Micro-maize and Cross-Polarisation

2013-01-15

 
 

I believe that many a photographer with interests in macro photography has tried to photograph through a microscope and found this a rather frustrating exercise. Even with the better microscopes, images become flat and the very small f-ratio will be merciless in revealing all kinds of impurities and flaws. That be in/on the sensor, the microscope optics or the object glass.

Anyway, I stumbled over this slide from a kids' plastic-fantastic microscope set that contains everything - apart from decent optical components.
 

 

Figure 1.
 

This is a thin section of maize tissue and I was tempted to photograph it. So thought, so done: A cardboard tube with a polarizing filter on top and an LED head lamp at the bottom constituted the base of my microscope and my faithful Tamron Adaptall-2 Model 01A Continuous Focusing Macro lens would serve as microscope optics together with a soligor 32mm extension tube and a Tamron SP F-series 2X Teleconverter. A polarizing filter mounted on the lens completed the set-up:

 
Figure 2. Still images here and in the following have been shot with my Pentax K200D. Sample images have been taken with my Pentax K-5.
 
Of course, this is only macro to a mild degree, (M = 2:1), but now take the AV-signal and feed it into your PC by means of a video grabber (or, feed the HDMI signal into your TV set if you want extra quality). With 1X to 10X magnification in live view you then have quite a decent digital microscope:
 
 
Figure 3.
 
And you may of course also photograph your object directly. With a bit of cropping, you will actually achieve something with the resemblance of a real microscopic photograph:
 
Figure 4. 100% crop, downsized.
 
But what about the cross-polarization then. Where does that come in? Well, on another page, I have shown, how cross-polarization may be used to remove or control reflexions in macro or close-up photography:

Cross-Polarization and the - Completely - Lost Reflexions

Many will also know the effect of cross-polarization from variable neutral density (ND) filters. But fewer may know that many transparent solids have the property that the can twist the polarization angle when polarized light is shone through the material. For example, take this transparent ruler placed on top of a polarizing filter:

 
Figure 5.
 
Now turn the polarizing filter on the camera lens until you have maximum extiction of the light passing through both filters. As you see, the light passing through the ruler has not been attenuated to the same degree. Only it has become more vividly coloured:
 
Figure 6.
 
The colours are due to inner tensions in the material that vary locally in the material. These local variations in material properties lead to different amounts that the polarization angle will be twisted for light of different wavelengths.

Could I use this effect for my "microscope" pictures as well?

And the answer is 'YES' so here comes a series of pictures from maximum extinction through maximum transmission of light. But first one note on the images in general:

The use of bluish, almost monochrome LED light results in rather flat images with a distinct blue colour cast.

 
Figure 7. Maximum extinction. JPEG straight out of the cameera. 100% non-resized crop.
 
Fortunately, the light isn't entirely monochromatic, and a simple stretching of levels will enhance both contrast and colour. So, this I have done in all of the following figures, as well as in Figure 4 above. You should also note the myriads of small withe dots: Unfortunately, Cross-polarization is also very capable of showing all the flaws in this cheap object glass.
 
Figure 8. Same image as Figure 7 above but with levels stretchhed
 
Figure 9.
 
Figure 10.
 
Figure 11.
 
Figure 12.
 
Figure 13. Very near maximum transmission.
 
Figure 14. Just passed maximum transmission
 
Note, how certain cells appear to be completely "dead" (which I acually also presume that they were, when the tissue section was made) while other cells appear to have active structures and components that twist the light and change colours in step with the degree of cross-polarization. One also notes how there are changing shadow effects that result in an almost 3-D like appearance in certain situations.
 
This way, one may further dissect one's samples and have structures and features enhanced that would otherwise not be so readily discernable.
 
Indeed, polarized light is a wonderful thing.

 

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Copyright 2013 - Steen G. Bruun