How do polarized test cards work?


[Like this, for example. When put through a polarized lense, you can see the fish, where it isn’t present otherwise. ]( How does it work?

In: 3

Huh. Never seen one of those, but it wouldn’t be hard to do.

The hidden image is printed with an uncolored polarizing ink. Light can pass through the ink, but is polarized. You don’t see anything, because light passes through the hidden image unimpaired.

The “lense” you’re looking through is another polarizer, oriented 90 degrees from the polarized image. “Crossed” polarizers block the light – the first polarizer only passes light in (say) the left-right polarization. The “lens” only passes light in the up-down polarization. So the image appears black, since the light can’t get through.

Try turning the lens 90 degrees. The image will disappear.

Light comes in different colors and polarization. You can see color because your eyes are sensitive to color. You cannot see polarization. All polarization looks the same to you.

Light scattered from most objects is unpolarized, it contains a mix of all polarizations, so seeing polarization is not particularly useful most of the time. Light reflected from a flat surface at a certain angle such as from a lake is polarized, i.e. predominantly one polarization.

Polarizing sun glasses filter one polarization and transmit others. Thus with polarized glasses blocking the reflection from the lake, you can see through the surface better and reveal the fish under water.

The test card simulates this. The paint appears the same color as the background when viewed normally but it’s polarized so it becomes visible with special glasses.