Calibration Tutorial

Preparation

First, let's open the Display Preferences panel to specify the default display and settings we like to work with when creating new experiments.

Click the Display tab to select the Display Preferences. Select the stimulus display and resolution to be used by default. Any new selection creates a Default calibration configuration that could be used to run an experiment without having to calibrate the setup. A warning message will, however, indicate the potential problem in this case. To proper calibrate the selected configuration, one needs to switch to the Calibration panel.

Tip: to directly jump to the Calibration panel, click the small colorwheel icon highlighted in blue.

To switch to the Calibration panel from the main panel, click the Calibration button in the toolbar.

Calibration Panel

The Calibration of your experimental setup requires the calibration of three aspects of the stimulus display: its geometry, its Gamma function, and the color properties of its phosphors. You need a ruler to perform the geometry calibration and a special device for measuring the emission properties of your display (photometer, colorimeter or spectrometer).

As highlighted in green, this panel can manage multiple configurations of a display (1) as well as multiple calibrations for a given configuration (2). In this example, the current calibration is named 'Default', and corresponds to the display at index 0 (this generally refers to the main display) for a resolution of 1024 x 768 pixels in 32-bits mode and with a frame rate of 60 Hz. Note that a 'Default' calibration is created each time a new configuration of the display is set in the Display Preferences.

3) Click the '+' button to create a new calibration, named 'NewCal*' by default. Rename it to better reflect the display type.

Step 1. Geometry Calibration

Let's start with the geometry calibration:

1) click the Geometry tab in the Calibration panel;

2) Set the size in pixels of the square patch used to perform the geometry calibration;

3) Click the button Perform Calibration;

This is the screen you should obtain on the stimulus display after clicking the Perform Calibration button: a square shape displayed in the center with the specified color and size.

4) Use the ruler to measure the sides of the square (make sure it's actually square);

5) Press ESC (escape key) to close the stimulus screen when the measurement is done;

6) Select the measurement unit, and report the measured value in the field Measured side.

Note in the bottom-left corner the presence of an indication about the time of the last calibration.

The Geometry calibration is done !

Step 2. Gamma Correction

To perform a proper Gamma calibration, you need a device that provides a photometer function. So far Psykinematix provides software interfacing with the Eye-One Display 2 from GretagMacbeth/X-Rite and the Spyder 2 from Color Vision. However, any photometer that provides its readings on a visual output can be used. Make sure your device is setup to provide luminance readings in cd/m^2.

If you don't have access to a photometer, you can still manually set the Gamma value for each gun, but your display may not be correctly calibrated.

To perform the Gamma calibration, follow the steps below:

1) Click the Gamma tab in the Calibration panel;

In this panel, you are presented with the stimulus properties (2), and with the measurement parameters (3). The actual calibration is performed after selecting the measurement mode (4) and clicking the measurement button (5). The plain curve in the graph shows the Gamma of the selected gun.

2) Set the number of luminance readings, the surround color, and the patch size in pixels;

3) Select the gun you want to calibrate: the Red, Green and Blue gun can be individually calibrated or combined to form the white gun. It is recommended to calibrate all 4 guns so the calibration configuration could be used in either the chromatic or achromatic mode set in the Display settings for each experiment.

4) Specify the source of the measurement: by default, it is set to Gamma and its value can be then directly edited in the field just below. We assume you have access to a photometer that provides the readings you will enter manually, so select the Readings option.

If you have a Eye-One or Spyder device connected to the computer, select it, and the readings will be automatically sent to Psykinematix during the measurements.

5) Select the action Gamma Measurement from the Perform... popup menu. Gamma measurement should be performed first, then followed by a checking of the linearity of the Gamma correction.

This is the screen you obtain on the stimulus display when selecting one of the Perform... actions: a square shape displayed in the center with the specified color and size, and surround.

6) Place the photometer head centered on the white square. The square should be larger than the light sensitive area of the photometer, if it is not then press ESC, increase the square size, and re-select the action Gamma Measurement;

7) Press ESC when the photometer head is in place. This starts the measurement process;

8) In manual mode, enter the photometer reading using the keypad after each change of the square luminance, followed by the Return key to move to the next luminance level. The reading index and entry are displayed in the bottom-right corner of the stimulus display. Use the Delete key to modify the entry before it is validated with the Return key.

When using a Eye-One or Spyder device, the reading is directly sent to the computer and the next luminance level is automatically presented. In this case, the reading index is displayed in the top-left corner of the stimulus display.

Press ESC to close the screen when the luminance measurements are completed (or press ESC for 1 second to abort the measurement process).

When the Gamma measurement for the selected gun is completed, the graph shows the luminance readings with the filled square symbols.

The Gamma field shows the Gamma value that best fits these data.

The inverse plain curve shows the resulting Gamma correction to be applied.

The graph always presents the normalized intensities, so note the minimum and maximum measured luminances displayed below the Gamma field.

