Infrared radiation has the capability of penetrating some types of opacities so that photographs can be made with infrared sensitive film of structures that cannot be seen by the human visual system. This technique was applied to ocular fundus photography in patients with various manifestations of diabetic retinopathy including retinal hemorrhages and exudates, neovascular tufts, marked proliferative tissue, and vitreous hemorrhage. In all such cases the infrared photographs depicted the optic disc, retinal vessels, and choroidal vasculature underlying the opacities when these structures could not be seen through the intervening opacities in routine photographs or on clinical examination. This technique has potential for revealing fundus abnormalities hidden from ordinary view by retinal or vitreous opacities.
Dallow RL, McMeel JW. Penetration of Retinal and Vitreous Opacities in Diabetic RetinopathyUse of Infrared Fundus Photography. Arch Ophthalmol. 1974;92(6):531-534. doi:10.1001/archopht.1974.01010010545019