As technology continues to advance in the medical industry, the importance of clear and reliable displays in medical devices becomes increasingly crucial.
The display screens must provide the necessary information to healthcare professionals to ensure proper diagnosis and treatment and maintaining and protecting these screens from damage can be challenging. That is where optical bonding comes in.
In this article, we will reveal the benefits of optical bonding when applied to LCD TFT screens for medical devices.
1. Introduction to Optical Bonding
Optical bonding is a process of attaching two pieces of glass or plastic together with a special silicon based optical material.
This process eliminates the air gap between the display screen and the cover glass / touch screen, which can cause reflection, glare, and reduced visibility.
By bonding the two pieces of glass or plastic together, the display screen becomes more rugged and resistant to environmental factors, such as moisture and dust.
There are several different types of optical bonding materials and processes available, including OCA and LOCA. OCA is a pre-cured adhesive that is applied between the cover glass and the LCD. and then cured using UV light or heat. LOCA, on the other hand, is a liquid adhesive that is injected between the front glass and the LCD.
While OCA is a suitable option for large-scale productions and especially in the consumer applications, LOCA is the preferred method for professional medical device displays. This is because LOCA provides a higher level of production flexibility for low and medium scale production and it allows for a more precise and consistent bond, which is crucial in the medical industry where even the slightest imperfection can have serious consequences.
The end result of this treatment is that the external light is optically directed, polarized, and utilized to enhance the color brightness of the display image. This enhanced color brightness maintains high contrast ratio levels, independent of the luminance intensity of the external light source. With the contrast ratio maintained and reflection “washout” eliminated, the display image is easily seen and thus becomes truly readable in direct sunlight conditions.
2. “Air Gap” is Devil
But why is this treatment necessary? Wouldn’t it be easier to avoid using a frontal glass?
One of the fundamental problems and inherent limitations with all LCDs in real-world environments is the delicate nature of the polarizer material. The frontal polarizer is easily scratched and physically damaged and these events can permanently destroy the quality of the display image, it is as well, very hydrophilic (absorbs water) and can be damaged with prolonged exposure to moisture, such as rain, blood and chemical products (cleaning, sanitizer and disinfectant etc.)
These fragile characteristics of the front polarizer deem it a necessity to protect the delicate frontal LCD surface with some type of protective window, be it cover glass or polycarbonate or touch screen. And here’s where everything starts to fall apart. Once a cover window or touch screen is placed in front of the LCD, an “air gap” is formed between the front polarizer of the LCD and the overlaying protective cover window.
This ‘air gap’, regardless of thickness, causes undesirable optical and performance conditions. From an ‘optical’ standpoint, this ‘gapped’ cover window causes reduction in display contrast, decreases in visible luminance from the LCD, and increases both specular and diffuse reflection levels.
The ‘air gap’ has such an adverse effect on the quality of the LCD image because of the optics of the different Index of Refraction (Refractive Index) of transparent surfaces.
The optical bonding eliminates the Refractive Index mismatch between different contacting surfaces (LCD, air, cover glass) and eliminates the significant light reflection that occurs at the interface between those surfaces. In fact the Refractive Index of the special silicon based optical material is very close to the polarizer index.
3. What are the benefits of optical bonding?
The bonding treatment has been a revolution in the realization of displays. There are many benefits that can be achieved by eliminating the air-gap and introducing the optical bonding.
Reflection: an optical issue. As already mentioned, Optical Bonding eliminates the reflections between display and touchscreen/ front glass creating a single index of refraction. In this way the refraction of light is reduced, thus increasing the contrast and the optical brilliance.
The same principle is applied when an external source of light hits the screen. With a non-bonded screen, the gap between the glass and LCD module creates opportunities for refraction which bounces external light back off the screen to the viewer as glare. When bonded together the light passes through the bonded layers and is absorbed somewhat into the screen. Optical bonding is therefore important in making screens sunlight readable.
Low reflective loss means higher contrast in high bright environments both indoor and outdoor.
Reflection: a safety issue – Surface reflections can be a minor nuisance to a safety hazard depending on the application. When the information from a display are mission or even life-critical, the display must be glare free and easy to read.
Environments with high ambient lighting can cause internal and external reflections that impair the visual quality of the screen. In medical displays a poor reading and visual quality of the screen can lead to wrong diagnosis.
That’s External light reflections decrease with anti-glare and anti-reflective top surface enhancements without additional power consumption. Air gaps cause internal reflections between the display and touch panel. Optical bonding fills the gap and eliminates the inner light reflection.
No Dust, Moisture, and Condensation – Air gaps are also susceptible to contaminants such as dirt, dust, moisture, and condensation. Presence of contaminants increases the risk of failure in the adhesive tape or other material used in the perimeter bond. Optical bonding eliminates the potential of contamination and condensation, protecting and extending product life.
The most obvious benefit to adding an optical bonding layer between the glass and LCD module is that it physically prevents dust and liquid ingress from getting between the two. The quality of manufacturing means that dust and water isn’t a big problem for screens in standard environments. What can be a problem however is condensation getting between the glass and LCD panel in environments with wide temperature ranges or fluctuating humidity. Condensation can cause screens to become foggy from moisture that penetrates the air gap. Again, the physical filling of the gap prevents this problem from arising. Optical bonding should therefore be considered for any outdoor application as well as indoor applications where consistent temperatures aren’t maintained
The touchscreen performs better. A touch monitor with optical bonding maintains a more accurate touch response because the pressure is maintained through the substrate, rather than being delayed by a gap. The touch controller does not need to be constantly recalibrated due to the static connection created.
