There are at least 2.2 billion people globally with visual impairment, and out of these, at least 1 billion cases could have been prevented with timely intervention. ANSI Z80.9-2020: Ophthalmics – Devices For Low Vision (And Errata 2023) applies to optical and electro-optical devices for persons with low vision.
What Is ANSI Z80.9?
ANSI Z80.9-2020 is applicable to optical and electro-optical low-vision devices that are used by visually impaired persons. The American National Standard specifies optical and mechanical requirements and test methods. Electro-optical devices are video systems for viewing objects at distance, intermediate, and/or near; they can incorporate a light source to illuminate the object and an optical device for viewing the monitor. Such devices capture images and display them on a monitor, display, or via projection onto a screen or directly into the eye.
ANSI Z80.9-2020 includes devices with optical, electrical, and/or electronic components used for image capture or display. Implanted low vision devices are excluded are not in the scope of the standard.
Low Vision Devices
ANSI Z80.9-2020 defines a low vision device as an apparatus that alters or enhances the image viewed by a person having low vision. The standard specifies two types of low vision devices:
- Optical low vision device is a lens or lens system, prism, or mirror, or combination of such optical components, that alters image vergence, size, and/or position.
- Electro-optical low vision device/ Opto-electronic low vision device is adevice or system that produces an altered or enhanced image through the interaction between light from the object and an electronic system.
Residual Vision VS. Low Vision
People with low vision have residual (partial) vision—where vision is lost but not impaired. Partial vision can occur in one or two eyes. People with residual vision have some light perception, but their vision loss does not require the use of spectacles or medical or surgical treatment. Such persons have the potential for enhanced functional vision if they receive appropriate low vision care services. On the other hand, low vision refers to a severe visual impairment in which visual acuity is 20/70 or poorer in the better-seeing eye and cannot improve with glasses or contacts.
Low Vision Devices: Magnifiers and Telescopes
Low vision aids or low vision assistive products are devices that aid people with low vision, allowing them to use their residual vision for better living. The two most common low vision devices are magnifiers for seeing objects close at hand and telescopes for seeing objects far away. ANSI Z80.9-2020 defines these two low-vision devices as:
- Magnifier: lens system designed to change the size of the image of a near object
- Telescope: Optical system composed of two separated lenses (or lens systems), the objective and the eyepiece, that forms a magnified image of a distant object when the eyepiece is the higher powered lens (or system).
How Do Low Vision Devices Work?
Low vision devices work by making objects appear bigger, brighter, and blacker or more closely, with improved contrast. These devices utilize the angular magnification effect by increasing the relative size and relative distance.
Angular Magnification
Angular magnification is apparent object size compared to the actual object size when seen without the device. More specifically, according to ANSI Z80.9-2020, it is the ratio of the angle subtended by the image of the object when viewed using a low vision device to the angle subtended by the object alone at a viewing point of reference, such as the entrance pupil of the eye. A telescope provides angular magnification.
Relative Size
Relative size makes the object larger and bigger without the need for any accommodation. It is magnification that is obtained by changing the object size. Relative size magnification can most commonly be achieved, for instance, by photocopying with magnification or increasing font or image size on a computer display. While the object is usually increased in size, a reduction in object size can be useful for patients with a small island of useful vision. Closed-circuit television, also known as video surveillance, is an example of relative size.
Relative Distance
Relative distance is achieved by bringing the object closer, requiring the need of accommodation, such as by the use of magnifiers. It is magnification that is obtained by changing the viewing distance. Relative distance magnification is used primarily for magnifiers positioned at or close to the spectacle. Moving the object closer to the eye increases the angle subtended by the object at the viewing point of reference and consequently increases the size of the retinal image. Moving the object farther from the eye decreases the angle subtended by the object and consequently decreases (minifies) the size of the retinal image. A spectacle microscope is an example of relative distance, as its lenses have greater power for near focus than found in typical spectacle corrections, creating retinal image magnification primarily by allowing for relative distance magnification.
ANSI Z80.9-2020: Ophthalmics – Devices For Low Vision (And Errata 2023) is available on the ANSI Webstore.