Image flicker reduction with fluorescent lighting

Details Image flicker reduction with fluorescent lighting Image flicker reduction with fluorescent lighting

1999-09-28

2001-08-07

Ho, Tuan (Department: 2612)

Television

Camera, system and detail

With object or scene illumination

C348S223100

Reexamination Certificate

active

06271884

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to digital imaging systems, and more particularly to a digital imaging system with reduced flicker caused by fluorescent lighting.
2. Description of the Related Art
Digital imagers have become commonplace in the last decade. For both video cameras and still cameras, semiconductor devices are commonly used to capture an image on a pixel-by-pixel basis and electronically process that image. Such devices as charge coupled devices (CCDs) and CMOS digital imagers have resulted in low cost video cameras, still cameras, and more recently cameras for coupling to computer systems for video conferencing and other image capture.
One problem pertaining to imaging systems generally and to digital imagers in particular is that of flicker. Flicker can arise from many sources, but in capturing digital video flicker especially results from a relationship between some periodic phenomena and the frame rate of the camera. Digital video cameras capture images on a frame-by-frame basis, typically at a predetermined frame rate. A common frame rate in the United States and in the computer industry is 30 Hz. But when such a frame rate is used in Europe, for example, flicker can result from fluorescent lighting systems employing the standard 50 Hz alternating current power. A 50 Hz lighting system yields periodic peaks of intensity at a rate of 100 Hz, or once every 10 milliseconds. Digital imaging systems often pick up “beats” associated with this 100 Hz intensity peak being captured at a 30 Hz rate. Beats can also arise from very slight differences in fundamental frequencies such as between 69.47 Hz video and 60 Hz lighting.
A number of solutions have been employed to eliminate these “beats.” These include filtering systems that filter out beat frequency, phase locking systems that attempt to lock on to the 100 Hz intensity peaks and synchronize frame capture, and a variety of other techniques.
SUMMARY OF THE INVENTION
A digital imager implementing the techniques according to the invention reduces flicker by setting an integration time for each pixel of the imager to an integral multiple of the period of lighting intensity variations. Digital imagers typically have a parameter known as integration time, which is simply the amount of time the electronic component of the pixel is allowed to capture light energy for each frame. By adjusting the integration time, the intensity of the image can be adjusted, enhancing images and preventing saturation at high intensities. In essence integration time can act as an electronic “iris.” By setting the integration time to be some integral multiple of an intensity period of a lighting source, however, flicker is reduced because the amount of light captured during an integration period is independent of where the integration period starts and ends relative to the variations in lighting intensity.
In one embodiment, an imager capturing video images at 30 frames a second (for a frame period of 33⅓ milliseconds) employs an integration time that is a multiple of 10 milliseconds, the period of the peak intensities of 50 Hz lighting. Thus, the amount of light captured during each integration will be essentially the same irrespective of where in the 50 Hz fluorescent lighting cycle the integration period begins.
Further features of various embodiments of the invention include detecting the period of the peak intensity and setting the integration time accordingly, and altering the integration time as part of a system to set the overall gain of a video camera. Further, the techniques can be implemented in a variety of cameras, including a computer coupled universal serial bus camera or a stand alone video camera. In the computer coupled system, overall gain can be controlled with a software device driver.


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