Novel image-capture design doubles vertical resolution, controls illumination
Designed for still-image-capture applications with CCD cameras, an illumination system from Videology (Greenville, RI) uses a novel design to simultaneously double the vertical resolution of NTSC cameras and control lighting conditions. Used with any PC-based frame grabber and NTSC camera, the Tru-Flash can be remotely controlled over an RS232 serial port. For real-time image capture, the camera is set for automatic exposure, but when an image is captured, the camera exposure time is set to 1/1000 s and the lens opening stopped down to f/8. "Because the exposure only occurs during the flash." says Herb Wise, video engineer at Videology, "the effects of ambient light are reduced." The lens opening of f/8 further re duces ambient light effects and provides a large depth of focus.
Triggering of the flash is controlled by the frame grabber and the NTSC camera. To expose both fields of the video frame, the flash fires twice in 1/60 s, with two separate flash capacitors discharging in 1/60 s one before the other. "Typically." says Wise, "a video camera set to 1/1000-s exposure will start its exposure eight to 15 lines before the vertical sync pulse and finish its exposure during or slightly after the vertical sync pulse. By synchronizing the flash to the camera video, it can be triggered to operate at these times."
In the Tru-Flash design, this is accomplished by separating the sync pulses from the camera video signal and counting video lines in a system controller. When a trigger is received, the system controller waits for the next vertical sync pulse and then counts horizontal video lines. When the 1/1000-s exposure region is reached, the flash is fired. This is repeated for the next field and the flash is fired a second time. After the first firing, a capacitor remains charged. A silicon-controlled rectifier is then turned on during the next video field and a second capacitor is discharged. When the cycle is finished, the system controller allows a dc/dc converter to recharge both flash capacitors.
"To obtain equal exposures for both video fields," says Wise, "the power of the two flashes must be equal. It is also desirable to vary the flash power while keeping the two flash powers equal. This is accomplished by using a photodiode to measure the light emitted by the flash as it is firing. The output of the photodiode is integrated and compared to a voltage set by the flash-power potentiometer. When the integrated output of the photodiode equals the flash-power potentiometer setting, the system controller turns off the transistor and extinguishes the firing of the flash tube." This results in equal flash power for both video fields, independent of the flash capacitor or other component variations.
