Voltage contrast is a phenomenon that allows you to see voltages with your eyes.
You may have heard of the scanning electron microscope (SEM). A beam of electrons is rastered across a specimen in a vacuum chamber, providing greater magnifications and depth-of-field than possible with a visible light microscope. For every high-energy "primary" electron that hits the specimen, thousands of low-speed "secondary" electrons are created. Those near the surface can escape into the vacuum, where they get sucked up by a positively changed collector. That's why the SEM is so good at imaging surfaces, and why rough textures looks especially bright. Since secondary electrons move slowly, they are affected by local electrical or magnetic fields. Make the specimen more positive and less secondaries escape. More negative and the secondaries accelerate towards the positive collector. In digital circuits, that means "on" looks black and "off" is white. Naturally this requires electrical connections into the vacuum chamber.
The video above explores functioning and mal-functioning random access memory (RAM) circuits. The surface passivation glass has been removed from some to expose the metal conductors. If the glass remains, only voltage transients are visible, thanks to capacitive coupling with the surface. But toggling the circuit at video rates, you can resolve every conductor present even thru the glass.
At the 1:20 mark, you can see a coupling fault: two memory cells respond instead of one. These are normally time-consuming to detect, requiring GALPAT tests which run in 4n^2 time. But here we see the fault with a single image.
The final minute shows another failure analysis technique: heat sensitive Liquid Crystal, applied to the surface, reveals the location of "hot spots" with a light microscope. In my hands, shorts as small as 0.1mA were located with a micrometer.
Ignore the sound. It was accidently overdubbed from off the air audio while copying 1 inch videotape to Beta but seems strangely appropriate.