CALIBRATING COLOR MEDICAL DISPLAYS: HOW TO DO IT RIGHT?

Color medical displays are in, and they are in for good. While with grayscale display calibration things are very clear due to DICOM Part 14 GSDF standard, requirements to color calibration of medical monitors are still somewhat uncertain.

Calibrating a display to read grayscale images normally means adjusting the luminance response curve. Luminance response, also known as gamma or transfer function, is a curve that describes the luminance output of an image producing device like a display or a printer.

DICOM Part 14 recommends calibrating a display to the Barten luminance response curve, also known as the DICOM curve. The DICOM curve is widely used and proved to work very well for grayscale images. It ensures that all gray shades are displayed with equal distances for the human eye.

Here’s what just noticeable differences mean on practice:

 

* Inconsistent image display. The lump visible on the left is almost invisible on the right.

* Same images. The image on the left is read on a high brightness, but uncalibrated display. Not all JND’s are visible. The image on the right is displayed by a low luminance, but calibrated monitor. More JND’s are visible.

Since DICOM Part 14 was originally developed for grayscale displays, it may not be a universal solution for color medical monitors as well. So, what luminance response do you calibrate a color medical display to if you use it to read grayscale images? Or color and pseudo color images? To answer these questions, let’s take a look at the available options for luminance response calibration.

What Are the Options for Luminance Response Calibration?

Barten/DICOM

The DICOM Grayscale Standard Display Function specifies exactly what luminance or density level should be produced for a certain input value, based on the Barten curve, which maps the values into a range that is perceptually linear. This means that input values are mapped into a space that is perceived as linear by a human observer.

With the luminance response adjusted to the DICOM curve, the display presents more just noticeable luminance differences to the viewer, and achieve similarity in grayscale reproduction for various displays.

DICOM Part 14 calibration makes sure all gray levels are produced in equal distance for the standard observer. Obviously, this type of luminance response calibration was introduced for grayscale images.

Gamma

Gamma (gamma correction, gamma encoding)  a nonlinear operation used to code and decode luminance. Gamma correction is defined by a power-law expression.

Gamma was originally developed to compensate for the characteristics of CRT displays.  In CRT displays, the light intensity varies nonlinearly with the electron-gun voltage. Altering the input signal by gamma compression can cancel this nonlinearity, such that the output picture has the intended luminance. So, Gamma correction is a good option for a CRT monitor, or to reproduce a CRT display’s image.

CIE L*

CIE luminosity function is established by the Commission Internationale de l'Éclairage (CIE) to describe the average sensitivity of the human eye to lightness. So in a certain way it is similar to the DICOM curve, but CIE L* does not only describe the equal distance in gray shades, but also the equidistant reproduction of colors and color shades. Calibrating color displays to CIE L* is currently recommended by the DIN 6868-57 standard (a German standard, discussing performance of medical displays).

Rec. 709

This luminance function is part of the Rec. 709 standard, developed for high-definition TV.
Thus this standard is highly adapted to pictures in movement. High definition endoscopy devices should follow REC 709.

EPD

EPD aim curve is part of a standard for geospatial imaging displays by National Geospatial Intelligence Agency (NGA). According to this standard, luminance response must match the EPD curve to ensure equal probability of detection for every shade of gray and every detail.

sRGB

sRGB is a standard developed cooperatively by HP and Microsoft  for the use on monitors, printers and the Internet.
sRGB uses a transfer function typical for CRT displays. This specification allowed sRGB to be directly displayed on typical CRT monitors of the time, a factor which greatly aided its acceptance.
The sRGB gamma can not be expressed as a single numerical value. The overall gamma is approximately 2.2, consisting of a linear (gamma 1.0) section near black, and a non-linear section elsewhere involving a 2.4 exponent and a gamma (slope of log output versus log input) changing from 1.0 through about 2.3.

This standard  color space is widely used as a default color space for various image reproduction devices, like displays, printers, or scanners. sRGB uses luminance response, typical of CRT displays and is close to Gamma 2.4.

CIECAM02

Here luminance, among other characteristics of a monitor, is adapted according to the CIECAM02 color appearance model, developed by the CIE.


NTSC

National Television System Committee is an analog television standard that defines certain luminance response among other characteristics of a TV.


PAL/SECAM

This is a standard color encoding system for analog television, defining luminance response among other characteristics.



Why Is Calibrating Only Luminance not Enough?

This image shows how two monitors, calibrated to the same luminance level, display the same shade of gray. The luminance is the same on both images, but for the human eye the grays look totally different. This difference can be disturbing and interfere with the way the viewer perceives and interprets an image.

* The difference you see is caused by the different color temperature of the grays, 
so even if you use the color display for reading grayscale images only, calibrating luminance to DICOM may prove insufficient. For the optimal performance of a color medical display, you need to calibrate both luminance response (Gamma) and color temperature of the monitor.

  
* All these levels of gray have the same luminance inside one row. 
But the human eye  would eventually see one gray shade as brighter or darker than another. That is why color calibration over the entire  luminance range is so important, and the highest accuracy is required with a delta Eab of at least < 2.

What Is the Right Choice?

As you see, there is no universal solution for any display and any purpose. If you use a color display for viewing grayscale images, the best option is to calibrate the display to DICOM GSDF and calibrate the color temperature, too.

For reading color and pseudo color images, we recommend calibrating the monitor to CIE L* and calibrating the color temperature. The recommended color temperature is D65: it corresponds to a Standard Norm light from the CIE (Commision Internationale D'Eclairage).

Finally, if you need to view moving images, it is best to calibrate your monitor to a video-related standard. We recommend calibrating color temperature, and calibrating luminance response to Rec.709.