An optical flat is a precisely polished flat surface, used as a reference against which the flatness of an unknown surface may be compared. Edmund Optics offers both single surface and dual surface optical flats in either ZERODUR or Fused Silica. The flatness of an optical flat is measured in fractions of a reference wavelength, 632.8nm. A λ/20 flat will have a maximum peak to valley deviation of 632.8/20 or 31.64nm. We offer several levels of flatness for our single surface flats: λ/4, λ/10, and λ/20. For our dual surface flats, ¼, 1/10 and 1/20 flatness are available.