Nat. Protoc. 9, 263–293 (2014); published online 9 January 2014; corrected after print 24 February 2016

In the version of this article initially published, units were incorrectly reported in Step 2 of the Procedure. They have been changed from 'mJ cm2' to 'mJ per cm2' in four instances. In addition, units that previously read '1 = 0.1 J per m2' have been changed to '1 = 1 J per m2'. These errors have been corrected in the HTML and PDF versions of the article.