately for an intended purpose of the linear light source. One LED may be used as the light source 2 or a plurality of LEDs may be used together for an increased quantity of light. The light from the light source 2 may be admitted into the light transmission tube 1 at one end or opposite ends thereof. By admission of light at opposite ends of the light transmission tube 1, the illumination from the light transmission tube 1 can be more uniform and of a higher intensity. As the light source 2, a single colour LED or two or more LEDs of different colours may be used. In case the linear light source is used as a go-stop control sign along a stop-line at a railway crossing, for example, two LEDs one of which emits yellow light while the other emits red light may be used. Normally or while no train is approaching the railway crossing, the yellow LED is used to emit yellow light from the line glower, thereby notifying walkers and car drivers that the site is a railway crossing and that they can cross the railway. Just before a train passes, or while the train is passing, by the crossing, the LED of yellow light is turned off and the LED of red colour is turned on to emit red light from the linear light source to warn the walkers and car drivers waiting until the train goes away. The drive unit 3 may be adapted to illuminate the LEDs continuously or intermittently.

The light transmission tube 1 is securely fixed at one end thereof with a joint member 20 by bonding or caulking. Also the LED light source 2 is integrally fixed to the light transmission tube 1 by means of the joint member 20.

The light source 2 and drive unit 3 are connected to each other with a rubber-, vinyl- or polyethylene-sheathed interconnecting cord 30. The joint member 20 is filled with a potting material 21 such as epoxy resin, silicone rubber or the like in order to insulate the connection between the interconnecting cord 30 and light source 2 and also to prevent water, steam, combustible gas or liquid from penetrating into the joint member 20. The interconnecting cord 30 may be protected or water-proofed by passing it through a flexible pipe made from a metal or resin, or a rubber or plastic pipe.

Furthermore, the light transmission tube 1 may be inserted into a transparent pipe 10A or the like as shown in FIG. 5 to protect the tube 1, or into a transparent heat-contractible tube 10B as shown in FIG. 6 to protect the tube 1 and seal the whole linear light source.

Moreover, the light transmission tube 1 may be provided on a part of the outer surface thereof, as shown in FIG. 7, with a further light reflective layer 16 formed from a reflective tape on which a metal such as stainless steel, gold or silver is evaporated, sputtered or plated, a coating of a reflective paint, a metal foil or a vinyl tape on which reflective particles such as titanium oxide are applied or which contains a pigment. Also, the light transmission tube 1 may be provided with a fixing channel 17 or 18 which reflects light (namely, serving also as a light-reflecting protective layer) as shown in FIG. 8 or 9. The channel may be formed from a metal material such as aluminium, stainless steel or the like, or from a plastic or elastomer in which highly light-reflective fine particles (powder) are incorporated and kneaded.

The drive unit 3 is provided to supply the light source 2 with power. It has an electric circuit (comprising a resistor, transistor, constant-current diode, etc.) fed from a power source such as a battery, solar battery, DC/AC source or the like to energize the LED or LEDs. A solar battery or a secondary source (storage battery) may be incorporated in the drive unit 3 itself or provided separately from the drive unit 3. The drive unit 3 has an interconnecting cord 30 of which the leading portion is appropriately sealed for a sufficient water-proofing.

Next, some embodiments (experiments 1 and 2) of the linear light source of the present invention will be described in comparison with a comparative example.

Embodiment 1:

A monomer solution (core solution of 0.92 in specific gravity) was prepared from MMA (methyl methacrylate) in an amount of 60 parts by weight, LMA (lauryl methacrylate) in an amount of 40 parts by weight, BPO (benzoyl peroxide) in an amount of 0.05 parts by weight. Silicon resin particles (by Toshiba Silicone) having mean particle size of 12 m and specific gravity of 1·32, as light-scattering particles, were dispersed into the monomer solution at a ratio of 0.15 parts by weight to 100 parts by weight. An FEP tube of 6 mm in outside diameter, 5 mm in inside diameter and 1.5 m in length was prepared and charged with the monomer solution, and sealed at opposite ends thereof. The FEP tube was left horizontally for 2 hours to allow the silicone resin particles to settle to the bottom of the FEP tube. After a lapse of two hours, the FEP tube was put into a hot water bath at a temperature of 65°C with care not to disturb the settled silicone resin particles. While the FEP tube was being applied at the opposite ends thereof with a pressure of 3.5 kg/cm², the monomer solution was polymerized and hardened for 3 hours.

The light transmission tube thus produced had a light reflective layer of the above-mentioned silicone resin particles formed on the core surface in a band-like shape along the length of the tube.

A piece of 30 cm in length was cut from the light transmission tube thus formed. A mirror-finished stainless steel sheet of 1 mm in thickness and 6 mm in outside diameter was bonded as a reflective plate to one end face of the light transmission tube piece with a transparent epoxy adhesive. An aluminium joint was used to connect a green LED (NSPG50 by Nichia Chemicals) to the other end face of the tube piece. Lead wires were soldered to the terminals of the LED. The exposed portions of the lead wires were sealed each with a silicone adhesive. Thus a linear light source was completed.

