AN AUTOMATIC LIGHT SPECTRUM COMPENSATION METHOD FOR CCD WHITE BALANCEMEASUREMENT

The light source spectrum of a test system has been used to compensate the CCD response under mid-range exposure for variations in the illum inator. The CCD white balance can then be calculated and screened base d on the ratios of the compensated R, G and B signals. First, a refere nce response is calculated based on the convolution of a reference lig ht spectrum with a representative CCD responsivity. Then a current the oretical response is calculated by replacing the reference spectrum wi th the current light spectrum. The ratio of the reference response and current theoretical response provides the spectrum compensation facto rs to correct measured CCD response in each of the three color channel s. With these corrections, a CCD white balance measurement will reflec t the true color filter characteristics. Error analysis shows that wit hout 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 b etween the CCD-under-test and the reference CCD.