Visualize hidden message with a phosphorescent card.
Phosphorescent materials can only be charged when it is exposed to light that is shorter than it’s excitation wavelength. And the excitation wavelength is often shorter than it’s emission wavelength (green in this case). Our monitor can emit three kinds of lights: red, green and blue. Only blue light has a shorter wavelength than green light. So only light from blue channel can charge the phosphorescent card.
This image encoded message in it’s blue channel. Random values in red and green channel are added to make it unrecognizable.
You can turn your screen in full brightness in darkroom and expose the phosphorescent card to it. Hold the card for 30s to 1 mins. Then you can see the hidden message. (First pic)
You can try the following image with a green phosphorescent sheet.