Abstract:
A vehicle headlamp in which an elongated tubular container having small openings in front and rear ends thereof and filled therein with a drying agent extends through the lamp housing. The size of the small opening at the end of the container which is interior of the lamp housing is smaller than the size of the small opening in the end of the container which is exterior of the lamp housing.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a vehicle headlamp having a vent hole. 
     (2) Description of the Prior Art 
     A prior art vehicle headlamp having a vent hole is basically constructed such as shown in FIG. 6. A drying agent 22 such as silica gel is filled in a container 21 having small holes 21a and 21b in opposite ends thereof, the container 21 extending through the housing (reflector) 23. Reference numeral 24 designates a lens of the prior art headlamp. 
     In the vehicle headlamp provided with the vent hole constructed as described above, a pressure rise in the headlamp is suppressed by the vent hole. Further, air coming from the outside is dehumidified by the drying agent 22. 
     The drying agent 22, which is in the state of moisture absorption by the dehumidification of air coming from the outside, is dried by heat generated when the lamp is lit, to thereby release water content thereof. That is, a recycle effect is provided. 
     To minimize the contact with open air, a porous resin film 25 having a permeability is disposed on the end of the housing 23 which is external of the container 21 as shown in FIG. 7, or means such as a mechanical valve 26 as shown in FIG. 8 is provided on the external end of container 21. 
     However, where the recycle effect is provided for the drying agent 22 as described above, it can be expected that the water content released from the drying agent 22 due to the temperature rise when the lamp is lit is discharged to the outside of the lamp due to the internal pressure of the headlamp. But, since a flow of air flowing out of the headlamp stops after a lapse of a fixed time, if the moisture absorption of the drying agent 22 is large, the release of water content continuously takes place thereafter, and the water content also flows into the headlamp through the small hole 21a in the end of the container 21 which is inside the housing 23. 
     At this time, the interior of the container 21 is in the state of high temperature and high humidity, and the air of the high temperature and high humidity flows into the housing 23 without the air of the high temperature and high humidity being humidity-absorbed into the drying agent 22, by the air flowing therein as the lamp is deenergized. 
     On the other hand, where the porous resin film 25 (FIG. 7) or the valve 26 (FIG. 8) is provided in order to cut off the open air, the discharge of the releasing water content to the outside of the lamp during the above-mentioned venting course is not only impaired by the film 25 or the valve 26 but also conversely the flow of water content into the inside of the lamp is accelerated. Moreover, the mechanical valve construction (of FIG. 8) must function to control the flow of air passing through the vent hole, as a consequence of which the construction becomes complicated and increased in size, leading to higher cost. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a vehicle headlamp with a vent hole which is lower in cost and which can provide a sufficient dehumidification effect. 
     According to the present invention, there is provided a vehicle headlamp in which an elongated tubular container having small holes in front and rear ends thereof and filled therein with a drying agent is disposed to be extended through a reflector, characterized in that the size of the small hole in the inner end of the container is made smaller than that of the outer end thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view showing one embodiment of a headlamp for a vehicle according to the present invention; 
     FIG. 2 is a characteristic curve showing the relation of the lighting and unlighting cycle and the humidity within the headlamp; 
     FIG. 3 is a characteristic curve showing the variation in time of the total outflow of the vent hole to the outflow per minute; 
     FIGS. 4 and 5, respectively, are sectional views showing another embodiment of the present invention; and 
     FIGS. 6 to 8, respectively, are sectional views showing a conventional device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in detail on the basis of the embodiments shown in the drawings. 
     Referring now to FIG. 1 showing one embodiment of the present invention, reference numeral 1 designates a reflector and a housing, 2a lens, 3 a bulb (electric bulb), and 4a container having a small hole 4a in one end thereof, the container extending through the housing 1 so that the small hole 4a is positioned internally of the housing 1. An L-shaped tube 5 is provided on the other open end of the container 4. This L-shaped tube 5 has a small hole 5a in the end joined with the container 4. On one end of the container 4 on the small hole 4a side is disposed a porous resin film 6 having a permeability, and on the other end is disposed a filter 7. The interior of the container 4 is filled with a drying agent 8 such as silica gel. 
     Preferably, the container 4 is mounted at a place where a change in temperature is rapidly produced when the lamp is lighted (turned on) and unlighted (turned off). This provision is made so as to respond to the moisture absorption of the drying agent, high humidity dependability of release of water content, change in outflow and inflow to the change in temperature quickly. 
     Where the headlamp provided with the vent hole constructed as described above and the headlamp provided with the vent hole according to the conventional construction are repeatedly lighted and unlighted, the change of humidity within the lamps is given in FIG. 2. In FIG. 2, curves l1 and l2 show the characteristic of the present embodiment whereas l21 and l22 show the characteristics of the prior art. The characteristic curves l2 and l22 are of the case in which the drying agent is in the state of moisture absorption. 
     In the case of the prior art, if the drying agent is well dried, the air of high temperature and high humidity flowing from outside is moisture-absorbed to some extent, whereas when the drying agent is in the state of moisture absorption, the drying agent acts to release water content without dehumidifying, and the headlamp becomes dim. 
