Rheinberg filter test

Jan l'Amie · Post by **Jan l'Amie** » Thu Nov 17, 2011 12:07 pm

No, I bought there a telescope lens ones. I will take a look at their site. Thanks.

Charles Krebs · Post by **Charles Krebs** » Thu Nov 17, 2011 2:38 pm

Jan,

You want the colored center to be a size that will just "fill" the rear aperture of the objective. Ideally it would be a different size for each objective (NA) but in practice you can usually use one size with a couple of different objectives, as long as the center color is large enough to at least fill the back aperture (but still allow "white" light to reach the subject from the outside edges. Be sure to leave the condenser aperture wide open). A center spot that is somewhat larger is OK, but too large and then you get no "white" light from the clear outside section hitting the subject... as in the "all blue" example you recently posted. (Too small and then "white" light can directly enter the front of the objective).

"Trial and error" will work, but as you have found it is no fun to cut those small circles. There is a way to test and find the best size for each of your objectives. This can be done pretty easily by making a test target.

First you want to be sure your condenser is properly centered and is also focused at the correct height. (Do you know how to do this?. And it should be pointed out that even if the condenser is the proper height, you may find that in actual usage you need to vary the condenser height slightly up or down to get good darkfield/Rheinberg)

I made and printed up a multicolored "bulls-eye" target of concentric circles ranging from 10mm to 30mm in diameter (on regular, lightweight printer paper). A single target can be cut out to the size of your filter drawer, and temporarily placed in the same location where the darkfield stop or Rheinberg filter will be. Put a "blank" slide on the stage and focus on a piece of dust on the slide top surface (or any small subject or mark you make on the slides top surface. You just don't want the view at the rear focal plane blocked by a large opaque subject). Then you remove an eyepiece and examine the back focal plane of the objective. You can pretty easily see (via the color coded rings) the size needed. (A small translucent rule in place of the target would work fine as well, but you might need a Bertrand lens or phase telescope to read it). You want to select a ring size where, as you move your eye around looking down the (empty) eyepiece tube, the ring is just outside the field of view from all angles. You can do this for all of your objectives. Typically the ones that can be accommodated with a dry condenser are 10X-40X. Lower powers are trickier, as are objectives with an NA over 0.65. (If you want to do darkfield with high NAs (over 0.65) you usually need to go to an "oiled" darkfield condenser.

I have a jpg of the targets I used posted here if you are interested (you can "right-click" it, save it, and print it with no resizing):
http://www.krebsmicro.com/forumpix/10to30.jpg

I gave up on lab "cork-borers" to make the center spots. Some of the acetate "gel" material is very tough stuff, and it was hard to get a clean edge without using so much force that it damaged the borer. Automotive "punch" sets are much stronger but they are not cheap and usually come in limited sizes. These days I just use a very fine "surgical" scissors and cut the circles by hand (after marking the circle on the gel with a template). They really don't need to be absolutely perfect circles, you just need to be sure they fill the back aperture and no light "leaks" into the objective from an uneven circle edge.

Jan l'Amie · Post by **Jan l'Amie** » Fri Nov 18, 2011 12:33 am

Charles thanks very much for this great info. I have copied your circles and will make some prints. This weekend I'm very bizzy with all kind of things (Sinterklaas comes to our town tomorrow) but next week I let you know the results. I will try to find some punch pipes for making the spots.

Pau · Post by **Pau** » Fri Nov 18, 2011 10:51 am

Mitch640 wrote:
I made this small circles with sharpened pieces of aluminium pipe and a hammer but this worked only once.
Punch Set?

This set seems to have all required diameters to make Reinberg filters:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0443028347

Jan l'Amie · Post by **Jan l'Amie** » Sat Nov 19, 2011 12:09 pm

I've tried a new combination of filters.
The first photo is made with a moonshaped oblique light with blue centre stop filter in combination with a blue outline circle.
The second is made with the same oblique light filter in combination with a red outline circle.

MikeShaw · Post by **MikeShaw** » Fri Dec 30, 2011 6:56 pm

About the punch sets - I've found two additional sets. I have an article that shows pictures and tells where to get these. One set is by Axminster (England),the other set is by Osborne (USA).

See my article in this month's Micscape Magazine:

http://www.microscopy-uk.org.uk/mag/indexmag.html

Scroll down to the 5th article - How to Make Rheinberg Filters.

Happy New Year to all!
Mike Shaw

rjlittlefield · Post by **rjlittlefield** » Fri Dec 30, 2011 9:18 pm

MikeShaw wrote:See my article in this month's Micscape Magazine:

http://www.microscopy-uk.org.uk/mag/indexmag.html

Scroll down to the 5th article - How to Make Rheinberg Filters.

Thanks for the article, Mike.

For future reference, the direct URL is http://www.microscopy-uk.org.uk/mag/art ... nberg.html .

--Rik

Litonotus · Post by **Litonotus** » Fri Dec 30, 2011 11:39 pm

I think printed filters are much easier to do... you can get the colour stop and every other coloured part at once (:

I achieved all those background colours using only ONE filter which is still not perfected yet - that's why the 3d effect and contrast are not as good as could be.

MikeShaw · Post by **MikeShaw** » Sat Dec 31, 2011 7:26 pm

Thanks for the link, Rik -

Nice assortment of colors, Litonotus. What did you print this on?

Rgds,
Mike

Litonotus · Post by **Litonotus** » Sun Jan 01, 2012 4:29 am

in the filter above I used colour foil pieces and that requires some work, but the same may be made by printing, and it is much easier - just a colour print on the transparent foil. I did that handmade one only to test my new solution.