The new design of the 50-peso banknote (Type F1) is the only one that have got 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 60 cm distance, and also be held at 10-15 cm from the eye.  

Warning: Do not use any laser light or sunlight.