In 2009, I was intrigued by Olympus objectives marked 'RP' on eBay and I wanted to know more about them.
They correspond to a technique called "relief phase contrast" whose description I found on an advertising brochure (ref. 1). Intrigued, I acquired them and I tested this technique before comparing it to others found useful under the same circumstances. I used an inverted microscope with plastic culture dishes which do not work with interference contrast.

I) Material

I am equipped with Olympus microscopes and for a long time, my main model is  the BHS. With the help of internet auction sites, I have added accessories and also bought other second-hand microscopes.

I bought in particular in the United States a few years ago, an Olympus CK2 inverted microscope. It is 110V but I added a 110-220V transformer for use in France!

 

I used it first with objectives A4Pl, A10PL with the phase rings supplied and a LWD20PL (this last one has a collar for regulation of thickness of object cover 0 - 2mm).

As remarked at the start of this article, last year I
bought two Olympus objectives:

10x achro RP NA 0,25 and
40x LWDCDPlanFRP NA 0,55

Note that the 40x is designed for a thickness of object cover of 1mm!
While the 10X is for a classic 0,17.

I made rings for phase corresponding to those in the original condenser of the CK2 but this one had an insufficient opening for the 40X.

Now at the same time, I bought second hand two partial Hoffman Modulation Contrast sets for the Nikon Diaphot. So I have a whole set of lenses and two Nikon condensers of NA 0,5.

One was associated with the objectives for contrast of Hoffman:
Plan 10x 0,25
LWD20x 0,40
LWD40x 0,50

It is now my usual set on this microscope.     

The other condenser with four openings in turret, serves for do-it-yourself work and in particular for this relief contrast. I made phase rings with cardboard. 

A brochure associated with the Olympus CK microscope (ref 1) gave me a model to make the phase rings adapted to two objectives for this "Relief Phase Contrast ( RPC)".

It is necessary to note that the brochure dedicated to the more recent microscopes CKX (ref 2) describes a different technique "Relief Contrast" which seems very close to the contrast of Hoffman ( HMC).
  

Did the patents of this technique become exploitable by Olympus? I have no details of the story, but my tests by setting up the two techniques cover two generations of equipment.

2) Results of the tests

To study the techniques images were taken with a digital Nikon camera that show.

the classical oral epithelium  cells.

Pictures are all resampled for 500 pixels length.

2nd series (this time the full field with the projection NFK2,5x eyepiece which was used, is kept):

3) Discussion
These techniques are consumers of light. A good part of the cone of lighting is blocked but there is a phase plate or a modulator in the objective. 

The images in RPC have an overall similar look to phase contrast, but the halo is present only with the latter one. It gives a light impression of relief but which has nothing to do with that seen with DIC. This technique RPC seems to me to bring little benefit. It was nevertheless proposed recently (2007) by a working doctor in Romania Joerg Piper (ref. 3).

It seems that Zeiss (ref. 4) also used a similar idea for a while (Variable relief contrast = Varel) before developing in 2003 an interference contrast method suitable for plastic dishes i.e. PlasDIC. (Ref. 5)

Differential interference contrast ( DIC) is much preferred every time we can use it.

The Hoffman contrast works with the plastic material dishes incompatible with DIC. It is the current technique with my Olympus microscope, named "relief contrast" by Olympus.

The techniques based on an oblique illumination are however interesting because they have only a low cost and can be implemented by amateurs.

The PlasDIC would certainly be interesting to try but it is reserved for Zeiss equipment. I wonder however if it would not be possible to realize it with a prism IC for reflected light? Perhaps it is a new aspect to explore by do-it-yourself.

Daniel Nardin.  Please feel free to e-mail me  if you have any questions or comments.

Bibliography and Internet references :

(1) Olympus relief phase contrast Brochure CK :  http://www.iolympus.cz/mikroskopy/prospekty/Relief%20phase%20contrast.pdf

(2) Olympus relief contrast Brochure CKX:  http://www.iolympus.cz/mikroskopy/prospekty/CKX2.pdf

(3) Relief phase contrast  http://www.relief-phase-contrast.com/index.html
Piper  J. : Relief-Phasenkontrast - eine universell einsetzbare Modifikation des Phasenkontrasts mit verbesserter Bildqualität, Mikrokosmos 96 / 4, 223-229, 2007

(4) Zeiss Varel  
http://www.zeiss.de/C1256F8500454979/0/877D55B12C3A9095C1256F8E003506DE/$file/varel.pdf
http://www.smt.zeiss.com/C12567BE0045ACF1/Contents-Frame/B14DDA7D7E2CA1D9C1256FB6002A5EA1

(5) Zeiss PlasDIC  http://www.zeiss.de/C12567BE00472A5C/EmbedTitelIntern/Article-PlasDic_e/$File/GIT_Arcticle.pdf