VAREL Contrasting for Light Microscopy

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Ken Ramos
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VAREL Contrasting for Light Microscopy

Post by Ken Ramos »

I recently received a message form a friend of mine who is a Zeiss Tech. He is sending me a prototype version of VAREL Contrasting objectives and condenser for my Axiostar. I thought I would post a portion of this message for those who may be interested in VAREL Contrasting. This is ICS optics so it may be possilbe that it may work on other microscopes as well, that have ICS optics, I don't know for sure. :D
Some of you may already be familiar with the well known contrasting technique called Phase Contrast. It is a technique utilized for thin, unstained specimens that are difficult to view under normal brightfield microscopy. One of the detriments of Phase is that it does not work well with thick, transparent specimens or through plastic vessels. To solve these problems, Zeiss has introduced and patented a contrasting technique known as VAREL (an abbreviation for VARiable RELief).

I would like to give a quick explanation of what is involved with resolving an image that has a proper balance of contrast and resolution:

Stained specimens of high absorption contain small and large structures that diffract, and also absorb, incoming light. We recognize this absorption when we see color. Detail and contrast are generated by the interference of non-diffracted wave fronts with those that are diffracted through the specimen. For microscopy’s purposes, the optimum contrast is generated when the difference between the diffracted and non-diffracted fronts is about 1/2 wavelength.

Previously, one who wished to view thick, transparent specimens, or use plastic vessels was limited to DIC (Differential Interference Contrasting) or Hoffman modulation, as these were the only techniques available that increased contrast without sacrificing resolution. However, both options are very expensive.

For those that are unable to dish out the big bucks for DIC or Hoffman, VAREL provides a satisfactory compromise between cost and contrasting performance. It utilizes a modified Phase objective and a specific crescent slit annulus. The VAREL ring allows the diffracted orders of light to pass right through the objective, without suppressing them.

Another nice aspect of this technology is that it allows for two contrasting techniques within one objective (VAREL and Phase). Switching between these two techniques is quick and does not disturb the specimen.

Normally, VAREL is an option that is only available on the Zeiss Axioskop 40, which in itself (without any of its available options) can be a pricey instrument. However, I’ve been working on it, and I believe I’ve gotten the VAREL components so that they work well with the basic Zeiss Axiostar Plus. I am excited, because this would allow someone—for around $1300—to add a very nice A-plan 10x, 40x Phase and VOREL package to their existing Zeiss Axiostar Plus microscope. (Or, for a bit more, one could include the LD 20x and LD 32x objectives as well).
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Kenneth Ramos
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