How to get image brightness ?

Hi everyone! I am new to MATLAB Image Processing and I need your help. I need to build a code that calculates this instantaneous apparent brightness m_inst, related to the straight line in figure. The exposure time of the frame that I have attached here is one second. Can anyone help me?

I have worked on a code like this to obtain the brightness of the pixels belonging to that straight line but I don't know if it is useful for the calculation of apparent brightness:

A=rgb2gray(imread('Img_0685.bmp'));

B=bwareafilt(imbinarize(A-medfilt2(A,[5,5])) ,1)  ;

pixelValues=A(B)
figure
imshow(B)

Answer 1

[yanqi liu]

clc; clear all; close all;
A=rgb2gray(imread('https://ww2.mathworks.cn/matlabcentral/answers/uploaded_files/766496/Img_0685.bmp'));

B=bwareafilt(imbinarize(A-medfilt2(A,[5,5])) ,1)  ;

res=mean2(double(A(B)))

res = 230.1644

figure
imshow(B)

Answer 2

[yanqi liu]

clc; clear all; close all;
A=rgb2gray(imread('https://ww2.mathworks.cn/matlabcentral/answers/uploaded_files/766496/Img_0685.bmp'));

B=bwareafilt(imbinarize(A-medfilt2(A,[5,5])) ,1)  ;

C=A.*uint8(B);

ci = unique(C(:));
ci_sum = 0;
for i = 1 : length(ci)
    if ci(i) > 0
        ci_num = length(find(C(:)==ci(i)));
        ci_sum = ci_sum + ci_num;
    end
end

n = length(find(C(:)>0));

csky = length(find(C(:)==0));

t = 1;

minist = 30 - 2.5*log10((ci_sum-n*csky)/t)

minist = 10.7431 - 3.4109i

figure
imshow(C)

[Gargolla9]

Thank you for your idea but I think that something is wrong: for this frame that I have attached here and also for others frame that I have, the value of ci_sum and n is always the same; another problem is the fact that the final result, m_inst, is a complex value but it must be a real value.