CRT and LCD monitors
properties and problems
Maarten Demeyer
October 27 2010
Overview
•
Cathode Ray Tube (CRT)
•
Liquid Crystal Display (LCD)
•
Comparing CRT and LCD
•
The future of computer displays
Overview
•
Cathode Ray Tube (CRT)
–
Electron gun and phosphor
–
Color
–
Properties and problems
•
Liquid Crystal Display (LCD)
•
Comparing CRT and LCD
•
The future of computer displays
Cathode Ray Tube (CRT)
Electron gun
fires electrons
at phosphor
Beam is controlled
through electromagnetic deflection
Phosphor emits
light when hit
by electrons
Electron gun and phosphor
•
•
Phosphor luminance decays over time after the electron
beam is gone
The shape and the parameters of the decay function depend on
the type of phosphor used.
E.g.: Color CRTs use ‘P22’ phosphor
•
A rapid initial luminance decay is followed by a long tail of
weak light emission
Electron gun and phosphor
Elze, T. (2010). PLoS One 5(9): e12792.
Electron gun and phosphor
The electron beam scans the screen systematically,
sequentially lighting up each location
Elze, T. (2010). PLoS One 5(9): e12792.
Electron gun and phosphor
•
•
The time between vertical blanks is the refresh rate
at which frames are drawn Typically: 60-200Hz refresh
The horizontal scan rate is the drawing speed of a
single line
E.g.: 85Hz refresh, 1024x768 resolution:
85 x 768 = 65 kHz (+vertical blank)
•
A slower refresh rate generally results in a brighter
display
Electron gun and phosphor
Thus, a snapshot of a CRT in action looks like this:
Electron gun and phosphor
The human visual system integrates this
flickering display into a stable image
Overview
•
Cathode Ray Tube (CRT)
–
Electron gun and phosphor
–
Color
–
Properties and problems
•
Liquid Crystal Display (LCD)
•
Comparing CRT and LCD
•
The future of computer displays
Color
• Color is created by using not one, but three
types of phosphor (red, green, and blue)
I.e.: RGB colors
• A separate electron gun fires upon each type
of phosphor, with different intensity
• To keep the three beams apart an extra
physical layer is present inside the display
●
Either shadow mask or aperture grill
Color
A shadow mask is a punched metal plate, separating
the projection of the three electron beams onto three
dots containing a different phosphor (a ‘triad’)
Color
The dot pitch is the distance between phosphor dots of the
same color
E.g.: 0.24 mm (40x30cm = 1600x1200 triads)
Color
Color
An aperture grill is a series of thin wires separating columns of
different colors
Damping wires prevent them from resonating
Color
Color
Shadow mask
Dimmer and less vivid, but sharper
Expands and contracts with temperature
Vertical and horizontal resolution limit
Aperture grill
Brighter and more vivid, but fuzzier
Damping wires are visible
Only horizontal resolution limit
Overview
•
Cathode Ray Tube (CRT)
–
Electron gun and phosphor
–
Color
–
Properties and problems
–
Resolution
–
Luminance values
–
Phosphor persistence
–
Exposure duration
–
Geometry
–
Other
•
Liquid Crystal Display (LCD)
•
Comparing CRT and LCD
•
The future of computer displays
Resolution
Temporal properties
Refresh rate setting, limited by
Scan speed of the electron guns
Spatial resolution
Phosphor persistence (see below)
Resolution
Spatial properties
Electron beam width and focus
Dot pitch (in color displays)
Still, a CRT has no ‘native resolution’
The beam can change state mid-dot
Therefore, a CRT scales well to different resolutions
I.e.: 640x480 to 1600x1200
This allows more flexible refresh rates
Resolution
Resolution
A low-resolution artefact: aliasing
= undersampling the stimulus
= lower device resolution than stimulus resolution
Spatial
Can be smoothed on PC
Resolution
Moiré aliasing: undersampling fine patterns
Resolution
Undersampling motion
Luminance values
A linear luminance scale on the PC (0-255) results in a linear
voltage signal to the CRT
…but NOT in a linear luminance increase on the screen
Therefore the monitor needs calibration
Luminance values
Measure different luminance levels, each color
separate
Determine the gamma correction function
Convert to correct linear values using a Look-Up Table
(LUT) or function
Caution: CRTs need to warm up in order to stabilize
their luminance output
Luminance values
Luminance resolution or bit depth
Bit depth is not inherent to the (analog) CRT, but to the digital device driving it
A 0-255 (= 28 = 8 bit) range of luminances can display a smooth luminance gradient
For RGB values: 3x8 bit = 24 bit
But, at low contrast such a gradient may be undersampled and lose the smoothness
Solution: Use a graphical processing device with a higher bit depth (e.g.,
Visage)
Phosphor persistence
Due to the gradual phosphor decay, a square wave
stimulus will always be followed by a fading afterimage in practice:
Phosphor persistence
Some phosphor persistence is unavoidable
Whether this poses a problem, depends on
- The duration of the decay
I.e., the phosphor type
- The stimuli used
I.e., the exact luminance sequence
- The research question
I.e., the importance of having an exact ED
Phosphor persistence
Phosphor decay can be measured with a (linear)
photosensitive cell (never trust official specifications!)
