Shear amount and resolution

It is important to keep in mind that the shear amount is extremely small. Since the shear amount is equivalent to the amount of horizontal deviation, if the shear amount is large, image resolution deteriorates. (Points and lines look fatter due to the horizontal deviation.) If the shear amount becomes too great, the horizontal deviation is such that it makes the image appear double.

Figure 2. Example of image appearing double due to excessive shear amount

Grid viewed under brightfield illumination

Grid viewed with small shear

Grid viewed with large shear

Images do not appear double when observing a grid micrometer with a prism that has little shear, but when a prism with a large shear amount is used, the calibrations appear double. Also, the image appears double only in the direction of the shear (along the diagonal from upper left to lower right), and the lines that are perpendicular to these lines do not appear double.

Sample: Grid micrometer
Magnification: 10x
Conventional reflected brightfield microscope images

[Note] To make the phenomenon that occurred in both grids viewed with small and large shear easy to see and describe, we only inserted a DIC prism into the brightfield optical path. Since interference was not induced, these are not true differential interference images.

To avoid this, in DIC (differential interference contrast) microscopes, their share amount is generally set to smaller than the human eye's resolving power. The difference in height between two points in the image being viewed in DIC observations is therefore sufficiently small spatially. In other words, we know that this is the gradient for each minute portion, or differential. This is the origin of the term “differential” in “differential interference.”