THE90 is an American startup that makes Gem, a jewellery style ultraviolet sensor worn as a pendant. It measures sun exposure and connects to an app that tailors guidance using a skin profile.

How it works

Light sensors measure ultraviolet A and ultraviolet B radiation reaching the pendant. Software adds exposure throughout the day and compares it with the profile. The app shows accumulated ultraviolet dose and provides alerts about sunscreen, covering or shade. It also estimates exposure related to skin ageing and vitamin D production.

Key considerations

The pendant measures light at its own position, which may differ from light reaching the skin. Clothing, shade, windows, reflections and body angle affect the dose. Sunscreen amount and wear cannot be measured directly. Vitamin D output is an estimate shaped by many biological factors. Gem supports sun awareness and cannot prevent burns or diagnose skin damage.