LED Displays 103: The Physical World

Physical Interactions

Should this be a whole Xxx level?

Mechanical Concerns

Seams

Z-axis Alignment

Curved/Faceted Displays

Viewing Position

Viewing Distance: Fill Factor vs. Pixel Pitch vs. Angular Resolution

Viewing Angle - off axis x/y, impact of z-axis alignment

Viewing Position and Compression Artifacts

Burn-in and Module Replacement

Thermal wear, rework/hot air damage

Colorimetry

White Point

Gamuts

Spectroradiometers

Test Patterns

Thermal Color Shift

Uneven thermal management (heat sink) can cause a spatial artifact, typically a pink/green or pink/cyan shift in a donut pattern around the module. Also shows PSU thermals if PSU is closely coupled or acts as an insulator to LDM.

LED on Camera

Timing: Frame Rate, Phase Offset, Refresh Rate, Shutter Angle, Multiplex Patterns

How to determine an LED display single refresh time

  • Set shutter speed as fast as possible until the dark part of the image is actually black
  • Increase shutter speed until the black lines disappear
  • The current shutter speed is the base time of an LED image refresh. Multiples of this shutter speed will be additional speeds that will create a smooth image without scan artifacts

Global & Rolling Shutter

Gamma, OETF/EOTF, OOTF

Genlock, Frame Lock, Frame Rate Conversion: When Do We Actually Need Genlock

Moire & Spatial Aliasing

Multiplexing

Brightness, bit depth decimation

LED Displays 102: Image Pipeline

LED Displays: Image Pipeline

An important aspect of all display technologies is how they reproduce image content, that is, the electronic signal that represents the image to be viewed on the display surface. We’ll start by looking at how a grayscale image is made, and then add color to that.

Luminance

At their most basic, emissive raster images are made up of pixels in a regular grid each having varying intensities. When viewed in aggregate, the eye fuses all of these individual pixels and their different intensities into what appears to be a continuous image.

Display Brightness

One of the fundamental properties of LEDs is that they can either be on or off. Unlike an incandescent light source, they don’t have an analog ‘brightness’ that can be adjusted. To achieve what appears as differing intensities, LEDs are flashed on and off at an extremely high refresh rate which the human vision system temporally integrates into a an apparent continuous brightness. By varying the duty cycle of the flashing pattern, different luminance levels can be created.

Bit depth decimation

Since an LED can only be on or off, it follows that its maximum intensity is achieved by leaving it turned on as long as possible per frame time. Conversely, what happens if we want to lower the maximum intensity of the display after the display's electronic properties have been defined, and the display has been produced? To do this, we must adjust the intensity digitally, by reducing the duty cycle.

$$ \begin{equation} a^2+b^2=c^2 \label{eq:pythagoras} \tag{1} \end{equation} $$

Referring to Eq. $\eqref{eq:pythagoras}$.

100% Maximum => 12.5% maximum $100/2^3$ = 3 bits lost. 16 bit maximum is now 13 bit maximum. Related to this, the smallest difference in luminance is always the same, regardless of what the maximum luminance of a display is set to. $$1 bit => 1500cd/m^2 * \frac{100\%}{2^{16}} => 0.02 cd/m^2$$ From this, we can see that applications that require lower maximum intensities are harmed by the numbers race of LED manufacturers in their quest for ever larger luminance ratings. If you know in advance the maximum luminance needed for your application, you’re much better off buying a display with 1x headroom than 10x headroom.

Full Range vs. Limited Range

The bit depth available to a digital image is made even more complex by full range vs. limited range codings. In order to simulate the way analog signals were handled, a limited range encoding was devised during the transition from analog to digital signalling. This coding uses a subset, or limited range, of the full bit depth available to reproduce images to better map to the analog signals that used a subset of the analog range to account for analog signal behaviors such as overshoot and ripple. Data outside of this range is used to maintain compatibility with analog equipment, but is expanded and thrown away when reproduced on a fully digital display technology.

HDR and Bit Depth

Color

White Point

Color Spaces

Accuracy vs. Precision

Uniformity compensation vs. Calibration

Standardized Color Spaces and Content Management

Color space standardization allows multiple devices to display images with the same perceived color. This allows an image that is prepared or manipulated on one device to be viewed on another, and have it appear as expected by the viewer.

Gamma, Transfer Functions, and Color Grading

OETF/EOTF, OOTF Rendering Intent

Thermal Color Shift

Synthetic vs. Sensor-based Image Content

RGB vs. YCbCr

Signal Compression

Chroma Subsampling

Progressive and Interlaced Formats

LED Displays 101: The Basics

LED Displays 101: The Basics

Overview

LED Insider is an article series intended to provide background information and working principles regarding the use of LED displays. Some of the concepts discussed may be foundational background that is applicable to any display type, while other topics will be very specific to contemporary LED display technology and may not transfer directly to other displays or media.

This series can be read through with each section building on the one before, to quickly come up to speed on all aspects of LED displays. If you’re already familiar with many of the concepts, you can skip through it and just use it as a refresher or a reference to fill in any gaps you may have. And of course, feedback is always welcome! If there’s a topic you’d like to see covered, or an area that can be improved on, let me know!

The LED Display

Throughout this series, we’ll be focusing on LED displays, a type of electronic display technology. To start off, we’ll look at how an LED display compares to other display technologies, and understand how and when some of these differences are important when selecting a display for a particular application.

Direct View

LED displays are considered to be direct view, which is a display type that emits light directly from the viewing surface. Direct view displays are contrasted with indirect view displays, which bounce modulated light off or through a passive surface. Examples of direct view displays are those using CRT, LCD, plasma, OLED, and LED technologies. These displays come in various sizes and have differing performance characteristics, but they all emit light directly from the surface being observed. Projectors are an example of an indirect view display, where the image generating surface such as an imaging chip or film frame is at least one step removed from the viewer.

Both display types have advantages and disadvantages, depending on the context or application. Direct view displays tend to require less controlled lighting in an environment, but are typically limited in physical size based on a fabrication technology. Projection can provide very large shared viewing surfaces, but due to the light required to come from a single* source, tend to be dimmer and require a controlled environment. Projected images can also be cast onto non-planar surfaces such as the side of a building, allowing for complex video

Emissive

Emissive displays emit light directly from the display surface. What kind of display is projection? Not exactly reflective compared to e-ink or other display types which modulates the light at the display surface, instead of relying on inbound pre-modulated light.

Individually Lit

In a display with individually lit pixels, each pixel or sub-pixel is illuminated independently of and other pixels in the display. This can increase contrast as ‘black’ pixels are truly off, instead of a dark shade of grey. The tradeoff can be increased cost due to material science process differences (plasma) or increased high power driver circuitry (LED).

Modular

Modular displays are bezel-less, and can be physically combined to compose a contiguous display that is much larger than it is feasible to create in a normal electronics fabrication process. TODO Compare to projection blending, bezeled LCD/OLED tiling.

Parts of a display

  • Module
  • Tile
  • Frame
  • Receiver card
  • Hub board
  • Processing (sender card)