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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
     The invention relates to a lighting system, and more particularly, to a lighting system for vehicles with steps or stairs. 
     BACKGROUND OF THE INVENTION 
     Many work vehicles such as, for example construction vehicles may be employed at night, i.e., under conditions of low ambient lighting. Such vehicles may be relatively large and may employ ladders, stairs, etc., for operator ingress and egress. Ingress and egress, under low natural lighting conditions, would be more convenient if supplemental lighting was provided to illuminate the stairs or ladders. 
     SUMMARY OF THE INVENTION 
     The invention includes a lighting system which provides supplemental lighting for vehicle stairs under conditions of low ambient lighting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a vehicle on which the invention would be useful and on which a first location of a stair light appears; 
         FIG. 2  illustrates a schematic of an exemplary embodiment of the invention; 
         FIG. 3  illustrates a schematic of an exemplary embodiment of the invention with an exemplary latching arrangement; 
         FIG. 4  illustrates a flow diagram for the exemplary schematic illustrated in  FIG. 2 . 
         FIG. 5  illustrates a second location for the stair light; and 
         FIG. 6  illustrates a plurality of stair lights directly illuminating the stairs, the door handle and the lock. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates an exemplary embodiment of a work vehicle  10  which could make use of the invention. The particular embodiment illustrated is an articulated dump truck (“ADT”)  10  employing an embodiment of the invention, i.e., the stair lighting system  100 . This illustrated ADT  10  includes: a front portion  20  which may have a front chassis (not shown), ground engaging front wheels  21 , a cab frame  22 , a roof  22   a , a door  22   b , a door handle  22   c , a battery  24  an ignition  25  and stairs  26 ; and a rear portion  30  having a rear chassis (not shown), ground engaging rear wheels  31 , and a dump body  32 ; and an articulation joint  40  allowing angular change between the front portion  20  and the rear portion  30 . An exemplary stair light  110  is shown located on the roof  22   a , in a position over the stairway  26  enabling it to illuminate the stairs  26  and the door handle  22   c.    
       FIG. 2  illustrates an exemplary schematic of the flow of communication for the stairway lighting system  100  which may include: a stair light  110 , a vehicle controller  120 , a light sensor  130  positioned to detect the intensity of ambient lighting, a motion sensor  140  which may be designed and positioned on a side of the cab frame  22  to detect operator motion or presence on or near the stairs  26 , a first switch  150  and a second switch  160 . It gives a simplified view of the lines of communication between elements of the system  100 . 
       FIG. 3  illustrates an exemplary schematic of power flow for the stairway lighting system  100  showing details of a latching arrangement which may include: a seven (7) volt power supply  116 ; a controller  120 ; a light sensor  130 ; a motion sensor  140 ; a first switch  150 ; a second switch  160 ; a door switch  170  and corresponding transitors  121 ,  131 ,  141 ,  151 ,  161 ,  171 . Electrical output for each of the transitors  121 ,  131 ,  141 ,  151 ,  161 ,  171  may flow from the line on the output side when electrical power is supplied to each of the two lines on the input side. The controller  120  may include: a five (5) volt power supply  120   a ; and a processing circuit  120   b  with a processor and memory and/or storage. Also included in the stairway lighting system  100  may be a battery  25  and, in this particular embodiment, an arming switch  115 . 
     The arming switch  115  may be mechanical or electronic and may respond to a remote signal such as a light or radio signal from a conventional remote control device (not shown) to which it may be designed or programmed to recognize and respond to. It may toggle to between an arming on state and an arming off state. Thus, a first signal from the remote control device may arm the system  100 , i.e., connect the battery  25  to the other switches  130 ,  140 ,  150 ,  160 ,  170  and a second signal from the remote control device may disarm the system  100 , i.e., disconnect the battery  25  from the other switches  130 ,  140 ,  150 ,  160 ,  170 . Arming and disarming the system  100  may also include disconnecting, i.e., interrupting the flow of electrical power to the seven (7) volt power supply  116  as illustrated. The arming switch  115  may, in some circumstances, be disarmed or armed by the controller  120  if a line of communication is established between the controller  120  and the arming switch  115 . 
     The first and second switches  150 ,  160  may be mechanical or electronic and may respond to physical manipulation or to a remote signal such as a light or radio signal from the remote control device to which they (it) may be designed or programmed to recognize and respond. They may also be designed such that activation of one or both of the first and second switches  150 ,  160 , via mechanical manipulation or remote signal, may cause it/them to change state from an off state to an on state for a predetermined minimum time and then revert to the off state. Finally, one of the first and second switches  150 ,  160  may be designed and located such that it is mechanically manipulated when a weight of the operator is applied to the stairs  26 . Such a switch may be purely mechanical or in the form of a strain gage and placed at an interface between the stairs  26  and the cab frame  22 . The weight of the operator may be applied to the stairs  26  when the operator steps on any of the stair steps  26   a.    
     The motion sensor switch  140 , upon sensing motion, may also enter an on state for the predetermined minimum time and enter into a standby state after the predetermined minimum time is exceeded or an off state if the controller  120  has disarmed the lighting system  100 . The predetermined minimum time for the on state of any of the switches  140 ,  150 ,  160 ,  170  may be set to a time required for the controller  120  to power up and latch the lighting system  110  to an on state via a constant supply of electrical energy from, for example, the five (5) volt power supply  120   b  to the input side of the transistor  121 . 
