This morning I made a few technical tests with the illuminator. I realized that there ought to be a conjugal image at the position of the aperture diaphragm [AD]. (As an aside: could anyone explain why in reflective köhler the field aperture comes after
the AD?). I tried to open the illuminator to place a screen at that position, but it turned out quite hard to do. The tube of the illuminator unscrews in three parts (careful! the aperture blades can become dislodged, so always keep the iris facing up). The AD is not the end of the first part, but the start of the second. It's therefore not feasable to place a disk of paper there and see what's projected.
Instead, I hung a sheet of note paper at a comparable distance after the first (lampside) part of the tube. But first I bent a piece of wire over the led to get a good shadow, for the image of a burning led is featureles and only hurts the eyes. Whith a little adjusting I got this:
File comment: Image of the LED with wire over it, near the aperture diaphragm
_WDW7612.jpg [ 125.76 KiB | Viewed 3848 times ]
The image moves forward and backward with contrary movements of the led relative to the collector lens. So, whatever the details, the concept seems sound to me.
Still I don't know how to position it exactly. Next I did the following - I don't know if it makes any sense, but if so, kindly explain me why:
The illuminator, when held in hand, projects a light circle on my desk. By moving it up and down I can get the blades of the FD into focus. This is about 10cm above the desk top, which judged by eye cannot be the back focal plane of the objective. I then adjusted the led position such that together with the iris blades I got the wire shadow in focus too.
With this adjustment I tested the evenness of the illumination. I made a photo of the surface of a piece of white acrylic and then plotted the luminace over the diagonal from top-left to bottom-right. Here's the result (blue line), compared with a similar test using transmtted light:
File comment: Luminance measured of the field diagonal. Blue: epi-illuminator; red: transmitted light.
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There's about one stop of vignetting towards the cut-off of the FD. It's asymmetric because my camera still is slightly off-centre. Note that I forgot to remove the iron wire...