How to remove correct illumination band in frequency domain image?

I ran this code for x-ray medical images and get high pass filtering and low pass filtering images in MatLab R2013a to remove illumination.But I don't know how to find correct illumination band to remove.How can I find it and remove illumination by filtering?

Then I tried to convert images into spatial domain using inverse FFT function.But didn't work correctly.Can you please suggest a code for this conversion also.

img = imread('cephalogram.bmp');

img2 = imnoise(img,'salt & pepper',0.025);

img3 = img2;

for c = 1 : 3

    img3(:, :, c) = medfilt2(img2(:, :, c), [5, 5]);%add median filter

end

I=rgb2gray(img3); % convert the image to grey

A = fft2(double(I)); % compute FFT of the grey image

A1=fftshift(A); % frequency scaling

% Gaussian Filter Response Calculation

[M N]=size(A); % image size

R=10; % filter size parameter

X=0:N-1;

Y=0:M-1;

[X Y]=meshgrid(X,Y);

Cx=0.5*N;

Cy=0.5*M;

Lo=exp(-((X-Cx).^2+(Y-Cy).^2)./(2*R).^2);

Hi=1-Lo; % High pass filter=1-low pass filter

% Filtered image=ifft(filter response*fft(original image))

J=A1.*Lo;

J1=ifftshift(J);

B1=ifft2(J1);

K=A1.*Hi;

K1=ifftshift(K);

B2=ifft2(K1);

%---------visualizing the results----------------------------------------------

figure(1)

imshow(I);colormap gray

title('original image','fontsize',14)

figure(2)

imshow(abs(A1),[-12 300000]), colormap gray

title('fft of original image','fontsize',14)

figure(3)

imshow(abs(B1),[12 290]), colormap gray

title('low pass filtered image','fontsize',14)

figure(4)

imshow(abs(B2),[12 290]), colormap gray

title('High pass filtered image','fontsize',14)

%%%-----------------Converting to spatial domain--------------------------------------------------------

%fftInverse = REAL_PART(FFT(ffTransform, /INVERSE))

% invimg = ifft2(A1);

%figure(7)

%imshow(abs(invimg),[12 290]), colormap gray

%title('Inverse lowpass image','fontsize',14)

Answer 1

[Image Analyst]

Generally changes in illumination / exposure are slowly varying so you'd want to use a high pass filter to remove low spatial frequencies. You can do this in the spatial, with conv2(), or Fourier domain with fft2(). Attach 'cephalogram.bmp' if you need more help.

[Nayana R]

Thank you so much Image Analyst for your valuable answer.I want to do it in Fourier domain. How can I use fft2() to remove illumination.I am attaching my 'cephalogram.bmp'.It's resolution is 1935*2400.I reduce it to 560 pixels.Can you give me a example code for this filtering technique?