Calibrating the most demanding professional displays www.qubyx.com
Why calibrate medical displays? Hospitals and clinics have quality programs for medicalequipment. However, often the displays used for diagnostics andreviewing are not part of those quality assurance programs. No monitor, including medical ones, displays images perfectly.Poor image quality results in fatigue of the doctors, more time
DICOM calibration of displays in medical environment•• What is light and how does it influence image?• What is DICOM GSDF Part14?• Why calibrate displays to DICOM?• How does calibration work?• What displays can be calibrated?• What are the required tools?
What is light?Visible light is the electromagnetic radiation that can be seen bythe human eye.Light is visible in the wavelengths from 380 to740 nm. When light is broken by a prism, different colors of thewhite light become visible.
How do we see light? Rod and con cells in the retina of the human eye are sensible tolight and can distinguish up to 10 million different color andluminance shades. These cells form the images. Non image building cells are the photosensitive ganglion cells,
What is DICOM Part14? The DICOM Standard is developed and published bythe US organisation NEMA (National ElectricalManufacturers Association). The Part 14 of this standard discusses how pixelvalues should be interpreted and displayed. Part 14 also specifies a function that relates pixelvalues to displayed luminance values. This function iscalled GSDF (Gray Scale Display Function).
GSDF Originally developed by the researcher Peter G.J. Barten and published as"Contrast Sensitivity of the Human Eye and Its Effects on Image Quality".
Why calibrate to DICOM GSDF?• Adapt display to human eye. Human eye is not an digital device like a computer is. The computer display, as the interface between the human and the digital machine, must be adapted to the characteristics of the human eye.• Re-calibrate over time. Computer displays tend to change over time, like any other light source. Backlight lamps and RGB filters change. That is why even a factory calibrated display needs to be re-calibrated over time to continue matching the sensibility of the human eye.• Make sure displays look the same. Consistency of images on various displays is important for diagnostics. Calibration iensures that an image looks the same in all services and on all workstation.• Standardise display outputs. Calibrated displays meet the key quality assurance standards in medical industry.• Safety and quality. Increase diagnostic safety and improve quality assurance.
Why calibratecolor of a display? • human eye perceives various color shades in a different way • a diagnostic image on a color display is influenced by color hues: reddish gray shade will look different from blueish gray, so adjusting luminance is not enough • color calibration ensures the same color temperature on all driving levels (different gray levels) of the display
Why calibrate a display regularly?•• display white level degrades• display back light changes over time• display color filters change over time• regular verification and adjustment of the white level of the display is necessary
How does calibration work? 1. Measurements taken.Reference colors and gray levels aredisplayed on the monitor and measured with a colorimeter or luminancemeter. 2. LUT generated. From the actual measured values and the knownBartens formula target values a correction table called Look Up Table(LUT) is generated.
What displays can be calibrated to DICOM GSDF?Almost any LCD, LED, or OLED display can be calibrated to DICOM GSDF tocompete with traditional medical displays.
Standardised Quality AssuranceIn some countries the legal bodies enforce acceptance testing of diagnostic and reviewing displays when they are installed, and constancy testing for quality assurance on a regular basis.Major QA standards are AAPM TG18, DIN 6868-57, DIN 6868-157, JESRA X-0093 , IEC 62563-1The tests performed within those standards consist of visual tests and measurement tests, which verify conformance to the requirements of the standards.
Controlled EnvironmentApart from calibration and constant verification of the display it is important to control the working environment: • ambient light must be stable and remain within the acceptable range • no direct reflection on the display panel should occur