How to find FWHM from this?

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hnn
hnn on 8 Oct 2022
Commented: hnn on 8 Oct 2022
I am very new to Matlab, and am trying to find the FWHM from this curve. I have tried findpeaks, and don't really understand any of it. I have these (time and power) as my variables and have tried but really don't know how to do it.

Accepted Answer

Image Analyst
Image Analyst on 8 Oct 2022
See if this is what you want
halfMaxValue = max(power) / 2; % Find the half max value.
% Find indexes of power where the power first and last is above the half max value.
leftIndex = find(power >= halfMaxValue, 1, 'first');
rightIndex = find(power >= halfMaxValue, 1, 'last');
% Compute the delta time value by using those indexes in the time vector.
fwhm = t(rightIndex) - t(leftIndex)
  1 Comment
hnn
hnn on 8 Oct 2022
Awesome thank you! That gave me exactly what I needed

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More Answers (1)

Chunru
Chunru on 8 Oct 2022
Your data has no half power points so you cannot find fwhm.
load(websave("fwhmdata.mat", "https://www.mathworks.com/matlabcentral/answers/uploaded_files/1149260/fwhm%20data.mat"))
whos
Name Size Bytes Class Attributes M 384x2 6144 double cmdout 1x33 66 char data 384x1 3072 double index1 384x1 3072 double index2 384x1 3072 double power 384x1 3072 double time 384x1 3072 double
plot(time, power);
[pmax, imax] = max(power);
hold on
plot(time(imax), pmax, 'ro');
i1 = find(power(imax:-1:1) <= 0.5*pmax, 1, 'first')
i1 = 0×1 empty double column vector
i2 = find(power(imax:end) <= 0.5*pmax, 1, 'first')
i2 = 0×1 empty double column vector
if ~isempty(i1) & ~isempty(i1)
fwhm = tmax(i2+imax-1) - tmin(imax-i1+1);
else
fwhm = nan;
end
fwhm
fwhm = NaN
  8 Comments
hnn
hnn on 8 Oct 2022
Your method did not work but I used a different function which gave me the correct FWHM value which also matches my python answer, as well as matching the physical beam size of the telescope used to gather this data. Thanks for your super super super helpful reply! Have a nice day
Chunru
Chunru on 8 Oct 2022
If you want the width at mid of max and min. Just add:
power = power - min(power);

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