Appendix B

Glossary

Algorithm: See perimetric algorithm.

Apostilb: A unit of luminance referred to in the SSA listings.

Applicability: The extent to which the results of a perimetry technique can be generalized to the target population and setting for which the test is intended. In other words, it assesses whether the study’s findings are relevant and applicable to the specific clinical context in which the test is meant to be used.

Automated (or automatic) kinetic perimetry: See semiautomated kinetic perimetry, a term used preferentially in this report to better reflect the role of the technician operating the test.

Automated perimetry: Automated presentation of the test stimulus and recording of patient responses (EyeWiki, 2024).

Blind spot: An area of the visual field in which a stimulus is not seen at any intensity.

Bowl perimeter (or bowl perimetry device): A perimetry device wherein an examinee looks into a bowl-shaped surface, upon which stimuli are projected. Includes most common perimeters used today, such as the Humphrey Field Analyzer and Octopus lines of products, as well as the original Goldmann perimeter.

Candela: The International System of Units (SI) unit of luminance. Referred to in the SSA listings.

Cecocentral area: The area of the visual field from the physiologic blind spot (approximately 15 degrees temporal to fixation) to central fixation (Araie et al., 1995).

Confrontation test: An examination in which a clinician moves their finger(s) in and out of the examinee’s field of view and asks when they can see and/or count the finger(s).

Contrast sensitivity: The minimum stimulus intensity (brightness) that a patient can detect at a specific location within the visual field, expressed in decibels (dB) on visual fields. Higher dB values indicate better function (i.e., detection of a dimmer stimulus) on visual field results.

Decibels (dB): “A measure of attenuation, where a higher dB score indicates the ability to perceive a stimulus of lower intensity (i.e., greater contrast sensitivity)” (Phu et al., 2025).

Defect severity: The degree to which an area of the visual field has lower contrast sensitivity than expected (Patel et al., 2007).

False-negative response: An indication that an examinee does not see a stimulus where one is present that the examinee would be expected to detect easily.

False-positive response: An indication that an examinee reports seeing a stimulus where one is not present.

Fixation loss: When an examinee’s gaze deviates from the fixation point during a perimetric exam.

Fixation point: A designated spot upon which a perimetric examinee is told to maintain their gaze.

Frequency doubling technology (FDT): A perimetric technology based on a flicker illusion, which essentially creates an image that appears double its actual spatial frequency. Although the stimulus does not move across the field, the flickering is a proxy for the stimulus intensity used in either static or kinetic perimetry (EyeWiki, 2023).

Goldmann stimulus size: A standard size scale for circular perimetric stimuli. The sizes, numbered I–V, each cover four times the area of the previous size.

Hill of vision: A term referring to a three-dimensional representation of the visual field, where the peak of the “hill” corresponds to the fovea (the point of highest visual sensitivity), and where the sensitivity gradually decreases away from the center of vision towards the periphery, creating a slope similar to a hill’s terrain.

Humphrey Field Analyzer (HFA): The primary perimeter used in the United States. The HFA is an automated static threshold perimeter.

Kinetic perimetry: A method of visual field testing using a moving stimulus where the stimulus is generally moved from a nonseeing area to a seeing area in a systematic way to map the central and peripheral visual field boundaries, in addition to any scotomas, including blind spots. This movement can be automated, semiautomated, or manual.

Luminance: The brightness of an object presented to the eye; perimetry devices report the maximum luminance of a stimulus and the background luminance (Phu et al., 2025).

Mean deviation or mean defect (MD): The average difference in visual field sensitivity across all measured locations, compared to a normal, age-matched reference field.

Meridians: Imaginary lines that divide the visual field into equal sections, like pieces of a pie. Typically, the visual field is divided into eight equal sections, with the meridians radiating out from the central focus point. These radii are labeled in degrees moving counterclockwise from the 3 o’clock position (0, 45, 90, 135, 180, 225, 270, and 315 degrees for an eight-meridian scheme). The radii may also be labeled using anatomical references: nasally (0 degrees in the left eye and 180 degrees in the right eye), up nasally, superiorly (90 degrees in both eyes), up temporally, temporally (180 degrees in the left eye and 0 degrees in the right eye), down temporally, inferiorly (270 degrees in both eyes), and down nasally (IMAIOS, n.d.).

Optical projection: Perimetry performed by projecting a light stimulus onto a background to present it to the patient’s eye in order to map the visual field.

Perimeter: A machine used to measure visual fields.

Perimetric algorithm: A statistical package that comes onboard with perimetric hardware and is used to mathematically estimate the boundaries of an examinee’s visual field. Generally independent of an individual’s results from prior testing sessions.

Perimetry: See visual perimetry.

