- Resistive touch: inexpensive, robust, and suitable for most conditions including gloved and wet
- No touch: inexpensive, can be put behind polycarbonate or cover glass for very harsh environments
- Capacitive: for higher end applications with bare finger and dry conditions; newer capacitive touch technologies are more resilient to small amounts of water or even food service or medical gloves.
LCD brightness, or more technically “Luminance”, is measured in millicandles of luminance per square meter, also known as “NITs”. Serious specifies a “minimum typical” brightness for each display, meaning that the described LCD has a typical luminance specification at least that amount. LCD brightness is measured at the LCD’s initial power-up. See backlight longevity, below, for how LCD backlights degrade over time.
As a simple rule of thumb:
- 200-399 NITs: good for almost all indoor environments, outdoor possible but may need shrouding to prevent bright sun
- 400-699 NITs: excellent for all indoor environments, considered mostly sunlight readable when not in direct sun
- 700-999 NITs: considered sunlight readable, some washout in direct sun
- 1000+ NITs: sunlight readable, direct sun readable
Luminance and Luminance Uniformity is calculated based on 9 measurements as follows:
- Luminance: Lv = avg(L1:L9)
- Uniformity: min(L1:L9) / max(L1:L9)
LCD backlight lifetime is a function of the specific LEDs selected for the backlight, and is measured in thousands of hours. If the backlight is maintained at 100% full power from initial LCD powerup fresh from the Serious factory, the typical brightness after that many hours will be typically 50% of its initial luminance.
Some common backlight longevity specifications are:
- 20,0000+: normal longevity, most cost effective
- 30,0000+: enhanced longevity, less common availability
- 40,0000+: enhanced longevity, uncommon
- 50,0000+: long life, nearly the longest you can buy, price premium
The technical definition of the test condition is as follows: The final brightness is at 50% of original brightness with an environment of ambient air flow at Ta=25±2C,60%RH±5% with LED forward current (If) at the recommended typical for maximum desired brightness.
Color LCDs get darker when operated below the minimum temperature range, and respond more sluggishly to visual changes. At higher temperatures, LCDs get washed out. At certain temperature extremes, the LCD driver chips may not function at all. The LCD temperature range is the minimum range where the LCD is deemed to perform acceptably without factoring in any backlight warming. LCD backlights do generate (when fully powered) several degrees of heat that can help obtain better LCD functionality at the low end of the LCD specification.
Note this range does not mean the SIM will cease functioning necessarily. Most SIMs are rated at -40 to +80C, so it is possible the SIM can continue to function even if the LCD is not visible to the user.
- -20 to +70C: most cost effective, most common
- -30 to +70 or 80C: harder to obtain, premium product
- -40: nearly impossible to buy LCDs rated at -40, typically requires heater modules which are expensive and require ~1W per square inch to power
This is the number of bits the LCD uses to represent one pixel. There is often a GUI performance penalty for going above 16 bits because 4 bytes (vs. 2) have to be used to represent a pixel on the driver MCU and typically this impacts memory and performance (or, alternatively, the power of the MCU required to deliver equivalent performance). Unless your GUI has extensive large shading gradients, 16-bit is almost always adequate for most applications.
- 16 bit / 65K colors: most cost effective, most widely available, suitable for most applications (RGB565)
- 18 bit / 262K colors: cost effective, adds slightly better Red/Blue (RGB666)
- 24 bit / 16M colors: excellent gradient support (RGB888 or YUV422), sometimes a price premium, MCU memory/performance implications
Viewing Angle and Technology
LCD technology is rapidly evolving. There are several types of “viewing technology” now available. Some are a function of the raw LCD panel fabrication process, others a result of optically bonded film techniques.
Viewing angle is a set of 4 measurements based on the 4 directions of deflection from straight-on viewing:
- TFT-TN Landscape: traditional landscape mode viewing. When the LCD is viewed with the long edge horizontally, has good viewing from 3 sides (typically 9, 12, and 3 o’clock) and reduced angle viewing from one (typically 6 o’clock)
- TFT-TN Portrait: traditional portrait mode viewing. When the LCD is viewed with the long edge vertically, has good viewing from 3 sides (typically 9, 12, and 3 o’clock) and reduced angle viewing from one (typically 6 o’clock)
- Optical Viewing Angle (OVA) Films: a film can be applied to the surface of a traditional TN-TFT panel and deliver typically 75%+ viewing angles in all 4 directions, similar to true as Multi-domain-Vertical Alignment (MVA) panel. Often OVA and MVA panels are categorized together as Multi Viewing Angle (MVA) panels
- Multi Viewing Angle (MVA): Multi-domain Vertical Alignment (MVA), often called “Multi Viewing Angle” is a new technology combining optics with LCD manufacturing techniques providing good viewing angles from all 4 directions without contrast or color shifting. MVA carries a price premium and is becoming less common as OVA is less expensive with similar results, and IPS delivers better performance and more common.
