Adjustment and inspection of headlamp aim may be accomplished by four methods: visual, optical, photoelectric, and mechanical.
Visual aim is the simplest but the least accurate. Lateral accuracy is restricted by the ability of the driver to align the vehicle perpendicular to the aiming board and the difficulty of determining the edge of the high-intensity zone of the low-beam lamps. The precision of visual aim is considerably less than the tolerances prescribed in the SAE J599b, Lighting Inspection Code.
An optical aimer projects a reduced image of the headlamp beam on a screen. Difficulty in visually determining the edge of the high-intensity zone of the low-beam lamps and inability to accurately align the aimer to the vehicle results in inaccurate measurement of headlamp aim.
Two photoelectric aim systems are available. The field photoelectric aimer is designed on the assumption that the left edge of the high-intensity zone of the low-beam lamp is 17 in (at 25 ft) from the point of maximum intensity of the light pattern, and that the top edge of the high-intensity zone of the low-beam lamp is 11 in (at 25 ft) from the point of maximum intensity of the light pattern. These conditions may sometimes occur by coincidence, but not by design. Furthermore, no accurate method is provided for aligning the aimer to the vehicle. Consequently, the system lacks the required precision and accuracy for aim adjustment and inspection. The “Third Derivative” photoelectric aim system is accurate and precise but very expensive. However, mechanically aimable lamps can be aimed and inspected with mechanical aimers, which are less expensive and more adaptable to field or shop aim adjustment or inspection.
Body variations and dog track reduce the accuracy of mechanical aimers. However, mechanical aimers, when used with mechanically aimable lamps, are more accurate than visual, optical, or photoelectric systems. Mechanical aimers are precise and are available at reasonable cost.
Therefore, only mechanical aimers should be used for aim adjustment and inspection in the field and for service.