How can I measure the brightness of a rotating object in a interval of time?

Hello, I have a code that rotates a thin plate around a random axis while being hit by a light source. After that I want to compute the brightness variation of the plate in an interval of time. My problem is that the light curve I obtain has strange "jumps" at some points and I don't know why this happens, it should be smoother. Here is the code I have. Am I measuring the brightness of the plate wrong? Thank you so much for your answers, I really need help because I don't know why this happens.

And if there is a way to generate the random axes in a better way it would be great to know too, thank you!

clc
clear all

%% THIN PLATE

vertex_matrix = [-1 0 -1; 1 0 -1; 1 0 1; -1 0 1];
faces_matrix = [1 2 3 4];

%% SIMULATE PLATE

hgtc = hgtransform('Parent',gca); 

patch('Vertices',vertex_matrix,'Faces',faces_matrix,...
      'FaceVertexCData',hsv(6),'FaceColor','White','FaceNormalsMode','auto','BackFaceLighting','reverselit','Parent',hgtc)      

axis([-4, 4, -4, 4, -4, 4])
axis square
set(gca,'color','k') %Plot black background 'k'

az = 0;
el = 0;
view(az, el);

xlabel('x') 
ylabel('y')
zlabel('z')

%% LIGHT

light('Position',[0 -5 0],'Style','local','Style','infinite');     %Place the light at infinity. The position specify the 
                                                                   %direction from which the light shines in parallel rays.   
                                                                   
%% ROTATION PLATE

beta = 0.003067961576; %2*pi/2048 ---> 1 lap in 2048s
t = 0:1:2047; 

JN=2; %JN - 1 = Number of axes you want to create (example: if JN = 3 ---> 2 random axes)
x = rand(1,JN-1);
y = rand(1,JN-1);
z = rand(1,JN-1);
axisvector = cell(JN-1, 1);
for i=1:JN-1
   v = [x(i) y(i) z(i)]; 
   axisvector{i} = v;
end


Faxis = cell(JN-1, 1);
for j=1:JN-1
 for i=1:length(t)
     RR = makehgtform('axisrotate',[axisvector{j}(1,1) axisvector{j}(1,2) axisvector{j}(1,3)],i*beta);
     set(hgtc,'Matrix',RR);
     F(i) = getframe(gcf,[120 60 320 300]) ;
     Faxis{j} = F;
     drawnow;
 end
end

meanGrayLevelsNEW = cell(JN-1, 1);
for j=1:JN-1
for i=1:length(F)
    % convert the image to a frame
    grayFrame = rgb2gray(Faxis{j}(i).cdata);
    % Calculate the mean gray level.
    meanGrayLevels(i) = mean(grayFrame(:));
    meanGrayLevelsNEW{j} = meanGrayLevels;
end
end
%% LIGHTCURVE

% Normalize the plot data 
for i=1:JN-1
    
minGray = min(meanGrayLevelsNEW{i});
maxGray = max(meanGrayLevelsNEW{i});
meanGrayLevelsNo = (meanGrayLevelsNEW{i} - minGray) / ( maxGray - minGray );

% Plot the mean gray levels.
        figure
		plot(meanGrayLevelsNo, 'k-', 'LineWidth', 2);
        axis([0 2048 0 1])
        xlabel('Frames') 
        ylabel('Relative Brightness')
		hold on;
end