Over-exposure to ultra-violet radiation (UVR) in the sunlight is the major cause of skin cancer, while under-exposure may cause vitamin D deficiency. Therefore, monitoring the invisible UVR is of significance. The design involves the development of a photo active ink that can generate a unique colour response to specific UV wavelengths. This ink was then used to fabricate low-cost wearable sensors that provide naked-eye monitoring of UVR, even at low doses typically encountered during solar exposure. As people with different skin types have different UV tolerance, these UV sensors can be customised to meet the specific need of different skin phototypes. These low-cost personlised wearable UV sensors offer remarkable potential in managing the impact of UVR in our day-to-day life. This design has been reported by over 200 media outlets in more than 15 languages and has got more than 50 million views across over 30 countries.