Looking Into the Dark-Field of Your Stereomicroscope

Those who follow the Instructable site have realized that amateur microscopy is one of our interests. Our most recent project, “Rebuilding the Focusing Mechanisms of Old Microscopes Using 3D Printing”, was about an easy and cheap repairing option for your old priceless microscope, a companion on your juvenile science adventures. Given the excellent reception, we thought we could share other ideas we have implemented on another type of microscope to add new or improved functionality. So, this new Instructable will show how to 3D print a simple accessory that can add a Dark Field view capability to your stereomicroscope (SM). For those who have never heard about Dark Field, we want to anticipate that it is not a Star War saga-related gadget or force (though it does sound like it) but a tool that allows spotting living microorganisms in a drop of water from your pond easier using oblique light illumination and so adding new insight into the hidden world. An informative YouTube video by Clive Bagshaw inspired the idea (here is the link to his video). He proposed a similar device using even more modest recycled materials such as a can of aluminium. Unfortunately, we could not find a can with a good fit for our SM to reproduce his model, but we had a 3D printer. So, we made a customizable device described in this Instructable that can be adapted to different microscope geometry.

For this project, you will need:

• OpenScad software (https://openscad.org). 
• A 3D printer slicer. We have used the great Prusa Slicer, but Unimaker Cura is also an excellent choice.
• A 3D printer. We have used a Creality Ender 5 PRO.
• A stereomicroscope (any model with a table provided with backlight illumination is suitable, you can adjust the device's dimensions if it does not fit in your microscope).
• A black tape (we have used a fibre reinforced 3M tape for packaging, but any other very dark coloured material also works).
• A reflective material such as an aluminium foil.
• Precision craft knife.

Dark-ground (or Dark-Field) is an illumination technique that exploits light diffraction to make an object on the microscope slide brighter than the surroundings. The principle is illustrated for a standard microscope on the left side diagram. The light coming from a light source is blocked by a collector shaped to allow only the light coming from the border of the cone to pass through. In an ordinary microscope, the condenser then focuses the transmitted light that passes through the opening in the microscope table at an angle that does not directly hit the objective aperture. That will create a dark field of view. Nevertheless, the light hits the specimen on the slide, reflecting or refracting (due to the contents inside the cellular organism) some rays straight to the objective, so the specimen results are illuminated. The bright objects moving on a dark background allow you to see the specimen much more clearly, making observations easier.

Stereomicroscopes are usually employed for magnification observation and typically do not have an optical system to shape and focus the light from the source. However, in essence, the darkfield requires a shaped filter trick to maximize the oblique transmission of the residual transmitted light through the slide with the sample.

Assuming that your stereomicroscope is equipped with a transmitted light source, in this Instructable, we will show how to fit it with the darkfield accessory that consists of such a cylindrical box with a shaped opaque light-stopper, reflective coating, and a collector opening of calculated size. In this Instructable, we have created one device that will allow you to do this without too much effort and works well. Read on to find out how to make it.

The device can be generated using the OpenScad program attached below.
The program consists of the following four modules used to generate the parts shown in the figure. The variable's names are used to customise the device, and their association with its geometry is indicated in the figure.
Bcylinder(): this module generates the cylindrical body of the device. The diameters and height of the cylinder and the position and aperture of the internal support rings can be adjusted.
Lid(): this one is for the lid to cover the top part of the body. It contains an aperture for the light. Also, in this case, the part size can be changed according to the stereomicroscope you are using.
LightStopper(): this is for the circular light stopper placed on the bottom of the device.
FixingRing(): this is used to generate a ring to fix the light stopper or a temporary stopper made using a transparent plastic sheet.
To show the module, uncomment their name at the end of the script. The explosion view can be obtained by assigning a value different by zero to the variable expView.

//########################################################################
// Script for the generation of the different component of the dark field 
// (DF) illumination acessory for stereomicroscope
// 
// MODULES:
// Bcylinder(): Generate the cylindrical body of the DF filter
// Lid(): Lid to cover the cylinder body with the light aperture.
// LightStopper(): Circular light stopper.
// FixingRing():Generate a ring to fix the light stopper.
//
// (c) Roccatano 2022
//######################################################################

$fn = 100;

outerCylinderHeight=28;
outerCylinderRadius=26.5;
innerCylinderRadius=outerCylinderRadius-2.5;
innerCylinderRadius2=innerCylinderRadius-2;
innerRingHeight=2;

// Light Stopper
StopperCircleRadius=10;
offsetStopper=0.0;

//Lid
apertureRadius=5.1; 
//offsetaAperture=2.5;
offsetAperture=0.0;

//Set the explosion view of the device
expView=20;

module Bcylinder() {
  // Bottom cylinder
  difference() {
    cylinder(outerCylinderHeight, outerCylinderRadius, outerCylinderRadius);
    translate ([0, 0, -50]) cylinder(100, innerCylinderRadius, innerCylinderRadius);  
  }
  difference() {
        color("red")  translate ([0, 0, 23.5]) cylinder(innerRingHeight, innerCylinderRadius+1, innerCylinderRadius+1);
      translate ([0, 0, -50]) cylinder(100, innerCylinderRadius2, innerCylinderRadius2);
  }
  difference() {
        color("blue")  translate ([0, 0, 11]) cylinder(innerRingHeight, innerCylinderRadius+1, innerCylinderRadius+1);
      translate ([0, 0, -50]) cylinder(100, innerCylinderRadius2, innerCylinderRadius2);
  }
  
