A method for measuring the thickness of a first absorbing material in the presence of a second absorbing material is provided. The method comprises the steps as follow. The thickness (tS) of the first absorbing material is fixed and the thickness of the second absorbing material is varied to obtain a calibration standard. The intensity of the transmissive energy passing through the calibration standard is detected by acquiring multiple pairs of image data comprising a foreground value (logn(Ic+s)) and a background value (logn(Ic)). The thickness (tSi) of the first absorbing material is changed and the above steps are repeated to obtain sets of image data. A fitting constant Id is determined to describe each set of the intensity data as μ s α ⁢ t S = log n ⁡ ( I c + I d ) - log n ⁡ ( I c + s + I d ) . A best fit of the proportional constant μ s α is determined to further calculate an unknown thickness of the first absorbing material (ts′) through the equation t s ′ = α μ s ⁡ [ ln ⁡ ( I c ′ + I d ) - ln ⁡ ( I c + s ′ + I d ) ] .