Contrast sensitivity and contrast threshold

Contrast sensitivity and contrast threshold

Learning objectives

To become familiar with two types of contrast chart
To understand the use of these charts
To obtain measures of contrast sensitivity and threshold
To determine the effects of haze glasses on visual acuity and contrast
To review the advantages and disadvantages of these charts

1. Vision Contrast Test System (VCTS)

Introduction

The VCTS uses highly controlled advanced photographic and printing techniques to present a series of sine-wave gratings at carefully calibrated levels of contrast. The size of the sine-wave gratings, the contrast levels, and their tilt are all based on current vision science and empirical data. Each row or circular grouping of patches tests at a specific spatial frequency (cycles per degree), which measures the observer's sensitivity to a particular object size. The low frequencies test sensitivity to very large objects, while on the other end, high frequencies measure sensitivity to very small objects. Each test frequency begins with a high level of contrast, which diminishes progressively with each succeeding patch. The sine waves, which appear as fuzzy grey bars, vary in their orientation within the patch and may be vertical or tilted left or right. The observer reports the lowest contrast patch visible in each grouping and describes the orientation. The tester records the results to produce a contrast sensitivity function (CSF) or curve.

Note

The VCTS is available in a distance and near format. This practical will use only the distance format. There are three configurations of the distance chart coded A, B, C after the patent number in the lower right corner of the chart. Use this to find the corresponding answer key on the reverse side of the laminated instruction sheet.

Practical assessment

Those that require refractive compensation should use glasses or contact lenses for all the following tests. Because of time constraints it is sufficient to conduct testing binocularly. In a clinical situation all tests should be done monocularly and binocularly.

Prior to assessing contrast sensitivity (CS) determine distance vision/visual acuity using the Snellen chart.

The following technique should be used to determine the CSF:

Place the observer at the other side of the room from the chart (approx. 3m).
Point out the sample patches at the bottom of the chart with the three possible responses (left, right or straight up).
Cover one eye with an occluder.
Instruct the patient to begin with Row A and look across from left to right.
Ask the observer to identify the last patch in which the lines can be seen and tell you which direction they tilt. If the response is incorrect have the patient describe the preceding patch.
Now have the observer guess which way the lines tilt in the next patch to the right even if he or she says that nothing can be seen in that patch. Guessing ensures the use of the three-alternative, forced-choice test method, known to improve the accuracy of contrast threshold measurement.
Each vertical column of numbers on the evaluation form corresponds to a horizontal row on the chart. Record the last patch the observer correctly identifies in each row by marking the corresponding dot on the evaluation form. Use a different colour to distinguish right, left and binocular.
To form the observer’s CSF, connect the points marked.

Determine the CSF under the following conditions:

1. With the normal eye binocularly

2. With haze glasses binocularly

3. With haze glasses ands a glare source binocularly

A glare source can be obtained by pointing an Anglepoise lamp chart directly towards the observer from a distance of about 20 to 30 cm. Three CSFs (one for each of the above conditions) marked with a different colour pen, should be plotted on one record sheet.

2. Pelli-Robson Contrast Test

Introduction

The letters on the chart are organised into groups of three i.e. triplets, with two triplets per line. Within each triplet all letters have the same contrast. The contrast decreases from one triplet to the next. The chart should be read from top left to as far down as possible. The observer should be made to guess even when they believe the letters are invisible. Several seconds should be allowed for the faintest letters to appear. Score miss-called Cs, Os or Ds as correct. Each correct letter has a contrast threshold value of 0.05 log units however scoring in terms of log units is reserved for research. A more user friendly approach is to score in terms of contrast percentage. See accompanying sheet to assist in contrast percentage scoring.

Practical assessment

The test is designed to be used at one metre. Determine the CS thresholds in terms of a contrast percentage under the following conditions.

1. With the normal eye binocularly

2. With haze glasses binocularly

3. With haze glasses (binocular) and a glare source as discussed above.

Note

For this technique a single number representing the contrast threshold in terms of log units or a percentage is obtained and not a CSF.

Make sure you are aware of the effects of a) haze glasses b) haze glasses and a glare source on:

vision/visual acuity
the CSF
contrast threshold percentage