High quality PD ruler.
Clear accurate measurements
Dust or wipe clean with a cloth dampened with water once a week. Be sure not to leave water spots on the surface. These water spots will dry and could possibly leave permanent marks. Clean stains/spots using the following steps: Dampen a soft cloth with a mixture of hot water and liquid dishwashing detergent. Wring the cloth as much as possible to remove excess liquid. Rub the surface lightly in a circular motion. Dry the surface immediately with a clean, soft towel.