LEA Symbols Low Contrast Test
10M Symbol Size
#251100 - #251200

In this test type the 10M size is convenient because the visual acuity as decimal visual acuity is is easy to calculate: at the testing distance of 1 meter it corresponds to visual acuity 0.1 (20/200, 6/60), at 2 meters to 0.2 (20/100, 6/30), at 4 meters 0.4 (20/50, 6/15) and at 0.3 meter distance 0.03 (20/600, 6/180) covering the low contrast visual acuity range of most persons with visual problems. The Snellen fractions corresponding different desimal visual acuity values are found on the list.

The contrast levels of the test lines on the five pages are 25%, 10%, 5%, 2.5% and 1.2%.

The same diagram that is used when testing with the low contrast visual acuity charts, can be used for reporting of the results from testing with tests with one symbol or number size. The number of correct answers is read on the left vertical axis and the cross is placed corresponding to the distance used when testing (distance is shown on the upper border of the recording form, top scale).

Luminance variation affects the threshold values in many disorders and even in normally sighted individuals. It is difficult to arrange high enough luminance on the test, which needs to be kept vertical. In a regular room this is possible only by directing a light source toward the test. Then the amount of light falling on the test is dependent on the distance of the test from the light source. For reliable follow-up measurements, the test should be at a fixed distance from the light source. In field surveys, the variation in the results caused by variation in illumination needs to be taken into consideration.

#251100-251200

The surface of the low contrast charts is easily damaged. Avoid touching the white test surface. If a person needs to point with his finger at the optotype to fixate on it, use tube gauze or a white glove to cover the finger.

• If the person has not seen the contrast tests before, explain that you are using a low contrast test the same four symbols that were used in the high contrast testing (" the same pictures that we just looked at and you used the names.../matched with the key card ").

• Start measuring at 3 meter distance if you expect close to normal visual functioning.

• Ask, which is the first picture on the line. If the child/person sees the first picture correctly with ease, move to the next page. Continue this way until the person makes an error.

• If the child/person answers incorrectly, ask, which is the next picture and then next going through the whole line. If the person sees three out of five symbols correctly and does see less than three of them on the next line, mark the result as the sum of all the optotypes on the previous lines + the three on the last line read.

• When coming to a new page, the child/person may say that "there is nothing" on that page. It is difficult to focus on optotypes close to the threshold values. Therefore, tell the child/person to look at the optotypes on the previous page and not to move the gaze when the page is turned. Quite often a child/person can see the whole line that he did not perceive when the image was not in perfect focus on his retina.

• For measurement at larger optotype sizes, move closer. If the first measurement was at 3 meters and 3 symbols were seen correctly at 2.5% contrast, a total of 18 correct answers is recorded, as in the case depicted in the diagram. Then move closer, in this case to 1.6 meters (corresponding visual acuity 0.16; 3/19 or 10/63), and measure until threshold, for example 3 correct at 1.2% contrast = a total of 23 correct answers.

• The results are marked on the recording form as the crossing of the lines marking the testing distance used and the number of optotypes seen correctly.

The result of the measurement is written down as the number of correct answers. Although only one optotype was tested on each of the lines at higher contrast, each line above the threshold line is credited for 5 correct answers.

Thus, if a person reads 3 of the 10M LEA SYMBOLS correctly on page 5 (1.2% contrast) at a distance of 1.6 meters, the result is written: Lea-S, 10M:23 at 1.6m.

The result can be marked on the diagram by locating the test distance on the upper border of the recording form (top scale) and marking the results (in this case 18 and 23) at the point where the line of the test distance (3 and 1.6m) and the line of the number of the correct answers cross each other. These points are marked with S and M in the diagram. In the diagram, it is easy to calculate that the result can also be written 0.16 (-2) at 1.2% contrast (0.16 = 3/19 or 10/63).

The number of correct answers is read on the left vertical axis and the cross is placed corresponding to the distance used when testing. In this case the result would then be 18 correct at 3m (marked M) and 23 correct at 1.6m (marked S). Line S-M-X depicts the slope of the contrast sensitivity

After having used the test for a while, you will not need the recording form any more, except for reporting your results to somebody who is not accustomed to using the test. You will have a mental image about where the threshold is located on the form. You will mark down the name of the test, the number of the correct answers and the distance at which you measured. These three numbers carry the necessary information for follow-up.

When both optotype and grating measurements are made, it is interesting to mark them on the recording form to see the relationship between the different threshold values. The luminance of the tests needs to be kept closely equal, otherwise the results are not comparable.

This contrast sensitivity recording form can be used to record any test results from contrast sensitivity tests. The comparison of test results from different tests is easy when they are plotted on this form. By combining results from optotype tests and grating tests the quality of the central visual field can be evaluated for low vision or occupational assessment.