military personnel, vision loss can be defined as the absence of vision
where it existed before, which can happen either acutely (i.e., abruptly)
or chronically (i.e., over a long period of time). Before the acquisition
of alternative adaptations and skills, the effects of visual loss can be
devastating, especially when a person's vision disappears over a short period
of time. Various scales have been developed to describe the extent of vision
and vision loss based on visual acuity, and early editions of the World
Health Organization's ICD (International Classification of Diseases and
Related Health Problems) described a simple distinction between "legally
sighted" and "legally blind.”
However, the ICD-9 released in 1979 introduced the smallest continuous scale which consisted of three tiers: normal vision, low vision, and blindness.
Anyone with non-correctable, reduced vision is considered to be visually impaired. The World Health Organization uses the following classifications of visual impairment:
When the vision in the better eye with best possible glasses correction is:
20/30 to 20/60 is considered mild vision loss, or near-normal vision
20/70 to 20/160 is considered moderate visual impairment, or moderate low vision
20/200 to 20/400 is considered severe visual impairment, or severe low vision
20/500 to 20/1,000 is considered profound visual impairment, or profound low vision
less than 20/1,000 is considered near-total visual impairment, or near total blindness
No Light Perception is considered total visual impairment, or total blindness
There are also levels of visual impairment based on visual field loss (loss of peripheral vision). In the United States, any person with vision that cannot be corrected to better than 20/200 in the best eye, or who has 20 degrees (diameter) or less of visual field remaining, is considered to be "legally blind" or eligible for disability classification and possible inclusion in certain government-sponsored programs.
It is critical that persons suffering vision loss be examined by an optometrist or ophthalmologist specializing in low vision care prior to other rehabilitation training to rule out potential medical or surgical correction for the problem and to establish a careful baseline refraction and prescription of both normal and low-vision glasses and optical aids. Only a qualified physician can evaluate visual functioning of a compromised visual system effectively.