On a per-mile basis, driving at night is more than three times as likely to result in a fatality as driving during daylight. While many factors contribute to this increase in collisions at night, such as fatigue, visibility certainly plays a significant role.

Now Cadillac introduces a technology designed to enhance night-time safety, an infrared Night Vision system that helps drivers "see" objects that would otherwise remain in the dark.

While Night Vision technology has been used in military and other applications, Cadillac will be the first automaker to offer Night Vision on a production vehicle — the 2000 DeVille. Cadillac is also the first to offer a Night Vision system that:

- Is fully integrated into a production vehicle
- Offers a head-up display to help drivers keep their eyes on the road and hands on the wheel

Overview

The technology selected for Cadillac's Night Vision program uses infrared imaging with an
uncooled staring-array detector for the sensor and an active matrix liquid crystal display IJIJD
(head up display). Raytheon Systems Co. developed the Night Vision sensor, while Delphi-
Delco Electronics developed the HUD for the Night Vision system.

The Cadillac Night Vision system is designed to help drivers see beyond the range of the headlamps. The system is not intended to be a drive-by system. A head-up display (HUD) projects a monochrome virtual image near the front bumper of the vehicle. That image is in the driver's peripheral vision. This allows the driver to view the normal road scene unobstructed by the virtual image, yet also look at the virtual image without refocusing or looking away from the road scene.

Having a HUD with the image at the lower end of the driver's sight line also helps the driver deal with headlamp glare from oncoming vehicles, since the driver tends naturally to look downward to avoid the worst of the glare and the sensor is unaffected by ambient light

The sensor utilizes Raytheon's uncooled infrared technology, which detects the thermal radiation of objects that are invisible to the human eye. All objects emit heat — humans, animals and running vehicles more so than surrounding objects. Thermal radiation is focused on to the detector using optics designed to pass infrared wavelengths. The energy is absorbed by a grid of detector elements, each of which responds with a change in capacitance. These changes are processed electronically to create a monochrome video image, in which hotter objects in the scene appear white.

The sensor is mounted in the grille, which is designed to provide an unobstructed view of the forward road scene.
This system is only operated at night, when the park lamps or headlamps are on and the photo cell indicates dark. The Night Vision switch has two main controls, a rocker and a slider. The rocker moves a mirror in the HUD, causing the virtual image to move up and down to the driver's desired position. The slider controls the virtual image intensity and has an "off" detent at the minimum position. The system has a 45-second warm-up time and will display a Night Vision logo until the sensor is ready. The logo is then replaced with the thermal image of the forward road scene.

Motivation for a Night Vision system

The motivation to develop an automobile Night Vision system is the desire to enhance a driver's field of vision beyond the headlights. The system is not intended to replace visual information obtained by looking through the windshield, but to provide additional information when it is dark or vision is otherwise obscured. For example, the driver may be temporarily blinded by the glare of oncoming headlamps, or may be unable to separate the shape of a deer from the dried brush at the edge of the road. In these and similar situations, Night Vision can help provide information that the driver needs and otherwise would not have.

Infrared: Seeing in the dark

In 1800, William Herschel, royal astronomer to King George III of England, discovered infrared radiation (IR). While measuring the temperatures of the various colors produced by passing sunlight through a prism, he discovered that heating occurred by rays he could not see. This part of the spectrum is called infrared, because these rays are below (infra) the frequency of red light.

The infrared spectrum begins at a wavelength of approximately 0.75 microns and runs to 1000 microns. Because the IR energy must pass through the atmosphere to get from the source to the detector, infrared imaging uses the ranges of BR energy that travels most easily through the atmosphere. This primarily involves two ranges: 3 to 5 microns, known as medium wave IR (MWIR); and 8 to 14 microns, or long wave BR (LWIR). In these two bands, IR photons are least absorbed by water molecules in the air.

Fortunately, much of the information that drivers need has peak radiation in these ranges. For example, human body temperature is 98.6 F/37 ^C; the spectral wavelength at which the maximum energy is emitted at this temperature is 9.3 microns.

Every object in the universe radiates IR if its temperature is above absolute zero. The amount of radiation depends on two things: the temperature of the object and its emissivity. Objects that are the same temperature can emit IR at different rates. Thermal images are produced by intercepting the radiation from various objects in space and focusing this radiation onto IR detectors.

