HP Color LaserJet CM4730 Service Manual - Page 188

Electrostatic latent image formation block The image formation process begins in the electrostatic latent image formation block. This block consists of three steps that results in forming an electrostatic latent image on the photosensitive drum: ● Pre-exposure ● Primary charging ● Laser beam exposure When the last step in this block is complete, areas of the drum surface that have not been exposed to the laser beam retain a negative charge. The areas that have been exposed to the laser are neutralized. This neutralized image is called an electrostatic latent image because it is invisible to the eye. 0 Surface potential (V) Exposed area Time (t) Unexposed area Pre-exposure (step1) Primary charging (step2) Laser beam Transfer (step6) Pre-exposure exposure (step3) (step1) Figure 4-18 Electrostatic latent image formation block Primary charging (step2) Step 1: Pre-exposure To prepare for primary charging, light from the pre-exposure LED strikes the photosensitive drum surface. This eliminates any residual charge on the drum surface to avoid density unevenness. LED Photosensitive drum Figure 4-19 Pre-exposure 168 Chapter 4 Theory of operation ENWW