       FIG. 4  illustrates an exemplary flow diagram  200  for actions of the controller  120  in the schematic of  FIG. 3 . The underlying assumptions for the flow diagram of  FIG. 3  is that the stair light  110  is initially off at step  210  and the operator is away from the work vehicle  10 . If the stairway lighting system  100  is not armed, via the arming switch  115 , at step  215 , the lighting system  100  returns to step  210  with the stair light  110  off. If the lighting system  100  is armed at step  215 , via the arming switch  115 , and none of the switches  140 ,  150 ,  160 ,  170  is in an on state, the lighting system  100  returns to step  210 . If the lighting system  100  is armed at step  215 , via the arming switch  115 , and at least one of the switches  140 ,  150 ,  160 ,  170  is in an on state, the controller  120  may check input from the light sensor  130  to determine if ambient light is less than a predetermined intensity. If ambient light is not less than a predetermined intensity at step  250 , the lighting system  100  returns to step  210 . However, if the ambient light is equal to or greater than the predetermined intensity, the controller latches the power, turns the stair light  110  on and begins to track the time the stair light  110  is on at step  260 . The controller  120  keeps the lighting system latched and the stair light  110  on at step  270  until the tracked time is greater than or equal to a predetermined minimum time. Once the stair light has been on for a time equal to or greater than the predetermined minimum time, the power is unlatched by the controller  120  ant the lighting system returns to step  210 . and the stair lighting system  100  includes the light sensor  130 , then the intensity of ambient light may be detected and if the light sensor  130  indicates the ambient lighting is not less than a predetermined intensity, the controller  120  returns to step  210  with the stair light  110  off. If the light sensor  130  indicates the ambient lighting is less than a predetermined intensity, the stair light is turned on and the controller  120  may begin to track the time the stair light  110  is on at step  260  and may then move to step  270 . If, at step  270 , the on time of the stair light  110  is less than a predetermined minimum value, the lighting system remains at step  270  and the stair light  110  remains on. Once the stair light  110  is on for a time equal to or greater than the predetermined minimum time at step  270 , the controller  120  may unlatch the power at step  280  and, at step  290 , return to step  210 . The predetermined intensity may be set at any value but may definitely include light intensities at which the operator would find it difficult to see such as, for example, an ambient nighttime light intensity. Likewise, the predetermined minimum time may be set at any value but may, generally, be large enough to allow the operator to safely board or exit the vehicle  10  such as, for example, one (1) minute. 
     As illustrated in  FIG. 4 , if the stair light is off at step  210  and the system  100  is armed at step  220 , it may determine if the operator is on or near the stairs  26  via the proximity or motion sensor  140  at step  280 . If the proximity or motion sensor  140  does not indicate that the operator is on or near the stairs  26 , the system may return to step  210  with the stair light(s) off. However, if the motion sensor  140  indicates that the operator is on or near the stairs  26  at step  280 , then the system  100  may move to step  240  in the process. 
     The stair light system  100  may be designed such that the controller  120  and all sensors and switches are directly powered by the battery  24 ; but in a standby mode when armed and the operator is not present. Thus, the controller  120  and all sensors and switches may consume a small amount of electrical power when armed but a minimum amount of power when the system is unarmed and mechanical switches are used to arm the system as neither the controllers nor the sensors need be powered until the system is armed. 
     It may be desirable that the system  100  be perpetually armed if the times of operator absence are relatively short. However, there may be times that it is more desirable for the system  100  to be unarmed, e.g., the vehicle  10  will not be operated for an extended period of time, the operator wishes to avoid playful tampering with the lighting of the vehicle  10  by third parties, etc. The system  100  may, once again, be armed by manipulation of the first switch  150  or the second switch  160  either mechanically, or if equipped, by the remote control device. 
     When the system  100  is equipped with a door switch  170 , an opening of the door  22   b  may cause the door switch  170  to change state from off to on and change the state of the system  100  from unarmed to armed. The underlying assumption here is that the operator is in the cab  22  and is opening the door  22   b  to exit the vehicle  10 . The stair light  110  may be turned on to aid the operator&#39;s exit either automatically upon opening of the door  22   b  or, upon both the opening of the door  22   b  and the detection of an ambient light intensity that is less than a predetermined minimum if the system is equipped with a light sensor  130 . The predetermined minimum intensity for ambient light may be set to any value via programming for the controller  120 . 
     If the system  100  is not equipped with a door switch  170 , it may be armed by a change in state of the ignition  25 , i.e., the system may be armed when the ignition  25  moves from an on state to an off state. The underlying assumption here is that the operator is preparing to exit the vehicle  10 . 
     The default state of the system  100  may be the last state of the system deliberately chosen by the operator. Thus, if a change in the state of system  100  from armed to unarmed, or vice versa, is last effected via a remote or mechanical manipulation of one of the first and second switches  150 ,  160 , the state of the system  100  may remain as such until a change in state is again effected by a remote or mechanical manipulation of one of the first and second switches  150 ,  160 . 
       FIG. 5  illustrates the stair light  110  located on a side of the cab frame  22 . Such a position may result in illumination of a greater intensity as the stair light  100  is located significantly closer to the stairs  26 . 
       FIG. 6  shows the stair light  110  as a plurality of lights  110   a  directly attached to, and illuminating, the stairs  26  at each step  26   a.    
     Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.

Summary:
A lighting system which may provide supplemental lighting for the stairs of a vehicle under conditions of low ambient lighting. Supplemental lighting may be provided for at least a period of time desired for ingress and/or egress of the operator.