The calibration of your system is an important step to make sure your experiments can be appropriately published as well as replicated on other systems. Full calibration is recommended each time the display settings have been modified. Click on the Calibration icon in the toolbar at any time to access the Calibration panel. Psykinematix can help you calibrate several aspects of your experimental setup:
Managing the Calibrations
Supported Calibration Devices
Several calibrations can be managed for each configuration of the display settings (display index where 0 refers to the main display, resolution, bit depth, and frame rate). Calibrated configurations appear in the pop-up menu at the top right of the calibration panel, while the calibrations attached to the selected configuration appear in the top left editable pop-up menu.
To create calibrations for a new configuration of the display settings (i.e. not yet present in the top right pop-up menu of the calibration panel), select the new settings in the Display Preferences and click on the small colored wheel icon to return to the calibration panel where the display settings and the list of available calibrations will automatically be updated. A calibration named 'Default' in the pop-up menu is created by default for any new configuration of the display settings. The currently selected calibration is the one used by default when creating new experiments; however, this can be changed in the Display settings of the Experiment properties panel.
Note that changing the display settings in the Display Preferences (used by default when creating new experiments) updates the current configuration in this calibration panel.
The '+' and '-' buttons add and remove calibrations for the current configuration of the display settings. Control-clicking on the '+' button adds a copy of the current calibration; otherwise, a default calibration is created. You should rename any new calibration to reflect the display brand and type as shown in the example above.
The unique name of the display obtained from its EDID information (Extended Display Identification Data) is also indicated at the bottom of the panel. This name consists of the display name, manufacturer code, the serial number and the month and year of manufacture. This unique display name is marked "undefined" if this information is not available (ie not EDID information available) or if the last calibration was performed with a version of Psykinematix earlier than 1.3 (if available the name will be updated at the next calibration). The unique display name is now used to further filter out the calibration configurations for displays not currently connected: this ensures that only the calibration configurations for the connected displays get updated and used by the experiments.
A calibration highlighted in red indicates it is out of date based on the deadline set in the Admin Preferences. The display should be then recalibrated for this configuration (Note that each calibration panel shows the number of days since the last calibration). The progress of the calibration process is indicated at the bottom of the panel under the form of a percentage and with the status of each calibration step (geometry, Gamma & color):
Further details about the progress and status of the calibration can be found under the Summary tab where a button is associated with each calibration step. Each step button is referred either as:
- "not completed yet"
- "completed and up-to-date"
- "completed but expired"
If a calibration step is not completed and up-to-date, clicking on its button jumps to the associated tab where the calibration can be performed or completed (see below). For the Gamma calibration, the checked boxes indicate which guns remain to be calibrated.
The button allows the calibration information to be locked to prevent unwanted changes. Once locked, you may need to enter a password to unlock the calibration information. (See the Admin Preferences to set this protection password.)
Geometry calibration ensures that stimuli look the same on different displays in terms of their spatial properties (stimuli size in degrees, spatial frequency in cycle per degree, etc). Geometry calibration is carried out through the following panel (Click on the Geometry tab):
All you need is a ruler:
Set the size of the square patch used to calibrate the display geometry, its appearance ("Square Color"), and its surround's appearance ("Surround Color").
Click on the "Perform Calibration" button and a square with the specified appearance will appear in the center of the calibrated display. Measure each side of the patch using the ruler to make sure it appears square; if it does not, use the display controls to adjust its aspect ratio. Note the size of the square either in inches or centimeters, and press the ESC key to return to the panel view.
Select the measurement unit used (cm or in), then enter the measured size in the text field entitled "Measured side" followed by RETURN. Important: Entering the measured value first and then selecting the unit will convert the entered value to the specified unit.
Note that the Geometry calibration is synchronized with the Display Preferences which indicate the field of view expressed in degrees and the maximal spatial frequency given a default viewing distance.
A successful Geometry calibration should be indicated by a green check mark in the progress status, you may then click on the "Next Step" button to move to the Gamma calibration.
