When powered, redstone lamps behave as if transparent, like a glass block. When not powered, they behave like opaque blocks, like stone.
Build an enclosed space such that the sky light level becomes 0. Replace some of the ceiling or walls with redstone lamp. Power the lamp.
Expected behavior: A powered lamp should act as a solid block and block sky light. The sky light level should be zero in the enclosed room.
Actual behavior: Sky light level becomes non zero.
To duplicate:
1) Build enclosure with a redstone lamp forming part of the enclosure.
2) Ensure that sky light level is 0.
3) Power redstone lamp.
4) Observe the sky light level.
Related issues
Attachments
Comments

This bug makes it very difficult to enclose a daylight sensor with redstone lamps to make a lamp that turns itself on at night!
Main issue: MC-5927.
I don't think this issue is a duplicate of MC-5927, it doesn't explain why redstone lamp behave that way.
Since this strange behavior is still present in 14w21b, I suggest to reopen.
I agree, but it is easier to demonstrate the redstone lamp opacity by pressing F3 and checking the sky light levels. If you create an enclosed space out of redstone lamps, you find the sky light to be 0. Then when you power the lamps, sky light gets in. I don't think this is intended behavior. But perhaps it is.

From the way the description is written, the ticket appears to focus on the behavior of the daylight sensor, rather than on the redstone lamp. If you rewrite the description to demonstrate the redstone lamp issue independently of the issue with the daylight sensor, I'll reopen it.

Very nice.

Aren't all light producing blocks supposed to be transparent? (So that the light that they produce can pass through the block itself, correct me if I'm wrong)
Also in 1.8.1-pre3 they don't seem to be fully transparent (like glass), instead they diffuse sky light like leaves (except they don't initially reduce the light level).
Note the color/sky light level changes in the screenshots (the 2nd underground screenshot has a stone block on top of the glowstone lamp, allowing no sky light to pass; the 1st doesn't have the stone block).

Can longer reproduce in 15w44b. A tall shaft closed with a lit redstone lamp is dark at the bottom while being brightly lit when the redstone lamp is destroyed.