The Goerz 3D Condenser – many Surprises?

The primary mystery surrounding the Goerz 3D condenser is, "What is this thing supposed to do, and, how is it supposed to do it?"

Fortunately, careful testing of this 'Mysterious' condenser has led to a better understanding of its intended workings. However, not only that but this testing has also uncovered a number of surprises surrounding its design and use.

All of this hinges on understanding the functions of strange plate located in built into the bottom of the condenser, a few mm above the Condenser Iris. This plate has a clear central strip (or, 'slit') about 3.5mm wide separating two partially-silvered areas, which exhibit about 25% light transmission, and together fill the remaining aperture area.

The key to understanding this Condenser seemingly lies in understanding the intended function of this central slit.

To better appreciate this, consider a normal condenser where the Iris is closed down such that the objective aperture (as seen in the rear of the objective) is only about 1/4 to 1/3 open.

Under these conditions the effective NA of the objective is reduced considerably (to just a bit more than 1/2 of its rated value, actually), the image contrast is increased, and also the 'depth-of-focus' in the Object plane is increased somewhat. However, all this is accomplished at the cost of object resolution, which is reduced to about 1/2 of its maximum value. Even worse, this form of masking results in an image that is objectionably harsh and prone to various undesirable artifacts.

Now, consider what happens if the normal, circular Iris opening is replaced with a slit opening of the same width, but whose length spans the full aperture of the Condenser…

In this case we might expect full resolution in the direction of the length of the slit but also an increased depth-of-focus in the direction of the width of the slit. In this way we might hope to achieve improved image contrast and 'depth-of-focus' while sustaining only minimal loss of resolution. However, although image brightness is somewhat better than with the minimally-open  iris method (above) the image is still a bit darker than normally desired and some image artifacts may still be observed.

Now, making the "sides" of this slit semi-transparent, instead of fully opaque, would seem to offer the prospect of moderating the least-desired effects – increasing image brightness and minimizing possible image artifacts, while also allowing the device to be used "normally", if desired, with little loss of imaging performance. This seems to be the Goerz approach, based on examination of the device and initial testing.

So, this is the current thinking on how the Goerz might have been intended to work – no magic, no Mystery; just some basic physical optics!

In other words, the "3D" effect seems to be mostly marketing hype for a slight, directionally-biased increase in 'depth-of-focus' resulting from the use of a somewhat unique, built-in slit aperture.

Since it was not practical to remove the critical 'slit plate' from the Goerz condenser, testing of this concept was performed on a similar (but totally normal) common NA 1.25 Abbe-type condenser.

Different Slit-type masks were compared with normal Bright field and with common COL-type masks in a preliminary effort to determine whether Slit-type masking provides any observable benefits, and, indeed, any detectable "3D" effect.

The initial results of these tests are summarized in the photos below…

(Click anywhere on the above image panel for a larger version.)

The there are two big surprises revealed in these photos: (1) despite its novel design and construction the Goerz design actually performs rather poorly as a "3D" device, barely equaling the depth-of-focus performance of a number of more common alternatives (as shown), and, (2) the real surprise lies in the number of alternative methods which easily equal, if not surpass, the performance of the Goerz device, both in terms of resolution and image contrast!

In other words, despite its novel design, elaborate construction, assortment of optics (optional tops) and unique optics-offset adjustment, the device seems to do nothing which cannot be accomplished by simple alternatives.

It only genuine attractions are its low-power (NA ~0.5) Brightfield mode (using no top), and its Darkfield modes, all of which are mildly compromised by the presence of the non-removable "3D" slit gizmo embedded in the basic design.

As far as possible "alternatives" are concerned, it may be noted in the photos that these all seem to offer results which are basically similar, differing mainly in the presentation of image details, as well as ease-of-implementation and ease-of use. All of this, of course, being potential food for a future post or two…

See the post of March 28, 2017 (below) for more on the Goerz '3D' Condenser. 

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