Gamma calibration corrects for the intrinsic non-linearities in the display luminance and in the digital-to-analog conversion of the graphics card. This is especially important to minimize luminance artifacts in contrast-modulated patterns (2nd-order stimuli). To carry out the Gamma correction, you first need to estimate the Gamma of your display by measuring the light intensity (typically the luminance in Candela/m^2 or Nit) as a function of the pixel intensity (typically between 0 and 255) for each gun (Red, Green and Blue) and their linear combination (White) using either a photometer or a colorimeter.
Psykinematix uses the standard gain-offset-gamma (GOG) model to perform the Gamma correction on CRT displays (for details see: Berns 1996, Brainard et al 2002, Georgeson 2007). Because the GOG model may be inappropriate for LCD displays that often show a saturating transfer function, Psykinematix takes care of discarding the saturating luminance readings before fitting the GOG model. This generally provides an effective Gamma correction over an extended luminance range even if the display transfer function does not conform well to the standard GOG model.
Gamma correction is carried out through the following panel (Click on the Gamma tab):
Before starting the calibration, you have to specify which gun will be calibrated and how the luminance readings will be measured. These selections are made through the use of two pop-up menus as shown below:
The first pop-up menu indicates how the luminance measurements are made: The "Readings" option specifies a mode where one visually reads the luminance measurement from a photometer (with an LCD display) and enters its value manually (the gun value is then increased to produce the next luminance reading). The "Gamma" option allows the user to directly specify a value in the Gamma text field (note that this text field otherwise displays the Gamma value resulting from the fitting of a Gamma function to the readings), and does not require a photometric device. The "Eye-One" option indicates that an Eye-One device is used to automatically measure the luminance readings (see the "Supported Calibration Devices" section to find out the list of supported devices).
The second pop-up menu ("Guns") indicates which gun should be calibrated: in addition to the standard red, green and blue guns, a white gun is available to measure the Gamma of the gray-level (ie: the 3 standard guns are activated with the same pixel intensity). The two remaining options (Mono 9.6 and Mono 10.8) are related to the bit-stealing chromatic mode and can only be used to check the Gamma correction produced by linearization of the red, green and blue guns (up to 766 and 1786 intensity levels are available for Mono 9.6 and Mono 10.8, respectively).
Other relevant parameters are:
the number of readings that uniformly sample the pixel intensity (the maximum number of readings is typically 256 on an 8 bits per pixel graphics card; an error message will be shown if more samples than allowed are requested);
- the size of the square patch which luminance is measured;
- the appearance of its surround.
Note that the bit resolutions for each pixel component and for its hardware Gamma lookup table provided by the graphics card are indicated, as they affect the quality of the Gamma calibration. More accurate Gamma corrections are obtained from a hardware Gamma lookup table with a bit resolution higher than the one of the pixel components (embedded 10-bit gamma correction is typically found in recent graphics cards and digital displays).
Gamma calibration and correction checking are accessed through the "Perform..." pop-up button:
To summarize, to perform the Gamma calibration with a photometric device, you should:
Set the measurement mode (to "Readings" or one of the supported devices), the gun to be calibrated, the number of samples, the size of the square patch, and its surround appearance;
Click on the "Perform..." button and select the "Gamma Measurement" option. When a white square will first appear in the center of the calibrated display, you will be asked to position the photometric device, and then its pixel intensity will be changed for each sample. Depending on the measurement mode, you will have to either manually enter each photometer reading or wait until the device finishes on its own. When it is completed, you will be asked to press the ESC key to return to the panel view;
The luminance readings will be fitted with a Gamma function and displayed in the graphing window (filled squares for the measurements, continuous line for the fit). The fitted Gamma value is displayed in the "Gamma" text field, along with the minimum and maximum luminances. The correction is then directly derived from a Gamma function with the reciprocal Gamma value;
Click on the "Perform..." button and select the "Correction Checking" option to verify the accuracy of the Gamma correction. This process is identical to step 2 above;
The luminance readings from the Gamma correction are then displayed in the graphing window (open circles) on top of the diagonal straight line which indicates perfect linearity;
Repeat the whole process for each gun. To compare the quality of the Gamma correction for the different measurements, use the check boxes on the left of the graphing window to select which guns are displayed (L for the RGB combination).
You can automatize the Gamma calibration process by selecting either the "Both for current gun" or "Both for all guns" options under the "Perform..." button to perform the Gamma measurement and the correction checking sequentially.
