Parchment paper (mine at least) still eats up about 1.5 stops of light. Probably enough to impact your vivarium if left on long term. By comparison my standard laser printer paper (75 g/m^2 density) eats up about 3 stops of light, so definitely better in terms of letting more light through and I find it diffuses the light sufficiently most of the time. Plus, since I bought a roll for photographic purposes I tried using it when cooking messy things like chicken wings. Cleanup is now a breeze.