In addition, in an optically bonded display, light passes through a single index of refraction, reducing dramatically parallax. Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight; that sort of “trick of the light” can certainly cause erroneous touch point selection.
The data gathered from one of the several Global Display Solutions’ tests, aimed at examining the advantageous effects of cutting-edge optical technologies, revealed that users experienced touch inaccuracies in 36% of instances on average, and up to 71% when faced with the worst viewing angles, due to refraction and parallax error in touchscreens with an air gap. Conversely, when utilizing an optically bonded touchscreen without an air gap, the error was up to 15 time lower, with zero errors observed under the most challenging angle conditions on the optically bonded display.
Impact Resistance – An optically bonded cover glass increases impact resistance on most LCD modules and helps protect it from physical damage. The touchscreen or any component on the front of the display can flex under pressure with a gasket or perimeter bond. Optical bonding reinforces these components and decreases the chances of field failures from use and abuse.
Adding custom cover glass, touchscreens, optical filters, optical coatings, and other enhancements with optical bonding creates a durable and complete rugged solution.
Bonded displays pass safety tests and even in case of a broken cover glass, this is held in place and will not cause harm to the user.
Vibration Resistance – Vibrations encountered in aerospace, military, transportation, medical (when displays are moved around on a cart for instance) and other industrial environments and applications weaken the integrity of a display product. Vibrations can rattle the LCD and its internal components causing premature failure.
Deflection Protection – The added strength of an optical bond decreases deflection of the LCD. For example, when a person is using a stylus on a digitizer unit, they may notice the effects of the point hampering display performance. Using an optical bond and a thin cover glass drastically reduces the risk of a poor performing LCD.
Optical bonding makes screens more physically durable. The additional silicon layer in optical bonding absorbs shock. This provides high durability for use in public access areas, factories, or other harsh environments.
Improved thermal management. By filling the insulating air gap between the components:
- Backlight power can be reduced for a low power draw thereby producing less heat. 500 nit displays become daylight viewable and 800 nit displays become full sunlight readable, that drastically reducing the power consumption.
- Bonding eliminates the air gap preventing the “greenhouse” effect which causes heat rise.
- The thermal transmission of the bonding material allows heat to conduct out of the display.
- Bonded displays with better thermal management will prevent displays from going isotropic (clearing) and turning black
Viewing angle. Optical bonding allows to see the full view angle of the original LCD in case of front glass.
So many advantages for medical grade displays!
All the advantages listed in this article, enable the realization of displays that are perfect for the medical sector. With cross-industry know-how and design method, the optical bonding process can be tailored to the specific requirements of each medical device, ensuring optimal performance and durability.
One of the most common applications of optical bonding is the patient monitor. Patient monitors are critical devices that provide real-time information about a patient’s vital signs. They are used in a wide range of medical settings, including hospitals, clinics, and emergency medical services. By using optical bonding, patient monitor displays can be made clearer, more readable, and more durable, thus increasing the accuracy and reliability of the information they provide.
Another common application of optical bonding is in the design of surgical displays. Surgical displays are used to provide real-time information to surgeons during procedures. They are often used in conjunction with other medical devices, such as endoscopes and laparoscopic cameras. Surgical displays are used in very high ambient light conditions. Optical bonding eliminates light reflections, offers enhanced optical performances and superior contrast quality that resulting in a highly readable display.
Bonding is important also to help maintaining the highest standard of disinfection in the work place which is vital in the medical environment. Using LCD displays with easy cleaning user interfaces is the ideal way to support this goal.
There are other two benefits of LOCA that are important to mention: the ability to make repairs and adjustments to the device without having to replace the entire display. This can save a significant amount of time and money for both the manufacturer and the end-user and the ability to upgrade the device with new technology. As technology continues to advance, medical device manufacturers may want to upgrade the display to take advantage of new features and capabilities. By using removable and re-workable optical bonding, manufacturers can easily replace the display without having to replace the entire device.
Since the beginning of years 2000, GDS, Global Display Solutions, has introduced optical bonding in its outdoor as well as in its medical grade displays. All the above-described benefits can be achieved thanks to the GDS’ G+Bond proprietary process.
Global Display Solutions has a wide range of technical solutions specifically developed for the medical market and which include optical bonding. All those products have the following characteristics:
- Bonded LCD/TS in a close system without airgaps
- No mechanical gaps and edges where dust and grime can accumulate
- Frontal glass that cover the whole surface
- IP56 (Front) Hermetic System
- a removable bonding process where cover glass and touch screen can be salvaged with an in-process rework yield exceeding 92% and reaching the 100% for the LCD TFT panel.
The display size will not be a concern either: virtually any display sizes can be bonded. The GDS G+Bond bonding process is scalable from LCD as small as 4” to 98”. In conclusion, optical bonding is an innovative way to enhance the performance and durability of medical device displays. By using LOCA and cross-industry know-how, medical device manufacturers can create truly unique and reliable displays that meet the specific requirements of each application. From patient monitors to surgical displays, optical bonding is the future of display technology in the medical industry.
