TNO Anaglyph Stereo Test

• The TNO Anaglyph Test is for binocular testing small children (21/2 to 5) as well as adults.
• This also works with our Fun Frame glasses.
• Features 3 plates for screening and 3 for diagnostics. This kit includes seven plates and Red/Green Glasses.

Reliable method to determine depth perception.In close collaboration with TNO we developed the TNO Stereotest. Since 1978 this has been the most accepted method to determine depth perception in adults and (very young) children. The 19th edition is now available. More than 9,000 ophthalmologists, orthoptists and optometrists use the TNO Stereotest.The test uses random dot stereograms where ‘half images’ are projected on top of each other in complementary colours.First test regarding dissociation.
Detecting microstrabismus.

The TNO stereo test has been primarily designed as an aid to the early detection of amblyopia in young children of between 2 1⁄2 and 5 years old [1]. However, apart from its employment in pediatric vision screening, it can also be used for Stereopsis testing in occupational testing. In fact, the TNO test has already been introduced in the field of aviation medicine, as part of the routine eye examination of pilot and aircrew applicants [2, 3].

The TNO test consist of three screening plates (Plates I – III), a plate for testing eye suppression (Plate IV), and two plates (Plate V and VI) for the quantitative testing of stereoacuity. The three screening plates show test images, a butterfly and geometric forms, that, when viewed through the red-green glasses, seem to float above the test plate. The quantitative plates are comprised of optotypes (standard symbols used for vision testing), shaped as discs with a sector missing.

As shown below, the optotypes are distributed over four different depth levels, specified in terms of seconds of arc (arcsec), the unit used for quantifying stereoacuity. They cover a range of 60 to 480 arcsec, values much lower than the 1980 arcsec that are used for the screening plates.

Failing to pass the screening plates indicates the complete loss of stereoscopic vision, also known as stereo blindness [4]. This carries the implication, that the test will come to an end after taking the suppression test (Plate IV). On the other hand, when the Plates I-IV are passed without problems, the next step is to start with stereoacuity testing (Plate V and VI). By using a pass/fail criterion of 240 arcsec, no amblyopes will be missed. Furthermore, lowering the criterion to ≤ 120 arcsec, warrants the prediction of normal or correctable normal vision [1, 5].

Plate I: In this plate two butterflies seem to be floating above the background. However, a child whith no stereoscopic depth perception will only see one. To check whether the child is possibly not paying attention, invite it to touch a wing of the visible butterfly. Thereafter, ask this once more, but first turn the plate upside down. This causes the butterfly to ‘fall’ through the background. If the child does nor seem to be aware of the depth reversal, there is no sense in further testing.

Plate II: When stereopsis is properly functioning, four discs (‘balls’, ‘plates’), of different sizes will be seen in depth. The largest one should be seen in the upper right corner of the plate. That disc will not be visible, however, to subjects with impaired stereopsis. They will see only two discs, with the largest one located in the upper left corner. Ask for the largest.

Plate III: The five test symbols shown on the left page, are diplayed in stereoscopic depth on the right page. Due to its different texture, the cross shown in the center, will be visible to everyone. However, the other four test symbols cannot be seen by subjects with impaired stereopsis. When explaining the response task, start by pointing at the cross shown on the left page, and then ask where it can be seen on the right page. Repeat this for the four other symbols.

Plate IV: This is a suppression test, showing a small white disc, flanked by two larger discs, colored red and green on the left and right side of the center, respectively. However, for a subject suffering from unilateral eye suppression, either the disc on the left, or the one on the right side will be invisible. Invisibility of the disc on the left indicates suppression of the left eye. Similarly, invisibility of the disc on the right side is an indication of suppression of the right eye. The test starts by asking how many discs can be seen.

Plate V-VI : Both plates comprise optotypes, that are distributed over four depth levels, covering a range of 60 to 480 arcsec (see figure on the left). When presenting Plate V (see page 7), start by pointing at the optotype (‘pancake’), shown on the left page. Also mention the gap. Next, ask the child to find the ‘pancakes’ in Plate V, and also to point at where a piece is missing. Use the same approach for Plate VI.

VIEWING DISTANSE

The test plates should be presented perpendicular to the line of sight, and squarely positioned (not rotated) in front of the subject. During the test, glasses should not be removed. The recommended viewing distance is 40 cm.

However, if wanted, one or more plates can also be shown at larger viewing distances. One may thus measure stereoacuities over the range 480 – 1980 arc sec. For example, when a child has passed the screening plates (1980 arcsec), but is unable to see any of the optotypes shown in Plate V (480 arcsec), one can go to a less difficult level by presenting a screening plate at, say, twice the viewing distance (80 cm). This implies a lowering of its associated stereoacuity value by a factor of 2. That is, an acuity level of 990 arcsec, instead of the 480 arcsec of Plate V. In general, increasing the viewing distance by a certain factor, reduces a plate’s stereoaccuity value by that same factor.

TEST ILLUMINATION

The TNO testplates are designed as red-green anaglyphs, the familiar two-color stereo pictures (stereograms) that, when viewed through matching red-green glasses; evoke the same perception as an image seen in depth.

The colors of the TNO anaglyphs were selected for testing under daylight (color temperature of about 650 Kelvin), at a lighting level of between 500 and 1000 Ix. However, any other light source with a similar Color Rendering Index (CRI ≥ 90), may be used as well. Whether a particular ‘daylight’ lamp qualifies as such, can be tested (whithin seconds) as follows: Go to a place where the test plates can be viewed under natural daylight, put on the red-green glasses, and view Plate IV. Next, alternately close the right and left eye. Note, then, that doing so produces the effect of ‘toggling’ between seeing only the red or the green disc. If this effect can also be observed when Plate IV is lighted by the substitute illuminant, then the latter can be used in combination with the TNO anaglyphs.

ABOUT THE TNO STEREO TEST

BACKGROUND

The type of stereogram employed in the TNO test is called a random-dot stereogram [8]. Its introduction in orthopthic vision screening dates from 1972, the year in which the ‘TNO test for stereoscopic vision’ was made commercially available.

At that time, the TNO test had already shown its potential as an aid to the detection of amblyopia in children aged 2-4 years. In amblyopia screening this is the most relevant, but also the most problematic age range, due to the relatively high percentage of children that are untestable (mostly the 2-3 year olds). Still, in a study [1] in which the TNO test was used for testing children in the 2-4 years category, only 10% was found to be not testable. This result should be compared to much higher ‘untestability’ scores, in the order of 30% [6] and 50% [7], that have been reported for visual acuity testing.