In order to obtain a variety of illumination modes, the Olympus MIC-D digital microscope is equipped with a rotation arm that travels through a range of 135 degrees. Using the various positions available with the rotation arm, the microscope can be configured for brightfield, oblique, darkfield, and reflected light observation. This tutorial explores the range of motion available with the MIC-D rotation arm.

The tutorial initializes with a diagram of the MIC-D digital microscope configured for brightfield illumination, and having the rotation arm positioned parallel to the optical axis of the microscope. In order to operate the tutorial, use the Rotation Arm Position slider to translate the microscope rotation arm through its 135-degree motion range. The current rotation radius is presented in degrees to the right of the slider bar. As the rotation arm enters radius values that are useful for off-axis illumination techniques, the appropriate lighting mode is presented beneath the microscope drawing.

In oblique illumination, direct light from the condenser light cone is restricted to a single azimuth, striking the specimen from only one direction rather than bathing it with an even distribution of light through a well-defined numerical aperture. The net effect is to reveal details in pseudo-relief to produce a shadowed effect on the side opposite the light source and bright specimen highlights on the side nearest the illuminator. In order to achieve oblique or off-axis illumination with the MIC-D digital microscope, the illuminator head is shifted by moving the rotation arm a few degrees away from the upright (brightfield) position.

The useful range of motion for producing oblique illumination with the rotation arm is between 2-3 and 15 degrees from the vertical (optical) axis of the MIC-D digital microscope. In addition, the off-axis angle of specimen illumination can also be modulated or fine-tuned by rotating the illumination head itself (while the arm is held stationary) on a pivot that attaches the head to the rotation arm. The illumination head can be rotated on the pivot to a maximum angle of 10 degrees. Because there are two variables that can be introduced when producing off-axis illumination with the MIC-D, rotation of the main illumination arm and independent rotation of the attached illumination head, the number of possibilities for varying the illumination angle is almost limitless.

In traditional optical microscopy, darkfield illumination requires blocking the central light that ordinarily passes through and around (surrounding) the specimen, allowing only oblique rays from every azimuth to "strike" the specimen mounted on a microscope slide. When the rotation arm of the MIC-D digital microscope is positioned at a distance greater than 15 degrees from the central (optical) axis, this instrument is capable of imaging specimens by a mechanism that produces results similar to those observed in true darkfield illumination.

In reflected light mode, the rotation arm is translated from the on-axis (vertical) position through a 135-degree angle to direct light onto the surface of a specimen, at an incident illumination angle of 45 degrees, by means of a port located beneath the stage. In this configuration, the illumination head is not rotated from its default position (coaxial with the rotation arm) on the pivot that secures it to the rotation arm. Correct positioning of the rotation arm is guaranteed by a pin attached to the backside of the arm adjacent to the microscope body. The pin rests in a crescent-shaped cavity that defines the range of motion allowed for the rotation arm. When the pin is firmly seated at one end of the cavity, the rotation arm resides in a vertical position for brightfield microscopy, while at the other end, the rotation arm is oriented for reflected light microscopy.