i refer to them as "scanlines" i duno if thats wht every1 calls em but its wht i do.
u open a transperent 3x3pxl document and with a 1pxl black pencil put dots from 1 corner to the other (which ever direction depending on which way u want ur lines to be angled) then go Edit>define as pattern.
once u have done this open ur siggy and apply the new pattern and lower the opacity to w.e suits ur design.
hope that helped u out.