Is there a way to extract the width/diameter of the contour lines?

I have this image which is an intensity plot of my laser beam:

Now I want to find the horizontal and vertical (1/e^2) diameter (distance is 100 µm/px). Do you have any suggestions on how to go about fitting an ellipse at the 1/e^2 threshold.

the countour function seams to draw nice interpolated lines would there be any idea in trying to extract the information from this plot? If I can specify the counter to only be drawn at the 1/e^2 level?

[Micke Malmström]

This is what I have so far which seams to do fine:

d=dir('*.cma'); & find all files from the Pyrocam III

for ii= 1:length(d)% 1:1%
    data=importdata(d(ii).name); 
    data=data(55:end,:); % remove bad pixels
    data=data./max(data(:)).*exp(2); % normalize and multiply by e^2
%     data(data<0)=0; % Remove negative values, (not needed here)
% 
%     figure(1);clf
%     imagesc(data); 
%     axis image; hold on
%     colormap('jet');
%     pause(0.01) % to see that it looks reasonable 

% Draw contour
     [C,h] = contour(data,'LineWidth',2, 'LevelList',1, 'LineColor','r');
% Calculate Horizontal and Vertical extent multiply by 0.1 mm/px
     Hw(ii)=(max(C(1,2:end))-min(C(1,2:end)))*.1;
     Vw(ii)=(max(C(2,2:end))-min(C(2,2:end)))*.1;

end

Answer 1

[Image Analyst]

Get a binary image of your data where it's more than 1/e^2

binaryImage = data > 1 / exp(2);

Now get the boundaries:

boundaries = bwboundaries(binaryImage);
x = boundaries{:,2};
y = boundaries{:,1};

then use the FAQ to fit an ellipse: http://matlab.wikia.com/wiki/FAQ#How_can_I_fit_an_ellipse_or_other_shape_to_a_set_of_XY_data.3F to those x and y values.

Answer 2

[Star Strider]

If I understand correctly what you want, you can draw a specific contour (with a second call to contour) at the 1/e^2 level with:

C = contour(data, [1 1]*(1/e^2), 'LineStyle','none');
contour(data,'LineWidth',2, 'LevelList',1, 'LineColor','r');

The ‘C’ variable will then have the (x,y) coordinates of the ‘1/e^2’ contour.