Test Chart 2022

Stereopsis refers to the ability of the visual system to interpret the disparity between the two retinal images leading to the perception of distance or depth. Stereopsis is quantified in terms of stereoacuity which is the minimum angular disparity which can be perceived. Normal stereoacuity varies between approximately 10 and 100 seconds of arc depending on the method/ test used.

Random dot stereograms provide a good method for assessing stereopsis. Pairs of red and green dots are placed randomly within the image and are viewed by the patient through red and green filters so that one eye sees the red dots and the other the green dots. The brain tends to fuse the red and green dots and the separation of the pairs of dots is interpreted as a disparity. Therefore by changing the separation of the dots in part of the image, an illusion of depth can be created and part of the image can be made to “stand out” from the screen.

Random dot stereograms

Good stereopsis requires good co-ordination between the eyes and good visual acuity in each eye. Assessment of stereopsis therefore provides the clinician with valuable information about patients’ binocular status and the standard of vision in each eye.

The Thomson Test Chart generates stereograms dynamically allowing a range of optotypes to be generated “in depth” within the stereogram. It also allows stereoacuity to be measured by varying the relative disparity within the stereogram.

There are a number of vision tests available for measuring stereopsis at near and a rich literature describing norms.  In contrast, there are very few tests capable of measuring stereopsis in the distance and relatively little information about norms and the optimum stimulus parameters.

Given the paucity of published norms for distance stereopsis, the clinician is expected to develop their own norms for the conditions selected.

The Thomson Test Chart is capable of generating stereograms of tumbling Es, a square or a circle. When the stereopsis test is selected, the Secondary toolbar is expanded to include 3 more buttons as shown.

The three stimuli may be selected by clicking on the corresponding icon in the Secondary Toolbar or by pressing O, U and M respectively on the keyboard or remote.

The stereo disparity may be changed using the vertical scrollbar on the Control Window or by pressing the vertical cursor control keys on the keyboard or remote.

The current stereo disparity is shown on the test chart screen and also on the Information Bar of the Control Window as shown below.

Summary