A TV camera converts the light and dark of an image into minute charges of electricity. A camera carries a screen or mosaic behind its lens. It contains tiny dots of photosensitive material called caesium. Though caesium dots are so close that it looks continuous, in reality the dots are separated from each other. Each dot acts as a photo-electric cell. Whenever light falls on the dots, they release electrons or small charges of electricity. When a picture is shot, the picture is focused on the mosaic screen. The caesium dots release electrons, i.e. become charged with electricity.

The number of electrons ( or the amount of electricity generated ) depends on the strength of  the light falling on each caesium dot. A bright light releases more electrons and produces stronger electrical charge from each dot. A dull light releases fewer electrons and produces weaker electrical charge from each dot.

In this fashion, the lighter portion of the image being televised will release more electrons, while the darker portion will release less electrons. The variations in picture tone are translated into variations of electric charge being generated by different caesium dots. The whole picture that enters the camera resembles an identical picture made of varying electrical charges.