an electrographic apparatus has a transparent photoconductor on which visible images are formed by applying toner to a latent electrostatic image . the toned image is subsequently transferred to a copy sheet and fused to the sheet . a post - development erase lamp is located on one side of the photoconductor . the density of toned images on the photoconductor is measured by light rays from the erase lamp traveling through the photoconductor and a toned control image thereon to a photodetector on the opposite side of the photoconductor . this eliminates the need for special lamps used only for measuring the density of toned control images .