Part 2: Mapping out the optical paths
There are two main optical paths that need to be set up. The first is the illumination path where the LEDs are combined and directed toward the DMD. The second is the projection path where the DMD is imaged onto the sample. The geometry has to be relatively precise, so in this section you’ll map out these light paths and start to place some of the components. This section is broken down into the following steps:
- Determine the height
- Place the DMD
- Map the projection path
- Map the illumination path
- Align DMD
- Lay out the LED paths
a) Determine the height
The openSFDI has a nominal optic height of 170 mm (6.75 inches). If you’re using 4 inch posts this can be adjusted down to ~5 inches or up to ~8 inches. If you want to use a lower height you will need to replace the 4″ post holding the DMD with a shorter one. Optically, increasing the height can give you either a higher focal plane, or a larger field of view. You should use caution trying to increase the field of view too much. The current system design should just overfill the 2″ diameter routing mirror along the long axis of the DMD. If you want to increase the field of view by raising the height, you’ll have to move the mirror closer to the achromatic lens. The entire system was designed to be quite compact, so there isn’t a lot of room to move the mirror closer. Decreasing the system height will decrease the height of the focal plane or give a smaller field of view, but will result in a more compact system. As designed, the Open SFDI system should yield a 7.5 x 4.5 cm field of view with a focal plane 50 mm above the table surface. Once you set the height, it will be annoying to change, so it’s worth thinking about now. The rest of the instructions will assume a 170 mm height, if you decide to go higher or lower mentally replace all instructions containing 170 mm with whatever system height you choose.
b) Place the DMD
The location and orientation of the DMD will determine where every other piece of the system will go. The projection axis will be perpendicular to the face of the DMD, while the illumination will be at a 24 degree angle from the illumination axis because of the tilt of the mirrors.
- Place the breadboard such that the 18 inch side is going left to right and the 12 inch side is going front to back
- Turn the DMD assembly so the post holder base is flush with the edge of the breadboard
- Loosely screw down the assembly using a 1/4-20 screw in the third hole from the front on the rightmost row of holes. It should be loose enough to easily slide forward and back, but tight enough not to tip over
- Loosen the set screw holding the post in place and rotate the DMD so it is parallel to the edge of the breadboard. While the set screw is loose raise or lower the DMD such that the center of the DMD surface is 170 mm above the table.
- Once the height and angle are correct tighten the set screw
- It’s convenient if the projection axis is along a row of holes in the bread board. To do this stand a ruler up on the table with one edge centered on the third hole from the right and the third hole from the front (ie the hole two to the left of the one used to attach the DMD base plate). Slide the DMD assembly forward and back until the center of the DMD is aligned with the straight edge.
- Tighten the screw holding the base plate to the board
Congratulations! You’ve just set up your first (and most important) component. Before moving on double check that the height above the table is correct, the face of the DMD is parallel to the edge of the breadboard, and the center of the DMD is aligned with the third row of holes. If these aren’t correct now it will cause problems later.
c) Map the projection path
This one is easy. The projection path will extend perpendicular to the face of the DMD along the third row of holes from the front. Using a straightedge and a marker draw a line connecting the 3rd row of holes. The achromatic imaging lens and one of the polarizers will be placed directly on this line. The camera and routing mirror will be suspended above the line.
d) Map the illumination path
For this step you’ll need the LED test circuit, and one of the LED assemblies. You’ll also need to use the DMD control software [link to download and install instructions] to set all the DMD mirrors to their “on” state. IT IS CRITICAL THAT YOU DO NOT TOUCH THE DMD DURING THIS PROCEDURE. Only move the LED for now.
- Using a ruler, set the center of the LED emitter to 170 mm above the bread board surface.
- Plug the DMD power supply into the control board. The DMD should boot up and display a splash screen
- Plug the USB cable into the DMD control board and into the laptop
- Boot the DMD control software and select the “Video Mode” tab
- In the “source” box select “Test Image”
- Select “solid field” from the pull down menu
- Click “Set.” If you look at the DMD, the splash screen should be gone. All the mirrors are now set to “on”
- Tape a blank piece of paper to a glove box (a tissue box or Lab Snacks box will also work) and stand it up at the leftmost edge of the breadboard centered along the projection axis. Using a straightedge, draw a vertical line on the screen that lines up with the projection axis
- Put one edge of your ruler against the DMD baseplate and the other end against the LED baseplate
- Turn on the LED
- Sweep the LED assembly in an arc with the origin at the DMD baseplate while watching the screen along the illumination axis. Make sure to keep the LED pointed at the DMD throughout the arc.
- When the reflected light from the DMD appears on the screen, make small adjustments until the light form the DMD is centered. Since there is no imaging lens this light will appear to be an amorphous blob. Just try to put the center of mass of the blob on the vertical line. If the angle is very different from the images below because you are constructing your system in a different geometry you may need to flip your DMD PCB over (so the connector ribbon is on the top) to make sure the “on” state mirrors are oriented in the right direction. If you do this, make sure to reset the height.
- Looking from the top place your ruler directly under the center of the DMD on one side and directly under the LED emitter on the other side. Use a marker to connect the two points. This will be the illumination axis. The angle between the illumination and projection axes should be 24 degrees.
e) Align DMD
- On the screen draw a horizontal line 170 mm above the bread board
- Adjust the vertical knob on the kinematic mount until the center of the light reflected from the DMD is at the intersection of the vertical and horizontal lines.
f) Lay out LED paths
The 660 nm LED will be placed along the main illumination axis. The other two will be at right angles to this axis.
- Starting from the point where the 5th row of holes intersects the illumination axis draw a line at 90 degrees to the axis extending toward the back of the breadboard. The line should be about 15 cm (6 inches) long. This will be the axis for the 850 nm LED
- Repeat the previous step except start from the intersection of the 6th row of holes and the illumination axis
That’s it for Part 2. On to Part 3 where you’ll place the components on the breadboard.