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Karin Reed, T. Designs
Organic Light-Emitting Diodes (OLEDs)
OLEDs contain organic compounds made of carbon and other substances to light up (Illuminate) and display characters, icons and graphics in a wide variety of colors that are readable in daylight and nighttime. Since they generate their own illumination, they are called an emissive display.
LCDs (Liquid Crystal Display) are readable in strong ambient light (sun, office light) but require a backlight for readable in dark environments.
Key advantages of OLEDs include:
OLEDs draw less power than LCDs containing a LED backlight
A faster refresh rate which produces a sharper contrast
Higher brightness without increased power consumption
An advantage for battery powered products
Thinner than TFT and character displays
Wider operating temperature allowing the display to operate to -40C
Wider viewing angle
Produce darker blacks than LCD technology
An OLED with an off pixel produces a dark black that is not possible for LCDs
LCDs can produce brighter white with their LED backlight
AMOLED vs. PMOLED
OLEDs are broken down into two categories: PMOLED (Passive Matrix Organic Light-Emitting Diode) and AMOLED (Active Matrix Organic Light-Emitting Diode).
The word ‘matrix’ just means how the display is driven. Below is a rough drawing of a matrix of pixels. Each pixel is either on or off to generate an image.
There are five rows with each row containing several pixels.
Passive displays control/refresh one row at a time. Once that row is refreshed, the control moves to the next row and so on. That means that the rows are on a small percentage of time which limits their size.
Another word for this is duty cycle which is the ratio of time ON vs. time OFF. This can be seen in refrigerators where the compressor is on for ten minutes for every thirty minutes off.
Duty Cycle = 10 minutes (on) / 40 minutes (total time on and off) = 25% duty cycle
The lower the duty cycle, the more time before the display is refreshed, the more power required to illuminate the pixel, and the duller the contrast.
Once control moves to the next line of pixels, the brightness of the previous line of pixels fades. To fix this and provide an even light for the human eye, more power is required to increase the brightness when the pixels are refreshed.
If your display has 5 rows, then you need to make the pixel 5 times brighter which equates to 5 times more power. This increased brightness reduces the display’s half-life.
Half-life is the amount of time in hours for the display to be half as bright as when first turned on. This is not the amount of time for it to burn out, but just to grow dim.
Even with this set back, PMOLEDs are popular because they are less expensive and easier to manufacture.
AMOLEDs produces a higher quality image by adding one capacitor for each pixel. A capacitor is a mini battery that stores power and illuminates the pixel after it has been refreshed. This allows the pixel to stay active and not fade away when refreshed.
The capacitors increase complexity and therefore, AMOLED’s are more expensive, but requires much less power and can produce larger displays than PMOLED modules.
Advantages of AMOLEDs:
Wider viewing angle
Thinner displays since there is no need for a backlight
Brightness of each pixel is independently controlled.
LCDs can independently control each pixel, but not the brightness of that pixel. The brightness is control by the LED backlight that sits behind all the pixels.
A sharper contrast
Contrast is the difference between the brightest and the darkest color
Disadvantages of OLEDs:
They may have a possible burn in due to half-life. If some pixels are on more often than others, the high usage pixels could dim over time.
The brighter each pixel, the more power required.
OLEDs contain an organic material that may lead to a shorter life-time than LCDs
Pixels (aka picture element) are very tiny dots to create the image or character or icon. The more pixels in an area, the sharper the resolution.
A 320 x 240 graphic display contains 320 pixels (or dots) along the x-axis and 240 pixels along the y-axis for a total of 76,800 pixels.
Each pixel is its own light source and consists of three sub-pixels that are Red, Blue and Green (RGB). Each pixel can be driven at different intensities to generate a wide possibility of colors.
Many displays can generate between 64,000 to 64,000,000 unique colors by mixing a combination of Red/Green/Blue.
OLEDs offer a much wider viewing angle than TFTS and LCDs (Liquid Crystal Displays). LCDs (FSTN) can provide up to a 120-degree viewing angle, but there will be a viewing bias. That is, one of the four views (top, bottom, left or right) will have the sharpest viewing angle.
At one time, the lifetime of an OLED ranged from 10K to 15K hours. Now, it is much closer to 100K hours.
Lifetime for an OLED is when the image becomes burned in.
Many graphic OLEDs contain a built-in touch screen. Touch screens are not standard on character displays but can be added.
There are two main types of touch screens
Resistive Touch Screens (RTS)
RTS consists of combining two layers of material with a small air gap between each layer. When the user applies pressure to the top layer, it makes contact with the lower layer and creates a short circuit. The touch screen is then able to calculate the exact location of the touch.
Resistive touch screen’s advantage is the freedom for the user to choose any type of stylus; A stylus is a device used to make contact and apply pressure to the top layer of the touch panel such as a bare finger, fingernail, gloved finger, edge of a credit card or the soft end of a pen.
They are cheaper than Capacitive touch screens and easier to integrate.
