The 10RP42-1 Achromatic Zero-order Wave Plate has λ/2 retardation over a 545 nm center wavelength. The standard 25.4 mm outer diameter is convenient for mounting into Newport rotary mounts. It is designed to operate with a high degree of retardation accuracy over a broad wavelength range. This 1 inch diameter true zero-order wave plate is constructed using a stack of birefringent polymer films laminated between two high-precision BK 7 windows, each with a high efficiency, broadband antireflection coating. This construction ensures excellent transmitted wavefront quality, while minimizing beam deviation and surface reflection losses. The assembly is mounted in a black anodized aluminum housing for protection and ease of mounting, with the retarder fast axis marked for alignment reference. Achromatic polymer wave plates have better angular acceptance and less sensitivity to wavelength change than achromatic quartz–MgF2 wave plates. Retardation changes by less than 1% over a ±7° incidence angle and is maintained to better than ±λ/100 over the entire bandwidth. Optical transmittance varies slightly from the precision zero-order wave plates because several polymer layers are used in each achromatic wave plate.