Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
If the objectives are not a parfocal set, the stage should be lowered and the image
refocused when changing from low to high power.
With a graticule or filar eyepiece, ensure that the lines in its field of view are sharp when
the eyes are relaxed. Remove the eyepiece and look through it with one eye while viewing
a distant object with the other. The eyepiece focusing can then be adjusted to make the
lines in the field of view appear sharp.
With a binocular eyepiece, ensure that the image is sharp for both eyes and that the
distance between the eyepieces is correct. Firstly, cover the eye looking down the
adjustable eyepiece and focus the microscope for the other eye using the stage fine focus
control. Then cover the other eye and obtain a sharp image with the eyepiece focus
control.
9 - Close the field diaphragm and focus it in the field of view using the condenser focusing
control.
Due to the imperfect correction of the condenser and further aberration introduced by
subsequent mounting plates, the image of the field diaphragm tends to be rather poor in
transmitted illumination.
10 - Centre the diaphragm image in the field of view, using the controls on the condenser.
11 - Open out the field diaphragm to just cover the field of view with uniform illumination
As mentioned before, opening it any further will introduce stray reflections that reduce
contrast.
12 - Remove the diffuser, then centre and focus the image of the lamp filament in the plane
of the condenser diaphragm. Use the source centring and field lens focus controls for this.
Strictly, it should be the diffuser that is focused in this plane but it is easier to see the
image of the filament.
13 - Close the condenser diaphragm down and focus the filament image on it or on a piece
of paper held against it. If this diaphragm is not accessible remove an eyepiece or insert
the Bertrand lens and adjust the focus of the filament image until there is no parallax
between it and the objective pupil.
14 - Replace the diffuser and adjust the condenser diaphragm to be 2/3 the diameter of the
objective pupil, as shown in Figure 15.
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