Image displays Flashcards
What are hard and soft copy display systems?
- Hard: Film with image on.
- Soft: Image on computer screen.
What are the main stages of the imaging chain?
- Acquisition device.
- Display processing software.
- Display controller.
- Softcopy/hardcopy display.
What does the display processing software do?
Transforms ‘image values’ to digital driving levels.
What does the display controller do?
Converts the digital driving levels produced by the display processing software to signals appropriate for the display device. These typically conform to a specific standard.
What are the two categories of soft copy devices?
- Secondary (e.g. review monitors).
- Primary (e.g. diagnostic workstations).
What kinds of soft copy display are available? Which are most common?
- Cathode ray tube (essentially obsolete).
- Liquid crystal display (still common).
- Emissive flat display e.g. active matrix organic light emitting displays (becoming more common).
How does a cathode ray tube work?
- Vacuum tube, phosphor and electron beam.
- Electrons emitted by cathode.
- Accelerated towards phosphor.
- Deflection coils used to scan electron beam across phosphor.
- Light produced in phosphor where beam strikes.
How does a liquid crystal display work?
- Polarised light from backlight enters liquid crystal cell.
- Orthogonal grooves in the two alignment layers sandwiching the liquid crystal cell cause preferential alignment of liquid crystal cell molecules to the grooves they are in close proximity to. This creates a ‘twist’ in the molecular structure.
- The degree of twist is altered across the liquid crystal cell by altering the electric field across the cell with the TFT.
- The twist causes the plane of polarisation of the light entering the cell to be rotated.
- The amount of light transmitted by the liquid crystal cell depends on how much the plane of polarisation is twisted with respect to the front polariser.
What affects the actual pixel size of of a liquid crystal display when compared to nominal?
Electronic elements (e.g. TFT switch) mean the actual pixel size is smaller than nominal.
What do the maximum and minimum luminance of a liquid crystal display depend on?
- Minimum: How opaque the liquid crystal cell is in its full off state (i.e. how good is it from preventing light from backlight passing to front).
- Maximum: Brightness of backlight, nature of polarisers (i.e. how much they transmit) and the transmission of the liquid crystal cell in its full on state).
What determines the uniformity of a liquid crystal display?
- Uniformity of the backlight.
- Uniformity of the liquid crystal elements.
What are some disadvantages of liquid crystal displays?
- Lower refresh rate than cathode ray tubes.
- Emission of light can change depending on viewing angle which can result in contrast loss and greyscale inversion.
What are the main components of an active matrix organic light emitting display? What does each component do?
- Backplate (substrate).
- TFT array (control the current to each pixel).
- Organic emissive layers (semiconductor material which produces light due to the recombination of electron-hole pairs).
- Cathode.
What is one of the main differences between liquid crystal displays and active matrix organic light emitting displays?
LCDs have a backlight whereas AMOLEDs are emissive devices which produce light themselves.
What are the advantages/disadvantages of active matrix organic light emitting displays?
Advantages:
- High resolution, brightness and contrast (blacker blacks than LCD).
- Wide viewing angle when compared to LCD.
- Lightweight, thin and low power.
Disadvantages:
- Limited lifespan of organic material (especially blue) which can cause a shift in colour balance.
- Susceptibility to water damage.
What are the typical properties of a common ‘consumer’ display?
- Low-mid resolution (~ 2 million pixels).
- Low maximum luminance (100-250 cd m^-2).
- Low bit depth (2^8 = 256 grey levels).
- No built in image greyscale standard display function (GSDF) calibration.
- Not too stable over time.
- Cheap and commonplace.
What options are available when displaying a large medical image on a small ‘consumer’ display?
- Display the full image. However, this would result in pixel binning and loss of information.
- Display the full resolution. However, this would mean only a certain portion of the image could be viewed at a time.
- Use an ‘in between’ of the above two methods.
What are the typical properties of reporting workstations?
- High resolution (3-5 million pixels and up to 10).
- High maximum luminance (600 + cd m^-2).
- Large bit depth (up to 16 000 grey levels).
- Built in image greyscale standard display function (GSDF) calibration.
- Stable luminance.
- Very expensive.
What is luminance?
Rate at which visible light is emitted from a surface in units of cd m^-2. It is independent of the distance from the source.
What is illuminance?
Rate at which visible light falls upon a surface in units of lux. Used as a measure of ambient light.
What is optical density?
Log of the ratio of the amount of light incident on a film to that transmitted.
What is the luminance ratio? What is L_amb?
- Luminance ratio = Max luminance/Min luminance.
- L_amb: Luminance added to display by ambient light.
What are some examples of hardcopy devices? When may these be used?
- Dry laser printer (backup if PACS is down).
- Thermal imager (on some ultrasound units).
- Conventional x-ray film (essentially obsolete except for dental).
What is the difference between a wet laser and a dry laser?
In both cases, a film is scanned to produce a latent image. For a wet laser, the film is then chemically processed to produce an image. For a dry laser, the film is heated to produce an image.
