Nine sensor systems scanned diquat symptoms on barley and oilseed rape ten days after spraying to compare sensor output. Crops were sown in four densities and diquat was applied with a logarithmic sprayer. Only two RGB image systems are used here to illustrate how to compare ED50 parameters in response to crop density. ED50 for the system measuring the excess green index by continuous samples from 40 m altitude dropped somewhat in response to density in both crop species. ED50 for green foliage coverage at discrete samples from 1.5 m altitude did not show the same trend for both crops, for oilseed rape, ED50, however, dropped with increasing density.