Monitor Calibration and Profiling

Understanding monitor calibration and profiling

The monitor is your window to the world (or at least to your photos). If you've ever been in a television store and looked at a row of sets playing the same program, you may have noticed that the color appearance can vary widely (or wildly) from TV to TV.

If you don't calibrate and profile your monitor, then your picture's appearance can vary widely from the way it would look on other monitors, and can mislead you about the actual colors in your image. To address this problem, monitors can be calibrated and profiled. This process makes the device show as accurate an image as possible.

Calibration

Calibration is the process of setting the monitor to the desired neutral output. It includes settings for luminance, white point and gamma. Once it's been neutralized as well as the monitor's controls allow, it's time to measure the color and help to perfect it with software.

Profiling

Profiling is the process of measuring the imperfections in the monitor, and creating a "filter" that compensates for those imperfections. Using the parameters set in the calibration step, profiling requires using a hardware device, often referred to as a "puck" (a Colorimeter or Spectrophotometer), that hangs over the monitor screen and reads several sets of red, green, blue and grey patches generated by the profiling software. The color patches are measured by the puck as they are displayed. The differences between the colors the monitor displays in its native state and the true colors of the patches are used to create a monitor profile which will cause the monitor to display the true colors more closely than it did in its native state.

Monitor types

Like high-end stereo speakers, the monitor is a place where you really do get what you pay for. The best systems are precision engineered for added fidelity and evenness across the screen.

Fast-changing marketplace

The old-school CRT monitors are pretty much out of service. They have been replaced by LCD flat panel monitors. The newest technology is a new form of flat panel that uses LED light behind the screen.

Important characteristics to consider when selecting a monitor to use for image editing:

Illumination type – backlit fluorescent or LED

In the 1990s, high-end CRT (cathode ray tube) monitors were considered to be state of the art. That technology has gradually given way to LCD panels. Up until now, these panels have used CCFL (cold cathode fluorescent lamp) back lights. CCFL technology is in the process of being replaced by LED (light emitting diode) backlights. LED backlights have many advantages over CCFL including requiring half the power consumption and, more importantly, they are free of mercury, a toxic material that is emitted by electronic equipment. LED backlights have another advantage: the white light they create is derived from pure red, blue and green LEDs, so the light is purer, brighter and can offer a wider color gamut than CCFL. Some LED panels that are currently available have up to 120% of the Adobe RGB (1998) color gamut, which is 25% more than the very best CCFL panels. Most likely your next LCD will be an LED backlit display.

DDC (display data channel) enabled

DDC allows adjustment of the display's brightness, contrast, white point and gamma through the profiling device and software. This will save time and improve precision when calibrating and profiling monitors. When shopping for a new monitor, it's best to choose a DDC-compliant display if possible.

Bit depth

Computer monitors started out as 1-bit depth displays – good only for looking at type. Monitor development quickly progressed and now all consumer displays are 24-bit and can display over 16.7 million distinct colors. Twenty four-bit color is based on eight bits per RGB channel, which is why you may see monitors described as having 8-bit depth color. High quality graphics monitors such as the EIZO CG series have 10-, 12-, or 14-bit depth. This results in smoother transitions between tone values and the possibility of using the extra bits to extend the dynamic range of displayed images, allowing more detail to be seen in extremely bright areas and better shadow detail all at the same time. This extra headroom allows for easier editing in wide gamut color spaces. Some of this advantage is still theoretical until Photoshop supports greater than 8-bit depth all the way from the display to the pixels in an image.

Gamut

Many high-end monitors advertise the fact that they can display all or nearly all of the Adobe RGB (1998) color space. At first glance, this seems like something that you would want, and in many ways it is, but there are a few trade-offs. One issue is that the visible difference between almost identical colors – meaning colors that vary by just one number in an RGB triplet, such as our example in the color management overview section of 255, 133, 1 and 255, 134, 1 –  is much greater. This can make color editing more difficult. Another unwanted side-effect of wider gamut displays is that untagged colors and images on the web (which is most of them) will be greatly exaggerated. This neon effect is the result of narrow gamut sRGB images displaying in the monitor gamut space, which is what happens with non-color-managed browsers and/or untagged images. This can make web viewing an unpleasant experience on these wide gamut displays.

