steel it may or may not be protected against the elements by any of a variety of synthetic varnish or resin base coatings which can be sprayed onto one or both surfaces or they can be protected by electroplating in this form the surface will have a reflectance of 65 if a higher reflectance value is desired the steel sheet can be coated with a metallic silver layer in the manner of a conventional glass mirror which will provide a reflectance value of about 95 with such a coating it would be necessary to apply some form of protective coating for the silver metallic layer since it would be susceptable to deterioration from airborne elements such as oxygen and sulphides other suitable known forms of metal or metal based materials with suitable specular surface properties are deemed to be within the scope of this invention as well as other forms of protective coatings such as polycarbonate resins which can be added adhesively to the metal sheet with a high reflective coating sandwiched between the plastic coating and the metal sheet a feature of the present invention not yet mentioned is the means by which a bending moment in the tensioning frame is overcome the bending moment.