Signal lens from the in-game notebook
Signal Lenses are a collectable in Horizon Forbidden West. There are six to be found in abandoned Carja watchtowers scattered across the Daunt. They can then be traded to Raynah in Barren Light after completing Signals of the Sun.
Lore[]
Designed by an Oseram, the signal lenses reflected sunlight across watchtowers in the Daunt to allow the Carja to monitor the Tenakth movements in the area and signal any approaching enemies. The watchtowers however were abandoned and fell into disrepair after the Carja were driven from the Daunt by the Tenakth.
No longer having use for them following the Liberation of Meridian, the Carja commissioned for their demolition. Upon hearing of this, Raynah the daughter of the Oseram designer, set out to retrieve the lenses as a way of remembering her now deceased father and his work. Raynah hired a Carja bodyguard to assist on her task, however he fell to his death defending her against machines upon approaching the tower southwest of Chainscrape. Injured and unequipped to carry the task out herself, Raynah employs Aloy to retrieve the six lenses.
Upon returning the six lenses to Raynah, she takes them with her back to the Claim.
Rewards[]
Each lens returned to Raynah at Barren Light awards 
15 Metal Shards, 1 Bronze Ingot, and 2 Greenshine Slivers. Once all six are returned, Aloy receives the Dune Shadow dye. It is possible to collect the lenses before encountering Raynah for the first time, at which point all rewards are given at once.