1. Dandona, L., Dandona, R.: Revision of visual impairment definitions in the international statistical classification of diseases. BMC Med. 4 (2006). doi: 10.1186/1741-7015-4-7
2. World Health Organization: Visual impairment and blindness. http://www.who.int/mediacentre/factsheets/fs282/en/
3. Erickson, W., Lee, C., von Schrader, S.: Disability Statistics from the 2013 American Community Survey (ACS). Cornell University Employment and Disability Institute (EDI), Ithaca (2015)
4. Canadian National Institute for the Blind: Fast facts about vision loss. http://www.cnib.ca/en/about/media/vision-loss/pages/default.aspx
5. Kim, D., Wall Emerson, R., Curtis, A.: Analysis of user characteristics related to drop-off detection with long cane. J. Rehab. Res. Dev. 47, 233–242 (2010)