With the measurement taken, the prism can now be cleaned. Samples should be removed from the
prism surfaces as soon as practical after measurement. Leaving sample between the prisms for long
periods, and allowing it to dry, can cause the two prisms to stick together.
The sample should be removed from both prisms using a suitable solvent; distilled water or alcohol,
depending upon whether the sample is water or oil based, and cleaning with tissue. The prisms
should then be finally washed with distilled water or alcohol and dried with clean tissue.
Note:
When cleaning the prisms, please remember that excessive rubbing with abrasive tissues could
scratch the prism surfaces. This would reduce the quality of the borderline and also cause sample
contamination. B&S do not recommend the use of aggressive solvents such as acetone – always use
alcohols or other non-aggressive solvents.
Reflection mode (High Accuracy models only)
If the sample layer is optically dense at the wavelength used, then insufficient light may not pass
through it to provide a visible borderline. This problem can generally be overcome by using internal
reflection of the light at the interface of the prism and sample.
To use internal reflection
•
Slide down the “Upper prism shutter” to prevent light entering into the upper prism.
•
Open the “Lower prism box window shutter” by turning the knob clockwise as seen from
the front of the instrument.
•
Adjust the light source to enter the “Lower prism box window.
The borderline as seen in the field telescope will however be inverted, the darker part of the field
being at the top. Usually the contrast is not so good as with transmission mode but in general a
sharp line can be obtained.
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