Homebrew PCBs

Have just posted a picture under vk4apn. There is a noticable ripple
effect on the straight lines. The image was crisp and straight and
made using vacuum lightbox. I made some other smaller boards at same
time and they are perfect. The PCB material is 0.8mm kinsten. It
looks like board itself is not perfectly flat as ridges conincide with
etched pattern. First time to use thin board so am I doing something
wrong?
Paul vk4apn