Test Chart Xpert 3Di Contrast Calibration Manual

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CONTRAST CALIBRATION Test Chart Xpert 3Di incorporates a number of tests which require accurate calibration of the screen contrast – contrast sensitivity and low contrast acuity in particular. The software is shipped with a default calibration which may be adequate for general clinical applications. However, where absolute measurements are important and for research purposes, a full calibration is recommended. SCREEN GAMMA Computers connect to display screens using a variety of interfaces including VGA, DVI, HDMI, Displayport etc. Computers specify the colour/luminance of each pixel in terms of three numbers between 0 and 255. These numbers determine the relative amounts of Red, Green and Blue. By varying the relative amounts of Red, Green and Blue (RGB) a complete gamut of colours can be generated. To display neutral colours (black to grey to white), the amount of red, green and blue is maintained constant. For example 0,0,0 (RGB) will be black, 128,128,128 will be a grey and 255, 255, 255 will be white. A graph showing the luminance of the screen as a function of the output from the computer (0-255) is known as the screen’s gamma. The gamma function tends to vary from one display to another which is why it is necessary to calibrate the contrast for each system. To achieve this, a light meter must be used to measure the “brightness” of the screen for each input value (0-255). THE LIGHT METER The light meter supplied is designed to measure illuminance in Lux. This refers to the amount of light falling on the sensor. This type of light meter can be used instead of a luminance meter (which are very much more expensive) because for the purpose of this calibration we only need to know relative light output (contrast) rather than the absolute luminance. 

Carefully unpack the light meter and ensure that all components are present as shown below. 2

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Photocell

Illuminance display

On / off/ units

Range switch

Backlight on/off Peak toggle Hold toggle

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Ensure that the battery is inserted and charged and turn the light meter on by pushing the switch upwards so that the arrow is aligned with Lux. Remove the photocell from the main meter by pulling it upwards firmly. Set the range switch to 2000 to begin with.

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CALIBRATION 

Run Test Chart Xpert 3Di and Select Setup from the menu at the top of the screen. Select the Contrast calibration tab. This will show the dialogue window shown below.

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Click on the Show contrast calibration screen button. This will display a white square in the centre of the main test chart screen as shown below. Select the name of the Gamma function you wish to create from the drop down box. You may either overwrite and existing function of create a new function by selecting Add new function.

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Set the screen brightness and contrast (using the buttons on the monitor) to the required settings. It is recommended that these are generally set to about 50-60% although the optimum setting will vary one monitor to another. NOTE: Some monitors incorporate automatic brightness adjustment to compensate for ambient light. This must be turned off. Remove the cap from photocell Turn the room lights off Place the photocell in the centre of the white square. You will need to get some help to hold it in this position or use masking tape to secure it.

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Set the Current graphics card value to 255 using the up/down arrows and enter the light meter value in the text box labelled Enter photocell value as shown below/

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Decrease the Current graphics card value to 250. You will notice that the luminance of the white square decreases a little. Allow a few seconds for the photocell to stabilise and enter the new value. Repeat this process in decrements of 5, all the way down to zero. You will see the new gamma curve building up on the screen as shown below. NOTE: As the meter reading decreases, move the Range switch as required. For example, if the value is below 200 - select 200, if it is below 20 – select 20 etc.

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On completion, click on OK. The new calibration data will now become the default Gamma.

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PLEASE NOTE: 

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Adjusting the monitor brightness / contrast controls will change the gamma and invalidate the calibration. It is recommended that a note is placed on the monitor to ensure that these controls are not touched. It is recommended that a calibration is carried out every six months to compensate for any drift in the gamma as the monitor ages.

HELP AND SUPPORT If you require any help, please contact our helpline on 01707 414700. It is generally easier if we can log in to your computer to resolve any issues. To do this, your computer will need to be connected to the internet and you will need to download a copy of Teamviewer from www.teamviewer.com. We are always keen to hear your suggestions about how we might improve the software.

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