Diagnostic Display Basics
In order to understand the methods used to assess digital image display quality, we will need to understand some basics on how a display works and how to evaluate a display’s performance.
Virtually all displays used in dentistry today are based on Liquid Crystal Dispay (LCD) technology. All of the following information relates to LCD based technology and does not apply to the Cathode Ray Tube (CRT) technology that it replaced.
Diagnostic display systems are composed of two basic components: a video graphics adapter (video card) and the display device (monitor). The computer acquires and stores the digital images as a binary file on a hard drive or computer server. When the digital image is to be displayed, it is output from the computer to the video graphic card which in turn outputs digital driving levels (DDL) to the display that will result in the variation in light intensity of color pixels on the display screen. Originally, commercial graphic adapters were somewhat limited in their capabilities, but with the advent of digital color cameras and computer gaming, they have more data processing power than all the computers used to send man to the moon. They are more than adequate for the diagnostic display of digital dental radiographic images. The most common display devices used today are LCD panels which are backlit with cold cathode fluorescent lamps (CCFL) or light emitting diodes (LED). In both types of displays, LCD elements in front of the backlight regulate the amount of light that is transmitted through the panel. Therefore, the backlight intensity determines the maximum display luminance or white level. The amount of light blocked by the panel determines the minimum luminance or black level.
Basic components of a color LCD display panel
The important thing to understand is that the backlight determines much of the display’s performance and that this light source is degrading from the very first time it is turned on. Just like any other light source, the intensity of light produced will decrease over time and eventually burn out. The display will eventually need to be replaced due to the fact that the backlight source will no longer provide enough light to display the digital radiographic image accurately. Another consideration is the warm-up time for the backlight source. While most displays with CCFL have been replaced by LED for backlight source, both require a period of warm-up to stabilize the intensity of the light emitted.
Before we go any further we need to know how to measure light intensity. Luminance is a measure of the photon energy that reaches the human eye. The unit of measure for luminous intensity is the candela (cd). One candela, which means candle in Latin, is roughly the luminous intensity of one common candle. The preferred system of measure for luminance is expressed as candelas per square meter (cd/m2). Luminance is measured with an electronic device called a photometer.
Why is luminance so important? Radiologic images contain up to 256 shades of gray that should be distinguishable to the average human eye. If the display is too dim, then the difference between one shade of gray and the next may be lost. The difference between one shade and the next has to be large enough to be perceived, and displays that are too dim do not have enough luminance to create 256 visually perceptible shades of gray. A display system can counteract this problem by adding significant capacity for brightness to the backlight, but running the backlight at perhaps 50% voltage on day one and then increasing the voltage over time to maintain the light output as the backlight decays. AAPM recommends a minimum for Lmax of >170 cd/m2 ideally 250 cd/m2 .
By measuring maximum and minimum luminance of the display over time, we can determine if the display is still adequate for displaying diagnostic images. This is part of a Display Quality Assurance (QA) program.
Luminance uniformity defines the emission of light across the surface of a display. Significant variation in the emission across the display can adversely affect the contrast of the displayed image. This non-uniformity can be caused by variation in the backlight source or in the liquid crystal layer itself. Non-uniformity in the backlight source is usually caused by degeneration due to aging. It is therefore necessary to evaluate the uniformity of light intensity across the entire surface of the display device. AAPM recommends the maximum luminance variance to be no greater than 30%. This is also part of a Display QA program.
The contrast of a display can be described at the range of luminance from a pixel with the lowest possible gray level to a pixel with the highest gray level possible. This can be expressed as the Contrast Ratio (CR) which is calculated by dividing the maximum luminance by the minimum luminance when measured under low ambient lighting conditions. For example: a maximum luminance of Lmax 350 cd/m2 and a minimum luminance Lmin of 1 cd/m2 would yield a CR of 350:1. Another factor that greatly affects contrast and the ability to discriminate small changes in brigthness is the ambient or room light in the viewing area. The Luminace Ratio LR is the same ratio as CR, only taking into account the ambient lighting conditions during normal operations. AAPM suggests that a LR of at least 250:1 is needed to accurately represent subtle changes in gray levels associated with normal anatomy and pathology. Measuring the luminance ratio over time is part of a Display QA program.
Ambient light or background room lighting comes from many directions and is therefore diffuse in nature. This light reflects off the surface of the display and adds to the minimum luminance perceived by the human eye. A display pixel with a gray level that represents a luminance value less that the ambient light will not be observed; it is effectively washed out by the ambient light. This is why the ambient light level should always be kept at a minimum. Illuminance is the term used to describe the amount of light reflected by the display surface. The lux is the SI unit of illuminance and is used as a measure of the intensity of light that hits or is reflected off a surface. Ambient lighting for proper image interpretation should be approximately 64 lux and ideally around 32 lux. Ambient light is measured by a photometer in lux.
As a point of reference, here are some examples of the illuminance under various conditions:
Measuring the ambient light level in the viewing area and ensuring that it is below the 70 lux threshold for primary radiographic interpretation is part of a Display QA program.
Display Response Characteristic
As described above, a digital dental radiographic image is composed of a matrix of pixels with 256 possible gray levels. The digital image is output to a video graphic card which in turns outputs digital driving levels (DDL) to the display. When a digital drive level (DDL) voltage is applied to an LCD display, it produces a corresponding output luminance. This output is a combination of light from the backlight going through a liquid crystal element and other optical elements to produce an image that can be viewed by the eye. Unfortunately, each individual LCD panel responds differently to each DDL that is input. This variance in response to different DDL is called the Display Response Characteristic. By plotting the digital driving level (DDL) to the resulting luminance, a display response characteristic curve can be generated.
Measuring the display response characteristic curve is a part of a Display QA program.
The important thing to remember is that a digital image that has good diagnostic value when viewed on one device could look very different and have greatly reduced diagnostic value when viewed on another device. This is why it is necessary to calibrate a display device to optimize the visual presentation for the human visual system and provide for consistancy between different display systems. The DICOM Grayscale Standard Display Function (GSDF) provides the standard to calibrate diagnostic display systems.