Advice please: High NA air objective with small WD

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Horst_S
Posts: 6
Joined: Sun Jan 03, 2021 12:57 pm
Location: Austria

Re: Advice please: High NA air objective with small WD

Post by Horst_S »

Chris S. wrote:
Mon Jan 18, 2021 2:20 am
...
To avoid the flat, “deer in the headlights” look others have warned of, I’ll likely, in most cases, mix light from outside the objective with light sent through the objective; in traditional photography terms, the outside light will be the key light, and the through-the lens light will be fill light. ..
This will give you a lot of illumination possibilities. With very small WD there might arise a problem because the light intensities from the two sources are so different. The light through the lens will be so much brighter that you might need to dim it with strong grey filters to see an effect from the oblique light source at all. The light sources should also have matched color temperature/spectral composition, otherwise you will get colorization effects. Another possibility might be to take separate exposures with different exposure times (illumination through the lens with typically fractions of a second and illumination from the side with several seconds) and merge them by image processing.
I have also been thinking about some kind of "light conductor" - a small glass plate (1-2mm thickness, approx. the WD of the lens) with a bore that just fits the end of the objective (to form a ring around it) top and bottom silver plated. :smt017
As Pau already mentioned - experimenting with the Epi-illuminaton may be an additional possibility.

These are some single shots (no stacks) taken with cross polarization on a metallurgical microscope with Epi-illuminator, to give you an idea what you can expect.
The images are corrected by automated flatfield-compensation only (which eliminates unevenness caused by the light source / vignetting /dust speckles), no contrast or saturation enhancement;
Objective MPlanFl 50x/NA0,8 WD1mm strong extinction:
Epi Pol 50xNA0_8 single full field.jpg
Crop 100%
Epi Pol 50xNA0_8 single crop.jpg
Metallic surface with long WD MPlanAch 5x/NA0,15; exposure time adjusted so that highlights just started to overexpose
Extinction approx. half of maximum:
coin half Ext small.jpg
Same object Extinction set close to maximum
coin max Ext.jpg

Cross polarization should also be doable with your FS70 according to the system diagram:
https://www.mitutoyo.com/wp-content/upl ... 9_FS70.pdf

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