Lights from the object in the scene passes through the cornea, iris, then through the lens and the clear gel (vitreous humour) in the body of the eye, and are focused on the retina [1]. However, as the optical system ages especially after age 60, visual acuity is reduced, contrast sensitivity is diminished, colour discrimination is reduced, the amount of time needed to adapt to large and immediate alters in luminance is increased, and the glare sensitivity is increased [2, 3]. Likewise, occasionally the lenses do not function properly, when they are not fitted and/ or adjusted correctly, which causes some defects in the optical system, including (1) focusing images behind or in front of the retina instead of on it, giving long sight or short sight, and (2) giving distortion and, in some directions, blurring of images [4]. Fortunately, nearly all optical defects can be corrected or improved through wearing prescription eyeglasses or contact lenses, which function as electric lenses [4, 5]. Figure 1 shows a cross-section of the human eye.
The retinal image process begins at the retina [6]. Retina involves two types of photoreceptors (cells that react to light), namely cones and rods [1, 3]. The cones are sensitive to high amount of light, the range of illuminance related with sunlight [7]. They also make colour discrimination possible due to their different photo-pigments [3]. The rods function under low illuminance while not providing any data about colour [1]. Moreover, research suggests that both cons and rods are active at high illuminance, whereas each contributing to different aspects of vision [4]. The photoreceptors at the retina absorb the lights and convert them to electrical signals [3]. They also link to form the optic nerve through joining the optic nerve from the other eye [8]. Finally, the optic nerve carries the visual data to the brain [3].
References:
1. Tregenza, P. and D. Loe, The design of lighting. Vol. 2nd. 2014, Milton Park, Abingdon, Oxon: Routledge.
2. Alexander, K.R., The Senescence of Human Vision. Retina, 1995. 15(2): p. 177.
3. IESNA and M.S. Rea, The IESNA lighting handbook: reference & application. 2000, New York, NY: Illuminating Engineering Society of North America.
4. Gordon, G., Interior lighting for designers. 4th ed. 2003, New York: Wiley.
5. Bogus W.J. and Sather R. Eyeglasses and Contact Lenses. 2015 July 6]; Available from: http://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=90&ContentID=P02089.
6. Boyce, P., Human factors in lighting. 2003, New York: Taylor & Francis.
7. Bean, R., Lighting: Interior and Exterior. 2012: Taylor and Francis.
8. Foley H.J. and Matlin M.W. The Visual System. 2015 July 4]; Available from: http://www.skidmore.edu/~hfoley/Perc3.htm.