How much energy is required to blind someone?
Light can affect retina in many ways [1]:


photothermal mechanisms - exposure to intense flashes of less than 20 microseconds that cause a rise in temperature of at least 10 degrees Celsius [5];
photomechanical mechanisms - exposure to flashes shorter than 5 nanoseconds [5];
photochemical mechanisms - light is absorbed by a chromophore and leads to the formation of an electronically excited state of it, resulting in chemical transformations [5].


The retina is more susceptible to light induced degeneration at lower wavelengths [2, 8]. The damage degree also depends on intensity and exposure duration. 12 hours a day of more than 500 lux light, more than three consecutive days can affect rat retina [3]. 2 hours  exposure to white light of 3000 lux has lead to apoptosis in rats retina [4].

Sunlight irradiance is about 11 W/cm2 (ranges between 1.5 and 122 W/cm2) and causes damage when exposure time is tens of minutes or even less [5].

Light induces apoptosis by different mechanisms:


  Intense light triggers cell death in cones and rods by two distinct mechanisms: rapid, necrosis-like cell death occurs in cones; prolonged, apoptotic death is displayed by rods. However, the areas of cone and rod loss coincide, suggesting some interrelationship [6].


Cones and rods have different vulnerability [7].



References:


Youssef PN, Sheibani N, Albert DM. Retinal light toxicity. Eye (Lond). 2011 Jan;25(1):1-14. doi: 10.1038/eye.2010.149. PubMed PMID: 21178995.
Rozanowska, M. B. (2012), Light-Induced Damage to the Retina: Current Understanding of the Mechanisms and Unresolved Questions: A Symposium-in-Print. Photochemistry and Photobiology, 88: 1303–1308. doi: 10.1111/j.1751-1097.2012.01240.x 
Jin X, Wu L, Zheng H, Mishima S. [Retinal light damage: I. The influences of light intensity and exposure duration at moderate and low intensities of cyclic light]. Yan Ke Xue Bao. 1998 Dec;14(4):215-9. PubMed PMID: 12579740. 
Remé CE, Weller M, Szczyesny P, Munz K, Hafezi F, et al. Light-induced apoptosis in the rat retina in-vivo. In: Anderson RE, LaVail MM, Hollyfield JG, editors. Degenerative Diseases of the Retina. Plenum Press; New York: 1995. Via http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2831109/
Malgorzata Rozanowska, Bartosz Rozanowski, Michael Boulton. LIGHT-INDUCED DAMAGE to the RETINA (2009). Available from http://www.photobiology.info/Rozanowska.html (accessed 20.07.2014).
Cortina MS, Gordon WC, Lukiw WJ, Bazan NG. Light-induced photoreceptor damage triggers DNA repair: differential fate of rods and cones. Adv. Exp. Med. Biol. 2004;533:229-40. PubMed PMID: 15180269. 
Okano K, Maeda A, Chen Y, Chauhan V, Tang J, Palczewska G, Sakai T, Tsuneoka H, Palczewski K, Maeda T. Retinal cone and rod photoreceptor cells exhibit differential susceptibility to light-induced damage. J. Neurochem. 2012 Apr;121(1):146-56. doi: 10.1111/j.1471-4159.2012.07647.x. PubMed PMID: 22220722. 
Kuse Y, Ogawa K, Tsuruma K, Shimazawa M, Hara H. Damage of photoreceptor-derived cells in culture induced by light emitting diode-derived blue light. Sci Rep. 2014 Jun 9;4:5223. doi: 10.1038/srep05223. PubMed PMID: 24909301.