The Contrast Sensitivity test is displayed by selecting the Contr icon on the Primary toolbar, pressing F11 on the keyboard of selecting Contr S on the remote control handset.


Measurements of contrast sensitivity have been shown to provide valuable information about patients’ visual status.

It has been shown that measurements of high contrast visual acuity do not always adequately reflect patients’ quality of vision in the “real world”. This is particularly true for patients with cataracts and various neurological conditions as well as contact lens wearers and patients who have undergone refractive surgery.



The traditional method of measuring a contrast sensitivity function using sine wave gratings is time consuming and difficult to implement on a standard PC. However, it has been shown that measurement of the contrast threshold for large letters generally provides sufficient information in the clinical setting.


The patient must view the screen directly from 1 metre.  


Letters are displayed in triplets of decreasing contrast. The contrast of the letters is decrease using the Vertical Scrollbar in the Control Window or by using the Vertical Cursor Control keys on the keyboard or remote. The lowest contrast where the patient can read 2 of the 3 letters is recorded.



The test can be performed using any of the optotypes. The required Optotype is selected from the pull-down list in the Secondary Toolbar.


There are five predefined sets of letters (the first three letters defined for the LogMAR charts).  To rotate around these 5 predefined sets, click on the O icon on the Secondary toolbar or press the O key on the keyboard or the remote.


To randomise the letters, click on the R icon on the Secondary toolbar or press the R key on the keyboard or the remote.


Contrast sensitivity is usually measured with large letters to provide an assessment at the low spatial frequency end of the contrast sensitivity function. the Thomson Test Chart provides the facility for changing the size of the letters to enable the contrast sensitivity function to be sampled at different spatial frequencies.  To change the size of the letters, press PgUp or PgDn on the keyboard or remote.


Contrast in this context is defined as:

C=  Lback - Lchar

                   Lback


Where Lchar is the luminance of the character and Lback is the luminance of the background. By this definition, contrast varies between 1 and 0 or 100% and 0%.


Contrast sensitivity is the reciprocal of the contrast threshold. Therefore, if the minimum contrast perceived by the patient was 0.01 (1%), the contrast sensitivity would be 100 and log10 contrast sensitivity would be 2.0.


Labels showing Log contrast sensitivity, contrast sensitivity or percentage contrast may be displayed in the left and right margins of the Contrast sensitivity chart.


The format of the labels can be specified via the Contrast Sens tab of the Settings screen. The size of the labels may also be changed by changing the Font size setting on the same screen.




NOTE 1: The contrast values are calculated from the contrast calibration graph (Gamma function) and will only be accurate if the contrast of the display has been calibrated carefully.


NOTE 2: At low contrasts the characters may appear to have a slight hue. This is a result of the bit-stealing method used to achieve more


Summary

Mouse

Select the corresponding button in the Secondary toolbar for default (O) or randomised letters (R)


To select an optotype, click on the corresponding button


Use the Vertical Scrollbar on the Control Window scroll the chart and display letters of different contrasts



Keyboard

O Sets chart to default arrangement of optotypes.

R Randomise optotypes


Use SHIFT + horizontal cursor control keys to rotate around different optotypes

Use vertical cursor control keys to display letters of different contrasts

Use PgUp and PgDn to change size of the optotype




Remote

O Sets chart to default arrangement of optotypes.

R Randomise optotypes


Use SHIFT + horizontal arrows to rotate around different optotypes

Use vertical arrows to change the contrast of the optotypes


Use PgUp and PgDn to change size of the optotype


 contrast steps. It should not adversely affect results.