Part 4: Projection alignment

In this part you’ll arrange the final optical components along the projection axis. It is broken up into the following steps:

  1. Roughly place the imaging lens
  2. Roughly place the polarizer
  3. Roughly place the routing mirror
  4. Roughly place the camera and polarizer
  5. Finalize optical component placement
  6. Install the screen

a) Roughly place the imaging lens

The achromatic lens is used to form a magnified image of the DMD on your sample.

  1. Use a ruler to set the height of the imaging lens to 170 mm.
  2. Make sure the flat side of the lens is pointing toward the DMD and the open part of the mount is facing the illumination axis. The clear edge mount is used to minimize the amount of illumination light blocked by the imaging lens mount.
  3. Orient the base (BA1) of the imaging lens assembly so it is parallel to the front of the bread board and aligned with the projection axis.
  4. Attach the base plate to the bread board so the lens is approximately 75 mm (3 inches) from the DMD. By loosening the screw holding the baseplate you should be able to slide the lens closer or farther from the DMD while keeping the face of the lens and the DMD parallel. This will be used to adjust the focus of the image in a later step.
  5. Verify that the face of the lens is parallel to the DMD and that the height is still 170 mm

Model of imaging lens
Model showing imaging lens in place. Note orientation of the baseplate for easy motion towards and away from the DMD.

b) Roughly place the polarizer

The polarizer immediately following the imaging lens is mounted on a rotation mount so the polarization state of the LEDs can be controlled. The goal is to place the polarizer so it blocks the minimum amount of light from getting to the routing mirror.

  1. Adjust the height of the polarizer to 170 mm/li>
  2. Orient the base plate of the polarizer assembly so it is parallel to the front of the bread board. It should be the same as the base plate of the imaging lens you set up in step b
  3. Screw down the polarizer asesmbly so it is about 50 mm away from the imaging lens. When you loosen the screw you should be able to move it forward and backward without changing the angle of the polarizer relative to the DMD face.
  4. Turn on the DMD and set it to “Test pattern” -> “solid field.”
  5. Turn on the 660 nm LED
  6. At this point the light from the DMD should be passing through the imaging lens and converging to a 2D patterns of spots. The patterning is due to diffraction from the DMD mirrors which act as a 2 dimensional grating. The light is focused because the achromatic lens is forming an image of the LED emitter.
  7. Place the polarizer where the light is focused. This will allow the maximum number of diffracted orders to pass through the polarizer which will increase the amount of light hitting the sample.

model showing first polarizer
Model showing approximate position of the first polarizer. Again, note the orientation of the base plate.

c) Roughly place the routing mirror

The routing mirror is used to bounce the downward so the sample can sit flat on the breadboard.

  1. Loosen the nut on the right angle bracket holding the post that the camera is mounted on and remove it for now
  2. Attach a base plate to a 4 inch post holder.
  3. Center this assembly on the second row of holes from the left and the fourth row of holes from the back and attach it to the bread board.
  4. Insert a 4 inch post into the assembly and tighten the nut so it doesn’t move around
  5. Mount the 2 inch mirror in the 2 inch kinematic mirror mount and attach the mount to a 6 inch post.
  6. Put a right angle mount on the opposite end of the 6 inch post and slide it onto the post/post holder/base assembly you just attached to the bread board. The mirror should extend over the bread board.
  7. Set the height of the mirror to 170 mm and the angle to slightly less than 45 degrees
  8. With the 660 nm LED on, follow the beam of light and make sure it hits the mirror fully. At this point it may overfill or underfill the mirror. That’s fine, final positioning of all the components will come in a later step.

