Erythropsia (from Greek erythros = red, opsis = vision) is the perception of a reddish tint over the entire visual field. The healthy crystalline lens absorbs ultraviolet radiation (below ~400 nm) and a portion of short-wavelength visible light (400-450 nm), providing a natural yellow filter that balances the relative stimulation of L-, M-, and S-cones. After cataract extraction without IOL implantation (aphakia), the removal of this filter increases retinal irradiance at short and ultraviolet wavelengths. Paradoxically, the resulting colour shift is toward red — because the S-cone and M-cone photopigments bleach more rapidly under the increased UV/blue-violet exposure, creating a transient relative L-cone dominance. The effect is most pronounced immediately post-operatively and under bright daylight. Erythropsia from snow blindness (photokeratitis) follows a similar mechanism: intense UV exposure bleaches S-cone photopigment preferentially, tilting the chromatic balance toward long wavelengths. Pharmacological erythropsia (sildenafil, digitalis) arises from direct modulation of retinal phototransduction via PDE6 inhibition or Na⁺/K⁺-ATPase disruption.