Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις (parallaxis), meaning “alteration”. Nearby objects have a larger parallax than more distant objects when observed from different positions, so parallax can be used to determine distances.
So what does this mean in English you might ask? Well, it’s very simple to describe when one relates parallax and parallax error to the headlight test in the MoT procedure.
Lets consider manufacturer A’s headlight tester:
As you can see on the headlight tester on the left; at the top of the vertical post is a mirror which is used in order to get the headlight aligner aligned with the vehicle that is being inspected. The process for achieving this is fairly straightforward. On the trolley/base is a clamp which allows the vertical post to rotate. Looking through the mirror the operator judges, when the beam setter is aligned to the vehicle so the measurement can be accurately made. But dependant on the position the operators eye, errors can creep into this alignment reducing the quality of accuracy of the test…
The next problem with this headlight tester is making sure the bulb is shining directly into the middle of the lens of the tester – a tape measure should be used for the vertical measurement but the “side to side” measurement is made by the operator judging when the centre of the body of the tester is in line with the bulb centre. For a second time in the test there is a risk of reduced accuracy.
Lets see now how we can use technology to eliminate those error altogether…
GETECH Headlight Aligner (GE5418L)
See the difference? At the top of the upright is a laser which is fixed into a position to maintain a known datum for the measurement. The system is essentially foolproof…. Now, by adding a secondary laser inside the body of the tester we are able to pick out the centre of the bulb with total accuracy and repeatability! See the full product video here and once convinced of the accuracy of measurement visit our webpage HERE where you can buy this unit online for a price comparable to a standard mirror visor model and thus eliminate your VTS’ risk of systematic errors.