DESIGN FOR MAXIMUM SENSITIVITY 



they still exhibit a dark cross which be- 

 comes progressively more prominent as the 

 NA is increased (Fig. la). Light from the 

 areas between the arms of the cross is in- 

 troduced by rotation of the plane of polariza- 

 tion at glass air interfaces in the lenses and 

 in microscope slides. The rotation is a result 

 of differential reflection losses of the parallel 

 and perpendicular components of polarized 

 light and may be as large as 7° ^^ 8° for 

 high NA objectives even after low reflection 

 coating and oil immersion (2, 10, 33). This 

 lowers the EF (^lO^ for NAobjective = 

 NAcondenser = 1.25) and furthermore dis- 

 torts the Airy diffraction pattern (14, 19). 

 Much of the rotation and therefore light 

 giving rise to the cross can be eliminated by 

 the use of "polarization rectifiers" built into 

 the objective and condenser lenses (Fig. 2R). 

 The rectifier consists of a X/2 BR plate which 

 reverses the above mentioned rotation and a 

 zero power meniscus which introduces suffi- 

 cient additional rotation to cancel out the 

 reversed rotation (12). 



Suitably rectified objectives and con- 

 densers used with a proper microscope (last 

 section) give verj^ high EF (2 X 10^ for 



NAcondenser = NAobjective = 1-25) aild Uiakc 



detectable very small BR (<0.1 A°) of ob- 

 jects which in themselves are barely resolva- 

 ble wdth the light microscope (<0.2yu). The 

 objective back aperture being uniformly dark 

 (Fig. Ic), spurious diffraction is reduced to a 

 minimum and the image is therefore more 

 reliable (12, 14). Determination of the polar- 

 ization angle and other parameters can be 

 made with great accuracy by using rectified 

 optics whereas such measurements with or- 

 dinary polarizing microscopes contain in- 

 herent and significant errors (33). 



Compensators. Many types of compen- 

 sators and half -shade plates have been used 

 for increasing the sensitivity and precision in 

 determining the ellipticity of polarized light, 

 polarization angle, etc. Their construction, 

 sensitivity and operation may be found in 

 references 3, 4, 7, 9, 11, 13, 15 to 18, 23, 24, 



25, 27, 28, 30, 31, 33. As mentioned above, 

 the property of the eye or other detectors 

 influence their performance. Photoelectric or 

 photographic photometry can also aid in in- 

 creasing the sensitivity. 



When algebraic computation of compensa- 

 tor actions are difficult, e.g., when more than 

 two anisotropic components lie between the 

 polars, advantage can frequently be taken 

 of the geometrical construction available 

 with the Poincare sphere or its two dimen- 

 sional analog (16, 29). 



Optical Layout of a Polarizing Micro- 

 scope with Exceptional Sensitivity. Fig- 

 ure 2 shows the essential optical layout of a 

 transilluminating polarizing microscope de- 

 signed to give maximum sensitivity and 

 image quality. The system illustrated is in- 

 verted with the light source S on top and 

 detectors (EM and E) at the bottom. 



Light from the bright source is filtered 

 and is focused by Li and L2 onto a pinhole 

 aperture A 2 . This restricts the size of the 

 source image so that after projection by L3 

 it just covers the condenser aperture dia- 

 phragm Ae . The polarizing Glan-Thompson 

 prism POL is placed behind the stop A4 away 

 from the condenser COND, to prevent light 

 scattered by the polarizer from entering the 

 condenser. Half-shade plates are placed at 

 the level of A5 , and compensators COMP 

 above the condenser. Both the condenser 

 and the objective OBJ lenses are rectified 

 by Ri and R2 and are low reflection coated. 

 The image of the field diaphragm (A3 or 

 Ag) is focussed onto the object plane OB by 

 the condenser, whose NA can be made equal 

 to that of the objective. 



Stigmatizing lenses Sti and St 2 are low re- 

 flection coated on their exteriors while their 

 backs are cemented directly onto the ana- 

 lyzing Glan-Thompson prism ANAL to pro- 

 tect the surfaces of the prism. Aperture 

 stops Ai-As are placed at critical points to 

 minimize scattered light from entering the 

 image forming system. The final image is 

 cast by OCi on a photographic or photoelec- 



483 



