The present invention relates to a television camera including an optical system, a photoelectric conversion element and an automatic iris control portion and, particularly, to a television camera capable of restricting flicker noise generated under a flickering light source such as fluorescent lamp and of obtaining an optimum imaging conditions even under illumination of light source other than fluorescent lamp.
In a case where an object illuminated with, for example, a fluorescent lamp is to be imaged by means of a television camera, it has been known that flicker noise is generated due to beat of field frequency of the television camera and frequency-dependent variation of amount of fluorescent light. In NTSC system, for example, the field period is 1/60 sec and flashing period of fluorescent lamp in an area in which commercial power source frequency is, for example, 50 Hz, is typically 1/100 sec. Therefore, when, in such area, an object under illumination of fluorescent lamp is imaged by a television camera, flicker noise having a period of 1/20 sec which is the least common multiple of the both numerical values, that is, having frequency of 20 Hz, is generated. The reason for generation of such flicker noise is discussed in, for example, Japanese Kokai (P) 1-204578 in which a method of reducing the effect of this flicker noise has been proposed. According to this method, flicker noise can be restricted by detecting a flickering period of a fluorescent light source by a flickering period detection circuit of a television camera and controlling a signal storage time of a photoelectric conversion element of the camera on the basis of the detected signal such that the signal storage time becomes an integer multiple of flickering period of the florescent light source.
In this conventional technique, however, there is no consideration made with respect to variation of illumination condition during imaging operation, and therefore, there is undesired sensitivity degradation of the photoelectric conversion element occurred when a light source is switched from a flickering light source such as fluorescent lamp to a non-flickering light source such as incandiscent lamp, or vice versa, during imaging operation. For example, in order to restrict flicker noise when an imaging is being performed with a television camera of NTSC system under illumination of fluorescent lamp in an area in which the flickering period of fluorescent lamp is 1/100 sec, it is necessary to regulate the signal storage time of photoelectric conversion element to 1/100 sec in which case the sensitivity of the photoelectric conversion element is lowered by about 4 dB compared with the imaging with image storage time of the photoelectric conversion element being 1/60 sec for normal non-flickering illumination.
In order to improve an image quality during imaging under fluorescent lamp illumination, it is better to restrict flicker noise while allowing degradation of sensitivity of the photoelectric conversion element. However, when the fluorescent lamp is switched to nonflickering lamp such as incandiscent lamp during imaging, the sensitivity may be degraded by about 4 dB since signal storage time of 1/100 sec is kept as it is.