In 2014 Turner and Goulden demonstrated that the ultraviolet B (UVB) irradiance from a Handheld Minimal Erythema Dose (MED) device varied depending upon the internal temperature of the device(1) . The Handheld MED device in question consisted of a single PL-S 9W/01/2P narrowband UVB fluorescent lamp located inside a plastic handheld enclosure. On the treatment surface of the device there were 10 apertures, each one containing attenuating foil to deliver a range of doses for a fixed exposure time. This article is protected by copyright. All rights reserved.