An automatic light spectrum compensation method for CCD white balance measurement

The light source spectrum of a test system has been used to compensate the CCD response under mid-range exposure for variations in the illuminator. The CCD white balance can then be calculated and screened based on the ratios of the compensated R, G and B signals. First, a reference response is calculated based on the convolution of a reference light spectrum with a representative CCD responsivity. Then a current theoretical response is calculated by replacing the reference spectrum with the current light spectrum. The ratio of the reference response and current theoretical response provides the spectrum compensation factors to correct the measured CCD response in each of the three color channels. With these corrections, a CCD white balance measurement will reflect the true color filter characteristics. Error analysis shows that without correction, a light source spectral shift can cause a first order error in the sensitivity measurement. However the proposed correction method compensates this spectrum-induced error while introducing only a second order error from the difference of the responsivity curves between the CCD-under-test and the reference CCD.