Patent ID: 7275847

Claim:
A headlight optical axis adjustment method for adjusting a low beam optical axis of a headlight that has a vertically adjustable optical axis in a state in which the headlight is installed in an automobile, the method comprising carrying out in sequence: a first step of, on an imaged picture of a light distribution pattern of low beam illumination from a master headlight whose optical axis has been adjusted, the imaged picture being imaged at a predetermined distance in front of the master headlight, determining a relative positional relationship between a reference point and a horizontal line and an oblique line of a light/dark boundary line and setting an acceptance frame based on an imaged position of an intersection point of the horizontal line and the oblique line; and a second step of, on an imaged picture of a light distribution pattern of low beam illumination from a headlight to be adjusted whose optical axis is to be adjusted, the imaged picture being imaged at the predetermined distance in front of the headlight to be adjusted, detecting a horizontal line and an oblique line of a light/dark boundary line based on the relative positional relationship with the reference point and adjusting the optical axis of the headlight to be adjusted so that an intersection point of the detected horizontal line and oblique line is present within the acceptance frame; wherein in the first and second steps a position on the imaged picture corresponding to a bulb center that is set in advance on a front face lens of the master headlight and the headlight to be adjusted is determined as the reference point, and wherein in the first step the reference point on the imaged picture is determined according to measurement of vertical and lateral positions of the bulb center, which is set in advance on the front face lens of the master headlight, and in the second step the position of the bulb center, which is set in advance on the front face lens of the headlight to be adjusted, is estimated based on an image of light reflected from an area of a reflector of the headlight to be adjusted and projected on a screen, and the estimated position of the bulb center is employed as the reference point.