A guy ran some tests in the KF Raid to determine the effects of LL, attack number and impact on damage dealt.
The conclusion may surprise you…
Your light level does not directly contribute to damage. It does, however, add a penalty if you’re below the recommended light. Going into the raid at 295 light will get you a ~15% damage penalty at Oryx, for instance.
The attack stat on your weapon matters. Each point of attack seems to add ~0.7% damage.
Each weapon contributes 12% of your light, each armor piece 10% and ghosts, class items and artifacts 8%.
Daily heroics and Nightfalls have extra damage penalties on top of everything else.