The effect of light illumination on CdS‐LiNbO3convolvers at 77 K and room temperature is investigated. The spectral response of the convolution output is closely connected with the photoconductivity spectrum, and at 77 K three exciton lines are clearly observed in the spectrum of the convolution output. Whether the convolution output is enhanced or quenched under illumination depends on the magnitude of the surface barrier of the semiconductor. The results are qualitatively explained on the basis of the increase in carrier density near the semiconductor surface by the light illumination.