Light-piping

Light-piping is a low-tech gimmick used on Transformers toys that allows lighting effects (usually for the eyes) without the need for electronics. Light-piping is achieved by using a clear (often colored) piece of plastic that is inserted into the body part in question (usually the head), leading from one side of the body to the other side. When the other (usually the rear) side is held against an external light source, the light will shine through the clear plastic and cause a lighting effect on the front side.

Generation 2

 * Optimus Prime (Hero)

Robots in Disguise

 * Scourge (G2 Laser Optimus Prime redeco)

Alternators

 * Shockblast

Energon

 * Treadshot
 * Terradive
 * Windrazor
 * Storm Jet, including the Superion Maximus head
 * Wideload
 * Steamhammer, including the Constructicon Maximus head

Cybertron

 * Overhaul
 * Clocker
 * Brakedown
 * Scrapmetal (all color variants)
 * Thundercracker (Deluxe Class)
 * Red Alert (well... supposedly. It doesn't really work, though)
 * Snarl
 * Unicron (Deluxe Class)
 * Dark Crumplezone (hindered somewhat by a large piece of opaque plastic blocking most of the light behind his head)

2007 Movie line

 * Protoform Starscream
 * Protoform Optimus Prime
 * Autobot Jazz (Deluxe Class), including Final Battle Jatt retool
 * Decepticon Brawl (Deluxe Class)
 * Wreckage sports very thin eyes, rendering the light-piping gimmick pretty borderline useless.
 * Swindle uses a very dark tint for the clear plastic, rendering the light-piping gimmick pretty downright useless.
 * Dreadwing features possibly the least impressive light-piping gimmick, considering the top of his head is made up of the jet mode cockpit's canopy.
 * Arcee (Deluxe Class)
 * Bumblebee (Deluxe Class '08 Camaro)
 * Longarm
 * Autobot Ratchet (Voyager Class) sports very thin eyes, rendering the light-piping gimmick pretty borderline useless.
 * Blackout (Voyager Class)
 * Megatron (Voyager Class)
 * Optimus Prime (Voyager Class)
 * Megatron (Leader Class)
 * Decepticon Brawl (Leader Class) combines a light-piping gimmick with electronics, thus allowing for two different methods of lighting..

Toys with disabled light-piping gimmicks
Some toys are designed to feature a light-piping gimmick, but the gimmick is rendered useless by having the parts in question (often the eyes) painted over or replaced with opaque plastics.

Cybertron

 * Ransack
 * Brakedown GTS not only sports painted eyes... but also uses opaque black plastic for the light-piping parts from the original release of the mold.
 * Override

Timelines

 * Darksyde Megatron

2007 Movie line

 * Ironhide (Voyager Class)
 * Dropkick