Introduction: Dark Background for Diffused Light Illuminator

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This instructable proposes a short report about a complement for my self-built diffused light illuminator: a darkfield photographic set, capable to provide an out-of-focus, solid black background.

It's a cheap solution that, based on the first attempts, may provide decent results.

For the sake of clarity, I provide two illustrations and some pictures.

The basic concept is building a "dark room" above which the subject, typically an insect, glued to the head of a pin, seems to float in the darkness although being surrounded by the diffused, indirect illumination provided by the LED strip illuminator.

The illustration above here lists the components to insert in the "dark room".

Tools needed include scissors, small and big, and adhesive tape. I also used a stapler and some glue to assemble the "dark room". A compass is needed to draw circles, as needed e.g. for the hole in the "ceiling" of the dark room.

Full list of parts follows here:

  1. An insect (or other subject) glued on the head of a pin;
  2. A "pedestal" where the pin will be inserted - the upper surface of the pedestal shall be covered with non-reflective black fabric or in any case painted matt black. The pedestal I use is illustrated at the bottom of this page;
  3. A black (or painted matt black) rubber "finger" of suitable size, needed because my pedestal is made by a small piece of threaded metal tube (usually available as a furniture accessory - the kind of bolt used to join kitchen cabinets side by side through the side panels) - the rubber "finger" both eliminates reflections, and keeps the pin steady. Probably, a small piece of black plastazote could carry out the same function;
  4. A double-face (white above and black below), easily built cardboard disc, that will be used to narrow the hole above the "dark room";
  5. A couple of smaller discs made of "mirror" reflective paper or of simple pure white cardboard, that will be used both to make the aperture under the subject narrower (thus limiting further the amount of light that may reach the "dark room", and reflecting part of the diffused illumination towards the base of the subject, whose contours will be more evident), and above the illuminator to leave a hole with a diameter as close as possible to the optically active part of the objective (thus limiting the amount of illumination that escapes from above, and making as small as possible the dark reflection of the objective on particularly shiny subjects): obviously the hole under the subject must be wide enough to provide a dark background all around the subject, and that above the illuminator must leave the optical field completely free;
  6. A diffused light illuminator;
  7. A "dark room" made with a cardboard box, black inside, about 2 inches high, and at least as wide as the illuminator that will be placed above it.

Step 1: Putting Things Together - a "two Stages" Photographic Set

The "dark room" should be jet black, and its inside should possibly be matt, non reflective. In my case, I found some black heavy cardboard in a discarded display from a perfumery, and I reassembled it outside-in, cutting the cardboard at the most suitable size for my microscope stand.

A circular hole is made in the top of the "dark room". Why did I make it so big, when the subjects that I'll photograph will be in most cases less than 2 cm? Because, as illustrated in the picture, the roof/cover of the dark room is pivoting and the edge of the hole, when closing the dark room, should stay well clear from the subjects. But also because when making several vertical stacks of pictures across the same subject, I need the possibility to slide the pin/pedestal assembly in every direction without hitting the edges of the hole. As explained below, a disc of paper with a narrow hole (barely fitting the subject) will be placed above the closed "dark room", so that, at the moment of shooting, the "floor" under the subject will be as reflective as possible, while preserving a "black hole" all around it.

Assembly

As per illustrations and photographs.

  1. Subjects are prepared by glueing the insects on the heads of entomological pins (obviously the subject can be something else than an insect...)
  2. The "finger" is puton the pedestal (if the pedestal is a small support of black Plastazote, this step may not be needed)
  3. The pin is inserted in the pedestal,
  4. The pedestal is put in the "dark room", checking that the subject sticks at least 1 inch above the "dark room",

Step 2: In the "clear Room"

  1. The "dark room" is closed, with the subject sticking out 2 or 3 centimeters,
  2. The double-face holed disc is placed above it, black side facing downwards,
  3. The second cardboard (or reflective "mirror paper") holed disc is cut out as tight as possible around the subject, and is placed carefully at the base of the subject, checking that in any case it's surrounded by the dark background,
  4. The illuminator is put above the "dark room", thus creating the "clear room",
  5. The upper opening of the illuminator is narrowed by cutting out another holed cardboard disc and putting it the most suitable position to minimize the reflections of the objective while keeping the optical field free.

Step 3: Possible Results of a Focus Stacking Session With Dark Background

Here are two preliminary pictures taken by applying my old Sony Alpha to my Short Unimac microscope (my first experiment with a T2 adapter to mount the camera in the place of the ocular). I still don't master this technique, but surely the dark, out-of-focus background is evident and the subject stands out quite crisply. Both the subjects are around 1.5 cm long excluding antennae. Chrysis ignita is the small wasp, picture is made with two Z-stacks of 64 pictures each, composed by Helicon Focus stitched with Microsoft ICE. Chrysolina graminis is the small beetle, picture is made with a single Z-stack of 89 pictures. Click on the pictures to enlarge.