Physiologic blind spot: The area where there is expected to be no visual sensitivity, corresponding to the location of the optic nerve.

Reliability indices: Indicators of the confidence with which one can ascertain whether the results of a single test are credible or if they require repetition or rejection.

Reproducibility: Also referred to in the literature as “reliability,” but for the purposes of this report, reproducibility is the degree to which results remain consistent (or are less variable) over repeated measurements.

Scotoma: An area that falls within the boundary of the visual field in which stimuli are not seen at an intensity expected in that location.

Semiautomated kinetic perimetry: A visual field test that uses a moving stimulus of a selected size and intensity, with the speed and direction of the stimulus being automated.

Sensitivity: In the context of this report, a test’s ability to correctly identify those with a qualifying disability. (Not to be confused with “contrast sensitivity” defined above.)

Specificity: In the context of this report, a test’s ability to correctly identify those without a qualifying disability.

Static perimetry: A visual field test during which stationary stimuli are presented at defined points in the visual field. Locations at which the stimulus is seen and not seen are recorded.

Static automated threshold perimetry (static or standard automated perimetry (SAP), “white on white” perimetry): Visual field test that uses the projection of a white stimulus onto a white background to determine the probable threshold at chosen locations in the visual field. Blue on yellow static automated threshold perimetry is also available.

Statutory blindness, federal definition: “Central visual acuity of 20/200 or less in the better eye with the use of correcting lens. An eye which has a limitation in the field of vision so that the widest diameter of the visual field subtends an angle no greater than 20 degrees is considered to have a central visual acuity of 20/200 or less.”¹

Subthreshold (stimulus): A stimulus below the estimated detection threshold of a normal visual field location.

Suprathreshold (stimulus): A stimulus “above the estimated detection threshold of a normal visual field location” (Artes et al., 2003).

Tangent screen test: A manual method of assessing the central and peripheral visual field using a flat black screen (usually 1 to 2 meters away) and a moving target. It is useful for detecting functional vision loss, neurologic field defects, and scotomas.

Threshold: The stimulus intensity that a person can detect on 50 percent of presentations.

Threshold testing: Perimetric examination that calculates the precise contrast sensitivity of various locations in the eye.

Validity: The ability of a perimetry technique to accurately identify whether an individual meets the criteria for disability.

Visual acuity: A measure of the sharpness or clarity of vision at a given distance where normal vision is 20/20 (CHOP, n.d.).

Visual acuity efficiency, SSA definition: “A percentage that corresponds to the best-corrected central visual acuity for distance in [the] better eye” (SSA, n.d.). It is based on a reference chart that aligns Snellen visual acuity metrics with visual acuity efficiency percentages.

Visual acuity impairment value, SSA definition: “Corresponds to the best-corrected central visual acuity for distance in [the] better eye” (SSA, n.d.). It is based on a reference chart using Snellen metrics.

Visual efficiency, SSA definition: “A calculated value of [the] remaining visual function, [combining the] visual acuity efficiency and [the] visual field efficiency[, expressed] as a percentage” (SSA, n.d.). For example, if the visual acuity efficiency percentage is 75 and the visual field efficiency percentage is 40, the visual efficiency percentage is (75 × 40) / 100 = 30 percent.

Visual field: The total area in which objects can be seen by either or both eyes while focusing on a central point; the right visual field is perceived by the right eye and the left visual field is perceived by the left eye.

Visual field includes both central vision and peripheral vision, which is the ability to see objects to the side or up and down while looking straight ahead (Medline Plus, 2023).

Visual field efficiency, SSA definition: “A percentage that corresponds to the visual field in [the] better eye. … the visual field efficiency percentage [is calculated] by adding the number of degrees [seen] along the eight principal meridians found on a visual field chart (0, 45, 90, 135, 180, 225, 270, 315) in your better eye and dividing by 5” (SSA, n.d.). For example, if the visual field is contracted down to 25 degrees in all eight meridians, the remaining visual field efficiency would be (25 × 8) / 5 = 40 percent.

Visual field impairment value, SSA definition: “Corresponds to the visual field in [the] better eye. Using the [mean deviation (MD)] from acceptable automated static threshold perimetry, [SSA calculates] the visual field impairment value by dividing the absolute value of the MD by 22” (SSA, n.d.).

Visual field index: A “global metric that represents the entire visual field as a single number. It is estimated by calculating age corrected defect depth at the test points identified as significantly depressed in pattern deviation probability maps” (Iutaka et al., 2017).

Visual impairment value, SSA definition: “A calculated value of [a person’s] loss of visual function, [combining the] visual acuity impairment value and [the] visual field impairment value” (SSA, n.d.).

Visual perimeter device: See perimeter.

Visual perimetry: Systemic measurement of the visual field.

REFERENCES