Aspects of the disclosure can provide a method of detecting a density of a pigmented layer on an object. The method can include emitting a first modulated light onto a first portion of the object having the pigmented layer, detecting a first reflected light of the first modulated light from the first portion of the object, and determining the density of the pigmented layer according to the first reflected light. Furthermore, the method can include emitting a second modulated light onto a second portion of the object, detecting a second reflected light of the second modulated light from the second portion of the object, and determining the density of the pigmented layer according to a relative ratio that is related to the first reflected light and the second reflected light.