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Message ID: 57     Entry time: Wed Jun 23 22:57:22 2010
Author: James K 
Type: Misc 
Category: Hartmann sensor 
Subject: SURF Log -- Day 6, Centroiding 

 So in addition to taking steps towards starting to set stuff up for the experiment in the lab, I spent a good deal of the day figuring out how to use the pre-existing code for finding the centroids in spot images. I spent quite a bit of time trying to use an outdated version of the code that didn't work for the actual captured images, and then once I was directed towards the right version I was hindered for a little while by a bug.

The 'bug' turns out to be something very simple, yet relatively subtle. In the function centroid_images.m in '/opt/EDTpdv/hartmann/src/', the function was assuming a threshold of 0 with my images, even though it has not long before been working with an image that Dr. Brooks loaded. Looking through the code, I noticed that before finding the threshold using the MATLAB function graythresh, several adjustments were made so as to subtract out the background and normalize the array. After estimating and subtracting a background, the function divides the entries of the image array by the maximum value in the image so as to normalize this. For arrays composed of numbers represented as doubles, this is fine. However, the function that I wrote to import my image arrays into MATLAB outputs an image array with integer data. So when the function divided my integer image arrays by the maximum values in the array, it rounded every value in the array to the nearest integer -- that is, the "normalized" array only contained ones and zeros. The function graythresh views this as a black and white image, and thus outputs a threshold of 0.

To remedy this, I edited centroid_images.m to convert the image array into an array of doubles near the very beginning of the function. The only new line is simply "image=double(image);", and I made a note of my edit in a comment above that line. The function started working for me after I did that.


I then wrote a function which automatically centroids an input image and then plots the centroids as scatter-plot of red circles over the image. For an image taken off of the Hartmann camera, it gave the following:


Zoomed in on the higher-intensity peaks, the centroids look good. They're a little offset, but that could just be an artifact of the plotting procedure; I can't say for certain either way. They all appear offset by the same amount, though:


One problem is that, for spots with a much lower relative intensity than the maximum intensity peak, the centroid appears to be offset:


Better centering of the beam and more even illumination of the Hartmann plate could mitigate this problem, perhaps.


I also wrote a function which inputs two image matrices and outputs vector field plots representing the shift in each centroid from the first to the second images. To demonstrate that I could use this function to display the shifting of the centroids from a change in the wavefront, I translated the fiber mount of the SLED in the direction of the optical axis by about 6 turns of the z-control knob  (corresponding to a translation of about 1.9mm, according to the user's guide for the fiber aligner). This gave the following images:


Before the translation:




 This led to a displacement of the centroids shown as follows:


Note that the magnitudes of the actual displacements are small, making the shift difficult to see. However, when we scale the displacement vectors up, we can get much more readily visible Direction vectors (having the same direction as the actual displacement vectors, but not the same magnitude):


This was a very rough sort of measurement, since exposure time, focus of the microscope optic, etc. were not adjusted, and the centroids are compared between single images rather than composite images, meaning that random noise could have quite an effect, especially for the lower-magnitude displacements. However, this plot appears to show the centroids 'spreading out', which is as expected for moving the SLED closer to the sensor along the optical axis.


The following MATLAB functions were written for this (both attached):

centroidplot.m -- calls centroid_image and plots the data

centroidcompare.m -- calls centroid_image twice for two inputs matrices, using the first matrix's centroid output structure as a reference for the second. Does a vector field plot from the displacements and reference positions in the second output centroids structure.

Attachment 5: 6turn_before.png  25 kB  | Hide | Hide all
Attachment 9: centroidplot.m  363 Bytes  Uploaded Thu Jun 24 08:33:01 2010  | Hide | Hide all
function centroiddata=centroidplot(M,N)
%a function to read the image matrix M and plot the centroids of each plot
%on the image
cd /opt/EDTpdv/hartmann/src/
centroiddata = centroid_image(H);
cd /opt/EDTpdv/JKmatlab/

... 6 more lines ...
Attachment 10: centroidcompare.m  762 Bytes  Uploaded Thu Jun 24 08:33:29 2010  | Hide | Hide all
function centroiddata=centroidcompare(A,B,M,N)
%compares the Mth image in 3D image matrix A to Nth in B
cd /opt/EDTpdv/hartmann/src/
cd /opt/EDTpdv/JKmatlab
... 16 more lines ...
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