White Paper - SX12 Tracking and Target Separation 6
https://geospatial.trimble.com
6
The tracker camera is focused to infinity while the target is at a closer range, affecting the size of the target's
image. This is the same effect as background blur in a camera image, where small light sources that are not in
focus get expanded into bright discs in the image. The bright defocused discs will become bigger the further out
of focus the light sources are, and the bright discs will also become bigger if the camera has a larger lens.
Figure 8a. The picture shows some bright light sources. The camera is focused at the light sources.
Figure 8b. The picture shows the same bright light sources as figure 8a but now the camera is focused at a much
longer distance. Note how the images of the light sources get expanded into bright discs in the image due to the
defocus.
For an SX12, the small light source is the tracker transmitter laser that illuminates the target and gets reflected
back by the prism to the instrument. The tracker detector sees the reflection of the tracker transmitter through
the prism and will perceive it as being two times further away than the prism itself. This is similar to when you
look into a mirror and the distance between you and your reflection is twice the distance between you and the
mirror.
The prism target is a mirror that allows the SX12 to see and aim to its own tracker transmitter. The tracker
transmitter laser is essentially a point source but since the tracker system is focused to infinity, the image of the
target will become bigger when the target moves closer to the instrument, due to defocus, as seen in figure 8b.
