Question

Explain the function of a vidicon camera tube.

Answer 1

Vidicon camera is a television camera which converts the light energy into electrical energy. If functions on the principle of photoconductivity, where the resistance of target material decreases, when exposed to light.
Construction : The Vidicon consists of a glass envelope with an optically flat face plate as shown in figure. A photosensitive, target plate is available on the inner side of the face plate. The target plate has two layers. To the front, facing the face plate, is a thin layer of tin oxide. This is transparent to light but electrically conductive. The other side of the target plate is coated with a semiconductor, photosensitive antimony tri sulphate. The tin oxide layer is connected to a power supply of $50V$.
Grid $1$ is the electron gun, consisting a cathode and a control grid. The emitted electrons are accelerated by grid $2$. The accelerated electrons are focused on the photo conductive layer by grid $3$. Vertical and horizontal deflecting coils, placed around the tube are used to deflect the electron beam for scanning the target.

Working : The light from a scene is focussed on the target. Light passes through the face plate and tin oxide, incident on the photo- conductive layer. Due to the variations in the light intensity of the scene, the resistance of the photo- conductive layer varies. The emitted electrons from antimony tri sulphate reach the positive tin oxide layer. So, each point on the photo- conductive layer acquires positive charge. Hence, a charge image that corresponds to the incident optical image is produced. As the electron beam from the gun is incident on the charge image, drop in voltage takes place. As a result, a varying current is produced. This current produces the video-signal output of the camera.