LightBoost settling

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Introduction

This page describes how the LCD settling behavior is specified and measured. Regarding the set of performance measures, we distinguish between two LCD operating modes: Strobed backlight (LightBoost and similar) and flicker-free backlight. This page is only about the settling performance in strobed backlight mode – for the settling performance in flicker-free mode see Flicker-free settling.

Side note: Both modes, flicker-free and strobed backlight, can be equally useful, but it all depends on the application. Strobed backlight is certainly useful when it comes to presenting motion stimuli and avoiding motion blur, no matter whether the stimuli are possibly tracked with the eyes. Strobed backlight also offers a basically instant and fully synchronous stimulus onset and offset, which comes, of course, with flicker that is usually not meant to be part of the stimulation. Synchronous stimulus onset means that the stimulus appears at all screen locations at the same time (sufficient settling performance assumed).
Flicker-free backlight, on the other hand, is useful for static stimuli or for stimuli which are animated in place, meaning for stimuli which do not move but are possibly switched on and off at fixed locations. The stimulus onset and offset is not as instant as with strobed backlight, and the screen is not updated all at once but, instead, from top to bottom. However, there is no flicker unless flicker is intended to be part of the stimulus.
Actually, both backlight modes have aspects that are similar to the operation mode of the good old CRTs. The pulse-like excitation of the phosphors is more similar to the strobed backlight mode, whereas the line-wise screen refresh is more similar to the flicker-free backlight mode.

The goal behind representing the settling behavior by a few graphs and numbers is to quantitatively rate the performance of a monitor and to compare it with other monitors or the requirements of the application at hand. It is difficult, however, to come up with a set of performance measures that is small, easy to understand, easy to measure, and of practical relevance. What is practically relevant depends, of course, on the application at hand and on the properties the monitors actually can differ in. Regarding the latter, an overshoot measure, for example, does only make sense if the monitors actually differ in overshoot behavior, which they only do since overdrive technologies have been implemented. So to some extent, the set of performance measures needs to be adapted to the ever changing monitor technology. Using an inappropriate set of performance measures is not only misleading us, the customers, when looking for a good monitor but might also make manufacturers optimize monitors in the wrong way.

In order to quantify things as objectively as possible, the numbers should be based on physical measurements. However, what matters in the end are not the physical properties directly but their impact on our perception as the signal travels through the visual system. Unfortunately, quantifying perception and inferring perceptual quantities from the physical properties is not as trivial as it might seem. We know a few things about the visual system, but this knowledge is derived from rather specific experiments that were run under well controlled lab conditions and do not necessarily translate well into real world scenarios where many more factors play a role. One example, which is also relevant for monitor characterization, is Weber's law, which basically says that the visual system's sensitivity for luminance differences scales with the luminance level at which the differences are observed. Sensitivity for a luminance change is higher for dark stimuli than it is for bright stimuli. However, in how far Weber's law holds for a given spatial stimulus configuration or even for dynamically changing scenes cannot be easily inferred just from the simple case/experiment for which Weber's law has been found true. Even if we had an accurate model of how the test stimuli are processed by the visual system, we still would not know how well the test stimuli adopted for the measurement taken here are representative for arbitrary image sequences in real world applications.


[2nd view] Matrix plots of settling parameters for the LCD being operated in LightBoost mode (example).


Settling error

Using the usual settling time for specifying the settling behavior in the backlight-strobed mode is not really useful as the luminance signal is not a time-continuous signal. Instead, the luminance signal is a pulsed signal with a rather low sampling frequency (meaning low time resolution) and with rather brief pulses (meaning long gaps without luminance signal)). Therefore, it makes more sense to look at the errors of pulse energies, the settling error for short. Some monitors, like the EIZO FORIS FG2421, pulse the backlight twice per refresh cycle (although with different pulse durations), which is more difficult to specify and to interpret. For the sake of simplicity, we just take the average luminance over the entire refresh cycle and relate this to the according value in the settled state in order to calculate the settling error.

