DESIGN FOR :MAXIMI >I SENSITIVITY 



28, 32, 33; see especially 5 for general refer- 

 ence and 33 for a thorough theoretical treat- 

 ment of the polarizing microscope). 



This article will give special attention to 

 methods and devices for obtaining maximum 

 sensitivity with the polarizing microscope. 

 As described later, the combined sensitivity, 

 resolution, and image quality of the polariz- 

 ing microscope has been vastly improved in 

 the past few years, and retardation (coefR- 



o 



cient of birefringence X thickness) of 0.1 A 

 unit can be detected on objects 0.2m wide. 

 These advances now permit application of 

 polarization microscopy to new realms such 

 as analyses of fine structure in living cells, 

 or experimental studies of rigorous diffrac- 

 tion theory of the kind which could not be 

 performed with equipment available in the 

 past. 



Specification of the System for Ob- 

 taining ^Maximum Sensitivity 



For the sensitive detection of birefringence 

 (BR) and dichroism, precise measurement of 

 retardation, optical activity (rotation of 

 plane-polarized light), extinction angle, etc., 

 detectable contrast must be introduced with 

 only minimal differences in a parameter. 

 Contrast is maximized for a given object bj^ 

 reducing stray light in the system (11, 30), 

 and by using appropriate compensators or 

 half -shade plates (3, 7, 11, 13, 15, 18, 27, 

 28, 30, 31). The degree of success in reduc- 

 tion of stray light, or increase in extinction 

 factor (EF), can be expressed numerically as 

 EF = intensity of light with parallel polars, 

 divided by intensity of light with crossed 

 polars (30) . The effect of various components 

 on the EF is discussed in the next section. 



The ability of the eye to perceive contrast 

 is drastically lowered at low levels of image 

 brightness, a condition which prevails at 

 high EF's. A maximally bright light source 

 is therefore required. Contrast discrimina- 

 tion can also be improved bj^ darkening the 

 room and by masking off unwanted sources 

 of bright Ught in the image (e.g., retardation 



can be measured to X/1000 with a quartz 

 wedge when the regions outside of the dark 

 fringes are masked). 



Specification of the Components 



Polars. ^Modern sheet polaroids (general 

 properties outhned in 21) can, but do not 

 always, have EF as high as 3 X 10^ for green 

 light at normal incidence. For most work EF 

 of this magnitude is adequate, but two addi- 

 tional factors should be borne in mind for 

 critical application. The parallel transmis- 

 sion of a pair of high extinction polaroids is of 

 the order of 10 ^ 15 %, hence a considerable 

 light loss takes place. Most sheet polaroids 

 have microcrystalline textures or inclusions 

 which can cause "hot spots" affecting the 

 diffraction image. 



Calcite polars coated with a low reflection 

 film may show virtually no light loss (paral- 

 lel transmission '=50^c) and an extremely 

 high EF (^3 X 10«). Of the various t>T)es 

 of calcite prisms available (17, Lambrecht 

 20 has supplied excellent quality prisms), 

 the square-ended Glan-Thompson prisms ap- 

 pear to be the most effective. The optical 

 qualitj' and EF of calcite prisms depend to a 

 great extent on the skill of manufacture and 

 subsequent care of handling since the soft 

 calcite surfaces are prone to scratching and 

 pitting. The beam passing through the 

 analyzing calcite prism should be made par- 

 allel by the stigmatizing lenses to minimize 

 the astigmatism resulting from the double 

 refraction of calcite. 



Condenser and Objective Lenses. All 

 optical components lying between the polar- 

 izer and the analyzer must be scrupulous^ 

 clean. Not only may any dust particle, 

 greasy surface, etc. scatter enough light to 

 lower the EF and seriously reduce object 

 contrast, but by acting as loci having high 

 transmittance, such entities would give the 

 effect of oblique illumination and distort the 

 diffraction image. 



The deleterious effect of large amounts of 

 strain BR in the condenser and objective 



481 