- In Plane Switching (IPS): an LCD fabrication technology which gives excellent viewing angles from all directions without contrast/color shift. It carries a higher price premium and is more common in larger panels (10.1″+) in the industrial/embedded space.
- Organic LED (OLED): other than very small (1″ x 1″ type) displays, OLED is unavailable in the industrial/embedded market due to its poor manufacturing yields and quality control challenges. It is unlikely OLED technology will become available, as the future looks to MiniLED technology as a more manufacturable and cost effective process.
Dimensions and Areas
A typical capacitive touch LCD is dimensioned similar to this drawing:
The terms on this drawing are as follows:
- COVER LENS/GLASS: outer dimensions of the LCD module
- BLACK SILK: outer area of the cover lens coated on the back to prevent light penetration
- TRANSPARENT HOLE: the silk region may include one or more round openings in locations shown to allow optical clarity for an LEDs, light sensors, etc.
- BEZEL OPENING: inner limit where your bezel should be contacting the surface of the touch panel & LCD with any gasket and not adversely affect touch performance
- LENS VISIBLE AREA (V.A.): opening inside the silk region for viewing the display
- ACTIVE AREA (A.A.): active pixel area of the LCD, and of the touch region; you will want to ensure visually the center (C) of this active area is visible to the end user, and accessible for touch interface
- ACTIVE AREA CENTER: center point of the active area of the display
Response time is the time for a display pixel to turn on or turn off. Specifically it is the slower of Tr and Tf, where:
- Tr is the time it takes to change from non-selected state (relative luminance 10%) to selected state (relative luminance 90%)
- Tf is the time it takes to change from selected state (relative luminance 90%) to non-selected state (relative luminance 10%)
As measured by a LCD-5100 luminance measuring device or equivalent.
Note that the specification of Response Time is at 25C; response times will often be progressively slower as the operating temperature drops below -10C for most panels.
There are two types of interpretations of Tr and Tf, depending on the nature of the panel technology: Normally Black (Negative) or Normally White (Positive).
Normally Black Type (Negative) Displays
Normally Black type, or “Negative”, displays include IPS type panel technologies.
Normally White Type (Positive) Displays
Normally White type, or “Positive”, displays are the most common and include standard TN-TFT display technologies.
Contrast ratio is the ratio between the display pixel fully white to the display pixel fully black:
- CR (Contrast) = Luminance of White Pixel / Luminance of Black Pixel
The measurement conditions are:
Most TN-TFT displays have typical contrast ratios in the 250:1 to 400:1 range and occasionally as high as 600:1.
IPS displays typically have higher contrast ratios, often 600 to 800:1.
Color can be technically and empirically measured, but how it is perceived by the human eye is the true goal of color appearance in an LCD. A color “gamut” is a specific range of color out of the full color palette most relevant to how the human eye perceives color.
While there are many methods used to show a color gamut, the industry standard for displays is the CIE 1931 Chromaticity XY Graph:
This diagram shows the “standard” color spaces we want to reference when we talk about what color appearance can be generated by a specific LCD:
- The colored area in the graph is the total range of color visible to the human eye. Not all these colors can be generated by an LCD. D65 is the white point
- Not to be confused with the old NTSC television format, the NTSC triangular area is the subset of colors generally most relevant to the human eye.
- The sRGB (saturated RGB) triangle represents a specific gamut (color range) described in the IEC 61966-2.1 standard. Most digital photography is taken sRGB space. Photo editing in tools such as Adobe Photoshop is generally performed in the sRGB color space. The sRGB space covers approximately 72% of the NTSC space.
- Adobe RGB was an attempt by Adobe to create a richer RGB color space than sRGB and as close to the NTSC space as possible. However this color space is unavailable on current display technology and can cause innumerable color translation issues into sRGB causing flat-looking images. Teams creating content for GUIs should not use Adobe RGB as a color space, but rather use sRGB throughout their editing and content creation flow.
- When colors are displayed on a specific LCD, they deliver colors in an area similar to the dashed triangle on the diagram above — in other words, a subset of the NTSC space, and often close to the sRGB space used to take and edit photographs digitally. In web and GUI development tools, colors are generally represented by a triplet of 3 hex values from 0x00-0xFF (i.e. 0-255 decimal, or 0-100%) in triplets such as 0x223344 where 0x22 is hex 22 for the red value, 0x33 for green, and 0x44 for blue. Each individual color (red, green. blue) when driven at 100% (0xFF) represents a corner of the dashed “LCD performance” triangle.
Because the NTSC gamut is the most relevant color space to the human eye, the standard mechanism for determining an LCD’s color performance is to compare the LCD’s performance (the dashed triangle) against this NTSC gamut:
- NTSC(gamut)% = area of dashed triangle / area of NTSC triangle * 100%
Most industrial LCDs have a typical NTSC% of approximately 50-52%.