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Video clips in 'avi' format included. Click here for an animated gif version
if your computer doesn't support 'avi'.
Viewing slides of crystals under the microscope between polarisation filters never ceases to appeal. However, one aspect that isn't fully appreciated in a still image, is the kaleidoscope of colour as the crystals and/or polarisation filters are rotated with respect to each other. A short animation in a web page can convey this aspect quite well and two approaches are as follows.
1) Capture a live sequence from a video camera attached to the microscope while the filters or crystal slide are rotated.
2) Take a sequence of stills at different angles of the slide to the filters and compile into an animated gif or movie with readily available software. This has the benefit that each frame can be 'tweaked' before compiling and potentially gives the higher quality but also more time intensive. A still frame capture device e.g. a digital still camera can also be used if a video camera isn't available.
The first method seems the most convenient and the quicker, but with my set-up I found it hard to achieve an even rotation of the filters in real time. Vibration was also a problem when the analyser filter was rotated. So I adopted the second method instead. (Other Micscape contributors have demonstrated that creating video clips from stills can work remarkably well e.g. see Martin Mach's animated radiolaria images).
The second method may require a lot of stills to create a smooth (high frame rate) video clip. But to roughly gauge the effect, something more modest can be tried e.g. say eight stills. I marked the base of the vertical monocular tube of my Biolam microscope (shown right) in ca. 45º (and 22.5°) divisions with the analyser filter mounted inside. This allowed eight or sixteen stepped image captures for one full rotation of tube/filter (e.g. 16x22.5 = 360º).
Two of the early trials using this method are presented below.
Musk ketone ('Biosil' prepared slide), 3.5x objective, no eyepiece. Eight stills at 45º intervals compiled into a four frames/second looped video clip (81kB). Compiling the same images into an eight frame/second clip and looping created a less pleasing result.
Cholesterol acetate, ('Biosil' prepared slide), 9x objective, no eyepiece. Sixteen stills at 22.5º intervals compiled into a four frames/second looped video clip (245kB). A smoother result with more frames. The original image is almost monochrome, (false colour added for effect), but the clip shows the attractive geometric sequence often seen in these sort of slides that a single image can't convey.
Before creating a video sequence from a chosen subject, it's well worth spending some time rotating the filter or slide while viewing by eye, to judge which subject field and magnification gives a good effect. An area of the slide which gives one stunning still image may not give a nice sequence for a full rotation. Large areas may go black or clear for example with rotation depending on the crystal thickness etc. Finding an area on the slide with one or more radial features (e.g. as above) can work well to give a feeling of movement in the animation. The sequences the author tried with e.g. just strong diagonals didn't work as well.
Hopefully, the trials above give a reasonable idea of the effects possible, but the really smooth dramatic result which the author was hoping for requires a lot more stills ... but at the expense of longer video clips and larger files.
If you have created video clips in this way or by other routes and would like to share them on Micscape or can provide the web link, please let us know.
Comments to the author Dave Walker are welcomed.
(N.B. No man-hauling of sledges, husky dogs or warm clothing is required for this sort of 'polar exploration'!)
Additional Notes
1) Most of the still image capture programs (e.g. the 'Snappy' software the author uses), allows extensive image control before taking the picture i.e. brightness, contrast, colour etc. It's a good idea to adjust the image to create the effect desired before image capture, as this saves extensive individual frame image-processing afterwards. The author increased the colour saturation beyond that seen in the real image for visual effect before capturing the stills.
2) There are a number of utilities available on the Web to allow images in e.g. bmp or tga format to be compiled into an avi movie clip. The author used Paul Roberts' bmp2avi programme (ca. 50kB) which works a treat. The still images just need to be named sequentially, and the programme will do the rest with the frame rate under user control. A freeware Windows or DOS command version is available.
3) For the above trials, the author rotated the analyser filter/monocular tube, while keeping the crystal slide and polariser (in the filter tray) stationary. A different approach is to keep the filters in 'crossed polar' mode and rotate the slide - if a rotating stage is available. This requires a very well centred stage, but has the benefit that if the stage is graduated in degrees this can be used to accurately step the angles between still captures.
4) If rotating the monocular tube, some way of maintaining a light tight seal to the digital camera is required. The author found the blue foam with square hole found in coverslip boxes fitted well round the tube and gave a good flexible seal to the video camera. This also allowed small focussing adjustments. (See image right).
Other Micscape articles
Articles introducing the use of polarising filters for studying crystals and other subjects are here and here. Typing in the keyword 'polar' in the Library Search engine (link below) will give many articles showing images using polarising filters.
Published in the June 2000 edition of Micscape Magazine.
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