When choosing a monitor for content creation, color grading or QC, one aspect that is often overlooked is color calibration. In this article, we'd like to provide some insight on the following:
 

• What is Display Color Calibration?
• Currently Used Calibration Standards
• Calibration Basics
• HDR Display Evaluation

What is Display Color Calibration?

Color calibration is the process of measuring and adjusting the color response of a display device to a known standard. The primary goal of color calibration is to preserve the creative intent of content creators when reviewed across different devices. When creators capture and master content, they adhere to certain image standards (brightness, black level, color gamut, gamma response, etc.). When viewing content, if your display is not configured and calibrated properly to adhere to these industry standards, what you’re viewing on the screen is not an accurate representation of the final image the creators wanted you to see. Likewise, for creating content, calibration is even more important – if you master on an uncalibrated monitor there’s little chance your content will have any consistent look across devices once distributed. Only through calibration can the artistic vision be consistent, accurate and thus fully enjoyed.

Currently Used Calibration Standards

Current calibration targets are split into two categories, those for standard dynamic range (SDR) and those for high dynamic range (HDR). While performing calibration to a specific target, you’re often referring to multiple and entirely different attributes of the image such as color gamut, white point, peak luminance, and electro optical transfer function (EOTF) or gamma response. An outline of common calibration targets can be seen below:

SDR Calibration Target

• Color Gamut: BT.709
• White Point: D65
• Peak Luminance: 100 nits (cd/m2)
• EOTF: BT.1886 (Gamma 2.4) or Gamma 2.2, 2.4, 2.6

 

HDR Calibration Target

• Color Gamut: P3 or BT.2020
• White Point: D65
• Peak Luminance: 1,000 nits (cd/m2)
• EOTF: ST.2084 (PQ) or Hybrid Log Gamma (HLG)

 

Calibration Basics

There are several different methods of calibrating your display. Each method aims to accomplish the same goal of measuring and then adjusting the color response of your device.

Display Measurement

In order to measure the current color performance of your display, you’ll need at least a colorimeter and pattern generator. A colorimeter is a tristimulus device – meaning that it uses three different color filters (red, green, blue) to mimic the way the human eye perceives color. During calibration, a colorimeter is placed against the flat surface of the display to measure designated test patches. The light from the display passes through to the colorimeter’s filters that then quantify the data into numerical values.

If your display does not come equipped with built in test patches, you’ll need an external pattern generator. Simply put, these devices generate high quality, accurate test patterns that are displayed on your monitor for measurement.

To learn more about colorimeters and other hardware relevant to color calibration, refer to our dedicated article here: Display Calibration: Colorimeters vs. Spectroradiometers.

Additionally, several software solutions exist to interface these tools together. Portrait Display’s Calman and Light Illusion’s ColourSpace are examples of applications that help streamline the calibration process – interfacing directly with a plethora of colorimeters, pattern generators and even display devices themselves. Likewise, several monitor manufacturers also have built-in calibration tools to help make calibration accessible to users.

To learn more about software solutions and calibration questions specific to your display device, refer to the Portrait Display’s knowledge base here and Light Illusion’s knowledge base here.

Display Adjustment

Once the measurements have been completed, the display needs to be adjusted to account for differences between the measured values and the calibration targets. There are two main methods to make calibration adjustments on your display – LUT (look up table) correction & manual CMS (color management system) control on screen display (OSD) menu adjustments.

Calibration LUTs are useful to store correction values and to counteract some display’s non-linear color rendering errors. Many newer high-end displays support direct 1D and 3D-LUT import. 1D LUTs are often used for broader overall white point or color balance changes (can only re-map an individual input to an individual output), whereas 3D LUTs are used for more complex adjustments (as they can re-map individual inputs to a multitude of outputs). LUTs are often much more accurate and faster than manual calibration. Additionally, users can set the amount of measurement passes, reducing interpolation between calibration points. Calibration LUTs are typically generated via software and then imported into the display. The display would then read this LUT and make the adjustments before an image is displayed.

Manual adjustment refers to altering a display’s built-in color management systems (CMS) controls via the on screen display (OSD) menu. Typically, this involves taking a live reading of a test pattern and making adjustments on the corresponding control within the display menu until the reading best matches the calibration target. Although this method is more involved and dependent on the specific display’s adjustment settings, it is more often straightforward and removes the complexity of LUT correction.

HDR Display Evaluation

Performing a display evaluation is critical to ensure a given product is adhering to industry standards and manufacturer's advertised specifications. To learn more about Dolby's recommendations for HDR display evaluations, refer to our new document: Dolby Vision HDR Display Evaluation Guideline.