Routing mirror model
Model showing current state of bread board including the routing mirror. The projection axis is also shown.

d) Roughly place the camera and polarizer

  1. In a previous step you placed the baseplate, post holder, and the TR8 post that the camera and polarizer will be mounted to. First raise the TR8 post so there’s approximately 25 mm (1 inch) left in the post holder (i.e. it is almost fully extended).
  2. If you haven’t alredy, mount the polarizer in the fixed mirror mount (FMP1) by loosening the set screw, dropping the polarizer into place, and re-tightening the screw
  3. Attach the mounted polarizer to a 6 inch post
  4. Slide a right angle adapter over the 8 inch post with the open end facing the projection axis
  5. Insert the post with the polarizer into the right angle adaptor. Adjust the angle of the polarizer so it is parallel to the surface of the breadboard. Adjust the height so the polarizer is a little bit higher than the routing mirror
  6. Slide the camera assembly over the 8 inch post
  7. Adjust the angle so the camera is pointed directly down.
  8. Adjust the height so the camera lens is almost touching the polarizer mount

Model showing the placement of the second polarizer.

Model of the camera and polarizer in position.

e) Finalize placement of projection components

The goal of this step is to make sure the image of the DMD is in focus at the proper position and is large enough, and that the image from the camera is in focus.

  1. Turn on the DMD and in the control software from Keynote Photonics select “Video mode” with the source as “Test Images” and then select “Checkerboard” for the image.
  2. Plug the camera into the computer and view a live image using MAX
  3. Turn on the 660 nm LED
  4. The system was designed to have the focal plane 50 mm above the surface of the breadboard so grab something that’s about 50 mm high and place a white piece of paper on top of it. If you followed all of the previous steps you should see a checkerboard pattern on the paper.
  5. Adjust the angle of the mirror until you can see the checkerboard on the image from the camera
  6. Loosen the screw holding the baseplate of the imaging lens and make small adjustments to the position until the image on the paper is in sharp focus.
  7. Switch the test pattern to “Grid” and repeat the previous step. The “Grid pattern is composed of very fine lines that will not be visible unless the image is in very good focus. You may have to turn off the room lights to see the image better. Don’t worry too much if you are unable to see the lines of the “Grid” image. As long as the “Checkerboard” image looks in focus is should be good enough for SFDI. Once you find the right position for the lens tighten it down firmly.
  8. Using a business card or scrap of paper follow the light as it exits the imaging lens and focuses to a series of points. Make sure that the polarizer is placed approximately where the size of the points is minimized, and tighten it down firmly.

  9. Keep following the beam until it reaches the mirror. The beam should have expanded by this point to just overfill the mirror along the long axis. Make sure the beam is hitting the mirror in the center. Adjust the height and/or the front/back position until the light is striking the mirror in the center. If the beam is much larger or much smaller than the mirror you may have to adjust the position of the post holder holding the mirror forward or backward. If you do you’ll also have to adjust the angle of the post holding the mirror to make sure it’s perpendicular to the projection axis. You may also need to adjust the position of the post holder holding the camera to ensure that the pattern is in the field of view.
  10. Adjust the height of the camera until the checkerboard image fills the field of view or the polarizer is so low it starts to block some of the incoming light
  11. Place a piece of paper with writing in the illuminated spot.
  12. Using the live image from the camera adjust the focus of the lens until the printing appears in focus. This will be easier if you open the aperture to f/2 or so, but you may have to decrease the exposure time to compensate. If you have problems with saturation, turn off the 660 nm LED and just use room light to focus on the printing. Once the printing is in focus, lock the focus ring and tighten all of the nuts on the post holders, and right angle adaptors.

f) Install the screen

Light from the LEDs that isn’t reflected by the dichroic can bounce around end up lighting the imaging plane which will decrease the signal-to-noise ratio of the system. In this step you’ll install a screen to block stray light from hitting your sample.

  1. Look at the image below and screw 3 8 inch posts at the indicated locations
  2. Take a piece of black aluminum foil about 50 cm (20 inches) long
  3. Wrap one end of the foil around the post closest to the imaging lens and tape it into place.
  4. Guide the foil around the other two posts you installed
  5. Wrap the foil around the third post and tape into place. Cut off any extra or just double it up and tape it down.

Screen mounting positions
Top view of the openSFDI system with screen mounting points indicated

completed openSFDI system
Model of completed openSFDI system including screen

Awesome. Your open SFDI system is now complete! In the next section we’ll make sure all the electrical connections are set up and collect our first SFDI images.