Note that all luminance readings and Gamma corrected intensities can be inspected in the Gamma calibration drawer available by clicking on the "light bulb" icon in the top-left corner of the panel. These values can be exported by control-clicking inside the table and selecting one of the options from the contextual menu.
Finally, it is recommended to regularly perform a correction checking and redo the Gamma calibration if the correction is no longer adequate. It is also recommended to wait for the monitor to warm up and stabilize for about 1/2 to 1 hour before carrying out a Gamma calibration (especially for CRT monitors).
A successful Gamma calibration is indicated by a green check mark in the progress status, you may then click on the "Next Step" button to move to the Color calibration.
Color calibration can be performed in two ways depending on the instrument used to characterize the color properties of your display:
- When using a spectrometer, color calibration is based on radiometric data (energy as a function of light wavelength).
- When using a colorimeter, color calibration is based on the xy chromatic coordinates based on the CIE 1931 standard.
Depending on the role of color in your experiment, either device may be preferable. By measuring the spectral emission of the display, radiometric data specifies how much light reaches each type of cone (assuming some human cone fundamentals for the spectral absorption), and characterizes the stimulus in terms of cone excitation (at the perceptual level). The xy chromatic coordinates provided by a colorimeter are display-independent and may be more appropriate when the perception of color per se is under investigation (at the cognitive level).
Psykinematix offers these two methods for performing color calibration through the following panel (click on the Color tab):
Each method consists in measuring the color properties of a patch of a given size and surround appearance for each phosphor activation (and their combination, i.e. the white point).
Note that the number of days since the last calibration is indicated next to the calibration mode (Radiometric or Colorimetric). While the chromatic properties are generally not expected to change over time, it is still recommended to regularly perform a color calibration each time the display settings have been modified. It is also recommended to wait for the monitor to warm up and stabilize for about 1/2 to 1 hour before carrying out the Color calibration (especially for CRT monitors).
When selecting the "Radiometric Data" option, you are presented with an "LMS to RGB" matrix on the left and a graphing window on the right. The "LMS to RGB" matrix defines a transformation from the LMS color space (excitation for the Long-, Middle- and Short-wavelength sensitive cones) to the display-dependent RGB color space. Such a matrix is obtained by multiplying the spectral emission of each phosphor by the spectral absorption of each cone. These phosphor emissions and cone absorptions are plotted in the graphing window as a function of the light wavelength (continuous lines for phosphor emissions, dashed lines for cone absorptions, the P and C check boxes enable/disable each plot).
The pop-up button below the matrix specifies how the matrix is generated based on the phosphor emissions. The matrix can be:
- manually edited with the "Custom Matrix" option (no curve is plotted in this case),
- derived from some spectral measurements stored in a custom "SPD file" (Spectral Power Distribution for each phosphor as a function of the wavelength as measured by a spectrophotometer), or
- derived from some pre-defined spectral emissions of standard phosphors ("P22-EBU").
When selecting the "SPD File" option, you will be presented with a panel to select a custom file. A custom SPD file is a simple tabulated text file where each line consists of the wavelength (in nm) followed by the power of the light emitted by the Red, Green and Blue phosphors as seen in the following example:
390 4.076190E-04 3.582270E-04 6.142650E-03
391 4.970680E-04 4.386600E-04 7.442800E-03
392 6.047130E-04 5.362300E-04 9.016610E-03
828 9.797880E-07 9.473490E-08 0.000000E+00
829 9.247250E-07 8.967180E-08 0.000000E+00
830 8.730080E-07 8.489020E-08 0.000000E+00
The computation of the "LMS to RGB" matrix also depends on cone absorption. Cone fundamentals which have been proposed to model the cones sensitivity in the normal human eye characterize cone absorption (see http://www.cvrl.org/cones.htm). The "Cone Fundamentals" pop-up button provides three models (Smith & Pokorny 1975, Stockman & Sharpe 2000 for 2 and 10 degrees). The selected cone fundamentals are plotted in the graphing window along with the phosphors' SPD.
The "LMS to RGB" matrix is automatically updated on selection of the phosphor spectral properties (SPD or phosphor type) or the cone fundamentals. In radiometric mode, all color spaces are computed on the basis of this "LMS to RGB" matrix.
