How to find the locations or indices of lower and upper ends of FWHM of a peak

How to find the locations and Index of lower and upper limit of FWHM of the largest peak shown in the pic below

Whn I use findpeaks function I am able to get the FWHM but I find it hard to get the location/index of the upper and lower limits of the FWHM

since there exists another peak in the range having a peak value above the half maximum of the main peak.

Answer 1

[Simon Chan]

Suppose y is the data, try this:

You may change the interpolation method, mine is only an example.

The location of the limits on the left side is between idx1 & idx1+1,

while on the right side, it is between idx2-1 & idx2.

[~,location] = max(y);
yn = y/max(y);                                                      % Normalized data
idx1 = location - find(diff(yn(location:-1:1)>0.5)~=0,1,'first');
idx2 = location + find(diff(yn(location:1:end)>0.5)~=0,1,'first');
xaxis_left = interp1([yn(idx1), yn(idx1+1)], [x(idx1), x(idx1+1)], [yn(idx1), 0.5, yn(idx1+1)], 'linear');
xaxis_right = interp1([yn(idx2-1), yn(idx2)], [x(idx2-1), x(idx2)], [yn(idx2-1), 0.5, yn(idx2)], 'linear');
FWHM = xaxis_right(2) - xaxis_left(2)

[Vinay Killamsetty]

Dear , Simon Chan

Thank you very much for your answer. Infact it helped me in solving the problem.

Do you have any suggestion for finding the location of the first minimum on both sides of the main peak

[Simon Chan]

Try this:

idx3 and idx4 are the minimum points on the left and right side respectively.

Hope it helps

idx3 = location - find(diff(yn(location:-1:1))>0,1,'first') + 1;
idx4 = location + find(diff(yn(location:1:end))>0,1,'first') - 1;

Answer 2

[KSSV]

This might be usefu: https://in.mathworks.com/matlabcentral/answers/862960-how-to-calculate-the-full-wave-half-maximum

[Vinay Killamsetty]

Deat KSSV,

Thank you very much for your suggestion.

I have used the below command as you have suggested

knnsearch(x,y,'k',2)

But this function has helped me in finding the location of higher end of the FWHM but it is giving an inappropriate value for the location oflower end FWHM.

Instead of loacting the lower end of FWHM it is giving me some location from 1st peak