An apparatus for determining the volume of a deposited skin-print 30 comprises a primary electromagnetic radiation source 40, an electromagnetic radiation detector 50, and a translucent waveguide 10 having a first surface 16 providing a waveguide interface coincident with a skin-print receiving region 20. EM radiation is transmitted from the source towards the interface at an angle of incidence relative to a line perpendicular to the interface such that when the interface interfaces directly with ambient the light reflects and when a deposited skin-print is present on the receiving region, at least a portion of the incident EM radiation is transmitted through the interface. The detector determines a primary output value representing an amount of EM radiation transmitted through and/or reflected by the interface, and the volume of deposited skin-print is determined using calibration data that shows the correspondence between the primary output value and skin-print volume. The calibration data may comprise volume data obtained for a population of calibration skin-prints and equated to corresponding primary output values. The EM radiation may be transmitted into the waveguide at such an angle as to cause the light to travel through the waveguide by total internal reflection.
