Illumina primer immobilization

[howiewang]

I'm just curious the primers are actually immobilized on the bottom wall only or they are immobilized on both top and bottom wall (and side wall as well) of the flow cell?
If primers are on bottom wall only, how the bottom wall is selectively activated?
If primers on both bottom and top wall, if the fluorescence signal from top wall will effect the final signal (fluorescence from out of focus plane)? or illumina actually image both walls?
Many thanks.

[james hadfield]

Oligos are attached al over the surfaces of the lane. The imaging is directed to one focus point to generate high quality data.