For each eXact aperture size, the target window size is 1 mm (on radius) larger than the spot measurement area size. This 1 mm over illumination of the measurement area is required by ISO 5/4. The white area around the patch does not influence the measurement data, but does require the operator to center the measurement sample in the target window opening.
The illustration below shows how this minimum measurement area appears in relation to the target window opening.
The black circle represents the target window opening.
The white circle represents the actual measurement area (which equals the aperture size).
The magenta square represents the minimum suggested patch size.
This can be used as a guide for operators when placing the instrument optics over patches which do not completely fill the target window opening.
The following aperture sizes are recommended:
It is always suggest to use an as large as possible aperture size.