Phosphor persistence
However, peak luminance of a CRT is not what is relevant to human visual
perception
It takes 30 ms to cross the 1% threshold!
Phosphor persistence
Any hard luminance threshold is arbitrary
A shutter test is better:
- Block the actual stimulus with a physical shutter
- Open it exactly at stimulus offset
- Measure whether the subject can use the afterimage
But, the brain might still work in complex ways:
Perhaps the preceding stimulus makes the afterimage less visible?
Perhaps it increases sensitivity to the afterimage?
…
Still, a shutter test convinces most reviewers
Phosphor persistence
How to minimize the problem?
- Use dimmed room lighting
Scotopic vision is more sensitive to weak light
- Use brighter backgrounds
Phosphor persistence is additive, but: Weber’s Law
- Use a filter for low luminances
For instance, a car window foil
- Use a faster phosphor
This will necessitate a monochrome display
(NOTE: colors do not decay equally!)
- Use a LED display
Only feasible for simple stimuli
Overview
•
Cathode Ray Tube (CRT)
–
Electron gun and phosphor
–
Color
–
Properties and problems
–
Resolution
–
Luminance values
–
Phosphor persistence
–
Exposure duration
–
Geometry
–
Other
•
Liquid Crystal Display (LCD)
•
Comparing CRT and LCD
•
The future of computer displays
Exposure duration
Refresh rate limits exposure duration
Reported EDs should be a multiple of the frame time
E.g.: 60ms ED @60Hz is not possible (16.67ms frames)
But actually, there is no single correct exposure duration due to the nature of CRTs
E.g.: 1 frame @60Hz versus 1 frame @200Hz?
Keep in mind that the exact drawing time of a stimulus is determined by its vertical position
E.g.: top versus bottom @60Hz = 17ms difference!
Geometry
The electron beam projects the image to the front of
the vacuum tube. Since pixels are not fixed, the
geometry of the image may need manual adjustments
E.g.: Compare to an LCD projector
The front of the tube may not be flat
Especially in shadow mask color displays
Beam dispersion occurs towards the edges
Often this results in a slightly higher dot pitch
Other
Degaussing is the removing a magnetic field from the shadow
mask / aperture grill
Color convergence of the electron beams requires manual
adjustment
Raster lines of an aperture grill may affect the factual thickness of
thin vertical lines
E.g.: Vernier stimuli
Also: Horizontal versus vertical gratings
Phosphor dims gradually with age
Prolonged exposure of a pattern can cause burn-in
Other
Monochrome monitors can achieve better quality
visual stimulation because:
1) The choice of phosphor is free
But: seldomly truly white
2) Shadow mask / aperture grille is absent
Spatial resolution only limited by beam width
No unnecessary artefacts
Overview
•
Cathode Ray Tube (CRT)
•
Liquid Crystal Display (LCD)
–
Polarizers and liquid crystals
–
Color, TFT, and backlighting
–
Properties and problems
•
Comparing CRT and LCD
•
The future of display technology
Polarizers and liquid crystals
Polarizers block light
Polarizers and liquid crystals
Liquid Crystals transmit light and twist
according to the local electrical field
LCs ‘rotate’ the light
beam when a voltage
is applied!
Polarizers and liquid crystals
Liquid crystals do not produce light
They only determine whether light
can pass through the polarizers
Polarizers and liquid crystals
To produce light, an LCD must be backlit by
another light source
Polarizers and liquid crystals
LCD monitors manipulate a separate electrical field for each pixel,
affecting only the local LCs
Different luminance levels are often achieved by switching the voltage to the liquid crystals
on/off very rapidly (pulse modulation)
Color, TFT, and backlighting
To be more exact, a pixel consists of three subpixels, each
with their own electrical field twisting the crystals.
A red-green-blue mosaic filter enables the mixing of these
three subpixels into millions of different pixel colors
Color, TFT, and backlighting
Why are LCD computer monitors a recent
phenomenon?
