CRT and LCD monitors properties and problems. Maarten Demeyer October 27 2010

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 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 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

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

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