Unit 7- image display

3 advantages to digital image display

1. Multiple people can view the image

2. Image display parameters

3. Can keep track where the photo is at

what is the weakest link?

monitors

aspect ratio

• the ratio of width to heigh of an object

  • used to describe the shape of the monitor

• resizes the image so it does not show the distortion on the screen

example of aspect ratio

4:3 on 16:9 display (cut off on the sides) aspect ratio will resize the image

spatial resolution

• The ability to distinguish two adjacent details as being separate from each other. The clarity of an image.

•Smaller the pixel pitch = sharper the spatial resolution

•Often describe resolution as “3 MP or 5MP monitor”

•When comparing resolution of monitors you need consider the physical display area

Compare apples to apples not oranges to apples

dot pitch/ pixel pitch

distance between the centers of any 2 adjacent hardware pixels

smaller the pixel pitch=

sharper the spatial resolution

when comparing resolution of monitors you need to consider

the physical display area (compare apples to apples not oranges to apples)

3MP vs 5MP

5MP= shorter pixel pitch, better spatial resolution

smaller matrix=___ pixel size

larger

larger matrix= __ pixel size__ resolution

smaller; better

monitor spatial resolution is deciphered by___

monitor size & pixel size

125× 125 vs 35×35

35×35 is worse resolution due to less pixels

brightness

the black level of a display screen

density relates to

blackness

photometer purpose

measures the brightness output of an LCD

ACR requires a minimum brightness capability for display systems to be ___ lumens

250

radiologists prefer monitors with how many lumens?

500-600

window leveling

controls brightness

increase= increased brightness

decreased= darker image

contrast resolution

Ability of the display system to distinguish between multiple densities in the radiographic image

what is the determining factor for contrast resolution in digital?

bit depth of pixels

higher vs lower contrast

high= can distinguish shades of colors (blacks & whites)

low= can not distinguish shades of colors

monochrome monitors

• Color images will appear as shades of gray

• Has only one phosphor of color, but may have the ability to change the brightness causing the illusion of depth

• Produces sharper text and image because 1 phosphor per pixel than color

color monitors

• Monochrome images can be viewed on color monitors

• uses alternating intensities of red, green and blue to produce the different color variations

• Uses 3 phosphors per pixel

if all 3 colors set at 0=

BLACK

if all 3 colors set at 255=

WHITE

if red was set at 225 & blue & green at 0=

RED

0 represents what within pixel intensities?

bottom range, little intensity of light passed through the monitor pixel (no light/ black)

255 represents what in pixel intensities

top range of light intensity to pass through the pixel (all light possible is passing through the pixels/ white)

pixel

point of location, numerical value, smallest element of the matrix

SQUARE SHAPE

LCD (liquid crystal diodes)

a single pixel consists of 18 individual bar-like segments

• 6 units of red, 6 of blue, 6 of green, that make up a pixel

•Consists of a layer of nematic liquid crystal between 2 pieces of polarized glass

MOST COMMONLY USED AT RH

hardware pixel monitor

intersections of flat, transparent wires crossing over each other to form an overall square shape

a single pixel consists of __ individual bar-like segments

18

2 domains=

subpixel

pixel size comparison

size of the dot within an I, or period

stuck pixel

permanent dark spot due to constantly receiving charge

LCD- liquid crystal diodes benefits

1. lightweight

2. portable

3. inexpensive

4. wide range of sizes

5. no glare

6. brightness is uniform & consistent over time

LCD- liquid crystal diodes disadvantages

1. restricted angle from which it can be viewed (ex: standing off to the side)

2. requires at least 15 min warm up time to reach full luminance

3. poking or applying pressure to the screen can cause dead pixels (very sensitive when touched)

like an x-ray, a ray of light is ___ radiation that consists of a __ wave

electromagnetic; double

LCD monitors use ___

polarized glass

polarized glass contains what

long chains of iodine molecules, aligned parallel to each other (just like grid strips in a grid)

electrical component of the double wave

can pass through a polarized glass with vertical string molecules because it vibrates parallel to it

magnetic component of the double wave

cannot pass through a polarized glass with vertical string molecules because it vibrates perpendicular to it

LCD monitor consists of what 2 things?

