A wetness sensor for incontinence products, such as nappies/diapers, has a light source 7 (may be an LED) and detector 8 (may be a phototransistor or photodiode). An expandable portion 6 (may be a hydrogel) increases in volume when in contact with a liquid, and has a reflective surface 18 which reflects light from the source 7 towards the detector 8. The detector 8 generates a signal when the distance between it and the reflective surface 18 decreases. There may be an array of sensors on a flexible substrate 9, arranged in a Y shape, an ellipse, a star, or grid configuration. The expandable portion 6 may be in a housing, and a cap may have a capillary array 13 facilitating liquid ingress. A transparent impermeable barrier 15 may seal the light source 7 and detector 8 from liquid. There may be an external wireless monitoring unit.L'invention concerne un capteur d'humidité pour un produit d'incontinence et un procédé de détection de salissure d'un produit d'incontinence qui comprend le capteur d'humidité. Le capteur d'humidité comprend un capteur optique et un élément expansible.
