The crystallization of new technology: helmet-mounted display

    Helmet-Mounted Displays (HMD) were first proposed by Ivan Sutherland of Harvard University in 1968, and he designed a helmet-mounted display called the Sword of Damocles that applied CRT. After 30 years of development, helmet-mounted displays have made great progress and are widely used. Especially in the gradual popularization of micro-LCD displays, virtual reality (VR), pocket computers, video mobile phones, and equipment of modern digital troops, helmet-mounted displays occupy an important position in these fields.

Whether the helmet-mounted display application

    requires seeing the required data simultaneously in the field of view of the real world, or experiencing the full sense of presence when the visual image changes, simulation training, 3D games, telemedicine and surgery, or using infrared, Microscopes and electron microscopes are used to expand the visual capabilities of the human eye, and HMDs have been applied. For example, in the military, HMD can be used to display information such as command transmission, battlefield observation, terrain viewing, night vision system display, vehicle and aircraft gun sighting systems during vehicle and aircraft pilots and individual combat operations. In terms of CAD/CAM operations, HMD allows operators to remotely view data, such as partial data lists, engineering drawings, product specifications, etc. HMD was used when Boeing used VR technology to design the Boeing 777 aircraft.

Helmet-mounted display

    HMD generally consists of the following parts: image information display source, image imaging optical system, positioning sensing system, circuit control and connection system, helmet and counterweight device.

    The image display information source refers to the image information display device, which generally uses micro-high-resolution CRT or flat-panel display devices such as LCD, EL, VFD, LED, FED, and PDP. Because CRT has high resolution, high brightness, fast response speed and low cost, it is currently more commonly used in the military, but it is generally a 0.6 to 1-inch high-brightness, high-resolution monochrome projection display. In order to realize color CRT display, light valve technology can be used for modulation or optical fiber transmission method. The disadvantages of CRT are large power consumption, large size, heavy weight, and high voltage. In helmet displays, weight is one of the important factors affecting the field of view and eye movement. At the same time, since helmet displays are mostly used in a separate power supply environment, Therefore, these factors limit the application of CRT in helmet displays.

    In recent years, the development of flat-panel display devices, especially the rapid progress of flat-panel display devices for high-resolution video display, has played a role in reducing the weight and power consumption of helmets and reducing the operating voltage. Currently, helmet displays made with 1024×768 high-resolution micro-LCDs are on sale, priced at around US$15,000. The disadvantages of HMD using LCD are low brightness and slow response speed, which are not suitable for occasions that require fast response. EL and VFD displays still need to be improved in terms of driving and color, and their application in HMDs still needs to be developed.

    In a helmet-mounted display device, the observer sees the image clearly through the eyepiece in front of him. Therefore, in the HMD, the design of the optical system is very important, which affects the quality of the image display. HMD can be designed as fully immersed or semi-immersed as needed. The fully-immersed type displays the image of the display device in front of the observer through amplification, correction of aberrations such as distortion, and optical systems such as relays; the semi-immersed type projects the corrected and enlarged virtual image to the observer's eyes. On semi-transparent optical glass, the displayed image is superimposed on the image of the external environment through the glass, and the observer can obtain the displayed information and the information of the external environment at the same time. Figure 1 is a schematic diagram of the optical system of the immersed and semi-immersed helmet-mounted display. Figure 2 shows Kaiser’s binocular and semi-immersed helmet-mounted displays, which change the helmet into a headband to reduce weight.

    The design of the optical system is not only related to the quality of the image, but also affects the volume and weight of the helmet display, and whether the observer is tired when watching for a long time. Therefore, the optical system and visual experience are an important part of HMD research. Foreign HMD companies generally use commercial optical design software to optimize the design of optical systems. For example, the currently better CODE V and Light Tools of ORA are commonly used visualization optical system optimization and light trajectory calculation software. However, they are also relatively expensive, such as CODE V, which is around US$60,000 and is generally used for optical design in aerospace systems. Depending on the level of application, there are also some relatively low-priced software, such as ZEMAX, which costs around US$4,000.

    The design of the optical system should also consider the observer's vision, interpupillary distance and other factors, and design it into an adjustable structure. The helmet's positioning sensing system is an equally important part as the optical system. It includes head positioning and eyeball positioning.

    The positioning of eyeballs is mainly used in aiming systems. Infrared image recognition, processing and tracking are generally used to obtain eyeball movement information. There are many methods used for head positioning, such as ultrasonic, magnetic, infrared, light-emitting diode and other positioning systems. The positioning of the head provides information on the position and direction of six degrees of freedom. The requirements for positioning sensing systems are high sensitivity and small delay. Low sensitivity is easily affected by the external environment. Long delay time will not only give incorrect position information, but also cause dizziness and nausea in the observer. Magnetic positioning sensors suitable for helmet-mounted displays are already on sale. Polhemus Inc.'s Istrak, 3Space tracker and FASTRAK are AC magnetic positioning sensors. Its design concept is fast and short delay, through digital signal processing and A/D conversion stages. This idea was realized in parallel, with a delay time of only 4ms without any form of filtering; Ascension's Bird, Flock of Bird and Extended Range Transmitter (ERT) are DC magnetic positioning sensors, of which Flock of Bird is the most DC magnetic positioning sensors that can work together in the same environment to extend the operating range. Compared with the AC magnetic positioning system, one advantage of the DC magnetic positioning system is that it is less affected by the eddy currents generated by external magnetically conductive metals in the alternating magnetic field during use, and is more suitable for environments with metals. Bird reportedly has a 24ms latency.

    The circuit control system is generally placed separately from the helmet-mounted display to reduce the weight of the helmet. The design of the connection system when mounted on the aircraft must consider the ability to quickly detach the helmet from the control system on the aircraft in an emergency to ensure the safety of the pilot.

    The helmet is a fixed part of the display. In airborne or vehicle-mounted applications, the driver's helmet can be directly modified and manufactured. In other applications, a sling can be used to fix the display on the head. Since the weight of the monitor is on the front of the head, the center of gravity of the head changes and fatigue easily occurs. Therefore, a counterweight should be added to the back of the head to keep the center of gravity unchanged. However, the weight of the helmet should be as light as possible, especially in an airborne situation. The weight of the helmet will produce a large impact when the pilot ejects from the aircraft, which can easily cause cervical vertebra fractures.

Basic parameters of helmet-mounted display

    The HMD market is expanding with the development of virtual display technology and wireless Internet technology. The marketization of low-priced and high-performance micro-LCD displays has made it more and more likely to be used for general entertainment and communication consumption. I believe that in Its application will have great development in a few years.