Mystery Microscope Objective Illuminator

Rummaging through the Big Box o’ Optics in search of something else produced this doodad:

Microscope objective illuminator - overview
Microscope objective illuminator – overview

It carries no brand name or identifier, suggesting it was shop-made for a very specific and completely unknown purpose. The 5× objective also came from the BBo’O, but wasn’t related in any way other than fitting the threads, so the original purpose probably didn’t include it.

The little bulb fit into a cute and obviously heat-stressed socket:

Microscope objective illuminator - bulb detail
Microscope objective illuminator – bulb detail

The filament was, of course, broken, so I dismantled the socket and conjured a quick-n-dirty white LED that appears blue under the warm-white bench lighting:

Microscope objective illuminator - white LED
Microscope objective illuminator – white LED

The socket fits into the housing on the left, which screws onto a fitting I would have sworn was glued / frozen in place. Eventually, I found a slotted grub screw hidden under a glob of dirt:

Microscope objective illuminator - lock screw
Microscope objective illuminator – lock screw

Releasing the screw let the fitting slide right out:

Microscope objective illuminator - lamp reflector
Microscope objective illuminator – lamp reflector

The glass reflector sits at 45° to direct the light coaxially down into the objective (or whatever optics it was originally intended for), with the other end of the widget having a clear view straight through. I cleaned the usual collection of fuzz & dirt off the glass, then centered and aligned the reflection with the objective.

Unfortunately, the objective lens lacks antireflection coatings:

Microscope objective illuminator - stray light
Microscope objective illuminator – stray light

The LED tube is off to the right at 2 o’clock, with the bar across the reflector coming from stray light bouncing back from the far wall of the interior. The brilliant dot in the middle comes from light reflected off the various surfaces inside the objective.

An unimpeachable source tells me microscope objectives are designed to form a real image 180 mm up inside the ‘scope tube with the lens at the design height above the object. I have the luxury of being able to ignore all that, so I perched a lensless Raspberry Pi V1 camera on a short brass tube and affixed it to a three-axis positioner:

Microscope objective illuminator - RPi camera lashup
Microscope objective illuminator – RPi camera lashup

A closer look at the lashup reveals the utter crudity:

Microscope objective illuminator - RPi camera lashup - detail
Microscope objective illuminator – RPi camera lashup – detail

It’s better than I expected:

Microscope objective illuminator - RPi V1 camera image - unprocessed
Microscope objective illuminator – RPi V1 camera image – unprocessed

What you’re seeing is the real image formed by the objective lens directly on the RPi V1 camera’s sensor: in effect, the objective replaces the itsy-bitsy camera lens. It’s a screen capture from VLC using V4L2 loopback trickery.

Those are 0.1 inch squares printed on the paper, so the view is about 150×110 mil. Positioning the camera further from the objective would reduce both the view (increase the magnification) and the amount of light, so this may be about as good as it get.

The image started out with low contrast from all the stray light, but can be coerced into usability:

Microscope objective illuminator - RPi V1 camera image - auto-level adjust
Microscope objective illuminator – RPi V1 camera image – auto-level adjust

The weird violet-to-greenish color shading apparently comes from the lens shading correction matrix baked into the RPi image capture pipeline and can, with some difficulty, be fixed if you have a mind to do so.

All this is likely not worth the effort given the results of just perching a Pixel 3a atop the stereo zoom microscope:

Pixel 3a on stereo zoom microscope
Pixel 3a on stereo zoom microscope

But I just had to try it out.

3 thoughts on “Mystery Microscope Objective Illuminator

  1. Weird coloration might also be due to the fact that some lenses stack-ups have an IR-cut filter built-in and if you remove that the ISP (image signal processing) produces off-color images.

    1. I was surprised to learn about the “image processing” going on inside what looks like trivial cameras, but that’s just a symptom of my not understanding how much processing happens while just getting any image out of any sensor.

      And for five bucks delivered, too!

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