Definitions of Low Vision
What is Low Vision? Some definitions include:
"Low vision refers to a visual impairment that is not correctable through surgery, pharmaceuticals, glasses, or contact lenses. This condition is often characterized by partial sight, such as blurred vision, blind spots or tunnel vision, but also includes legal blindness. Low vision can impact people of all ages, but it is associated primarily with adults over the age of 60. It is estimated that between 3.5 million to five million people in the U.S. suffer from low vision." - lowvision.org
"If you have been told you have low vision, it means you still have some good usable vision..." - American Foundation for the Blind
"Low vision means that even with regular glasses, contact lenses, medicine, or surgery, people find everyday tasks difficult to do." - National Eye Institute
"Low vision is a condition caused by eye disease, in which visual acuity is 20/70 or poorer in the better-seeing eye and cannot be corrected or improved with regular eyeglasses." - (Scheiman, Scheiman, and Whittaker)
"...vision between 20/60 and 20/190 it is called being partially sighted or having low vision. If the change in vision is to 20/200 or worse, the person will still keep some vision but will be classified as "blind" (some people may be classified as blind if their field of vision, or the area that they can see, is less than 20° across--even if their vision is better than 20/200)." - Canadian National Institute for the Blind
"Anyone with noncorrectable reduced vision is visually impaired, and can have a wide range of problems. 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"
Luminance notes
WCAG:
- uses "luminance", e.g., in Understanding contrast
- "Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction.
Luminosity function Wikipedia:
- "The luminosity function or luminous efficiency function describes the average spectral sensitivity of human visual perception of brightness."
- "In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction"
- "Luminosity function, in photometry, describes the average visual sensitivity of the human eye to light of different wavelengths, also known as the luminous efficiency function"
- "In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye."
Wayne's perspective:
- brightness & luminosity are the same thing - quantity of light energy
- luminance is the quantity of light / area - that's the important metric for contrast