How to calculate the width where the intensity is higher than 20% of the maximum intensity?

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To do this, I think I have to utilize the findpeaks function. Then interpolate the 20% to the full width at half maximum (FWHM). So we can find its width?
Below is the code for an attempt to get the max intensity and its FWHM:
[pks,widths] = findpeaks(Int,x2,'NPeaks',1,'WidthReference','halfheight');

Accepted Answer

Mathieu NOE
Mathieu NOE on 29 Apr 2022
hello Angelo
I have no doubt you can use findpeaks
but sometimes I prefer my own way.
see demo below - this is to find W20 and extract the portion of data between the two crossing points
NB that the function crossing_V7 use linear interpolation which is more accurate (on the x axis) then simply finding the first x data element above 20% (this depends of your x spacing)
hope it helps
% data
x = x2; clear x2
y = Int; clear Int
threshold = 0.2*max(y); % 20% of peak amplitude
[t0_pos1,s0_pos1,t0_neg1,s0_neg1]= crossing_V7(y,x,threshold,'linear'); % positive (pos) and negative (neg) slope crossing points
% ind => time index (samples)
% t0 => corresponding time (x) values
% s0 => corresponding function (y) values , obviously they must be equal to "threshold"
% depending of the threshold level we have either one or three peaks
% if we have 3 peaks then pick the second one
if numel(s0_pos1) >1
s0_pos1 = s0_pos1(2);
s0_neg1 = s0_neg1(2);
t0_pos1 = t0_pos1(2);
t0_neg1 = t0_neg1(2);
% x difference between t0_neg and t0_pos = W20
W20 = t0_neg1 - t0_pos1
% extract the data and do interpolation
xx = linspace(t0_pos1,t0_neg1,100);
yy = interp1(x,y,xx);
plot(x,y,'b',t0_pos1,s0_pos1,'dr',t0_neg1,s0_neg1,'dg',xx,yy,'linewidth',2,'markersize',12);grid on
legend('signal','signal positive slope crossing points','signal negative slope crossing points','selected data');
xlim([-6e-4 6e-4]);
function [t0_pos,s0_pos,t0_neg,s0_neg] = crossing_V7(S,t,level,imeth)
% [ind,t0,s0,t0close,s0close] = crossing_V6(S,t,level,imeth,slope_sign) % older format
% CROSSING find the crossings of a given level of a signal
% ind = CROSSING(S) returns an index vector ind, the signal
% S crosses zero at ind or at between ind and ind+1
% [ind,t0] = CROSSING(S,t) additionally returns a time
% vector t0 of the zero crossings of the signal S. The crossing
% times are linearly interpolated between the given times t
% [ind,t0] = CROSSING(S,t,level) returns the crossings of the
% given level instead of the zero crossings
% ind = CROSSING(S,[],level) as above but without time interpolation
% [ind,t0] = CROSSING(S,t,level,par) allows additional parameters
% par = {'none'|'linear'}.
% With interpolation turned off (par = 'none') this function always
% returns the value left of the zero (the data point thats nearest
% to the zero AND smaller than the zero crossing).
% check the number of input arguments
% check the time vector input for consistency
if nargin < 2 | isempty(t)
% if no time vector is given, use the index vector as time
t = 1:length(S);
elseif length(t) ~= length(S)
% if S and t are not of the same length, throw an error
error('t and S must be of identical length!');
% check the level input
if nargin < 3
% set standard value 0, if level is not given
level = 0;
% check interpolation method input
if nargin < 4
imeth = 'linear';
% make row vectors
t = t(:)';
S = S(:)';
% always search for zeros. So if we want the crossing of
% any other threshold value "level", we subtract it from
% the values and search for zeros.
S = S - level;
% first look for exact zeros
ind0 = find( S == 0 );
% then look for zero crossings between data points
S1 = S(1:end-1) .* S(2:end);
ind1 = find( S1 < 0 );
% bring exact zeros and "in-between" zeros together
ind = sort([ind0 ind1]);
% and pick the associated time values
t0 = t(ind);
s0 = S(ind);
if ~isempty(ind)
if strcmp(imeth,'linear')
% linear interpolation of crossing
for ii=1:length(t0)
%if abs(S(ind(ii))) >= eps(S(ind(ii))) % MATLAB V7 et +
if abs(S(ind(ii))) >= eps*abs(S(ind(ii))) % MATLAB V6 et - EPS * ABS(X)
% interpolate only when data point is not already zero
NUM = (t(ind(ii)+1) - t(ind(ii)));
DEN = (S(ind(ii)+1) - S(ind(ii)));
slope = NUM / DEN;
slope_sign(ii) = sign(slope);
t0(ii) = t0(ii) - S(ind(ii)) * slope;
s0(ii) = level;
% extract the positive slope crossing points
ind_pos = find(sign(slope_sign)>0);
t0_pos = t0(ind_pos);
s0_pos = s0(ind_pos);
% extract the negative slope crossing points
ind_neg = find(sign(slope_sign)<0);
t0_neg = t0(ind_neg);
s0_neg = s0(ind_neg);
% empty output
ind_pos = [];
t0_pos = [];
s0_pos = [];
% extract the negative slope crossing points
ind_neg = [];
t0_neg = [];
s0_neg = [];

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