FASA Games, Inc.

how astral sensing interacts with the lighting conditions of the real world? what happens, if a normal human, who is capable of the spell astal sense or the talent astral sight finds himself in a dark cave, without any torches or the like? if he "switches" to astral seeing will he be able to sense his surroundings? can he fight something efectively this way? without darkness penalties perhaps?

without darknes penalties for sure, but with other penalties instead. Astral can be misleading- dimensions could be a little twisted, it happen that astral spaceis "bigger" than physical world, also its bright there- in cave earth will glow, and magical energy will glow, and enemy will glow, any any active magic will glow. Third penalty is from any tain thta could be present which can be darker than dark of the cave.
So all this conditions can impose penalties.

Astral Taint does impose Perception penalties. Also, each form of astral viewing has it's own limitations (range, detecting one-pattern-at-a-time vs everything-within-range, etc).