How can I get log Average luminance value?

I want to calculate Log Average Luminance Value from my image. In attached image, it shows equation from my reference paper to calculate LAVG. I also change my image to YCbCr format, divide 8x8 block and put it in Cell Array.

For RGB,

R=rgb_img(:,:,1)

G=rgb_img(:,:,2)

B=rgb_img(:,:,3)

But now, I don't know how to access Y value and how calculate Log Average Luminance. Can anyone please help me to do this? Because I'm really lost for my thesis.

I would be very grateful if anyone can help me in this matter.