19
4.7
Compensators
All polarizing microscopes are fitted with a slot in the intermediate
tube above the nosepiece and between the polarizer and analyzer. This
is intended for insertion into the optical path of a compensator.
Compensators also referred to as retardation plates and are sections of
optically anisotropic material with plane faces which, when inserted
diagonally in the microscope between crossed polarizers, produce a
specific optical path difference of mutually perpendicular plane-
polarized light waves.
I. 1/4λ-plate
- The 1/4λ-plate is called a mice plate because the
compensator plate is mostly made of mica. It has an optical path
difference in yellow light of around 140nm (yellow light has λ =
580nm, therefore 1/4λ = 580/4 = 145nm). It changes plane-polarized
light into circularly polarized light.
II. 1λ-plate (sensitive tint or first-order red)
-
The 1λ-plate is called
a gypsum plate of a thickness to give1λ optical path difference for
green light of 550 nm. This wavelength is therefore extinguished with
the resulting interference colour having the typical tint of the first-
order red/violet. This magenta colour is sometimes termed ‘sensitive
tint’.
III. Quartz Wedge –
This device
has a range of 4 orders and is commonly
employed for qualitative retardation measurements of petrographic
specimens or other birefringent materials whose retardation value falls
within the wedge limit. The quartz wedge is simplest form of
compensator where the optical path-length difference is varied by the
degree of insertion into the optical axis to match the optical path
difference of the specimen.
Bertrand Lens Turret
Bertrand Lens Focus Ring
Bertrand Lens Centering Screws
