Differential Interference Contrast (DIC)

Image: DIC 3D Rendering

To oversimplify a rather complicated process:

In reflected light situations, one places a polarizer in the illumination axis and an analyzer (another polarizer) in the viewing axis, above the illuminator. The polarizers are crossed at 90 degrees such that when looking at a perfectly formed mirrored surface, all light would be extinguished by the second polarizer.

Next a prism, made from two pieces of very carefully ground and oriented quartz, is placed between the illuminator beamsplitter and the objective. Because of the optical properties of the quartz, it takes the polarized beam, reflecting from the beamsplitter, and splits it into two beams that are physically separated by a minute amount, polarized at 90 degrees to each other, and one beam is shifted in time with respect to the other (phase shift). If the prism was centered and the subject was the perfect mirror discussed above, these two beams would go down thru the objective, bounce off the mirror, return thru the objective, re-combine passing thru the prism (into their original state), and be blocked by the analyzer.

If the subject exhibits any properties (surface profiles, optical densities etc.) that change the optical path lengths of either of the two beams, the beams now experience further phase shifts. When these beams cross back thru the prism, because they have experienced a phase shift, the prism cannot re-combine them into their original state of polarization. Now, instead of being totally blocked, portions of each beam pass thru the analyzer and interfere with each other at the image plane.

By sliding the prism sideways, the phase shifting performance of the prism may be accentuated, which will modify the final image. Likewise, by adding a polarization modifier, such as a 1/4 wave plate after the first polarizer in the illumination axis, the final effects may be modified.

For a detailed, interactive, presentation of the technology, it is suggested that you visit http://www.microscopyu.com or http://www.microscopy.fsu.edu/primer.


Navitar's DIC module may be used on any ultra coax versions (zoom or non-zoom) of the 12X, or Zoom 6000.

Note: The combination of the fixed lens and objective simulates the normal results of a standard microscope. Going to a "zoom vision" mode will magnify the image, at a cost of contrast due to reduced image NA and optical effects of added glass. It is well to remember that magnification does not improve resolution, just sometimes the ability to see that which was resolved by the "taking lens".

The module works with "taking NA's" ranging from 0.05 to 0.50, with optimum performance in the range from 0.15 to 0.4. Lens attachments, operating in the above range will serve for macro. Any infinity corrected objectives, with similar NA's, designed for incident light will suffice for micro. Objectives, designed for very long working distances, produce marginal results due to the relocation of the pupil. Operating parameters (mag., FOV, etc.) will be the same as Navitar's existing tables. Because of the two polarizers in the system, which "eat" a lot of light, one must balance entering NA and final magnification to maintain image intensity.

Image: DIC with lens

Image: DIC with the prism out
Prism Out
Image: DIC with the prism in
Prism In

PDF: DIC System DiagramView the DIC system diagram (36.2k)

The wizard first looks at your type of camera mounting, then at the sensor size (based on line scan length or area sensor diagonal).

Next the wizard will look for a potential solution to satisfy your magnfication requirements, as defined by image length (area sensor vertical dimension or line scan length) divided by the pertinent object dimension.

Because the DIC is coax based, the spot it can evenly illuminate varies between macro and micro, and also with the pupil match of the individual objectives. As the sensor gets larger, the magnification must get larger to spread this same size spot over the larger sensor. Frequently, requirements for large objects, with large sensors, cannot be accommodated.

A c-mount is limited to a 20mm sensor as that is the maximum that reasonably can get thru the mount. An f-mount is limited to a 30mm sensor because of optical constraints, and requires that a 2X adapter be used. This 2x magnification factor further limits the size of the object viewed.

NON-ZOOM MICRO 13MM - 30MM 4.5X - 36.4X 5X - 20X OBJECTIVE
ZOOM 6000 MICRO 16MM - 30MM 1.74X - 90X 5X - 20X OBJECTIVE
12X ZOOM MICRO 8MM - 16MM 3.57X - 133X 5X - 20X OBJECTIVE
* Maximum sensor length (area diagonal or line scan) depends on the equipment employed.

See Also:
DIC Operating Instructions