In a staring-array detector, like that used by Cadillac Night Vision, the IR radiation is focused on a flat solid-state array that is 320 sensing elements wide by 240 sensing elements tall. Each element is a temperature-dependent capacitor that changes capacitance depending on how much IR it is receiving. A chopper rotates in front of the detector to modulate the scene's
energy. It rotates in phase with the detector read-out circuitry timing. The circuit under each element samples capacitance on a regular basis, and these readings are converted into a video signal. Each element becomes a pixel in the video image, giving a 320 x 240 pixel image.

The detector used on Cadillac's Night Vision system is a room-temperature or uncooled focal plane array (FPA) detector, which also has been used for military and commercial applications. The detector temperatures are generally stabilized with thermoelectric coolers (Peltier effect devices), but there is no attempt to achieve cryogenic temperatures. No mechanical scanner is required to serially trace out object space to produce an image. Instead, each pixel of the FPA detector "stares" out into space continuously, the scene energy is modulated, and the image is then produced by electronically scanning (reading out) the detector array.

Raytheon's uncooled system focuses the thermal radiation of a scene onto a 1-inch detector (a solid state array (320X240) of infrared sensing elements that images heat much like a CCD images light). Each pixel responds to the thermal energy radiated by objects in the scene. The energy is modulated by allowing the pixels to view the scene and than an absence of scene (a chopper is used to modulate the signals). Each pixel is read out at video rates and processed to create a monochromatic video image. The video image is then sent to the HUD for display to the customer.

Sensor installation

Some considerations for installing the sensor on the vehicle are the sensor size, temperature tolerance, mechanical tolerances (e.g., to vibration), dirt tolerance and vulnerability to stone and weather damage. For the 2000 DeVille, the sensor is placed in the center of the grille, where it is designed to have an unobstructed view of the road and where it can be kept reasonably clean by car washes, etc. The sensor is temperature controlled for peak performance and is equipped with a window to protect the optics as well as give a durable surface to clean.

Optics

The lens system uses refractive optics (lenses), similar to those used in visible range cameras that bend (refract) the BR rays from the object to the detector. However, to pass the flux in the IR range of interest, which is 8 to 12 um, typical optical glasses will not suffice, because they act as a filter that ceases to transmit electromagnetic flux at about 1 to 2 um The major advantage of a refractive system is that the sensor can be made relatively small.

Refractive optics were chosen for the 2000 DeVille because of their smaller package. The optics will be mechanically controlled internally to allow for the system to remain in focus over the automotive temperature range.

Detector assembly

The Raytheon uncooled focal plane array (UPPA) is based on the properties of ferroelectric materials. The ferroelectric material family chosen is barium-strontium-titanate (BST). This material and composition has a phase transition near room temperature where it is most sensitive to functions of temperature. An applied field across a capacitor having such properties will respond with a current flow for any change in temperature. This is how the BST detector
operates.

The detector consists of a BST reticulate structure bonded to a readout integrated circuit. Each reticulated section corresponds to a single detector pixel. The pixels are on 48.5 jim centers and are less than 20 jim thick.

Electronics

The circuit card assemblies perform all of the video processing, timing and controls needed to implement a thermal imaging system using the uncooled detector array. The uncooled detector provides analog single-line video that consists of two interlaced fields of video, which each field containing 240 lines. Each line of video contains thermal information for 320 consecutive pixels. The video processing circuitry receives the detector analog video, digitizes and processes it, and converts it back into an analog video signal.

Image polarity

The Night Vision system can present an image as "white hot" or "black hot" In "white hot" mode, hotter objects in the scene appear white. In "black hot" mode, hotter objects in the scene appear black. "White hot" mode was chosen for Night Vision so that objects the driver cares about most — such as people, animals and running vehicles — stand out from the black background of the night.

The display

The head-up display (HUD) is integrated into the dash in front of the driver. The HUD projects a virtual image near the front edge of the hood having a horizontal field of view of 11 degrees and a vertical field of view of 4 degrees. The image-to-object ratio remains at about one-to-one. Keeping the image near life-size helped the driver related objects in the HUD image with objects in the forward road scene. It also helps the driver judge the distance to an object.

A HUD helps allow the driver to maintain head and eye positions to view the road and the display simultaneously. The HiUD, producing a virtual image, has the advantage of providing an image that is at far-field focus and, therefore, does not require the driver to refocus his eyes as the eyes move to the display and returns to the driving scene.