A transparency to be reproduced is positioned in register with copy material having respective sensitivity maxima in the blue, green and red wavelength ranges of the spectrum. An illuminating unit for exposing the copy material includes an array of cells such as an LCD. The density of each cell can be varied individually to thereby change the illumination intensity, and the densities are adjustable in such a manner that, in the spectral ranges corresponding to the neighborhoods of the sensitivity maxima, the density of each cell changes by approximately the same factor.