Note that in this mode, clicking on the "Perform Calibration" button still allows the presentation of the red, green and blue patches with maximum luminance; and their spectral distributions can be measured by an unsupported spectrometer.
As all displayable colors are formed from an additive mixture of the red, green and blue phosphor emissions, the color gamut of a display is a subset of all visible colors and is defined by a triangle in the CIE xy chromaticity diagram. When selecting the "Colorimetric Data" option, you are presented with an "xyL for RGBW" matrix on the left and a graphing window on the right representing the gamut of your display in a standard CIE chromaticity diagram. The "xyL for RGBW" matrix specifies the xy chromatic coordinates and the maximum luminance (in candela/m^2) for each phosphor as well as the white point. This set of xy coordinates defines the limits of the gamut triangle represented in the chromaticity diagram.
The xyL coordinates are generally measured through the use of an attached colorimeter. You can specify the chromaticity in several ways using the provided pop-up button:
- The "Custom..." option allows the user to manually edit the matrix (in case the device is not supported by Psykinematix).
- One of the supported calibration devices (see the "Supported Calibration Devices" section) is used to automatically measure the xy coordinates and maximum luminance.
- The remaining options are several pre-defined chromatic coordinates according to some standards set by display manufacturers.
The gamut of the display can be displayed according to two standard CIE chromaticity diagrams: the CIE 1931 in terms of xy chromatic coordinates or the CIE 1976 in terms of u'v' chromatic coordinates. As a colorimeter generally measures the xy coordinates, selecting the CIE 1976 option converts the xy diagram to a u'v' chromaticity diagram (as well as the RGB coordinates inside the table): the advantage of the 1976 diagram is that the distance between points is now approximately proportional to the perceived color difference, something definitely not true in the 1931 diagram.
To summarize, to perform the Color calibration with a colorimetric device, you should:
Set the measurement mode (to "Custom..." or one of the supported devices), the phosphor to be calibrated, the size of the square patch, and its surround appearance;
Click on the "Perform Calibration" button: a white square will first appear in the center of the display being calibrated, you will be asked to position the colorimetric device, and then the patch chromaticity will be set to the requested phosphor. Depending on the measurement mode, you will have to press the ESC key or wait until the device finishes its chromaticity reading to return to the panel view;
In custom mode, you will have to enter the chromaticity coordinates for the specified phosphor in the "xyL for RGBW" matrix (CIE 1931). When using a supported colorimeter, the matrix will automatically be filled in with the chromaticity coordinates and maximum luminance. In both modes, the gamut triangle in the CIE chromaticity diagram will be updated with the new settings;
Repeat the whole process for each phosphor if necessary.
Display calibration devices are photo-sensors that are mounted on the surface of the display screen and connected to the computer via a USB interface or some other link. During calibration they measure the light properties of test patterns that are displayed on the screen in sequence, and then send the data to the computer. Three types of devices are of interest for display calibration: photometers, colorimeters, and spectroradiometers.
Photometers are instruments used to measure the light intensity emitted by each phosphor, estimate its transfer function (Gamma), and produce individual Gamma correction lookup tables so a Gamma-corrected display appears linear to the human eye. Colorimeters characterize color samples to provide an objective measure of color characteristics, ie: the chromaticity coordinates of each phosphor as well as their luminances. As a matter of fact, they can be used as photometers but not as spectroradiometers. Spectroradiometers are instruments used to measure the properties of light over a specific portion of the electromagnetic spectrum, more specifically spectral power distributions of individual phosphors that compose a display. They can also be used as colorimeters and photometers.
Psykinematix aims to support as many calibration devices as possible, the only limitations being they have to be USB devices (or possibly connected through a Serial-to-USB adapter) and compatible with Mac OS X. If you own a device you would like to see interfaced with Psykinematix, please make a feature request using the feedback menu.
The current version of Psykinematix supports:
When a calibration device is selected at any step of the calibration process, information about the device (identity, type and connection status) and its settings are presented in a sliding drawer:
Before using the device, make sure to select the proper settings (in particular screen type and measurement mode as they directly affect the measurements). If the requested device is not connected, the drawer appears showing the device icon with an X through it, as illustrated below:
Attempts to perform a calibration without the device being connected will result in an error message.
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