The invention relates to a reflector comprising a reflector cavity (110) having a front opening for receiving EMR into the reflector cavity (110) for subsequent reflection of the EMR by at least one reflector element (120) arranged within the reflector cavity (110), characterized in that the reflector further comprises a front cover (130) that has high EM R transmittance in at least parts of the EM spectrum, the front cover (130) being arranged to cover the front opening of the reflector cavity (110), and that the front cover (130) and front opening form a gastight seal (140) impermeable to gas to prevent transport of material into and out of the reflector cavity (110) through the front opening.