For example, Fig. 2 shows the intensity spectrum of a phosphor- based white LED which has two peaks in its intensity spectrum and the corresponding correlogram with the distinctive features high- lighted in yellow boxes. With the distinctive features, the envelope of the interference signal cannot be modeled as a single Gaussian function. The effects of the distinctive feature to the reconstructed height profile vary depending on the working principle of the reconstruction algorithm, however it has been shown that the measurement repeatability will degrade if the spectral effect is not treated properly [15].