Finding and Graphing FWHM (Trouble with Find Function)

Hello! After looking at many other FWHM question i've come up with the code below. I have attached that data and the plotted data as well as the data ploted with FWHM.The FWHM is obviously incorrect, it is failing in the find function. hmLeft is giving me 1 and hmRight is giving me 500.Let me know your suggestions!

load data_10micronslit.txt;
xdata10 = data_10micronslit(:,1);
ydata10 = data_10micronslit(:,2);

ymax = max(ydata10);
halfmax = ymax/2;

hmLeft = find(ydata10 >= halfmax, 1, 'first');
hmRight = find(ydata10 >= halfmax, 1, 'last');

figure
plot(xdata10,ydata10,'bo');
xlabel('Wavelength (nm)');
ylabel('Signal');
title('Signal vs Wavelength');

hold on
line([hmLeft hmLeft], [0 halfmax], 'Color', 'r', 'LineWidth', 1);
line([hmRight hmRight], [0 halfmax], 'Color', 'r', 'LineWidth', 1);
line([hmLeft hmRight], [halfmax halfmax], 'color', 'r', 'LineWidth', 1);
hold off

Answer 1

[Star Strider]

Without ‘data_10micronslit.txt’, it is impossible to provide a specific solution. However the findpeaks function will return the FWHM of each peak it identifies (if you request that it do so). Additionally, the islocalmin and islocalmax functions can help to define the beginning and end of peaks as well as the amplitude of the peaks, and from that you can then use interp1 (twice) to calculate FWHM.

[Nichole Peterson]

I have attached the data set and a bit more infomation! Thank you for your help.

Though I dont think i understand how to use the functions you have given.

[Star Strider]

This findpeaks call:

D = load('data_10micronslit.txt');
xdata10 = D(:,1);
ydata10 = D(:,2);

[~,~,fwhm,prom] = findpeaks(ydata10, xdata10, 'MinPeakProminence',1E-3)

produces:

fwhm =
   0.228209696888428

EDIT — (27 Mar 2020 at 17:28)

The code to plot it becomes a bit more involved because interp1 has problems with the ‘ydata10’ vectors not being monotonic:

ymin = min(ydata10);
[pks,locs] = findpeaks(ydata10, 'MinPeakProminence',1E-3);
idxrng = [locs-20:locs; locs:locs+20]';             % Calculate ‘x’ Values To Plot ‘FWHM’
xm = xdata10(idxrng);
ym = ydata10(idxrng);
B1 = [xm(:,1), ones(size(xm(:,1)))] \ ym(:,1);
B2 = [xm(:,2), ones(size(xm(:,2)))] \ ym(:,2);
hmv = (pks-ymin)/2;
hm(1) = (hmv+ymin-B1(2))/B1(1);
hm(2) = (hmv+ymin-B2(2))/B2(1);

figure
plot(xdata10, ydata10)
hold on
plot(hm, [1 1]*hmv+ymin, '-r')
hold off
grid
text(hm(2), hmv+ymin, sprintf('\\leftarrow FWHM = %.5f',fwhm))

producing:

[Nichole Peterson]

Is there any way to use the find function so I can plot it using "line" and use hmleft-hmright = fwhm ?

[Nichole Peterson]

I am also finding that as I use this with othe data sets, like the one i have attached, I am getting the following as an output:

pks =

    0.2038
    0.2678
    0.2966
    0.3091
    0.3441
    0.3543
    0.3452
    0.3648
    0.3676
    0.3686
    0.3677
    0.3686
    0.3588
    0.3472
    0.3454
    0.3258
    0.3158
    0.2393
    0.2301


locs =

   251
   262
   267
   271
   279
   281
   283
   286
   288
   292
   294
   296
   299
   301
   303
   308
   310
   328
   330


fwhm =

    0.6763
    0.6262
    1.1284
    0.5787
    0.8014
    0.9827
    0.7835
    0.6270
    1.0751
    9.1444
    0.7254
   87.7117
    0.6506
    0.6384
    0.9176
    0.6596
    0.7704
    0.6441
    0.7670

[Star Strider]

For the second data set, the code changes slightly:

[pks,locs] = findpeaks(ydata10, 'MinPeakProminence',0.05);
[~,~,fwhm,prom] = findpeaks(ydata10, xdata10, 'MinPeakProminence',0.05);

idxrng = [locs-90:locs; locs:locs+90]';             % Calculate ‘x’ Values To Plot ‘FWHM’
xm = xdata10(idxrng);
ym = ydata10(idxrng);
B1 = [xm(:,1), ones(size(xm(:,1)))] \ ym(:,1);
B2 = [xm(:,2), ones(size(xm(:,2)))] \ ym(:,2);
hmv = (pks-ymin)/2;
hm(1) = (hmv+ymin-B1(2))/B1(1);
hm(2) = (hmv+ymin-B2(2))/B2(1);

and produces:

NOTE — There are two findpeaks calls in my code, one to calculate the FWHM value, and the other one to return the data necessary for the plot.