The Fusarium head blight caused mostly by Fusarium graminearum (F.g.) is the most important disease of wheat because it not only leads to yield loss, but the toxin contamination makes the yield harvested. First, visual assessment of the heads was made, then the ratio of Fusarium damaged kernels (FDK) becomes the attention, and since introduction of the toxin limits for wheat, the deoxynivalenol contamination has gained significance. However, the FDK has a greater practical significance, as the identification of Fusarium damaged kernels is the precondition of their separation.For this reason a more exact and more sensitive method was developed by using updated spectroscopy methods. The infection sensitive spectral index (ISSI) function has been developed to characterize spectral features of images of grains with different infection severities. The green and red color ranges could be best used in this analysis. It was also found that the way how different spectra from different grains or samples can be normalized and compared. This histogram analyzing method uses scanned images and it seems to be useful in describing the infection severity of heterogeneous samples better than available before.This might serve as scientific background to develop new instruments for rapid tests.