9) The last step for the Gamma calibration is the checking of the linearity of the Gamma correction. Select the action Correction Checking from the Perform... popup menu, and follow the same steps than above to perform the measurement process.

When the checking process in completed, the corrected Gamma is added to the graph with the empty circle symbols. The closer they are to the diagonal line, the better is the correction.

You should repeat the Gamma calibration for all other guns, after which the Gamma calibration is done !

Step 3. Color Calibration

For proper color calibration, you will need a device that provides the colorimeter or spectrometer functions. So far Psykinematix provides software interfacing with the Eye-One Display 2 colorimeter from GretagMacbeth/X-Rite and the Spyder 2 colorimeter from Color Vision. However, any colorimeter that provides its readings on a visual output or any spectrometer with a file output can be used.

If you don't have access to a colorimeter or a spectrometer, you can still set color properties that may be close to your display characteristics, but your display may not be correctly calibrated.

To perform the Color calibration, follow the steps below:

1) Click the Color tab in the Calibration panel;

In this panel, you are presented with the stimulus properties (2), and with the measured color properties represented in a table and graphically (4). The actual calibration is performed after selecting the phosphor, the measurement mode and clicking the measurement button (3).

2) Set the surround color and the patch size in pixels;

3.1) Select the phosphor you want to calibrate: the Red, Green and Blue phosphors can be individually calibrated or combined to form the white point. It is mandatory to calibrate all 4. If you have one of the supported devices, the 4 can be automatically calibrated in one step by selecting "All Phosphors".

3.2) Select the type of measurement to perform for each phosphor: radiometric or colorimetric properties. For best calibration, both types of measurement should be carried out.

You need a device with a spectrometer function for radiometric measurements, and with a colorimeter function for colorimetric measurements.

Colorimetric Data

Select the Colorimetric mode.

This mode allows you to specify the source of the colorimetric measurement of your display (1) and the measured chromatic properties are presented in the table (2): the xy chromatic coordinates and maximum luminance (Lmax) for each phosphor which form the Maxwell triangle (or gammut) of the display (3).

1) This popup menu provides several choices: if you own a colorimeter then use one of the two top options, Custom... allows you to manually edit the table (2) with readings obtained from a device not supported by Psykinematix, while Eye-One or Spyder 2 allows to automatically obtain the readings from the colorimeter. Click the Perform Calibration to perform the colorimetric measurements for the specified phosphors.

All other choices in this popup menu correspond to some standard displays, and should be selected only if you don't have access to a colorimeter, and have some clue about your display characteristics. On selection, the table (2) is filled with some pre-defined colorimetric data for the specified display type and the gammut updated (3).

Radiometric Data

Select the Radiometric mode.

This mode allows you to specify the source of the spectral measurement of your display (1) and the cone fundamentals (2), both shown in the graph (3), in order to generate a LMS to RGB matrix (4).

1) The source of the radiometric measurement is specified through a popup menu which offers several choices: provide a custom LMS to RGB matrix by editing the matrix (4), select a file containing the spectral power distribution for each phosphor, or select a phosphor standard that closely matches those of your display.

2) The cone fundamentals specify the spectral absorption for each cone, and are used in combination with the spectral emission of the display to produce the LMS to RGB matrix which is automatically updated with the selection.

3) The spectral power distributions of both display emission and cones absorption are represented graphically (plain for phosphors, dashed for cones) and updated with any change in the selections;

4) The resulting LMS to RGB matrix is displayed in the table, and updated with any change in the selections.

Note that Psykinematix does not support any spectrometer yet, but can import a data file with the data produced by the spectrometer (this file should simply be a text file conforming to a specific format described in the documentation). However, Psykinematix can present an appropriate stimulus patch so the spectrometer can perform its measurements for each phosphor. To do so, click the Perform Calibration button.

Specify a Calibration Configuration for an Experiment

After finishing an experiment design, you should specify the calibration configuration to be used during each session. To do so, select your experiment in the Designer table (of category Experiment with the 'Psy' red mini-icon), and click the INFO button to inspect its properties.

In the properties sheet, click the Display tab to view/edit the Display settings of the experiment. By default, the display settings (stimulus display and resolution) are those defined in the Display Preferences. To change them for the currently edited experiment, use the two provided popup menus. Each time these display settings are changed, the content of the popup menu entitled Calibration Name (highlighted in blue) is updated with all the calibration configurations available for these settings.

If none is available, the selection in the popup menu remains blank with a small warning icon attached to it. Click this warning icon to reset the display settings to the ones in the Display Preferences, or close this panel and return to the Display Preferences and Calibration panel to create a configuration for the new display settings (see beginning of this tutorial).

Conclusion

In this tutorial, you learned to calibrate the geometry of your display, to measure and linearize its Gamma, measure its color properties, and set the calibration configuration used by an experiment.