Thermal imager brings TV news out of the dark
Battle coverage during the 1991 Gulf War made television news history, exposing audiences to images from night-vision systems and thermal cameras, staples of modern combat. Aerial broadcast camera systems combining both daylight and infrared (IR) sensors now make those capabilities available to television news stations around the world. Using miniaturized electronics and powerful microprocessors, designers have developed charge-coupled-device (CCD) and IR cameras small enough to be housed in compact gimbals, some of which can be mounted on helicopters. Microprocessor-based controls incorporating "smart" software make the systems easier to operate. Logic circuits in gimbal control systems average the effect of sudden joystick movements, reducing the likelihood that the operator will overshoot the scene.
Last spring, aerial IR camera manufacturer FLIR Systems Inc. (FSI; Portland, OR) introduced a dual-sensor system targeting the electronic news-gathering market. The ULTRA 4000 camera system incorporates a high-resolution IR imager for low-light or no-light conditions, while an industry-standard broadcast camera is used for daylight news coverage. The cameras are mounted together in a compact, three-axis, gyro-stabilized gimbal, ensuring the kind of "jitter-free" image required by the broadcast industry and extending the value of the helicopters many stations use to cover news during daylight hours (see Fig.1).
Advanced IR camera
The visible-wavelength camera mounted in the ULTRA 4000 system is an industry-standard Sony Corp. (Tokyo, Japan) three-chi¥color CCD camera with a 16:1 zoom lens. A motorized 2X extender on the lens provides an effective magnification ratio of u¥to 32:1. The Sony camera body has been modified to fit within the 17.7-in.-diameter, 20.7-in.-high gimbal package, the smallest commercially available for a dual-camera broadcast-quality system.
The second camera, a thermal imager built by FLIR systems and also mounted in the gimbal, provides low-light/no-light coverage. Currently in use by military and law-enforcement agencies, SAFIRE is a digital camera operating over the 8- to 12-µm spectral range. It has a mercury cadmium telluride detector array maintained at 77 K by a miniature Stirling cooler. With thermal sensitivity of better than 0.1°C, the imager has a minimal resolvable temperature difference (MRTD) of less than 0.33°C, resulting in high-resolution images with visual definition comparable to black-and-white television (see Fig. 2).
A polygon mirror operating at 39,375 rpm horizontally scans the field of view (FOV) across the 4 ¥ 4 detector array, generating the standard 63.5-µs television line; a galvanometer vertically scans the scene to generate a standard RS170 or CCIR television image. Time-delay-integration (TDI) signal processing sums the outputs of the multiple detectors, improving the signal-to-noise ratio of the incident infrared energy. The detector signals are electronically delayed to coincide with the mechanical delay generated when physically separated detectors are mechanically scanned. Additional processing operations provide automatic gain and level and such options as electronic zoom and freeze frame.
The two fields of view, common to both the CCD and IR imagers, allow the airborne news teams to transmit both wide-angle and zoom images of a scene. The wide-angle setting provides a 28° horizontal FOV, corresponding to a spatial resolution of 0.93 mrad; the zoom setting horizontal FOV is 5°, yielding a system spatial resolution of 0.166 mrad. This results in an effective array of 525 ¥ 400 pixels.
Compact, stabilized mounting
The compact, 95-lb (43 kg) gimbal that houses the two cameras is a three-axis stabilized platform. Miniature gyroscopes, first developed for a missile program, take u¥very little space and flight payload while offering image stability of 50 µrad, or less than 0.001 of the screen width in jitter. Motion at this level is imperceptible to the human eye. The system is thus space-stabilized so that the image remains steady even in the face of straight-down image capture and 360° rotation.
Controllers for the CCD and IR imagers have been consolidated and incorporated in the gimbal rather than in the cockpit panel, eliminating wiring and freeing u¥cockpit space. The addition of the microprocessor to the hand-control panel allows the system to take out the "overshoot" and "jerk" that often occur with a standard or direct-control method. The joysticks that operate the daylight and IR sensors in these systems are virtually the same as those used in sophisticated television studio equipment, shortening the learning curve for camera operators.
The dynamic range of the image signals captured by the detector exceeds the dynamic range of the cockpit display. Automatic gain and level adjustments use this full signal range to generate clear, bright thermal images. By default, cockpit controls for gain and level (brightness and contrast) are placed in automatic mode at system initialization. The operator can, however, override the automatic gain and level to make manual adjustments.
Because the CCD and IR cameras can operate simultaneously, camera operators can toggle from one sensor to the other, reviewing image quality and selecting the source that best images events on the ground. With the addition of microwave downlinks, the broadcast signal can be sent to the station in real time. Feed from the digital cameras can be viewed at the television station as the operator in the helicopter sees the action, allowing station engineers to process and enhance the images as they are broadcast. In addition to live transmission, the camera operator back at the news scene can route the signals from the IR sensor and the daylight color camera to separate VCRs for simultaneous recording.
In addition to allowing news organizations to cover stories and events as they transition into night, the dual-camera sensor can enhance daylight stories by providing thermal information not available to conventional news cameras. For example, the thermal imager can clarify a story about a warehouse fire by showing details of the structure or the blaze itself that are hidden from visible-range cameras by the smoke.
While dual-sensor systems have been available for several years, this is the first year they have been tailored to fit the needs of network news broadcasters. NBC-affiliate KGW-TV (Portland, OR) was the first to use the ULTRA 4000 camera for news coverage. A night flight during winter floods in the Northwest last year convinced the news director that a dual-camera system was not only feasible, but necessary. At that time, KGW used an previous-generation IR imaging system to show its viewers the changing flood conditions around the city during the evening, in contrast to other stations whose reporters spoke to the cameras while surrounded by darkness.
Television news has become intensely competitive. Stations that meet the challenge will not only report on breaking news quickly but will present it in a visual format. Dual-camera broadcast systems now put 24-hour news coverage within the reach of any television station already flying a helicopter. o
FIGURE 2. High-resolution thermal sensors can provide images through darkness, smog, smoke, and dust--conditions that defeat ambient-light cameras. Dual-sensor systems have thermal sensitivity of less than 0.1°C, which results in visual definition similar to that of black-and-white television.