Disclosed is a crystalline form of N-4-fluoro-2-phenylmethyl-4,6,7,9-tetrahydro-3-hydroxy-9,9-dimethyl-4-oxo-pyrimido2,1-c1,4oxazine-2-carboxamide, sodium salt monohydrate. According to the invention, at least one first transparent dielectric layer is deposited, followed by a functional layer based on a material which reflects infra-red radiation. The chamber is in fluid communication with the compliant layer via the at least one aperture.