1. 2 thin sheets of glass

2. each with a light polarizing layer that are perpendicular to the other sheet

perpendicular sheets=

all light being blocked!!

nematic liquid crystals

• inserted between the polarizing glass sheets/lenses

• these crystals twist back & forth to allow light to pass through

what are nematic liquid crystals made of?

hydrated amorphous silicone

hardware pixel

• made of 2 wires & aligned perpendicular to each other

• has a scratch on it

• scratches are perpendicular to one another

• liquid crystals line up in a SPIRAL PATTERN

• light twists 90 degrees

the crystals line up with the “scratch” direction when no electrical charge is present, known as “____” and we see ___

on state- light passes through

“On” State

• like a light- when it is on, light can be seen

• NO ELECTRICAL CHARGE

  • twisting

• Since the scratches are perpendicular, the liquid crystals line up in a spiral pattern that twists 90 degrees between the two glass plates

• This lets light waves pass through the nematic liquid crystal and follow the spiral

• Light is able to pass through the second glass plate to the viewer of the monitor

a slight charge does what?

will untwist the crystal slightly, blocking a little bit of light from coming through

full charge does what?

untwisted, a dark spot, no light can pass through

how does shade of color relate to the charge of the pixel?

more charge=less twisting= more dark

less charge= more twisting= more light

OFF state

• charge is applied

  • NO TWISTING

  • NO LIGHT CAN PASS THROUGH

  • DARK

• as charge is applied to the crystals, they all align, preventing light to get through

• Light passing through is blocked by second polarizing glass

LCDs DO NOT produce __ by themselves, they need illumination to produce a visible image

light

backlight

without a backlight, an LCD display device will remain black, rendering is unviewable

• light diffusing filters that scatter light evenly over the monitor so it looks like one general light

backlighting types:

• fluorescent bulbs or LEDS

• LED is most common!!

how is a pixel activate when there are so many in a matrix?

• each hardware pixel (where cross hair wires meet) possess its own TFT

• allowing each column line to access a pixel simultaneously so that entire rows of pixels can be read out at ONE time

pros of pixel read out at one time (TFT)

• Allows electrical current to be turned off and on at a faster rate

• Speeds up response and refresh times (pixel can change its brightness)

• Higher quality images

cleansers that are acceptable on monitor screen

• vinegar diluted w/ water

• isopropyl alcohol

• petroleum benzene

  • DO NOT USE A PAPER TOWEL (can cause scratches)

quality control

system of maintaining standards of products based on QC recommendations by AAPM

minimum monitor matrix size for an acceptable digital radiograph is

1536×2048

grayscale standard display function (GSDF)

• Allow medical images to be transferred according to the DICOM standard with a consistent gray scale appearance.

• Mandated for all digital display devices.

Most images display ___different shades of gray

256

luminance

the rate of light (brightness) emitted from a source such as an LCD monitor

maximum luminance (max brightness)

checked at same monitor setting over a period of time with a photometer

luminance response

monitors ability to accurately display different shades of brightness from a test pattern

luminance ratio

compares maximum luminance to the minimum luminance (Contrast check)

luminance uniformity

consistency of a single brightness level displayed across the area of the screen

do LCD monitors have self correcting capabilities?

yes

luminance color

a measure to describe the perceived brightness of a color

Colors can be defined by __,___, & ___

hue, brightness and saturation

each hue of a color has its own___

luminance value

luminance ratio checks

contrast

digital imaging spatial resolution is primarily determined by:

• matrix size

• pixel size

high frequency= ___ wavelength & __ resolution

shorter; high

low frequency= __ wavelength & ___ resolution

long; low

how is evaluation of spatial resolution done? (2 things)

1. SMPTE (tests pattern & checks for blurring)

2. AAPM TG18

ACR & AAPM minimum resolution is what?

2.5LP/mm

TG18 tests for what?

uniformity (screen with all black & all white, look at the corners)

CX & SMPTE test for?

resolution

viewing angle/ on-axis viewing

• When you look at an LCD monitor from an angle, the image can look dimmer or even disappear.

  • 120-170 degrees are good ones

ambient lighting (peripheral glare)

• Light present in an environment from all light sources

what is the number one cause of degraded display performance in a system?

ambient lighting

specular reflection

reflection of a specific light source in the room  (specific light source & like a mirror- it reflects right back)

diffuse reflection

occurs on rough surfaces (like paper), scattering light in many directions, making objects visible without forming a glare or image.

veil glare

• Unexposed borders around the collimation edges allow excess light to enter the eye

• Shuttering or applying a black surround mask–effectively eliminates veil glare

While diagnosis is taking place, the maximum ambient lighting in a reading room should never be more than___

25 lux

(approx. ¼ of typical brightness of normal office lighting- 75-100 lux)

what is ambient lighting evaluated by?

TG 18 AD test pattern

Which characteristic of a monitor is defined the ability of the display system to distinguish between multiple densities in the radiographic image?

contrast resolution

On SMPTE pattern, a white on black bar and black on white bar are added specifically to check this with a monitor.

luminance ratio