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On the accuracy of microinterferometric measurements of optical‐path differences by means of the half‐shade method
Author(s) -
Pluta M.
Publication year - 1971
Publication title -
journal of microscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.569
H-Index - 111
eISSN - 1365-2818
pISSN - 0022-2720
DOI - 10.1111/j.1365-2818.1971.tb02270.x
Subject(s) - optics , optical path length , eyepiece , birefringence , moiré pattern , phase difference , optical path , microscope , mathematics , materials science , phase (matter) , physics , quantum mechanics , lens (geology)
SUMMARY Using a shearing double‐refracting interference microscope with a half‐shade eyepiece, the accuracy of optical‐path‐difference measurements has been examined. For the half‐shade elements, birefringent strips with phase difference equal to 180°, 110° and 20° as well as a half‐wave plate with a narrow slit, cemented between two glass plates, were used. It is known that different relative intensities of the background inside and outside the half‐shade strip introduce a subjective error in the correct visual assessment of the matching point of the interference image of the object under examination. Applying test objects made of thin strips of dielectric evaporated in vacuum onto glass slides, it has been demonstrated that this error depends upon the optical‐path difference and image size of the object being measured, as well as upon the half‐shade phase difference. For a small half‐shade phase difference the error is practically imperceptible and for the half‐shade strip giving half‐wave retardation it attains maximum values. In the case of homogeneous objects, the images of which are as large as 1.5–2° in the field of view of the microscope, this error does not usually produce inaccuracies greater than ± λ/100 in optical‐path‐difference measurements. Using the half‐shade strip with a diaphragm which masks the misleading background and enables an observer to see the matched image areas only, the optical‐path difference can be measured with accuracy ± 0.003λ or better.