Can you measure the darkness of a room or the lightness?

Answer 1

Yes, you can measure the lightness of a room as light is composed of photons which are a quantized form of energy. Spectrophotometers measure that energy relative to a standard.
Darkness is by definition the absence of light.

Why is this in Religion and Spirituality?

Answer 2

Only lightness, because darkness is the absence of light. In other words, darkness isn't a thing, it's an absence of a thing.