1. H. von Helmholtz, Handbuch der Physiologischen Optik, 3rd ed., 1909, pp. 198–199
2. ASTME 179-91a, Selection of Geometric Conditions for Measurement of Reflection and Transmission Properties of Materials
3. The Guide to Practical Measurement of UVA/UVB Ratios, The Boots Co. Plc., Nottingham, UK
4. C.J. Irwin, The current status of UVA testing and labeling in the USA, Presented at Broad Spectrum Sun Protection: The Issues and Status, London, UK, March 1997