An Article In Meteorite-Times Magazine
by Tom Phillips
A New Wave of Meteorite Examination
This is too cool to keep secret!
I often say "with the addition of a 1/4 wave retardation filter" in my cross polarized light micrograph descriptions and I use retardation filters in nearly all my cross polarized light work. One of my micrographs using a quarter wave retardation filter is in the Zeiss 2008 Calendar. It is of a Lunar thin section, Jim Strope's NWA 482 and the colors are what makes it eye catching.
I have been asked what a wave retardation filter is. First off, it is also known as a wave plate, retardation filter, glimmer plate, tint plate, quartz compensator and a few others as well.
There are such things as a full wave plate, half wave plate and quarter wave plate. A wave plate acts on light of a certain frequency and slows down (or more accurately, delays) that frequency of light the amount (in wavelength fractions) it was designed for. Light travels more slowly in certain materials (Birefringence), and the thickness as well as the composition of the wave plate determines which light frequencies are acted upon (plate composition) and by how much (plate thickness).
More information on how the retardation filters work can be found on this great Wikipedia page http://en.wikipedia.org/wiki/Wave_plate You can link to other related articles and read for hours, which is undoubtedly, better than me trying to relate it.
Wave plates have been rather expensive for microscope enthusiasts for the simple reason of economies of scale. If they were ever mass produced, they could be made affordable but with such a limited use, who would mass produce them?
I have hunted down surplus wave plates on eBay only to find they were designed for optics bench laser work and function in the 1500nm range which is great for red lasers and infrared but they didn't do much in the visible light spectrum which was what I was looking for in my meteorite examination.
I finally found a solution, and I have looked for years! The Sony Play Station 3 uses a Blue Ray disk reader. These devices have a laser diode that emits light at 405nm (Violet) as well as one that operates at 660nm (Red) combined with 780nm (Near IR). This combination through one reading lens (objective) requires some cool optics. In the combination of cube polarizers, and many lenses, there is a quarter wave plate. The plate is small (5mm) but that is big enough to use on a microscope if you apply a little ingenuity and the best part is, it retards the light at the violet frequencies. This makes colors come alive when used in conjunction with cross polarized light. It will draw out complex structures that are completely hidden in ordinary cross polarized light.
Sure PS3 systems are expensive but enough reader parts are removed for repair work to be occasionally sold reasonably on eBay. I have seen them often and, if you don't mind that the laser diodes have been removed, they are not expensive. (The diodes are the sought after part of this device for most buyers.)
Right now I am working with a beautiful thin section of NWA 2977 (Lunar) and NWA 2986 Shergottite (Martian). The material was supplied by Jim Strope and the thins were arranged (To his new standard of 1/4 micron DPU) by Jeff Hodges. This is a story in it's self and I will cover it with many more micrographs in an upcoming Micro Vision. Right now, just a big "Thank You" to Jim and Jeff.
I also am including a couple shots of NWA 4734 (Lunar) a thin section of Jeff Hodges.
This set of images is of NWA 2977 Lunar at 400X with (and with out) the wave plate.
Standard bright field (no polarization or wave plate).
Standard full cross polarized light (Xpol) with no wave plate.
These are with the addition of the PS3 quarter wave retardation plate. The color change is a result of rotation of the wave plate, polarizer or sample (some times all three) Yes it gets time consuming, but any color hue is possible!
This next set of images is NWA 2986 Shergottite at a magnification of 400X with the wave plate. I can fit in only so many images in one article so these are just some of my favorites. Keep in mind that the extreme variations in color are obtainable in almost all material on the thin section and not just certain materials such as olivine (which is the most responsive to standard cross polarized light).
This final set is NWA 4734 (Lunar) also at a magnification of 400X using this wave plate. This is a slide that will warrant it's own Micro Vision write up but I had to show off some of these lunar melt glass shots. This glass is nearly clear in standard methods.
So, what does it look like? Glad you asked! Here it is with a cm cube for scale.
This is all it took to use it. I started out by just setting it on the polarizer but once I found out the idea was sound and the results so impressive, I placed it on a 1 1/4" rubber washer with a 3/8" hole and fastened a lever handle to it so I could easily rotate it while keeping it centered. Fabrication took less than 5 minutes. The microscope I am using it on is an aus Jena Fluoval but the idea is the same for all polarization microscopes. I did find, however, the wave plate must be between the polarizer and analyzer (Top polarizer). If it is placed before or after the two polarizing filters, it has no effect.
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The Tom Phillips Microscopic Meteorite Photography and Gallery