This page describes how to build an optics rig based on board cameras for droplet microfluidics.
This is a DropletKitchen project.
If not stated otherwise, the designs and documentation in this repository is Copyright (c) 2016 Martin Fischlechner and DropletKitchen Contributors, and made available under a Creative Commons Attribution 4.0 International (CC BY 4.0) License.
Image of the rig at several stages of the build. A] fully assembled rig. The inset shows the LED with inexpensive focusing optics. B] image of the openSCAD model used to design the first version. Use this model as a starting point for your own designs. C] setup without the x,y,z-stage. The inset shows the board camera with the flipped objective removed. D] x,y,z-stage.
Many droplet microfluidic experiments have simple imaging requirements, for example as a real-time quality control to ensure droplet generation is running smoothly. This page shows how to build simple microscopes with inexpensive board cameras. Flipping the objective of the camera transforms the setup to a small microscope. Magnification of the system is dependent on the distance between the objective and the CCD or CMOS chip of the camera. In contrast to scientific cameras, inexpensive consumer electronics cameras or board cameras do not offer much control. They usually self-regulate exposure times depending on the light available and without frame rate adjustment, binning, or camera-triggering is possible. BUT - they are incredibly cheap and often offer surprising performance. Here we use a PS3 Eye camera and a CCD-camera that outputs analog PAL video used for First Person View (FPV) drone flying. While the PS3 eye is a rolling shutter camera, the board CCD camera is global shutter, which is well suited for imaging fast events.
This image compilation shows the performance of both cameras for observing a simple microfluidic droplet generator. The PS3Eye camera shows distortion effects due to its rolling shutter, but still allows to estimate droplet monodispersity (by virtue of width) up to velocities of 400 mm/s. The Sony CCD global shutter board camera lacks distortions, and yields useful images at higher velocities. With global shutter cameras the lightsource can additionally be strobed to enhance image quality. Due to the different chip sensor-objective distances of the setups used (see image below), the images of the PS3Eye have higher magnification. In its ‘fast mode’, the PS3Eye has a resolution of 320x240 (up to 180 fps) and is connected via USB. The Emax Sony 811 outputs analog video (PAL, 25fps) with a resolution of 976x582 pixels. Video can be displayed directly on a monitor. To use the camera with a computer, a framegrabber is needed.
The model was first developed in openSCAD (http://www.openscad.org/) and sliced for lasercutting (./files/RigSonyCCDPS3eyeBoardCameraOpenSCAD.scad). Later modifications were done in a .dxf-file (./files/RigSonyCCDPS3eyeBoardCamera.dxf) with Draftsight (http://www.3ds.com/products-services/draftsight-cad-software/free-download/). Starting with a 3D-model is helpful to play with overall dimensions and shapes, but the design is quicker to modify in 2D-CAD at later stages.
Parts for laser cutting.
| Item | number | bought from | part number | approx cost (£) |
|---|---|---|---|---|
| Structural components | ||||
| M6 Hex Screws (different lengths, eg l=16,20,25,30mm for flexibility) | many | RS or other hardware shop | NA | |
| M6 Nuts, Washers, ‘shake-proof washers’ | many | RS or other hardware shop | NA | |
| M6 Threaded Rods | 3x (~150mm) | hardware shop | NA | |
| M3 Screws, Nuts and Washers (different lengths) | many | RS or other hardware shop | NA | |
| 3mm Acrylic Sheet (410x280mm) | 1 pack of 5 | www.techsoft.co.uk | TAAC3-R3-5-COL | 18 |
| 5mm Acrylic Sheet (410x280mm) | 1 pack of 5 | www.techsoft.co.uk | TAAC3-R5-5-COL | 25 |
| Double-Sided Tape (3M, 468MP or 467MP), optional but nice to have | 1 | RS | 842-1089 | 50 |
| XYZ-stage | ||||
| Fine Adjustment Screw with Knob, 1/4”-80, 2.00” Long | 3 | Thorlabs | FAS200 | 17 |
| Locking Phosphor-Bronze Bushing with Nut, 1/4”-80, L=0.50” | 3 | Thorlabs | N80L6P | 21 |
| XY-stage from eBay (search ‘microscope stage’) | 1 | eBay | max 20 | |