}

module Lid() {
  // lid of the cylinder.
    translate ([0, 0, 0]) difference() {
    translate ([0, 0, 25]) cylinder(2.5, innerCylinderRadius-0.2, innerCylinderRadius-0.2);
    translate ([0,offsetAperture, 10]) cylinder(25, apertureRadius, apertureRadius);

  }
}

module LightStopper() {
  difference() {
    translate ([0, 0, -5]) cylinder(innerRingHeight, innerCylinderRadius, innerCylinderRadius);
    translate ([0, 0, -6]) cylinder(100, innerCylinderRadius2, innerCylinderRadius2);
  }
  translate ([0,offsetStopper, -5.0]) cylinder(innerRingHeight, StopperCircleRadius, StopperCircleRadius);
  translate ([0, 0, -4]) cube([1.5, innerCylinderRadius*2, innerRingHeigh], center = true);
}

module FixingRing() {
  difference() {
    translate ([0, 0, -5]) cylinder(innerRingHeight, innerCylinderRadius, innerCylinderRadius);
    translate ([0, 0, -6]) cylinder(100, innerCylinderRadius2, innerCylinderRadius2);
  }
}

// Comment the parts that you do not want to print.
// Lid
translate ([0,0,0.5+expView]) Lid();
// Body
Bcylinder();
// Light topper
translate ([0,0,14-expView]) LightStopper();
// fixing ring
//translate ([0,0,12-expView*2]) FixingRing();

CYLINDRICAL BODY

Before printing all the parts, you need to check if your microscope is the same as the one I am using. Our stereomicroscope (SM) was bought more than 20 years ago from a German retailer. We have also modified the power-supplied halogen lamp illumination system with a LED battery-based one (we will show in another instructable these modifications). The variable's value in the scripts fits with this microscope, but you need to check the geometry of your SM before proceeding with the printing.

You need to check the size of the opening in your SM's table (two times the variable outerCylinderRadius) and the table's deepness (variable outerCylinderHeight). Note that in our SM, we use LEDs as our light source as an alternative light source rather than the original halogen light (we will show in a future Instructable how we have converted our microscope). If you are using a halogen light source in your stereomicroscope, we recommend caution against the possible melting of our device if it is in too close contact.

Once the parameters are modified to fit your microscope in the script, you can 3D print the body (function Bcylinder(). Next, add a strip of aluminium foil between the two internal rings as shown in the first figure (Maybe doing a better job than ours!).

TOP LID

You also need to check the size of the aperture of the top and, if you need it, offset it to the centre to cover the field of view of your objective. To do that, you need to measure the diameter of the aperture by placing a ruler on the microscope table and measuring the diameter of the view's size, as shown in the figure. You can use half of this value for the variable apertureRadius. However, before proceeding with the 3D printing, I suggest creating a lid using carton paper and checking if you also need to offset a bit the aperture center to perfectly align with the objective. Once you print the top, paint the internal surface in black color.

Another essential part of the device is the "light stopper". Its size and position are as crucial as the lid aperture, requiring accurate measurements. To help find the optimal size and position for the light stopper circle on your device, we recommend building it with cheap materials that you can modify easily. This allows you to find the best dark field effects without wasting 3D printing time and material. You can simply cut a round piece of black tape of a size ~2 times larger than the lid aperture on a round transparent plastic acetate sheet with a diameter equal to the internal diameter of the device. Plug it into the bottom of the device ( to hold it in place, you can either 3D print the part called the "FixingRing" or make a ring out of cardboard, as shown in the Figures).

Insert the device into your SM and check if no direct light comes from the LED. You can vary the diameter and the offset of the black stopper circle to find the best effect. Once you are happy with the result, transfer the size in the script parameter (StopperCircleRadius and offsetStopper) and 3D print it. Then you need to cover both the size of the stopper using black tape. As shown in the figure, first you add a square of taking and then, using a sharp knife (careful with the fingers!), cut the excess following the shape of the stopper (see last two Figures).

We are now at the end of our construction, and it is time to look at the results. We have reported some examples of observation. It is important to note that the dark-ground effect for small objects to low resolution can be obtained by using oblique light from a window or a lamp, keeping the background illumination switched off. For example, we could get a similar effect with the mouse tooth using just oblique illumination from a window. However, our device turns very useful when this source is unavailable for tiny organisms or higher magnifications, as shown in the video and the mould picture. The device can be further improved to maximise the oblique light and reduce the leaking of direct sunlight. We will be delighted to hear from you if you have improved it.

And so we have reached the end of another build. We hope it will be helpful to learn more about Dark Field and for your home observations. In conclusion, we will leave you with this: Have Fun!