Due to the sequential update of the LC cells on one hand and the simultaneous backlight strobing on the other hand, the settling behavior is different at different vertical screen locations. In addition, manufacturers might choose to adjust the overdrive strength according to the vertical LC cell position. Therefore, performance measures for more than one vertical screen position need to be assessed. Here, we measure at the top, the middle, and the bottom of the screen. Because of the mechanical constraints imposed by the measurement method, the top and bottom positions are actually shifted away from the very first and the very last pixel line by 5% of the total screen height.

Plot description

The luminance levels used for the step matrix measurements were uniformly distributed across the pixel value space, and the monitor was calibrated to a gamma value of 2.2. Level 0 corresponds to black, level 8 corresponds to 100% white. Using a gamma different than 1 means using a non-linear luminance spacing. This causes the difference in physical luminances between Level 0 and 1 to be way smaller than the difference between level 7 and 8, whereas the perceived differences between these luminance steps should appear much more similar, according to Stevens' law (or similar laws). See also Luminance scaling & settling errors.

Each bar in a matrix plot represents the luminance error averaged over N=5 measurements when going from level "FROM" to level "TO". Only absolute (i.e., positive) values are presented, but the sign of the underlying signed values is indicated by the colors of the bars (red codes for negative or undershooting). Regarding overshoot and undershoot, overshoot means over-stepped, i.e., too much into the step direction. For example, if the nominal FROM luminance is higher than the TO luminance, overshoot means that the measured luminance was actually too low. The thin green bars, if not too small to be visible, show variation of the value and extend by one standard deviation above the corresponding main bars. Do not confuse the standard deviation with the standard error, which is, for N=5, only about 45% of the standard deviation (45% ≈ 1/sqrt(N)).

The gray and flat bars drawn on the back walls of the coordinate system cube are the projections of the main bars along the FROM and TO direction respectively. For some plots, bars more in the front of the plot might completely cover bars behind them. Where this is the case, an additional plot might be provided that shows the matrix from a different angle.

For each matrix plot, two maximum values are provided, one being calculated by taking 100% of all step sizes and directions into account (without counting the FROM=TO cases along the diagonal where the step size is zero), and one taking only the best 80% into account, i.e., leaving the worst cases unconsidered.

Most plots are organized in 3x4 subplots where each row contains the luminance errors measured at the different screen locations (top, middle, bottom) and each column contains the luminance errors measured for the different refresh cycles (#1,#2,#3,#4 after the luminance change).

Measurement method

Monitor setup

Unfortunately, the LCD settling behavior depends quite a lot on the monitor settings, and it is impractical to measure and report the results for all the possible setting combinations. The monitor's factory settings should give a good starting point though, assuming that these are the settings the monitor has been optimized for. However, if the factory settings are too far off from any reasonable monitor calibration, more realistic settings should be used. One particular problem arises when colored scenes and color calibration comes into play. This is because the gains for the color channels (Red, Green, and Blue), along with the Contrast setting, define the maximal nominal range of operation, i.e., the maximal luminance per primary color. If these settings are not maxed out, the remaining range can be used internally by the monitor for overdrive. But even if overdrive is not active, the settling behavior of the LC cells still depends on the chosen operating range.

This makes it difficult to choose good settings for the measurements. On the one hand, the measurements (or derived parameters) should provide information about the maximal available operating range, allowing the user to make an educated choice about the extent the operating range should be possibly delimited for best performance given the application in mind. On the other hand, the measurements should reflect the monitor's performance under the most commonly used circumstances.

The Brightness setting, which controls the backlight PWM ratio or flicker-free luminance level when the monitor is operated in non-strobed backlight mode, is possibly replaced by the LightBoost setting or has a different meaning. It controls the duration of the backlight pulse and sometimes even the amplitude. Unfortunately, the pulse width has some impact on the settling behavior which is bad insofar as it requires another free parameter to be chosen appropriately. In terms of motion blur reduction, shorter pulses are better, so we use the shortest possible pulse which still provides an average luminance of at least 100 cd/m2.

