What's the effect of breaking lights?

There is no effect.

Part 1: Does light affect the visibility of the player?

To test this, I soloed my way to Framing Frame Day 3. With my silenced pistol in hand, I waited for a guard to come by. When he spotted me, I killed him. Then I shot out all the lights on the roof. When the next guard came by, I stood in the same place and waited. He was standing in the same place as the previous guard when he died.

Result of stealth test: Light has no effect on player visibility.

Part 2: Does light (or the absence of it) cause guards to become curious?

After my previous test, no guard seemed to want to come up to the roof. I reloaded the mission and again soloed to the third day. When a guard arrived on the roof, I used my water arrows silenced pistol to shoot out the lights near him.

Result of attention test: Broken lights have no effect on guard attention.

Part 3: Extra Credit:

Then, since I was there, I started to wonder if there was anything I could shoot that would bother him...

List of things that guards do not notice .45 caliber bullets going into:

• Lights
• Tables
• Roofs
• Plants
• The floor right in front of them
• The space between their knees

List of things that guards do notice .45 caliber bullets going into:

• Cameras
• Windows
• Themselves