The below article contains excerpts from The Display Calibration Guide.
Dynamic light control is mainly used by displays to improve contrast performance and contrast perception by the user. The only exception to this is ABL or Automatic Brightness Limiter on OLEDs that are used to protect the display from overheating and burn-in.
Lowering light control improves contrast perception for two reasons:
- it allows the display to lower the black floor
- It can also allow the display to achieve higher contrast within the same frame of video by approaching full on/off contrast performance (and getting away from ANSI) as we talked about contrast in this article. However, some strategies, and some display types, are better suited to this than others.
In addition to improving contrast, dynamic light control can also have the benefit of improving the life expectancy or usable life of the light source – whether that’s LEDs used as a backlight or lamps and lasers used in projectors. Dynamically lowering light output preserves image brightness over time.
However, dynamic light output also makes it harder to calibrate a device, since the light-source can shift in colour as it changes brightness. This is why some displays will include internal tables that tell the colour management system within the display what those shifts are so the colour can be dynamically updated by the display as it shift brightness.
However, for the purpose of calibrating the display, we must disable any dynamic action – such as dynamic light control – first then turn it back on after calibration if desired.
Next, let’s have a look at each dynamic light control strategy displays use!
Projectors: Lamp Dimming
Some projectors that use lamps can dynamically dim the lamp dependent on the brightness of the scene. As the ADL (Average Display Luminance) gets lower, the lamp is dimmed, as the ADL gets brighter, the lamp is driven harder and therefore gets brighter.
Lamp dimming is not very quick, therefore it is likely to be more noticeable than other strategies. However, this also depends on the algorithms that control dynamic processing within the display.
In some instances, where lamp dimming is not aggressive and dynamic processing is in step with the brightness changes, lamp dimming can be effective in improving contrast perception without too many noticeable artefacts.
Lamp dimming is sometimes only engaged in “lamp eco” mode on some projector models and on medium or high lamp on some other models without the ability to switch it off independently from a dynamic iris (e.g. Epson UB5050/6050).
Projectors: Dynamic Iris
A dynamic iris behaves very much like lamp dimming in that it reacts – in this case expands or contracts – to brightness (ADL) changes dependent on the scene or frame of video.
Dynamic iris can be implemented with or without lamp dimming. However, most projectors will only employ one or the other. For example, Epson has employed a combination of both on some of its 3LCD projectors, while JVC used a dynamic iris only for dynamic action on its lamp-based projectors.
Dynamic irises are very effective at controlling the light that hits the screen and will both lower the black floor and allow higher intra-scene contrast. However, they can also send some light back into the light path as they block the light escaping, which means the theoretical maximum contrast performance is not usually achieved.
This is more of an issue with a manual iris system. Manual irises can be right after the lamp (but before the imaging chips) or after the imaging chips. If a manual iris is in front of the imaging chips and is clamped down, ANSI contrast is lowered while on/off contrast is increased so a trade-off must be found that suits the user. The same trade-off is not true if the iris is right after the lamp as there is no light spill back into the light path when clamping down the iris.
Projectors: Laser Dimming
Laser dimming works exactly like lamp dimming, but lasers can be modulated a lot faster and a lot more precisely than either a lamp or a dynamic iris. Hence laser-dimming has the chance of producing the most natural-looking image as long as dynamic processing is implemented well and keeps in very precise lock-step with the laser dimming.
While laser dimming has the chance of improving the black floor – and therefore perception of contrast – it doesn’t necessarily increase actual contrast performance of the display. That depends on the characteristics of the optical path and imaging chips.
Flat Panels: Backlight Dimming
On flat-panel displays, backlight dimming can be implemented to the same effect as laser dimming on projectors. Since the backlight consists of LEDs on modern TVs, the backlight scanning frequency will determine how fast the backlight can react to the image displayed on the screen. The faster the backlight scanning, the faster this reaction time can be.
However, it is normally the electronics and processing that need to keep up with the backlight scanning frequency, and these may be slower to react than the actual backlight itself. This is why the backlight scanning frequency is not necessarily indicative of backlight response time (not talking about refresh rate or pixel response time here – those are different and unrelated!).
The same idea is at play as dimming on projectors: the display will analyse each frame of video and determine how much to dim the backlight to lower the black floor while also minimising artefacts.
On displays where the backlight is controlled as one monolithic unit (whether it’s edge-lit or back-lit), the backlight is dimmed the same everywhere on the screen. This means that highlights or bright areas of the image are also dimmed. To combat this, the display will employ dynamic image processing.
On some edge-lit displays, the backlight is divided up into rows or colours (not both) that can be dimmed independently. This can work well in some instances, but the rows and columns will light up dependent on what’s showing on the screen. This can look distracting in a dark room especially if it is not well controlled and there are not many rows or columns for the backlight.
Flat Panels: FALD
Full Array Local Dimming (FALD) is the most sophisticated form of local dimming. The LED backlight that is behind the screen is divided up into zones and can dim independently. While the backlight is divided up into rows and columns as well, the individual zones – not a full row or column only – can dim.
The more zones, the more effective FALD can be. However, it is also dependent on how good the algorithm is for dimming and the related dynamic processing that’s taking place.
The main advantage of FALD is that bright areas can remain bright while dark areas can be dimmed as much as possible to increase intra-scene contrast (within the same video frame). This is even more advantageous for High Dynamic Range (HDR) content that can have very bright highlights next to deep darks.
Flat Panels: ABL
ABL here stands for Automatic Brightness Limiter.
ABL is employed on OLEDs to stop overheating and burn-in. ABL will dim the display as the screen gets brighter or gets bright for too long as determined by the algorithm.
Unfortunately, ABL can only de defeated within the service menu – or not at all. This is partly why it is good practice to use smaller windows to display the calibration patterns – as opposed to using full-screen patterns. However, even then, the display might dim after a while so it is important to keep an eye on this during calibration.
There is another protection built into OLEDs that needs mentioning here. Some of the models will dim small bright areas of the screen if those are left on for a longer period. This is to protect logos or game HUDs (head up displays) burning into the screen. However, this can also kick in with smaller patterns after a while so it’s best to keep this in mind.
Local Dimming on Projectors
On high-end projectors, something akin to Full-Array Local Dimming can be implemented by stacking multiple imaging chips, with one set controlling light intensity while the other set controlling colour intensity.
This is currently done by Christie’s high-end cinema projectors such as the Christie Eclipse. The Eclipse uses 1080p DLP chips for light intensity control and 4K imaging chips for colour control. This is why the Eclipse is a 6 DLP design – as opposed to the more usual 3 DLP design.
A similar effect could be achieved by Epson by stacking LCD chips, and in fact the design would be easier to achieve because LCD chips employed by Epson are transmissive, making the light path much simpler than a 6 DLP design.
This can only be achieved with a laser light source, however, as correct light polarisation is critical to ensure the LCD chips are not heat-damaged. This is because LCD chips capture some of the light as heat – especially light that is not correctly polarised. By using highly polarised light (which is what laser light is by definition), LCD chip transmittance can be improved with less of the light converted to heat.
The Display Calibration Guide
If you would like to learn more about displays, and display calibration, you can get The Display calibration Guide here.
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