3D Image Enhancement made EASY – Part II.

Comparing Simple Contrast Methods

A recent post (April 25, below) has revealed a very simple method of optical contrast enhancement, "Radial 3D Effect", which also adds a measure of 3D-like effect to an overall contrast improvement.. The method is very easy to implement and to use, functioning with a wide range of common objectives and yet requiring no special adjustments with objective changes.

Yet, in spite of these these and other operational advantages (described below), the basic question remains, "Just how well does this new method work?"

In this post we begin to address that question, starting with a direct comparison of images from this new method with those from two common alternative methods: (a) Simple Brightfield, and (b) Circular Oblique Lighting (or, COL). (These other methods also share the traits of simplicity, ease-of-use, and of not requiring any special objectives.)

So, how do these methods actually compare? 

To see, we can begin by examining the image set below… 

Click anywhere on the above image set for larger versions. 

The top image is in Brightfield mode, with illumination by the Kohler Method, with the Condenser Iris set to about 100% of the full objective aperture. This is to serve as the "Reference" image. It reveals a reasonable amount of detail but one could hope for greater contrast. Also, the image appears "flat," revealing little of the object surface texture.

The middle image is basically identical, but with the addition of the Radial "3D" Effect mask discussed earlier. Note that this image shows not only increased object contrast and detail, but also reveals object surface contours, especially evident in the specimen on the left.

The final image was made by replacing the "3D" mask with a standard Condenser phase ring (annulus) to produce COL contrast. Note that here there is also increased contrast (relative to Brightfield), but the "3D" effect seen in the middle image is absent.  However, much unlike the Radial 3D Mask method, image results with COL can be quite dependent upon the exact choice of annulus, as well as 'annulus-specimen' matching. Thus, with COL, each objective potentially requires using a different annulus. (The 'Radial 3D Mask' technique has no such sensitivities.)

One important factor, not shown by the above images, is the difference in overall image brightness which occurs with each of these methods.

Brightfield, of course, offers the highest image brightness level, with the Radial '3D' Mask method running second with somewhat less than half that level (depending on the exact diffusion material used). The COL method, however, is far behind this with overall brightness at 15% or lower, depending upon the size and width of the Condenser annulus selected. (Note that some specially-made COL rings may support slightly higher overall image brightness levels.)

Condenser iris Effects

Both Brightfield and Radial 3D methods also support a basic level of user control over "depth-of-focus" in the specimen plane, simply by adjusting the Condenser Iris opening. Reducing the opening, even by a small amount (e.g: from 100% to 80%) can result in a significant increase in not only depth-of-focus, but also in overall image contrast. These effects are depicted in the image set  below:

Click anywhere on the above image set for larger versions. 

As expected, reducing the Condenser Iris opening in either Brightfield or 3D mode provides a slight increase in image contrast, but in the 3D mode this more significantly enhances the apparent depth-of-focus for the specimen. In general, Iris openings from 100 percent open, down to 50 or 60 percent open, can prove useful. At less than about 50% open, image resolution can begin to suffer noticeably.  

Unfortunately, the COL method (described above) does not directly support such control – with COL the only recourse is to switch to using a smaller diameter annulus, which often can have unpredictable and undesirable effects on the overall appearance of the specimen. 

Be aware that the image set presented above is not intended as definitive basis for comparing these methods, but merely as a basic indication of the results produced by each method. More precise imaging techniques for these three methods, as well as for some additional methods, will be needed to permit extending this evaluation to a more definitive level. 

Note that the above images are limited by minor variations in focus between modes, as well as a slight residual camera motion which may obscure finer detail. Both these issues will be addressed in the test setup before any additional comparison photos are produced.

This comparison will continue is a subsequent post, or two…

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