Snap-On UHR Ultra, Service Manual

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15
Theory of Operation
TARGETS 2D
The target’s visual surface is a series of circles or “dots” made of a retroreflective material. This material
returns any light that hits its surface at a 180-degree angle directly back towards the source. If we are look-
ing at the reflected light, it is brightest at the center of the light source. As we move away from the source the
brightness of the light diminishes rapidly. Street signs have retroreflection material on certain parts of their
surface. This material reflects light from a vehicle’s headlights back to the motorist’s eyes. The portion of the
sign that traffic control wants the driver to pay attention to is reflective, while the rest of the sign fades into
darkness. Drivers in other cars cannot see the reflected light from the headlights of other cars because the
angle of view is too large. In the case of the 3D Aligner, light is generated by the LED’s, hits the target, and
returns along the same path to the camera when placed in the center of the LED array.
The retroreflection material is placed on an copper backing plate to insure the reflective pattern remains flat.
This assembly is then mounted into a hard ABS plastic housing for shock and damage protection. Finally,
a special piece of high quality precision exotic plastic is placed on top of the retroreflection material for final
protection from damage.
The dot pattern created allows the cameras to see a very unique item that the 3D Aligner recognizes. The
various dimensions of this pattern of dots is a known quantity that is stored within the program and is used as
a constant, forming the basis of our vision-based measuring system.
There are 33 dots on each front target and 31 on rear targets laid out in a very
distinct manner. A great deal of time and effort was placed on designing this pat-
tern of dots. In some ways the dots appear to be randomly placed, but in other
ways appear to have some symmetry. Some of the dots are larger than other
dots, and there may also be “donut” circles. It must be noted that these circles
we see are geometrically the “roundest” circles we will ever encounter this is
necessary for the interpretation of angles, as we will learn later. Note also that
the dots on the rear targets are larger than the front targets to allow vision at a
further distance.
Since the engineers designed the target dot pattern with great precision, they know the exact dimensions of
each dot and the distance from any one dot to another. It is possible then to create a computer model of this
pattern that the 3D Aligner software could use as it looked at camera images. This would be usable in the
program only if we were certain that the target attached to the vehicle matched the computer model. Since it
is impractical to have an engineer hand-make each target to the required precision, a detailed manufacturing
process was created to make sure each target was an exact duplicate of the original design.