An apparatus for use with a treatment device, wherein the treatment device is configured to apply light pulses to skin of a body part of a user to perform the treatment operation, wherein a light pulse applied to the skin treats an area of the skin, the apparatus comprising a processing unit configured to: receive a first measurement signal from a first sensor, the first measurement signal comprising information about positions and/or movements of the treatment device over time; for a light pulse previously applied by the treatment device to the body part during the treatment operation, process the first measurement signal to estimate a previous treatment position as a position of the treatment device relative to the body part when the light pulse was generated; based on the estimated previous treatment position, estimate a previous treatment area for the previously applied light pulse corresponding to an area of skin of the body part that the light pulse was applied to when the treatment device was at the previous treatment position; process the first measurement signal to estimate a current position of the treatment device relative to the body part; characterised in that the processing unit is further configured to: based on the estimated current position of the treatment device, estimate a current treatment area corresponding to an area of skin that the treatment device would apply a light pulse to while in the current position; and generate a feedback control signal for a feedback unit, wherein the feedback control signal is configured to cause the feedback unit to generate feedback indicating whether the current treatment area corresponds, or substantially corresponds, to a previous treatment area, and provide the generated feedback to the user during the treatment operation. receive a first measurement signal from a first sensor, the first measurement signal comprising information about positions and/or movements of the treatment device over time; for a light pulse previously applied by the treatment device to the body part during the treatment operation, process the first measurement signal to estimate a previous treatment position as a position of the treatment device relative to the body part when the light pulse was generated; based on the estimated previous treatment position, estimate a previous treatment area for the previously applied light pulse corresponding to an area of skin of the body part that the light pulse was applied to when the treatment device was at the previous treatment position; process the first measurement signal to estimate a current position of the treatment device relative to the body part; characterised in that the processing unit is further configured to: based on the estimated current position of the treatment device, estimate a current treatment area corresponding to an area of skin that the treatment device would apply a light pulse to while in the current position; and generate a feedback control signal for a feedback unit, wherein the feedback control signal is configured to cause the feedback unit to generate feedback indicating whether the current treatment area corresponds, or substantially corresponds, to a previous treatment area, and provide the generated feedback to the user during the treatment operation. based on the estimated current position of the treatment device, estimate a current treatment area corresponding to an area of skin that the treatment device would apply a light pulse to while in the current position; and generate a feedback control signal for a feedback unit, wherein the feedback control signal is configured to cause the feedback unit to generate feedback indicating whether the current treatment area corresponds, or substantially corresponds, to a previous treatment area, and provide the generated feedback to the user during the treatment operation. <EOS>