Contrast

Test Chart 2016 allows the contrast of the LogMAR, Snellen and Single letter charts to be varied to allow the measurement of high and low contrast acuity  It also includes a Contrast Sensitivity test similar to the Pelli-Robson test.

In order for the software to accurately reproduce specific contrasts on the screen, the system must be calibrated using a light-meter.  A suitable light meter can be purchased from Thomson Software Solutions.

Test Chart 2016 includes a number of default calibration sets (known as the Gamma curve).  In the absence of a light meter, a suitable Gamma curve should be selected from the list.  NOTE: Without calibration, the contrast values displayed will only be approximate.

Calibration

The graph shows how the contrast of the display varies with the output from the PC graphics card. This is sometimes referred to as the Gamma function.

To measure the Gamma function for your graphics board / monitor, first select New and enter a name for the new Gamma curve.

Tick the Display Calibration screen check box.  This will display the window below on the main test chart screen. Place the photocell of the light meter on the square.

Set the Computer value to 255 using the scroll-bar below the graph. Enter the Light Meter Reading.  While keeping the photocell in the same position, click on the LEFT arrow on the Computer value scroll-bar. This will decrease the Computer Value by 5 and darken the square.

Repeat this for all Computer Values from 255 to 0. A graph of the contrast as a function of Computer Value will appear automatically.

When you have completed this process, click on Save. This Gamma function will then be stored and used to calculate the contrast of all subsequent displays.

Existing Gamma functions may be overwritten or deleted by selecting Delete.

The current Gamma function will be used in all calculations of screen contrast.

NOTE: The luminance and contrast of most monitors takes some time to stabilise when first turned on. It is advisable to turn the monitor on for 30 minutes before using it, especially when measuring low contrast acuity or contrast sensitivity.