Commercial and makeshift retarders
Alan Wood
Like many amateur microscopists, I use polarising filters and retarders to add colours to crystals, minerals and various biological subjects. I started with a photographic polarising filter as the polariser (below the specimen), a disc of polarising film as the analyser (above the specimen), and plastic film in slide mounts as retarders (just above the polariser).
Simple polariser, analyser and retarder
Over the years, I have found bargain-priced Olympus polarisers and analysers at Club meetings and on eBay:
Olympus A-POL polariser (left) and B-AN analyser
However, until recently I have only used makeshift retarders that I have cut from a variety of plastic films; they go between the polariser and the analyser. Then at Microscopium I was fortunate to be given the Olympus AH-TP530-2 full-wave and AH-TP147-2 quarter-wave tint plates (retarders), so I wanted to compare them with my makeshift retarders. The Olympus tint plates fit my BH2-KP intermediate polarising attachment (from the 2018 Microscopium) and they also fit my BH2-MAN360 Precision Analyzer for reflected light.
Olympus full-wave and quarter-wave tint plates
The tint plates slide into the analyser that slides into the BH2-KP intermediate attachment that goes between the head and the stand.
Olympus tint plate, analyser and BH2-KP attachment
Olympus BH2-KP intermediate polarising attachment on a BHT stand
I make the card annuli for my makeshift retarders from Kellogg’s Corn Flakes packets using a compass cutter (easy to find on eBay or Amazon or in art shops), and attach the plastic film using PVA adhesive or double-sided sticky tape.
Compass cutter and annular card mounts for retarders
Makeshift retarders
The plastic film comes from various sources, including the wrappers of CDs and greetings cards and the windows on boxes containing cakes and quiches. It is easy to experiment with films because you do not need to mount them, you just put them on top of the polariser and rotate them. If you like the effect, then you can mount them.
It is easy to rotate the makeshift retarders because they just sit on the light output in the base of the microscope. The Olympus tint plates cannot be rotated, which limits their use for pictorial effects, but I can rotate the specimens on the BHT microscope that I use at home because the standard BH2-SVR stage rotates through 270°.
Makeshift retarder resting on polarising filter
I used to use glassless 35 mm slide mounts to hold my makeshift retarders, but they are a bit too small and I have to be careful not to cut off part of the image. Larger slide mounts used to be readily available, but they are scarce and expensive now.
Makeshift retarders in 35 mm slide mounts
The slide that I used for the following examples was a thin section of quartz diorite by an unknown mounter.
Thin section of quartz diorite
I took the photomicrographs with no polarisers, with crossed Olympus polariser + analyser, and with crossed Olympus polariser + analyser plus various retarders, using my Olympus BHT microscope with an SPlan 4× objective and Canon EOS 5D Mark II digital camera.
Quartz diorite, no polarisers
Quartz diorite, crossed polarisers
Quartz diorite, crossed polarisers plus Olympus quarter-wave retarder
Quartz diorite, crossed polarisers plus Olympus full-wave retarder
Quartz diorite, crossed polarisers plus makeshift retarder
Quartz diorite, crossed polarisers plus makeshift retarder
Quartz diorite, crossed polarisers plus makeshift retarder
Quartz diorite, crossed polarisers plus makeshift retarder
Quartz diorite, crossed polarisers plus makeshift retarder
Some of the makeshift retarders give interesting results, but none of them are as spectacular as the full-wave retarder. The Quekett Shop sells 2-inch squares of full-wave retarder that should provide very similar results to the expensive Olympus version.