Abstract: A copier/duplicator has transparent photoconductor with a first surface on which a latent electrostatic image is formed. The latent image is developed by toner particles applied as the photoconductor is moved through a development station. A toner concentration monitor includes an emitting element and a detector. The emitting element directs a beam of radiation through the photoconductor from a second surface thereof toward an area of the development station where a developer mixture including toner particles is being provided to the first surface of the photoconductor. The detector is located adjacent the second surface of the photoconductor to receive rays emitted by the element and reflected from the developer mixture through the transparent photoconductor.