     On the other hand, in the case of the present invention, sufficient dehumidifying effect is obtained not only when the drying agent 8 is dried, but also when it is absorbing moisture content. Moreover, the recycle effect clearly appears in the form of a reduction in humidity inside the lamp when the drying agent 8 is in the state of moisture absorption. 
     FIG. 3 shows the variation in time of the outflow of air flowing outside through the vent hole when the lamp is lit. In FIG. 3, curve l23 and curve l3 show the characteristic of the prior art and the present embodiment, respectively. 
     In the prior art, the outflow immediately after lighting is the maximum, and thereafter it rapidly reduces. The water content released from the drying agent which continues to rise in temperature no longer flows outside, but stays within the vent hole or flows into the housing via the small hole 21a.  The variation in time of the inflow of air flowing into the interior via the vent hole when the lamp is unlighted (turned off) is exactly the same as the variation in time when the lamp is lit. Accordingly, a large quantity of air flows in within a short period of time. At this time, the drying agent still is in the state of high temperature at which the dehumidifying effect cannot be expected, and therefore the air flows into the housing without being dehumidified. 
     On the other hand, in the case of the present embodiment of the present invention, the variation in time of the outflow and inflow takes the form of a peak (curve l3 in FIG. 3) after a fixed time and thereafter gradually reduces. This results from the effect of the permeable porous resin film 6 positioned between the inner small hole 4a and the drying agent 8. 
     As a result, when the lamp is lit, the release of water content from the drying agent 8 starts and then the outflow of gas starts; as the release of water content increases, the outflow also increases; even after the outflow reaches a peak, the outflow is maintained over a long period of time; and the discharge of the released water to the outside of the lamp is maintained. On the other hand, when the lamp is unlighted, the inflow of the open air starts after the drying agent 8 starts to cool; the inflow is average; and the inflow time is long. Thereafore, a further effect is attained in removal of water content which stays within the vent hole immediately after unlighting and lowering of the water content in air flowing in from the outside. 
     In the above-described embodiment, the container 4 is separately formed, and this container is made to extend through the housing (reflector) and is retained in place. It is to be noted that a container portion 4&#39; can be formed integral with the housing 1 as shown in FIG. 4. In addition, while the filter 7 disposed on the end of the container 4 is provided to prevent outflow of the drying agent 8 and inflow of dust or the like while maintaining a permeability, it is to be noted that the filter 7 can be made of a material similar to that of the permeable porous resin film 6 on the inner small hole 4a side, provided that it has a higher permeability than that of the resin film 6 on the inner side. 
     FIG. 5 shows another embodiment of the present invention, in which a small hole in the inner end of the container 4 is in the form of a threaded hole 4&#39;a, and a screw 9 is screwed into the threaded hole 4&#39;a in place of the permeable porous resin film 6 in the aforementioned embodiment. In this case, a spring washer 10 is provided. 
     With this arrangement, the interior of the lamp (interior of the reflector 1) and the interior of the container 4 are in communication with each other through a clearance between the threaded hole 4&#39;a and the screw 9 and through an extremely elongated passage in the form of a slit of the spring washer 10 to assume a vented state. For this reason, the amount of air moving in and out of the lamp when the latter is lighted and unlighted is the maximum after a fixed period of time, and thereafter gradually reduces. That is, the flow of air continues for a long period of time. 
     Accordingly, when the lamp is being lit, the water content released from the drying agent 8 is discharged outside over a long period of time, and the remaining water content is not left within the container 4. On the other hand, when the light is unlighted, the flowing-in open air contacts with the low-temperature drying agent 8 for a long period of time, and dehumidifying thereof is satisfactorily carried out. 
     In the prior art, the flowing-in and flowing-out quantity of air immediately after lighting and unlighting is large and thereafter rapidly reduces. As the result, the recycle action exerts a bad influence (i.e., is not satisfactorily carried out). 
     While in the above-described embodiment of FIG. 5, the threaded hole 4&#39;a is provided in the end of the container 4, and the screw 9 is screwed therein from outside of the container 4, it is to be noted that the screw 9 can be screwed in from the inside of container 4. It is to be further noted that a through-hole in the end of the container 4 need not be formed into a threaded hole, but a nut can be used on the screw 9 to form an integral configuration. 
     As described above, according to the present invention, a permeable porous resin film (layer) is provided in the end of the container encasing therein the drying agent on the internal side of the housing, or a screw is inserted into a small hole in the inner end thereof, to form an extremely elongated vent passage to make the outflow convenient with respect to dehumidification. A sufficient dehumidifying effect may be expected, and the humidity test in accordance with FMVSS with a vent hole may be cleared. In addition, a headlamp with a vent hole has a smaller rise of internal pressure as compared with the totally closed type headlamp. Therefore, the pressure resistance requirement of parts constituting a lamp is relieved to considerably enhance reliability, reduce cost, reduce weight, and increase freedom of design. A further advantage is that the construction of the vent hole is simple, the number of parts is small, and the shape is compact.