The intensity of light from the front side of the tube piece when a current of 20 mA was supplied to the LED is shown in Table 1. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="4" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col4" align="center">Light intensity at tube front side (in cd/m²)</entry></row><row><entry namest="col1" nameend="col1" rowsep="0" /><entry namest="col2" nameend="col4" align="left">Measuring points (distance from light-incident end of tube)</entry></row><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="left">50 mm</entry><entry namest="col3" nameend="col3" align="left">150 mm</entry><entry namest="col4" nameend="col4" align="left">250 mm</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Embodiment 1</entry><entry namest="col2" nameend="col2" align="left">120</entry><entry namest="col3" nameend="col3" align="left">100</entry><entry namest="col4" nameend="col4" align="left">100</entry></row><row><entry namest="col1" nameend="col1" align="left">Embodiment 2</entry><entry namest="col2" nameend="col2" align="left">200</entry><entry namest="col3" nameend="col3" align="left">180</entry><entry namest="col4" nameend="col4" align="left">175</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Comparative example</entry><entry namest="col2" nameend="col2" align="left">20</entry><entry namest="col3" nameend="col3" align="left">10</entry><entry namest="col4" nameend="col4" align="left">10</entry></row></tbody></tgroup></table></tables>

As seen from Table 1, the intensity of light obtained with the embodiment 1 is higher than that with the comparative example. The power consumption of the LED was 0.06 W.

The above test piece was left immersed in water for 6 months and then tested. The test results showed no problems such as current leakage and showed the same intensity of light from the test piece as in the above.

Embodiment 2:

In this embodiment, a highly reflective resin (VANLIGHT LD-1000R by Teijin) was used to form a channel 17 (or 18) as in FIG. 8 (or 9). The light transmission tube test piece as in the embodiment 1 was fitted in the channel 17 (or 18). Thus the channel itself has a light reflectivity.

The comparative example used for comparison with the embodiment 2 was formed in the same manner as the one used for comparison with the embodiment 1 except that no light-scattering particles are incorporated therein.

The embodiment 2 and comparative example were measured concerning intensity of light from the front side of the light transmission tube at points predetermined distances away from the light-incident end of the light transmission tube. The results are shown in Table 1.

As seen from Table 1, the intensity of light at the front side of the embodiment 2 fitted in the channel which is light-reflective itself is the highest, that of the embodiment 1 having the light reflective layer 13 is the second highest, and that of the comparative example having neither the light reflective layer 13 nor light-reflecting protective layer 14 is the lowest.

Next, applications of the linear light source of the present invention will be described herebelow:

A traffic control sign 4 can be edged with the light transmission tube 1 of the linear light source for contribution to traffic control at night, for example, as shown in FIG. 10.

As shown in FIG. 11, the light transmission tube 1 can be installed on a side wall of a tunnel (in addition, underpass, passageway in an office or hospital building, evacuation guideway in a picture theater, public hall, etc.) to provide guidance for direction, etc. In this case, the linear light source may be supplied with power in two ways, namely from a commercial power source for driving the light source, and from a battery such as a standby power source upon a power failure. Generally, a battery can energize an incandescent lamp or fluorescent lamp for about several tens of minutes. According to the present invention, however, an LED or LEDs are used in place of such lamps. The battery can energize it for a longer time. The linear illumination provided by the linear light source will permit a ready visual acknowledgment of such a guideway, and thus the linear light source can provide smooth guidance to a safe place.

The linear light source according to the present invention can be used on a staircase. The light transmission tube 1 is installed on the top of each riser 6 of the staircase as shown in FIG. 12. The illumination of the light transmission tube 1 will permit to effectively prevent a person from missing his or her foot on the stairs. The linear light source installed on an emergency staircase will allow a person to know the stairs upon an emergency at night, thus providing a safe and positive guidance for evacuation.

FIG. 13 shows the light transmission tube 1 used along the profile of a signboard or trade sign, and FIG. 14 shows segments of the light transmission tube 1 used as indication segments of an indicator panel 4' provided along a highway, showing an allowed maximum speed. Seven segments of the light transmission tube 1 are used for each digit to provide a numerical indication, and they are adapted to turn on and off to form a number as the limited speed is changed. With an increased number of such segments, it is possible to provide a variable character indication.

FIG. 15 shows the light transmission tube 1 installed on the inner lateral side of a car 8A to provide an indirect illumination. FIG. 16 shows the light transmission tube 1 provided as a foot light on the lower portion of a car door 8B. Also FIG. 17 shows the light transmission tubes 1 used as side marker lamps on the rear ends 8C of a car. The light transmission tube 1 is used along the profile of a triangular stop sign 9 on the rear side of a trunk lid of a car stopping on the road to signal to another approaching car.

In addition to the above applications, the linear light source of the present invention can be used as follows: <ul id="ul0003" list-style="none" compact="compact"><li>(1) Illuminated nameplate</li><li>(2) Illuminated area marker or area bar for use during nighttime work</li><li>(3) Illuminated walking stick</li><li>(4) Illuminated sword (for sports and toy)</li><li>(5) Illumination of tent guy rope (to prevent stumbling, falling down)</li><li>(6) Illumination of water bath or the like</li><li>(7) Lane markers or decoration of swimming pool</li><li>(8) Illumination of buoy, pier, levee and marine hose for improved visibility</li><li>(9) Railroad crossing barrier</li><li>(10) Safety indications such as tunnel height marker, entrance height marker, etc.</li></ul>

As having been described in the foregoing, the linear light source according to the present invention comprises a light transmission tube including a transparent core and a clad having a smaller refractive index than the core, a water-proof light source disposed at at least one axial end of the light transmission tube, and a drive unit to turn on and off the light source, the light emitted from the light source and incident upon an end of the light transmission tube being emitted from a lateral side of the tube. The light source is water-proofed and connected integrally with the light transmission tube so that the light source may be disposed outdoors or in water separately from the drive unit. Therefore, the main body of the linear light source can be used without any problem in water, rain, snow or in a place where an explosion may possibly take place. The linear light source according to the present invention can provide ample illumination from the lateral side thereof even with small power, and its compact design permits it to be placed in a limited space.