What is campimetry?
Campimetry is a method to examine and evaluate the central part of a person’s visual field.
The subject (in an experiment, or patient in a clinical situation) concentrates an eye on a target in the center of a black screen that is two meters ahead.
Then a small object with a lighter color is introduced into its field of vision and the subject informs when he first sees it.
This process is repeated with the object that is taken to the field of vision of the subject from different directions in order to build a map of the field of vision of the subjects using only that eye.
The procedure can be repeated for the other eye since the size and shape of the subject’s visual field may not be the same for each eye.
White noise field campimetry
The field noise field white noise, also called snow field fieldimetry, is a method that directly visualizes scotomas, allowing patients to interactively detect and describe their visual field defects.
This is done by presenting an equal number of small black and white square dots randomly distributed with a blink frequency of 30 Hz in a video display unit.
With the campimeter you can examine an area of 35 ° (horizontal) and 24 ° (vertical) of the central visual field, at a distance of 30 cm between the eye and the surface of the screen.
An equal number of randomly distributed black and white square dots with a blink frequency of 30 Hz is used, comparable to the snowfield of a TV screen without reception (static).
Each point has a visual angle of 12x 12. The luminance of the light points is 200 cd / m 2 and that of the dark ones of 0.8 cd / m 2, respectively.
A stimulus is presented in the center of the screen to stabilize the central fixation. The direct inspection and video monitoring of the examined eye is used to control the fixation.
Suitable thin lenses are used for close correction if necessary. The other eye is covered with an opaque patch.
The examiner asks the patient in a standardized way about the perception of the field. Most disturbances are described as “clouds.”
The clouds can be differentiated from the surrounding normal snowfield by the following qualities:
- Change in the perception of brightness. The possible states are: darker, equal or brighter.
- Change in the perception of noise, that is, in the “apparent” movement of the points. The possible states are: increased, unchanged, reduced and deficient.
The edge of the “cloud” is indicated by the patient with the tip of the finger directly on the surface of the monitor and can be documented directly by a touch screen option.
White field noise field test times vary from 1 to 5 minutes per eye, including standardized questions about the perception of the noise field and the documentation of the result.
Reliability of white noise field campimetry
The field noise of white noise field is a subjective method in two ways: on the one hand, it depends on the intellectual capacities of the subject examined to perceive and adequately describe the phenomena observed; on the other, it is influenced by the recognition and interpretation of the examiner.
Additional errors can be introduced due to the fact that patients, who for the first time in their lives perceive their scotomas, will tend to observe these phenomena directly.
This spontaneous looking movement inevitably changes the scotoma, which can then dislocate outside the examination area and disappear. By this method, continuous and careful control of the fixation by the examiner is mandatory.
Subjects with inadequate correction, inability to fix or deep central scotomas may not be able to perceive the stimulus from the noise field adequately.
In a recent version, the stimulation of the noise field is generated exclusively by software and is presented on a calibrated and high resolution color monitor with integrated touch screen; the results are recorded on a separate exam screen.
This allows patients to delineate their scotomas on the examination monitor without any disturbance from the visible edges.
The characteristics of the noise field (ie, brightness, dot movement, and color characteristics) within the delimited scotoma region or within the entire scan screen may vary interactively to evaluate other quantitative characteristics of the scotoma.