If you are klutz like me, it’s hard to use most of the techniques one can find on-line for making your own Rheinberg and other microscope filters. Many involve cutting bits of electrical tape or colouring in bits of plastic with marker pens. All credit to those who have the skills to make neat filters this way. The best technique that I came across was to print the filters on overhead projection (OHP) film. OHP films are almost universally available in shops that sell artists’ materials and stationery. Most microscope DIYer’s suggest printing filters with an inkjet printer, cutting them out with a pair of scissors and then placing them in your microscope’s filter holder. This technique works and using a free design programme like Inkscape (it’s wonderful!!), it is possible to create almost any pattern for a filter. However, I thought the OHP technique could be improved; the filters could be denser in colour, the ink on their surfaces protected from smudging/fingerprints and it would be great if they were less fiddly to handle. Even better, it would be nice to be able to slide the filters in place, have a set on a single piece of plastic and thus switch between them as one changes magnification.
The first ‘problem’; the density of the colour, is easily solved. Even the best of colour inkjet printers will produce colours that are a bit anaemic. The way round this is to pass the OHP film through the printer two or more times – I found three to be optimal. To do this you need to adjust the printers paper guides so the paper always follows exactly the same path. The other tip is to always let the OHP film dry before passing it through the printer again. The drying time is likely to vary with the film ones uses and also the printer. The first pass through the printer loads the surface of the film with ink but it dries quite quickly, on the 2nd and 3rd pass, the surface begins to saturate and things take longer. I hastened the drying of the ink by using a warm stream of air from a hairdryer. Using this multi-pass technique, you can get some truly intense colours.
The next problem is that the bits of OHP film are floppy, difficult to handle and eventually end up smudged with finger prints or the ink becomes smeared. I reasoned that it might be possible to protect the inked surface by gluing the discs to some acrylic. I had a cheap picture frame with acrylic ‘glass’ in it. This stuff is about 1mm thick and can easily be cut by repeatedly scoring and snapping. However, if you want circles, it can be cut with a compass cutter or if you know someone who has a laser cutter, it cuts like butter. I later bought a pack of 10 sheets of 1mm acrylic via Amazon. These sheets were protected with polythene on both surfaces – this is great because you can remove it as you need to while leaving it on the other surface, stripping it off only when the filter is finished.
My Bresser Infinity Microscope (see review at: https://petermobbs.com/micro-2/ ) has one intensely annoying feature; it has a push in filter holder and it is a pain-in-the-bottom to constantly be taking this off to drop in a new filter. To fix this, I designed and 3d-printed two filter holders; a swing-in one and another to allow for homemade sliding filters. The later consists of a slot through which a rectangle of acrylic can be slid. In fact, I created a filter holder that can house two such sliding filters, one oriented at 90 degrees to other. This arrangement allows for a gradient filter in one slot and an adjustable stop for oblique illumination in the other. This arrangement mimics what is called a Universal Gradient Filter (https://www.photomacrography.net/forum/viewtopic.php?t=15142). The two filter holders are shown in the photo below and I have placed the STL files on Thingiverse.
I cut the acrylic into strips 34mm wide; the slots in the filter holder that I printed are 35mm wide. In order to give myself something to handle the strips by while I glued the OHP film to them, I made them longer than the space that would be occupied by the filter patterns ie each strip had two sacrificial ends.
So, how to attach the OHPs to the acrylic glass and how to take advantage of the ability to slide them in? I used Inkscape to generate the filters I wanted and printed three of them side-by-side on OHP film. I included a rectangular outline to enable me to align the OHP film with an acrylic strip. I cut out the OHP film about 0.5cm bigger than the alignment rectangle. This margin will provide somewhere for the excess glue to go. I placed the OHP film ink-side up on a sheet of paper and ran a line of Loctite Activator for Plastic along the left- and right-hand margins of both it and the surface of the acrylic sheet. Then, I very carefully ran a *very, very* thin line of non-blooming thin super-glue near the LHS and RHS edges of the OHP strip and holding the acrylic by the sides and taking care to avoid the glue, I pressed it down onto the OHP ensuring that the OHP and acrylic were properly aligned. The bond will be very quick so its important to get the alignment right. I then set the assembly aside for the glue to harden and after a few minutes, cut the OHP film away from the edge of the acrylic strip with a craft knife. If a little super glue gets onto the filter pattern it has little or no effect on the function of the filter. If one is feeling less brave, I think two small tabs of Sellotape could be used to achieve the same end.
There is no end to the filters you can make this way. I have made Rheinberg, darkfield, graduated and oblique filters and if you use a sliding system you can have filters the intensity or pattern of which changes as you slide them through the holder. If they are assembled with reasonable accuracy, the filters should be well-centred. It is very handy to have a sliding filter with say dark-field patches for 4, 10, and 20x objectives and be able to slide the new filter in place as you change objectives. I have plans to make a filter holders with a detente to aid in positioning but I haven’t found that necessary – I just slide the filter in until I get the effect I want. If necessary it can be fixed there with a bead of Blue Tak.
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