"Orientation imaging through the light microscope"

1.2

WHAT IS ORIENTATION IMAGING ?

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Orientation imaging through the light microscope

Ultrathin section of Carrara marble
crossed polarizers, lambda plate

Without knowing - or without calling it so - we are all quite familiar with orientation imaging. Everytime we use a polarizing microscope to observe a thin section, we look at an orientation image.
Depending on

the birefringence of the mineral,

the orientation of the optical axis with respect to the polarizers,

the thickness of the thin section,

the number of polarizers and their relative orientation,

the spectral transmission of the filters and

the number and types of compensators

placed in the optical path, the resulting intereference colours are different and have to be interpreted differently. If we keep everything but the orientation of the crystal lattice constant (as is the case in a monophase material, and constant thin section thickness), the interference colours vary solely as a function of the orientation of the optical axis of the mineral.
Unfortunately, however, this dependence of interference colour on c-axis orientation is not unique: there are always a number of orientations that produce the same interference colour. If we think of ordinary polarization microscopy in terms of "analogue orientation imaging", we would say that it is a type of orientation imaging where the "choice of look-up tables" is "somewhat unfortunate" or limited. True orientation imaging always seeks to find a unique representation of crystallographic orientation, asigning each possible orientation exactly one colour and vice versa.
In this section, we will look at differently polarized and filtered images of an experimentally deformed quartzite with the aim of extracting whatever orientation information we can from the observable interference colours.

1.2	WHAT IS ORIENTATION IMAGING ?
	top / contents / section 1 / pages 1.1 -- 1.2 -- 1.3 -- 1.4 -- 1.5 -- 1.6 -- 1.7

		Orientation imaging through the light microscope
Ultrathin section of Carrara marble crossed polarizers, lambda plate		Without knowing - or without calling it so - we are all quite familiar with orientation imaging. Everytime we use a polarizing microscope to observe a thin section, we look at an orientation image. Depending on the birefringence of the mineral, the orientation of the optical axis with respect to the polarizers, the thickness of the thin section, the number of polarizers and their relative orientation, the spectral transmission of the filters and the number and types of compensators placed in the optical path, the resulting intereference colours are different and have to be interpreted differently. If we keep everything but the orientation of the crystal lattice constant (as is the case in a monophase material, and constant thin section thickness), the interference colours vary solely as a function of the orientation of the optical axis of the mineral. Unfortunately, however, this dependence of interference colour on c-axis orientation is not unique: there are always a number of orientations that produce the same interference colour. If we think of ordinary polarization microscopy in terms of "analogue orientation imaging", we would say that it is a type of orientation imaging where the "choice of look-up tables" is "somewhat unfortunate" or limited. True orientation imaging always seeks to find a unique representation of crystallographic orientation, asigning each possible orientation exactly one colour and vice versa. In this section, we will look at differently polarized and filtered images of an experimentally deformed quartzite with the aim of extracting whatever orientation information we can from the observable interference colours.