Evenness of illumination across the screen

We can say unequivocally that this is an essential feature for a monitor to have. When you buy a new widescreen monitor you should check it for evenness. This can be most accurately done by creating a monitor profile, and then use the "validate current profile" function of the monitor profiling software to measure the four corners and side-to-side areas. If you have a spectrophotometer handy, this can be used as well. A quick-and-dirty approach is to bring up an image in Photoshop and move it around on the screen and see if it appears to get brighter or darker as you move it around.

Pixel response time

This refers to how quickly a pixel can change colors, measured in milliseconds (ms); the fewer the milliseconds, the faster the pixels can change, reducing the ghosting or streaking effect you might see in a moving or changing image. This feature is important for watching videos or gaming but not very important if you use your monitor for editing images in Photoshop. What is important for image editing is quick screen redraw, which is governed by the amount of RAM available on the graphics card and the image cache level settings.

Built-in calibration tools

The best monitors come with calibration tools as part of a package. These include calibration and profiling software, and some include the hardware device or puck. These tools include DDC communication, which allows the calibration tools to adjust the monitor hardware settings directly as opposed to just adjusting the graphics card. This preserves the monitor's dynamic range better than adjusting only the graphics card.

LCD panel architecture

LCD (liquid crystal display) monitors come in several architectures. One term you may see is TFT (thin film transistor) LCD. Thin film technology improves image quality and is used in all high quality computer monitors.

In addition to TFT, other terms you may see are TN, VA, and IPS. TN (twisted nematic) technology is the most basic LCD architecture. However, TN panels are not the best for photo editing due to a limited viewing angle (meaning that the color and contrast change fairly dramatically if your viewing angle is not dead center), and low bit depth (6-bit depth is typical).

VA (vertical alignment) architecture is an improvement over TN with improved viewing angles and support for 8-bit depth. There are several variations such as MVA (multi-domain vertical alignment), PVA (patterned vertical alignment) and ASV (advanced super view).

IPS panels are the best for photo editing

VA architecture and its variants are an improvement over TN, but arguably the best architecture for high-quality displays (the type required for accurate photo image editing), is the IPS (in-plane switching) architecture. There are now several variants of in-plane switching as the technology continues to advance.

These are the key advantages of IPS architecture:

• True wide viewing angle: Accurate and consistent colors at any viewing angle
• Real color: IPS reproduces images that are closest to the actual color ( original image = digital camera = IPS monitor)
• Eye comfort: Less eye strain in comparison to VA and TN
• Stable quality: Image onscreen is designed to remain stable when you touch the screen

Many professional IPS-based LCD monitors also feature high bit-depth and ultra wide color gamuts, which we define as equal to or greater than the Adobe RGB (1998) color space.

One interesting variation of IPS is the AH-IPS architecture developed by LG Display. This type of IPS panel uses the AFFS (advanced fringe field switching). This architecture gives the best performance for smaller screens with high resolution, such as those used in handheld devices like the Apple iPad.

AH-IPS has the advantage of greater light transmission, which translates into lower power consumption. So far, this technology has been limited to small screens due to higher manufacturing costs for larger screens.

What luminance, white point and gamma do I use?

Luminance

Luminance or brightness of the monitor is the most important setting (outside of creating a good color profile) for screen-to-print matching. Monitor brightness is measured in candelas per square meter (cd/m2), also sometimes referred to as "nits". The acceptable range is 80 cd/m2 to 120 cd/m2, with 100 cd/m2 being the most commonly recommended brightness for pre-press work. The brightness of the monitor is driven to a large degree by the brightness of the working environment. The brighter the working environment, the brighter the monitor will need to be.

