光度视网膜亮度单位，相当于每平方米有一烛光亮度的表面，通过一平方毫米的瞳孔区域，到达视网膜的光线数量 A unit of retinal illumination, equal to the amount of light that reaches the retina through 1 square millimeter of pupil area from a surface having a brightness of 1 candela per square meter.
屈光正常当平行光线恰好聚焦于视网膜并形成清晰视觉时正常眼睛的状态 The condition of the normal eye when parallel rays are focused exactly on the retina and vision is perfect.
由於光线射到视网膜上，眼睛才看得见。 Sight is made possible by rays of light impinging on the retina.
