Area under a peak

Hi, I am struggling with calculating the area under the peak shown in the graph by red color. I want to calculate the area under the peak as well as area of the entire curve (including the peak).

The data file in.txt format has x and y axis data.

Answer 1

[Mathieu NOE]

hello

based on @Davide Masiello excellent proposition, I tried to make the selection of the peak zone a bit more automatic...

the computation of the area of the peak may significantly vary based on where start and stop the selected segment , as how much of the root (being wider) will impact the result

this is how is works with the code below :

you can change the threshold (threshold_above_curve) to see the effect

clear,clc
A = readmatrix('875nm.txt'); % faster
x = A(:,1);
y = A(:,2);
[~,imax] = max(y);

% Total area
area_tot = trapz(x,y)
%area_tot = -4.5442e+03

% Area below peak
window = 200; % must be wider than expected peak
threshold_above_curve = 0.25; % above 0 and less than 0.5

[val,ind] = max(y);
x_tmp = x(ind-window/2:ind+window/2);
y_tmp = y(ind-window/2:ind+window/2);
y_tmp_d = detrend(y_tmp);
y_tmp_d = y_tmp_d - min(y_tmp_d);
ind = find(y_tmp_d>threshold_above_curve);

area_peak = trapz(x_tmp(ind),y_tmp(ind))
figure
plot(x,y,x_tmp(ind),y_tmp(ind),'r');

[Davide Masiello]

Very nicely done. +1 :)

[Mathieu NOE]

thanks! :)

[Sam Chak]

Another +1

[Mathieu NOE]

thanks again !

it's the beginning of the glory ! haha

Answer 2

[Davide Masiello]

clear,clc
A = readtable('875nm.txt');
x = A.Var1;
y = A.Var2;

[~,imax] = max(y);

% Total area
area_tot = trapz(x,y)

area_tot = -4.5442e+03

% Area below peak
area_peak = trapz(x(imax-50:imax+50),y(imax-50:imax+50))

area_peak = -26.6612

figure
plot(x,y,x(imax-50:imax+50),y(imax-50:imax+50),'r')

Mind that the areas are both negative because your curve is almost all below the x axis.

[Haris Ashraf]

Here you assumed that peak will start and end 50 points ahead and back from the index of the peaks respectviely. Is there any particular reason for this assumption?

[Davide Masiello]

No, you can vary that number depending on what you decide your peak is. I have added a plot in my answer to show you what 50 points around the maximum actually encompasses.