How to find the wavelength from a given photo?

I want to find the wave-length of few spectral lines like the picture below

Is it possible since i want to use these kind of pictures to find wave length and use those wavelength for using in formulas.

I don't have any other software other than mathlab like simulink.

[Star Strider]

Do you have known reference spectra using the same instrument?

[Pijush]

no its just a image from Wikipedia. I just want a approximate value so that what ever image i use i can get a wavelength from it and i don't have to put them manually. @starStrider

[Star Strider]

That’s a nonstandard format (.svg), at least for MATLAB. When I tried to load it with imread, it threw an error.

Image Analyst may have some thoughts...

Answer 1

[Image Analyst]

You'd have to know what column corresponds to what wavelength. For example, is column 1 defined to be 400 nm and the last column is 700 nm? If so, you can just basically take the column, turn it into a percentage of the way from left to right, then multiply by 300 (=700-400) and add 400.

[rows, columns, numberOfColorChannels] = size(rgbImage);
allColumns = 1 : columns;
wavelength = (700-400) * allColumns / columns + 400;

To get the wavelength at some particular column, say column 250, just do

wavelengthAtCol250 = wavelength(250);

[Pijush]

that was a great method it will be of great use for me even in future. on this project can i make the mathlab recognize that on which coloum we get the spectral line, like lets say on 50th colomn there is a spectral line and i write a code such that the mathlab it self recognise that coloum have a spectral line and save the value of corresponding wavelength to a variable.

[Pijush]

like can we recognise the particular coloumn is black or some coloured?

[Image Analyst]

To find indexes, you can just check if the line is zero or not. To find the logical index (true or false):

logicalIndexes = yourImage(1, :, 1) ~= 0;

which assumes that the black is zero. Or you can get the actual indexes (20, 38, 193, or whatever):

linearIndexes = find(yourImage(1, :, 1) ~= 0);

[Pijush]

[rows, columns, numberOfColorChannels] = size(rgbImage);
allColumns = 1 : columns;
wavelength = (700-400) * allColumns / columns + 400;

can you explain this code?

[Walter Roberson]

You cannot "load" a .jpg image: you have to imread() it.

[Image Analyst]

See if these extremely verbose comments describe the simple operations better for you.

% First somehow read your image into variable rgbImage.
% Then find out how many rows tall, columns wide the image is,
% and how many color channels there are (which will be 3).
[rows, columns, numberOfColorChannels] = size(rgbImage);
% Make a list of numbers [1,2,3,4,5,6,......#columns]
allColumns = 1 : columns;
% Get the percentage of the way the column is from 1 to #columns.
% This will be the column number divided by the number of columns.
% We can do this for all columns at once by computing allColumns/columns.
% Then multiply by the wavelength range, like 300 nm.
% But this will give a number from 1 to 300 nm.
% So we need to add the first wavelength to shift the
% wavelength from the range 0-300 to 400-700.
wavelength = (700-400) * allColumns / columns + 400;

[Pijush]

i am getting error on line 2

    invalid constant left hand side of assignment

>>> [1,40,3] = size(rbgImgage);

with a small arrow below the 3

the code i put was

rbgImage = imread ("s.jpg")
[1,40,3] = size(rbgImgage);
allColumns = 1 : 3;
wavelength = (700-400)*allColumns / 300 + 400;

wavelengthAtCol250 = wavelength(450);

and how to implement this code

logicalIndexes = yourImage(1, :, 1) ~= 0;

[Image Analyst]

Now, I didn't do that, did I? What I did was

[rows, columns, numberOfColorChannels] = size(rgbImage);
allColumns = 1 : columns;

Why did you change it? You can't put constants on the left hand side because size() is going to try to fill up those spots with the values. Also you misspelled rgbImage.