If you wanted the whole scene to flash, you could do something like this in raytracing:
ifelse ticks > 200 and ticks < 203
[ raytracing:add-light x y z white ]
[ raytracing:add-light x y z color ]
This would make your scene flash white for 3 ticks, you could make multiple of these, or add more tick ranges if you would like it to flash multiple times. If you wanted a brighter flash just add more white lights!