cardboard iPhone brightness issue

Hi all,

I'm managing to create fairly decent VR stuff using the cardboard sdk package for iOS, but for some reason when I run the app in the viewer the imagery starts dark-ish then after about 15 seconds suddenly turns bright (a bit like screen timeout in reverse) I'm using a single cgi image rendered with a spherical camera, and imported as a cubemap on a skybox. Can anybody throw any light on what might be causing the issue (pardon the pun)?

Thanks