| Eclipse 20mm Threaded Hole Neodymium Ring Magnet | 1 | RS | 792-4565 | 7 |
| Eclipse 20mm Neodymium Disc Magnet (3each, we use 5 for assembly) | 2 | RS | 695-0184 | 22 |
| Short Steel Rods (magnetic) 20-40mm long, 5-6mm in diameter | 2 | NA | NA | NA |
| Light | ||||
| RECOM POWER RCD-24-0.70/W/X3 LED Driver Adjustable | 1 | Farnell | 1793223 | 13 |
| OPULENT REBEL-STAR-ES-NW200 High Brightness LED | 1 | Farnell | 2110405 | 4 |
| Polymer Optics 141/180, Lens Assembly | 1 | RS | 665-6573 | 3 |
| Stripboard, 10kOhm Potentiometer | eg Farnell, RS or other | NA | ||
| Male/Female 0.1” PCB Connectors | eg Farnell, RS or other | NA | ||
| Sugru for isolation of wire connections… | 1 | cpc Farnell | MK00041 | 6 |
| Epoxy Glue | NA | NA | NA | |
| Optics/Cameras | ||||
| Sony Playstation 3 Eye Camera (=rolling shutter) | 1 | amazon.co.uk | ~7 | |
| Emax SONY 811 700TVL 1/3-Inch CCD Video Camera (=global shutter) | 1 | www.unmannedtechshop.co.uk | CC7-E5A-55E | 19 |
- disassemble the PS3 eye camera, you will have to remove some plastic on the objective (glued) to be able to flip it. The SonyCCD camera can be used as is, just remove the objective.
- attach the flipped objective to the camera in a way that no light can enter from the sides. This can either be done by clamping or glueing (see image below). If you want to improve magnification, enlarge the distance between camera-chip and objective by using a 3D-printed tube or stack of acrylic rings. In this design adapter-plates were used that allow cameras to be easily exchanged. On the other hand they can also be just attached with some Sugru or hot melt glue.
Left: Playstation3 Eye camera with inverted optics. Magnification is controlled via the length of a 3D-printed tube. Right: Sony CCD board camera with inverted optics mounted with a sheet of acrylic.
Assembling the rig is straightforward; have a look at the image above. Make sure that your components fit - if you change the design, just measure components with a caliper and use exactly those dimensions for holes etc. Laser cut parts usually results in a tight fit. The camera with the inverted objective is held in place by clamping it with acrylic sheets and M3 screws.
The stage is mounted on three optics screws with spherical ends in a kinematic fashion - which defines the position of the stage by resting on 6 points of contact. This is ideally 3 points on one, 2 on the next, and one on the third. Here strong magnets are used to fix the stage on its support - a ring magnet on the first side (a small cheat but works well), 2 stacked disk magnets that magnetize two small steel rods on top, and a stack of three disk magnets on the third point. This assembly allows you not only to control z distance (focus), but you can also tilt the stage and therefore have some movement in x,y. For convenience, adding an inexpensive x,y stage is recommended. They all differ a bit, so make sure you adapt the files accordingly.
For the lightsource, a star-shaped white high power LED is used which is controlled by an adjustable LED driver. A M3 screw is used as a heat sink and also holds the LED in place. The LED driver, a potentiometer used to control brightness and an on/off switch is soldered onto stripboard and then packaged in an acrylic enclosure. It does not have to bear a lot of load, so the enclosure is based on cut acrylic slides with double-sided tape added on one side before cutting. This allows the structural components to be bonded together. To focus the light of the LED onto the chip, inexpensive plastic lenses are used. For assembly, just use a dab of epoxy glue. Do not use ‘Superglue’, as cyanoacrylate vapours render optics components cloudy over time. Change the distance between LED and the stage such that the image on the camera is overexposed when the LED is fully on and the camera is set automatically to the smallest exposure time possible. This allows you to get the best possible images of fast-moving objects with your setup. Below is the circuit diagram for wiring if you use the Recom LED-driver. If you want an on/off switch, just wire it to the voltage or ground wire. Almost any power supply will do (>=1A, 5-36V).
Circuit diagram for the Recom LED driver.