Our recommendation would have the White UV Standard validated at HunterLab Reston Service at least once every three years. Correct usage is to only use the spot labelled 3 to do the UV Calibration. Only use spot labelled 4 if there is ever doubt about the validity of spot 3. We would not recommend checking with spot 4 more than once every 6 months. Repeated checking of spot 4 will cause it to age at the same rate as spot 3 and diminish its usefulness as a reference check.