Find local min point in plot

Hello,

I am trying to find the local min on my plot but i want it to find only the minimun of the local min point.

this is my code:

clear all;
clc;
Wavelength = xlsread('Calcite.xlsx','A6:A2156');
reflection = xlsread('Calcite.xlsx','B6:B2156');
lmin=islocalmin(reflection);
figure;
plot(Wavelength,reflection,Wavelength(lmin),reflection(lmin),'ro')
title('(111b Sample) Relative Reflectance--Wavelength')
xlabel('Wavelength[nm]')
ylabel('Relative Reflectance')
grid on
grid minor

as you can see i get all the min points but i want to get only the minimum of the local min.

Thank you for your help.

Answer 1

[Cris LaPierre]

I suggest trying out the 'Find local extrema' live task. This will allow you to interactively set criterial for finding maxima and minima in your signal. Once you have the correct parameters, you can convert the task to code if you want, or keep the task in your live script.

[Anton Vernytsky]

Thank you ! it helped me.

Answer 2

[Voss]

clear all;
clc;
Wavelength = xlsread('Calcite.xlsx','A6:A2156');
reflection = xlsread('Calcite.xlsx','B6:B2156');

% lmin=islocalmin(reflection);
lmin = find(islocalmin(reflection));
[~,idx] = min(reflection(lmin));
idx = lmin(idx);

figure;
% plot(Wavelength,reflection,Wavelength(lmin),reflection(lmin),'ro')
plot(Wavelength,reflection,Wavelength(idx),reflection(idx),'ro');

title('(111b Sample) Relative Reflectance--Wavelength')
xlabel('Wavelength[nm]')
ylabel('Relative Reflectance')
grid on
grid minor

[Anton Vernytsky]

Thank you for your help.

[Voss]

You're welcome. If this solution works, please mark the answer as accepted. Thanks!

[Anton Vernytsky]

It only showed me the lowest point,what i did was the first solution,this is the code and plot:

clear all;
clc;
Wavelength = xlsread('Calcite.xlsx','A6:A2156');
reflection = xlsread('Calcite.xlsx','B6:B2156');
lmin = islocalmin(reflection,"MinSeparation",300);
figure;
plot(Wavelength,reflection,Wavelength(lmin),reflection(lmin),'ro')
title('(111b Sample) Relative Reflectance--Wavelength')
xlabel('Wavelength[nm]')
ylabel('Relative Reflectance')
grid on
grid minor

[Voss]

I interpreted "find only the minimun of the local min point" to mean "find the lowest local minimum". So yeah, if you want to ignore certain local minima because they are close to another local minimum or whatever, you can't use my code for that.