Last but not least, because the settling behavior depends also on the temperature of the liquid crystals – after all, liquid crystals are a fluid with a varying viscosity over temperature –, it is crucial to let the monitor warm up thoroughly before doing the measurements (1{{unit|hr} minimum).

Photodiode placement

Photodiode with rubber sleeve put directly on the screen for measuring the stripe.

The measurements have been taken with a photo diode PDA36A (Thorlabs), the gain of which was set to 60dB providing a bandwidth of 37.5kHz and a minimal rise/fall time (10%-90%) of about 9µs. The photodiode was placed at about 4.5 from the screen surface at a straight angle. Ambient light was kept from the measurement by a rubber lens hood of 3cm diameter which also limited the maximal incident angle to about ±20°.

The vertical extent of the measured screen area was not only limited by the rubber sleeve but also by the stimulus being just a small horizontal stripe covering just 5% of the screen height. Note that the LC cells are updated sequentially from the top of the screen to the bottom, which results in different delays for the settling curves depending on the vertical measurement position. Of course, due to the pulsed backlight, actually only part of the settling curves become visible. Nevertheless, the luminance profiles still look different for different pixel lines. Measuring more than one pixel line in order to get a sufficiently high S/N ratio comes with some signal smearing being caused by the averaging over a few pixel lines. However, limiting the stimulus to only 5% of the full vertical screen size should be sufficient to keep the smear effect negligible.

As mentioned above already, the photodiode is placed over three different vertical screen positions: top, middle, and bottom. Ideally, the top and the bottom position would be representative for the first and the last pixel line, but this is not possible when more than just one pixel line needs to be measured for S/N purposes. Moreover, the photodiode with its rubber sleeve needs to be placed with some distance from the monitor bezel to sit on a flat surface. Therefore, the top and bottom positions are shifted away from the very top and bottom of the screen by 5% respectively, i.e., the 5%-wide stripes are vertically centered at the 5% and the 95% screen position (and at 50% for the middle position).

Signal recording

The photodiode was connected to a PC oscilloscope PicoScope 4224 (Pico Technology). At the hardware level, a 1MHz sampling rate was used, but in the recording software the signal was strongly low-pass filtered (Gaussian with a -3dB frequency of 70Hz) and down-sampled to 5kHz.

Triggering

The beginning of the refresh cycle was defined by the falling edge of the luminance trigger pulse (black-white-black frame sequence) plus 5% of the refresh cycle duration (about 400µs at 120Hz). A trigger time was identified for two trigger pulses, one before and one after each frame sequence comprising a luminance step. Measuring two trigger pulses per sequence allowed determination of the exact refresh rate in terms of oscilloscope samples and also provided some means of checking timing consistency. When searching for triggers, pulses shorter than 400µs were ignored, which allowed to cope with the brief pre-pulses present with some monitors (EIZO FORIS FG2421).

After having identified the triggers and, thus, the refresh cycles, the signal was collapsed by simply averaging the signal values within the refresh cycles, resulting in one average luminance value per refresh cycle.

Frame sequence

In addition to the trigger pulses, the frame sequence for each particular luminance step consisted of 11 frames with the pre-step pixel value and 11 frames with the post-step pixel value. The average luminance over frames 9+10 of the pre- and post-step sequence defined the pre- and post-step luminance respectively. Directly measuring the settled pre- and post-step luminance levels allowed to get rid of rather high offset fluctuations of the photodiode (not clear whether these fluctuations were present all the time and where they came from – the photodiode, the amplifier, or the power supply).

What about the 11th frame? In order to give the bottom part of the screen as much settling time as possible before the backlight is pulsed, the backlight is, on purpose, pulsed so late that it actually might still be on when the next refresh has started already. For the top part of the screen this means that part of the information that is made visible by the backlight pulse belongs to the next frame already. Obviously, this is not what we want when measuring a settled state, and that is why the 11th frame has been added.

The complete luminance step matrix (stepping from/to 9 luminance levels) was measured systematically, cycling through the "from" levels in the outer loop and through the "to" levels in the inner loop. This matrix measurement was repeated for N=5 times in order to assess a measure of variation (shown as standard deviation by the thin green bars in the plots).

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