There is provided a hair cutting device for cutting hair on a body of a subject, the hair cutting device comprising a light source for generating light at one or more specific wavelengths corresponding to wavelengths absorbed by one or more chromophores in or on hair; a cutting element that comprises an optical waveguide that is coupled at a first end to the light source to receive light, wherein a portion of a sidewall of the optical waveguide forms a cutting face for contacting hair; a light sensor that is coupled to the optical waveguide away from the first end, wherein the light sensor is for measuring the light level in the optical waveguide and for providing an output signal representing the measured light level; and a control unit that is coupled to the light source, and coupled to the light sensor to receive the output signal, wherein the control unit is configured to determine a measure of the amount of input light transmitted across the optical waveguide from the measured light level and an input light level at the first end of the optical waveguide; and to control the power of the light generated by the light source based on the determined measure.