The white point

The white point is the calibration setting on a monitor that determines the color temperature of the brightest white. Color temperature is expressed in Kelvin, eg 6500K. A more accurate unit of measuring color temperature is the so-called standard illuminant, expressed as D50, D65, etc. For most practical purposes you can use either unit of measure with your monitor calibration software. 5000K/D50 and 5500K/D55 are commonly used in CMYK reproduction, and 6500K/D65 is commonly used for general purpose and images on the web.

Native white point

The native white point is the default white point of a monitor. Most high-quality LCD monitors are very close to 6500K. Less expensive monitors and many Windows operating system monitors are quite a bit bluer, having a native white point between 7300K and 9300K.

Gamma

Gamma is less important than white point and luminance for photographers because color managed image editing applications such as Photoshop automatically adjust for gamma and display all images the same, regardless of monitor gamma. The Windows operating system default is a gamma close to 2.2, while Apple operating systems have used a native gamma of 1.8 up until the Snow Leopard version of OS 10. Most monitor hardware is designed to have a native gamma of 2.2 so many Mac users calibrate and profile their systems to 2.2 gamma. A few calibration/profiling software applications use a variable gamma curve called L* gamma. It is a slightly less linear version of 2.2 gamma which can result in slightly more shadow detail.

Quality controlling your color management system

You can run the calibration routine, but you don't actually know that everything is set up optimally until you check it against some reference standards.

There are three elements to this:

• • the reference file
  • the reference print
  • reference lighting

Reference print and reference file

In order to confirm that the monitor is showing you accurate color, you need to compare it to a print that is of known quality. The best way to do that is to purchase a reference print that is a certified proof of the file that produced the print. The color of these two versions of the image should match. Of course, in order to make sure you are seeing the colors in the print correctly, you need to view it under high-quality lighting. This exercise can help you fine-tune your choice of monitor white point.

A GATF RHEM indicator affixed to the proof will show when you are viewing your color with a light source with an accurate color temperature. The way that this works is that when the print is viewed in 5000K lighting, the indicator will appear to be all one color. In anything less than 5000K quality light the indicator will exhibit a striped appearance

	Figure 2 A GATF metamerism sticker in light that is not 5000K.
	Figure 3 A GATF metamerism sticker in light that is 5000K.

Reference lighting

In order to check the quality of your calibration, you need to examine a print under a standardized lighting setup which we refer to as reference lighting. Reference lighting is a high quality light source that is close to the D50 standard for graphic arts. This can be a lightbooth such as those made by GTI or others, or it can be a relatively inexpensive SoLux lamp. The SoLux lamp is very color accurate, but the lightbooth will produce less glare. Some lightbooths are equipped with brightness controls and some even have USB connections to the computer to provide complete control over monitor and reference lighting equalization.
Reference lighting can confirm the quality of your calibration sequence.
You can also use it to determine the best light source.

For those who really want to get things perfect, the reference setup can be used to determine which white point is most appropriate for your eye and your workspace. Profile your monitor to D65 and compare the screen to the reference print viewed under the reference lighting. If the print appears to be slightly warmer than the monitor, try D60, D55 or even D50 until you find the best match of monitor to print for your system and your working environment.
To really ensure that your workflow is color consistent, you can print out a copy of the reference image and check it against the reference print to see how well your printer is calibrated.

Read more in Desktop Printer Profiling

Figure 4 This image shows how the color of the light source changes your perception of the colors in the print. Use a known reference light source like SoLux lights to check the print color.

Ambient lighting

Ambient lighting, otherwise known as your working environment lighting, is a critical component of your color management setup. To a large degree it will influence your choice of monitor white point and monitor brightness, the two most important variables for monitor calibration. We recommend a reasonably dim unchanging ambient light level. Avoid working in conditions where strong sunlight streams in, as it will be too bright and will change continually throughout the day and from day to day.