LED GET THE FACTS

With such a huge number of LED manufacturers and no industry uniformity with regards specifications and production standards, it is difficult to know what factors are best to judge a display on.

We have produced this guide to answer all the commonly asked questions and help you establish a framework so you can compare different brands against your unique solution requirements.

• What is pixel pitch and how does it relate to resolution?

Pixel pitch is the distance from the center of one pixel to the center of its neighboring pixel. The resolution is the length and height of a screen divided by the pixel pitch.

Lower pixel pitches typically result in higher resolution displays, with really fine pitches giving an image quality and resolution of a video wall made from LCD panels, while eliminating distracting seams, gaps and bezels.

• Why does the relationship between power and heat matter?

An ever-present challenge in display system design is selecting suitable power for the given brightness requirements. A big part of that process is making sure that LED components stay within their safe operating limits in terms of current & heat. The more power required in the system, the greater the heat generated which means that the LED diodes will operate at a higher temperature, shortening their lifetime. If one product operates at 1.5 times the power of another, (for a given brightness), the higher power product will see image degradation due to a faster aging process. These effects will manifest in poor uniformity, reduced brightness and reduced color saturation.

Displays with less heat emissions will have a much longer Effective Lifespan.

• What causes a display to emit heat?

Power is directly related to heat. Most displays convert less than 10% of operating power into light with the remaining being output as heat. A build up of heat will affect the way a display performs through various factors such as degradation of the data bandwidth, resulting in the effective grey levels being reduced along with refresh rates and brightness. These critical factors, as mentioned previously, are all interrelated and play a key role in the overall performance of the display.

There is a growing need to evaluate a display solution based on the Power Efficiency Merit – the amount of power needed to generate an image at the required brightness, refresh rate and bit depth, for the particular solution and specific environment.

• How is lifetime estimated?

This is one of the most fundamental and yet trickiest elements of display specification as there are no industry standards or a common definition of what constitutes ‘lifetime’ of a display. Typically LED manufacturers take it to mean the time until display brightness, as measured from a vertical angle, drops to 50% of the specified maximum. And yet, may others in the industry presume that lifetime is when the display effectively wears out.

This can be overcome by looking at the Effective Lifespan – that is how long the display can operate at the project specified brightness, refresh rate and data throughput. If for example, a display is specified to be run at 900 candela/m2 brightness for a project installation, the lifetime can then be determined by calculating when the display can no longer be able operate effectively at this brightness level, while maintaining calibrated uniformity. Ultimately the lifetime of an LED is dictated by its power efficiency, as displayed in how many watts of power are required to create a candela/m2 (NIT) of brightness.

• What defines image quality?

The quality of a displayed image is the consequence of many design parameters and factors.

Performance elements to consider are:

• Brightness – specific brightness needs will vary by ambient conditions and application but a brighter display operating a lower brightness will be under less stress than a lower brightness display operating a full power. Hence higher brightness is preferred
• Contrast Ratio – how black is the black and to what intensity can the white level be driven
• Color Gamut – a wide color gamut will enable the display to produce highly saturated, bold colors
• Grey Scale – look for the ability to display 16-bit grey levels or greater to ensure detail in the low grey levels can be differentiated
• Refresh Rate – there must be no flicker visible to the eye or through a camera lens using the highest shutter speed. As LED’s have a low persistence, a higher refresh rate is required to avoid such effects
• Scan Mode – in LED displays, the image is generated in blocks and the number of blocks is called the Scan Mode. The higher the Scan Mode numbers the better. I.e. 1/8th Scan is much better than 1/12th Scan. The ‘Effective Refresh Rate’ is determined by multiplying the published Refresh Rate divided by the scan mode. I.e. 2000 Hz multiplied by 1/8th Scan mode = 250Hz.
• Viewing Angle – if a wide off axis viewing angle is needed, look at how the image is seen from all angles, vertically and horizontally. Many displays show image degradation at wider viewing angles and also exhibit discoloration or color shift
• Resolution – ensure that you have the right pixel pitch for the viewing distance. For displays that are going to be viewing from a distance, the pixel pitch can be larger whereas close proximity will require a smaller pitch

• What key factors are important for scaling?

Ideally, a display should be the pixel pitch AND same aspect ratio as the desired content. LED is a modular technology so can it be built to an end user’s exact specification. If you need to scale an image, crop or stretch an image to fit a display with a different aspect ratio or resolution, the image quality will suffer.

• What is Common Cathode vs. Common Anode?

Common Cathode Technology is a new way of driving the LEDs within a display and we at Silicon Core Technology are the exclusive manufacturer & patent holder for this technology. The design is much more efficient at generating a given brightness for a given amount of power, ensuring that less power is wasted as heat, reducing power consumption and increasing the lifespan of the display. This results in market leading total cost of ownership.

In typical Common Anode LED design, the ballast resistor required by the red LED wastes energy by dissipating power and generating heat. Common Cathode based displays eliminate the need for the ballast resistor by having separate, dedicated power supply voltages to the red, green and blue LEDs, enabling the power supplied to the red LED to be controlled separately. The significantly reduced power requirements mean that Silicon Core’s proprietary Common Cathode LED can address up to 10 times the LEDs of a Common Anode driver. This creates more space allowing the LED board layout to be optimized for performance and reduced radiated emissions and for the creation of much finer pixel pitches enabling higher resolution displays that are suitable for close proximity viewing.

We welcome comparisons and work with clients to perform a power analysis as a factor in decision making when choosing an LED display vendor. We predict that the Common Anode approach will be replaced over time as it does not scale well for sub-3mm pixel pitch LED panel designs. Systems based on this approach suffer from poor performance, higher running costs and reduced service life.