Physical Optics Lab
LAB #11
Optics 505
Page 1
Lab #11, Student’s Choice: Coherence Microscopy
In this lab we will examine how spatial coherence affects the quality of an image, and we will
see how to make a source spatially coherent or incoherent. We will observe how images
propagate through Fourier planes, and see how to implement an optical “frequency filter”.
Preparation:
Read class lecture notes #15, and understand the coherence transfer function (CTF), and
cutoff frequency.
Read Goodman “Statistical Optics”, Chapter 7.2 (image propagation, Koehler illumination,
CTF), Chapter 7.3.2 (propagated image of grating), section 5.2.4 (mutual intensity function
J12).
Read Goodman, “Introduction to Fourier Optics”, 6.1
Setup:
source
stop
aperture
stop
grating
lens 1
lens 2
image
plane
lens 3
source
CCD
f
f
f
f
This is described in Goodman “Statistical Optics”, Fig. 7-11.
f
f
Optics 505
Physical Optics Lab
LAB #11
Page 1
Procedure:
1. Set up the lenses and apertures so that their separations are correct.
2. Insert the Ronchi grating #1, and focus the CCD until you are imaging the grating
lines clearly. Use a 20x objective on the CCD.
3. Set the source stop to be very small. (You may need to make it a little larger to let
enough light through.)
4. Vary the aperture stop size, and observe what happens to the image.
5. Vary the aperture stop size, and look just past the plane of the aperture stop. (A tissue
works well for this.) Notice how what you see in this plane relates to what you see in
step 4. What’s happening here?
6. Remove the Ronchi grating #1, and insert grating #2. Observe the difference in the
plane of the aperture stop.
7. Measure visibility of the image pattern, as a function of aperture stop size. (You may
want to use the Matlab framegrabbing programs.) Convert aperture stop size into
object-space numerical aperture (NA), and make a plot of visibility vs. NA. Label the
cutoff NA on the plot.
8. Now increase the size of the source stop. Look at what happens just past the plane of
the aperture stop, as you vary the source stop size.
9. Repeat step 7 twice more, using a different source stop size each time.
10. Now remove the grating, and put in a mesh screen. Make the source stop very small,
and open up the aperture stop all the way. Observe what you see in the plane of the
aperture stop. It helps to use a 10x objective with the CCD.
11. Remove the aperture stop, and put a slit in its place, so that the slit only allows through
exactly one line of foci. Observe the image.
12. Now increase the source stop size and observe what happens to the image.
Optics 505
Physical Optics Lab
LAB #11
Page 1
Questions:
1. Why is this experimental setup called a “4f” setup? Is this a telecentric system?
2. Is there a minimum diameter that the lenses can be? What determines the minimum
diameter?
3. How does the mutual intensity function J12 depend on our source stop size?
4. In step 7, you plotted visibility vs. NA, for a fixed grating frequency. How would you
convert this into a plot of visibility vs. grating frequency, for a fixed NA? This new
plot would be the system modulation transfer function (MTF). Convert your three
drawings into MTFs, and sketch them on the same plot.
5. Is the cutoff NA higher in the coherent system or the incoherent system? How about
the cutoff frequency?
6. How would the shape of the MTF change if your lenses had aberrations?
7. How does the coherence factor affect the image visibility in step 12? (The coherence
factor is defined as the width of an order in the aperture stop plane divided by the
aperture stop width.)