The F1 50-peso banknote is the only one that has this security element.

The numeral 50 can only be detected in the upper clear window by focusing on a speck of light. To identify the hidden numeral, the banknote should be placed against a speck of light, at least at a 60 cm distance, and also be held at 10-15 cm from the eye

Warning: Do not use any laser light or sunlight.