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

Low Contrast Test
10M Number Size
#270400 - #270500

If you have not used low contrast tests before, please, read the Introduction. It will give you some theoretical background that helps understanding the Instructions

In this test type the 10M size is convenient because the visual acuity as decimal visual acuity is easy to calculate: at the testing distance of 1 meter 10M 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 decimal 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%.

Luminance variation affects the threshold values in many disorders. It is difficult to arrange high enough luminance on the test, which needs to be kept vertical. If illumination in the examination room uses two lines of long day light fluorescent tubes, the luminance level in the middle of the lines is the same at different distances from the person to be tested. In field surveys, the variation in the results caused by variation in illumination needs to be taken into consideration.

Figure 1. 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 to cover the finger.

• If the person has not seen the contrast tests before, explain that you are using a low contrast test with the same four symbols/numbers that were used in the measurement of visual acuity at high contrast, 3 m, which corresponds to visual acuity 0.3 (20/60, 6/18).
  
  
• The tests with one optotype size require close attention on luminance level and direction of the light falling on the test. You can experience the variation of the visibility of the optotypes by tilting the test so that the lower edge is closer to the child/person to be tested than the upper edge: the forms are seen with ease even if they were not perceived when the test page was vertical. Since the test is a fast test of contrast sensitivity in the assessment of vision of visually impaired people in the whole area visual acuity values, it is important to use it so that the visibility of the optotypes is kept constant and the test functions as it was designed to function.
  
  
• Start measuring at 3 meter distance if you expect close to normal visual functioning and the visual acuity value measured at full contrast was normal.
  
  
• Ask, which is the first picture on the line of the page with 25% contrast. If the child/person sees the first picture correctly with ease, ask which the second picture is. If that is correctly recognized, move to the next page. Continue this way until the person makes an error.
  
  
• If the child/person answers incorrectly on the 2.5% page, ask: “Which is the next picture/number and then next going through the whole line”. If the person sees three out of five symbols/ numbers correctly and does not see any of them on the next page with 1.2% optotypes, mark the result as the sum of all 15 optotypes on the previous lines (25%,10% and 5%) + 3 on the last  line read =18. VA at 2.5% contrast = 0.3 (20/60, 6/18).
  
  
• 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, in this case to 1.6 meters (corresponding visual acuity 0.16; 3/19 or 10/63), and starting from the 2.5% page measure until threshold, for example 3 correct at 1.2% contrast = a total of 23 correct answers. VA at 1.2% contrast = 0.16(-2) (0.16 = 10/63 or 20/120; 3/19 or 6/38).
  
  
• 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).
  
  
• 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 (Figure 2).

The result of the measurement can be written down as the number of correct answers. Although only one or two optotypes were tested on each of the lines at higher contrast levels, each line above the threshold line is credited for 5 correct answers.

Figure 2. 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 3 m (marked M) and 23 correct at 1.6 m (marked S). Line S-M-X depicts the slope of the contrast sensitivity curve..

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 m and 1.6 m) 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 as VA at 2.5%: 0.3(-2) and at 1.2% contrast  0.16 (-2) (0.16 = 3/19 or 10/63).

If a patient with visual acuity 0.63 at full contrast has following VA values at lower contrast levels, the results are documented as in Figure 3:

Figure 3. At the distance of 4 meters the person saw all 5 optotypes correct at 10% and none on the 5% page; the result is: VA 0.4 at 10% contrast (A). At the distance of 3 meters all 5 optotypes correctly on the 5% page and 3 on the 2.5% page; the result is 0.3 (-2) at 2.5% contrast (B). At the distance of 2 meters the patient read 4 correct on the 1.2% page; the result is 0.2 (-1) at 1.2% contrast (C).

By choosing the distances 4 m, 3 m, 2 m, 1.6 m, 1.25 m, 1 m, 0.8 m, 0.6 m, 0.5m, 0.4 m the visual acuity values are 0.4, 0.3, 0.2, 0.16, 0.12, 0.1, 0.08, 0.06, 0.05 and 0.04.  (The corresponding values as Snellen fractions are in the VA list.) The use of metric system makes these measurements much easier than if Snellen fractions are used.

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 or the visual acuity and the contrast level. These two 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. In this diagram there are contrast sensitivity slopes of children with Cerebral Palsy. The slope of the curve based on LEA Symbols charts (blue line) and the slope based on the measurements with the LEA Grating Acuity Test and the LEA Low Contrast Grating Acuity test (red line) of a student with tetraplegic condition are placed on the background of other students’ results to demonstrate that the values measured with the large grating tests (stimulus varying between 4 degrees to 20 degrees in diameter as a function of the test distance) usually are several steps better on a logarithmic scale than values measured with optotype tests. The black slopes are based on measurements with optotype tests. Note that in this cohort of students with severe motor problems, mostly Cerebral Palsy, only 4 students did not reach the 2.5% level; two of them reached 5% level, one 10% and one 25% level.

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.

In order to record the shape of the curve, it is good to get at least three measures. One measure is at the highest contrast - the typical visual acuity threshold measure and the other two at lower contrast levels. In clinical measurements it is often acceptable to measure only one measurement at 2.5% contrast to shorten the test time. If the person does not perceive the test at 2.5% contrast, then 5% and higher contrast optotypes are used.

The threshold value (A) and the straight “slope “ (dashed line - -) from A to X does not give the same information about the form of the slope as the measurements at intermediate visual acuity values, which define the position of the curved slope (B, C).

In this cohort of students with severe motor problems due to brain damage, variation of the declination of the CS slope is great: visual acuity values 1.6 and 0.16(+2) at full contrast can have the same 0.1 VA value at 2.5% contrast (red dots mark the VA values)

Measurement of the threshold VA values at full contrast and at very low contrast is typical to many clinical examinations but these extreme values are not functionally interesting, except in some rare occupational tasks.