Capacitive Touch Screens (CTS)
Capacitive touch screens are found in many consumer products such as cell phones, tablets and medical equipment. The touchscreen panel contains an insulating material, in many cases it is glass, coated with a transparent conducting material such as indium tin oxide (ITO). The human skin is also an electrical conductor, so that when the surface of a finger contacts the surface of the glass, the electrostatic field is interrupted. This disturbance locates the position of contact.
Capacitive is growing in popularity since it can pinch and zoom. Two features that are not available in most RTS.
CTS is more expensive, more integrated and not as rugged as RTS.
Character OLEDs can display letters, numbers and punctuation marks. They contain a controller driver with a built-in character table that reduces programing steps when developing the display’s firmware.
Character LCD Table
Displaying a letter is as easy as sending the ASCII number to the controller. The controller than draws the letter at the specified location and automatically refreshes each character to maintain a sharp contrast.
OLED vs. LCD Power Consumption
OLEDs are an emissive technology, meaning that the characters/segments glow when the display is on. This is also known as a negative mode display meaning the background is dark and the letters/segments are light colored.
Negative mode modules stand out to catch the user’s attention and allow the display to be readable at night but draw additional current than a Liquid Crystal Display.
LCDs do not glow and without a backlight they can operate on as little as 1mA. The addition of a LED backlight is necessary for nighttime operation but will increase current by 15mA to 90mA depending on the number of LEDs and how brightly they are driven.
Graphic OLEDs can display letter, numbers, images, video etc. They use dots (pixels) in a matrix pattern. The dots are turned on and off by the customer’s firm wear.
Graphic displays are identified by the size of the glass that is measured, in inches, along the diagonal. Common sizes include .91”, 1.54”, 2.42” and others.
The resolution of graphic displays is measured by the number of dots along the horizon (X-axis) by the number of dots along the vertical (Y-axis). The more dots, the sharper the contrast. Examples are 320x240, 180x180 etc.
Monochrome vs. Multicolor Display
Why aren’t all OLEDs multicolor?
Most people think all OLEDs as a multi-color displays that can generate between 64K to 64M unique colors from the combination of Red, Green and Blue pixels.
But there is a demand for monochrome displays because they produce one color background (normally black for an OLED) and a different color character, normally white for OLEDs but can be other colors.
Monochrome displays produce a sharp contrast that makes it easy them easy to read. Many non-consumer industries such as industrial, construction, test and measurement and even medical applications use monochrome to provide the sharpest image possible.
Heaters for OLEDs
OLEDs will operate down to -40C which is better performance than TFTS (-30C) and monochrome displays such as character and segments (-25C).
But there will be times when the display must operate below -40C.
Here are a few ideas to maintain performance of the OLEDs as the mercury continues to drop.
Enclose the display in something that retains heat. Wind blowing across an unprotected display quickly lowers the temperature that results in poor performance.
Heat film is a clear membrane applied to the top, bottom or both layers of ball. It is the most expensive option. Its main advantage is its ability to provide even flow of heat while drawing the least amount of power. There are found on military, aviation, and many medical products. Its higher price may be justified for the improved performance and power savings.
One of the oldest, but still best methods to generate heat is to outline the display with ¼ watt, through hole resistors. They generate heat and last many years. One major downside is their power consumption and is not recommended for battery power products.
OLEDs are bright enough for most applications. It is possible to increase brightness by increasing current. This shortens the lifetime of the OLED and the batteries.
If you need a very bright, multi-color graphics display, TFTs may be your best option since they can be modified to produce brightness in excess of 1K Nits.
Note: One Nit is the amount of light produced by one candle. Sunlight readable displays need to meet or exceed 700 Nits.
Dark mode is where OLEDs can turn off individual pixels to save power. This is one key power saving advantage that OLEDs have over LCDs (Liquid Crystal Displays). LCDs are not able to turn off an independent pixel.
This is one reason why Google maps in dark mode reduce power consumption by 63% : https://www.zdnet.com/article/google-heres-why-dark-mode-massively-extends-your-oled-phones-battery-life/
Faster Response Time
OLEDs offer a faster response time than LCDs. LCD response time averages around 1mS, whereas OLEDs can refresh 1,000 times faster.
Available Interfaces (bus)
OLEDs are available in SPI, I2C and 8080 4-bit/8-bit and other interfaces. Many displays will accept more than one interface (bus). SPI is the most popular option though not the best for fast frame rate.
Industry Segments Utilizing OLEDs
Portable health monitoring equipment such as Fitbit and smartwatches, mp3 players. VR headsets make use of OLEDs as they may reduce eye strain, last longer per charge and are thinner than TFTs.
Many medical product incorporate OLEDs since the displays are rigorous and must perform in a demanding environment. These displays are readable in low light and high brightness locations. Their wide viewing angles making them an idea solution for outdoor and indoor environments. Anti-reflective coatings can be applied to improve readability in direct sunlight locations.
OLEDs provide the widest viewing angle needed for all environments and perform well down to -40C.
OLEDs with low power backlights extend the battery life of the product. They are highly durable to withstand extreme temperature shock and vibration. Handheld products integrating OLEDs include data loggers, instrument displays, handheld meters (measuring anything from light to temperature to gas composition) to handheld thermometers.
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