In practice it proved to be hard to control so many
pixels fast and accurate (without blocking the
backlight)
E.g.: 1600x1200x3 = 5.76 million
Thin Film Transistor (TFT) solved this: a transparent
layer with a separate controller directly at each
subpixel
This is a delicate technology:
broken transistor = ‘dead pixel’
Color, TFT, and backlighting
Up until recently, most LCD backlights were
fluorescent tubes (CCFL)
Color, TFT, and backlighting
More recently, LED backlights have taken over
- Brighter, thinner, more efficient
- Local dimming for uniformity control
Full lit
Edge/side lit
Color, TFT, and backlighting
For a greater range of colors, the backlight
array can consist of RGB LED triads
Liquid Crystal Display
Overview
•
Cathode Ray Tube (CRT)
•
Liquid Crystal Display (LCD)
–
Polarizers and liquid crystals
–
Color, TFT, and backlighting
–
Properties and problems
–
Resolution
–
Response time
–
Exposure duration
–
Uniformity and contrast
•
Comparing CRT and LCD
•
The future of computer displays
Resolution
Spatial properties
A LCD has physically fixed pixels
Pro:
The geometry of the screen is fixed as well
No manual adjustments required
Contra:
There is a native resolution for presenting stimuli
Changing resolution requires interpolation, drastically affecting the quality of the displayed images
Example: 17” = 1280x1024
Less control over physical stimulus size
Less control over frame size (drawing speed)
Resolution
Temporal properties
A LCD has no scanning beam: all pixels change state simultaneously rather
than sequentially
It does have a refresh rate for presenting frames
Typically: 60Hz
Unlike a CRT, every pixel remains active for the entire duration of a frame
The backlight however does flicker @200Hz
Unlike a CRT, a LCD monitor suffers from input lag
Typically: 10-50 ms
Response time
Liquid crystals take time to reorient themselves for every transition
in luminance
Response time = time to cover 10% to 90% of the luminance change
from black to white
Typical in recent TFT monitors: 2-8 ms
Response time
Problem #1:
0-10% and 90-100% are also relevant
Problem #2:
Response time is much higher for gray levels
Complex transitions!
Elze, T. (2010). PLoS One 5(9): e12792.
Response time
Problem #3:
‘Overdrive’ technique boosts response times
- Greater input lag
- Possibility of overshoots
Exposure duration
The exposure duration must still be a multiple of the frame duration
However onset and offset dynamics are slow, making the real exposure duration
arbitrary
The exact ED of a figure (or part of a figure!) depends on the exact luminance
levels
On the positive side, refreshes do happen in parallel
Input lag is a concern for synchronizing with events
- Measuring the input lag allows for post-hoc synchronization
E.g.: button presses, EEG,…
- But real-time synchronization is harder to achieve
Overview
•
Cathode Ray Tube (CRT)
•
Liquid Crystal Display (LCD)
–
Polarizers and liquid crystals
–
Color, TFT, and backlighting
–
Properties and problems
–
Resolution
–
Response time
–
Exposure duration
–
Uniformity and contrast
•
Comparing CRT and LCD
•
The future of computer displays
Uniformity and contrast
Inside a LCD monitor, the backlight is always on.
Therefore ‘true black’ is hard to achieve, and contrast and
colors are less pronounced than on a CRT.
Moreover, the backlighting may not be uniform across
the screen
Local backlight dimming has been applied to improve
uniformity and contrast in recent years
However this removes control over absolute luminance!
Looking angle can be of influence
Overview
•
Cathode Ray Tube (CRT)
•
Liquid Crystal Display (LCD)
•
Comparing CRT and LCD
•
The future of computer displays
Comparing CRT and LCD
LCD
CRT
Cheap
Becoming expensive
Easy to find
Becoming rare
Easy to handle
Heavy and bulky
Fixed geometry
Projected geometry
Fixed color positions
Color misconvergence
Simultaneous
Sequential
Comparing CRT and LCD
LCD
CRT
Slow, complex transitions
Input lag
Faster, simple transitions
Native resolution
No input lag
Overshoot artefacts
No black, bad contrast
Scalable resolution
Complex corrections
No overshoots
Dead pixels
Black, good contrast
Simple analog device
No dead pixels
Comparing CRT and LCD
So, when should I use a CRT?
Real-time synchronization with events
Short EDs, fast transitions
Precise luminance control
Low-light presentations
Within the class of CRT monitors, (specialized) monochrome monitors allow for even
greater accuracy
LCD projectors
LCD projectors work like LCD monitors, but
with a strong concentrated backlight and a
separate LCD for red, green and blue
Overview
•
Cathode Ray Tube (CRT)
•
Liquid Crystal Display (LCD)
•
Comparing CRT and LCD
•
The future of computer displays
The future of computer displays
Plasma screens
- Each pixel consists of 3 small fluorescent lamps
- Thus, phosphor-based
- No backlight, better contrast
- Low response time
- Only large sizes
Digital Light Processing (DLP)
-
Backlight
-
Array of micro-mirrors switches pixels on/off
-
Mostly popular for movie projectors
The future of computer displays
Surface-conduction Electron-emitter Display (SED)
-
Thin CRT with electron gun for each sub-pixel
-
No backlight, great contrast
-
Fast response times
-
Development seems to have stopped
Active Matrix Organic Light-Emitting Diode (AMOLED)
-
TFT controls a layer of light-emitting material
-
No backlight, great contrast
-
Fast response times
-
Thin and flexible
-
Projected to overtake LCD in 5-10 years
The future of computer displays
AMOLED
Thank you