crt and lcd monitors - gestalt revisionlcd monitors manipulate a separate electrical field for each...

CRT and LCD monitorsproperties and problems

Maarten DemeyerOctober 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




Cathode Ray Tube (CRT)

Electron gun fires electronsat phosphor

Phosphor emits light when hit by electrons

Beam is controlledthrough electro-

magnetic deflection

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


Elze, T. (2010). PLoS One 5(9): e12792.


The electron beam scans the screen systematically, sequentially lighting up each location

Elze, T. (2010). PLoS One 5(9): e12792.


• 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


Thus, a snapshot of a CRT in action looks like this:


The human visual system integrates this flickering display into a stable image

Overview

• Cathode Ray Tube (CRT)– Electron gun and phosphor

– Color





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

An aperture grill is a series of thin wires separating columns of different colors

Damping wires prevent them from resonating

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


– Electron gun and phosphor

– Color


– Resolution

– Luminance values

– Phosphor persistence

– Exposure duration

– Geometry

– Other




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 after-image in practice:


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 decay can be measured with a (linear) photosensitive cell (never trust official specifications!)

However, peak luminance of a CRT is not what is relevant to human visual perception

It takes 30 ms to cross the 1% threshold!



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


How to minimize the problem?

- Use dimmed room lightingScotopic vision is more sensitive to weak light

- Use brighter backgrounds Phosphor persistence is additive, but: Weber’s Law

- Use a filter for low luminancesFor instance, a car window foil

- Use a faster phosphorThis will necessitate a monochrome display

(NOTE: colors do not decay equally!)

- Use a LED displayOnly feasible for simple stimuli

Overview


– Electron gun and phosphor

– Color


– Resolution

– Luminance values

– Phosphor persistence


– Geometry

– Other




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 ageProlonged 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


• Liquid Crystal Display (LCD)– Polarizers and liquid crystals

– Color, TFT, and backlighting



• The future of display technology

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!


Liquid crystals do not produce light

They only determine whether lightcan pass through the polarizers


To produce light, an LCD must be backlit by another light source


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


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’


Up until recently, most LCD backlights were fluorescent tubes (CCFL)


More recently, LED backlights have taken over

- Brighter, thinner, more efficient

- Local dimming for uniformity control

Edge/side litFull lit


For a greater range of colors, the backlight array can consist of RGB LED triads

Liquid Crystal Display

Overview





– Resolution

– Response time


– Uniformity and contrast



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

Elze, T. (2010). PLoS One 5(9): e12792.

Complex transitions!

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





– Resolution

– Response time


– Uniformity and contrast



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





Comparing CRT and LCD

LCD

Cheap

Easy to find

Easy to handle

Fixed geometry

Fixed color positions

Simultaneous

CRT

Becoming expensive

Becoming rare

Heavy and bulky

Projected geometry

Color misconvergence

Sequential


LCD

Slow, complex transitions

Input lag

Native resolution

Overshoot artefacts

No black, bad contrast

Complex corrections

Dead pixels

CRT

Faster, simple transitions

No input lag

Scalable resolution

No overshoots

Black, good contrast

Simple analog device

No dead pixels


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





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


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


AMOLED

Thank you

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