The principle of camera flash

If you have read the article on the working principle of SLR cameras, you will know that it takes a lot of light to expose clear images on film. For most indoor photography (the surrounding light is relatively weak), it takes a long time to expose the film or increase the light intensity instantly to get a clear picture. For most subjects, it is not appropriate to increase the exposure time, because any rapid movement (including the movement of the camera itself) will blur the photo.

Electronic flash can solve the inherent problems of photography, and it is simple to use and cheap. Their only purpose is to emit instant light when you press the shutter, so as to light up the room when the film is exposed.

As introduced in the automatic camera, the basic camera flash system has three main parts.

A small battery used as a power source

A gas discharge tube that actually produces a flash.

A circuit (consisting of multiple electronic components) connecting a power supply and a discharge tube.

The two components at both ends of the system are very simple. When the two poles of the battery are connected to the circuit, the battery will force electrons to flow from one pole to the other through the circuit. Moving electrons (that is, current) provide energy for various components connected to the circuit (see how the battery works for details).

Discharge tubes are very similar to neon lights or fluorescent lights. It consists of a tube filled with xenon, with electrodes at both ends and a metal trigger plate in the middle. Its basic principle is to conduct current (moving free electrons) from one electrode to the other through xenon gas in the lamp tube. When free electrons move, they will charge xenon atoms, thus making them emit visible photons (see the principle of light for details).

Xenon in normal state can't conduct electricity, because there are few free electrons in it, that is, almost all electrons are combined with atoms, so there are almost no charged particles in xenon. In order to make xenon conductive, free electrons must be provided to the combination of electrons and atoms.

This is the role of the metal trigger plate. If a high positive voltage (electromotive force) is applied to the plate instantly, it will produce a strong attraction to the negative electrons in the atom. If gravity is strong enough, it will suck electrons out of atoms. This process of removing electrons from atoms is called ionization.

Since the free electrons are negatively charged, they will move from the negative electrode to the positive electrode after being in a free state. When electrons move, they will collide with other atoms, so that these atoms also lose electrons, thus ionizing xenon. Electrons moving at high speed will collide with xenon atoms, charging xenon atoms and generating light (see the working principle of fluorescent lamps for more information).

A relatively high voltage (electrical "pressure") is required to complete this process. It takes hundreds of volts to move electrons between two electrodes, and thousands of volts to generate free electrons enough to make xenon conductive.

Because the general camera battery only provides 1.5 volts, the voltage of the flash circuit needs to be greatly increased.