Phototube
With a photocell or photocell, the intensity of light can be measured with a suitable wavelength. It consists of two electrodes in an evacuated glass bulb and is counted in a broader sense to the electron tubes. Due to the incident light, electrons are accelerated from the photocathode due to the external photoelectric effect. If these electrons hit the anode, they are derived and can be measured as a photocurrent. Otherwise they will be attracted to the cathode again.
The photocell was invented in 1893 by Hans Geitel and Julius Elster. It was largely replaced by semiconductor optical sensors. For the measurement of very low light levels the photocell has been further developed by integrating a secondary electron multiplier to the photomultiplier (PMT).
Demarcation: photoreceiver constructed from semiconductors are among the semiconductor detectors, it tends to be Photodiodes, photoresistors and solar cells - these are not referred to as photocells.
Construction
A photoelectric cell consists of two electrodes in a usually evacuated glass enclosure. The two electrodes differ in structure and arrangement:
- The cathode consists of a metal, electrons can be released from the through light, if the energy of the light is sufficiently large (outer photoelectric effect). For this reason it is called the photocathode. The work function may be reduced, for example by coating it with cesium, or a cesium compound to make the photocell for longer wavelengths of the visible spectrum.
- The anode is usually a wire ring, is not to be taken by the light. The anode is to collect electrons from the cathode triggered. So that they can no longer leave the wire ring, but if it is struck by light, it is made of a metal having a high work function such as particularly copper.
- When it comes to high current, the glass vessel can be filled with rarefied gas. It can lead to an avalanche effect by impact ionization, whereby the measurable current increases significantly.
Operation with the driving potential
Is between anode and cathode a voltage is applied, wherein the positive pole of the external voltage source is connected to the anode and the negative pole to the cathode, the released of the light electrons to the anode are accelerated toward, and an electric current ( photocurrent ) of few microamperes are measured.
- With a small voltage of a few volts, the photocurrent is approximately proportional to the applied voltage. At low voltages, the electric field strength between the cathode and anode is not enough to suck out all emitted electrons from the cathode through the anode, and thus can contribute to the photocurrent. The other "fall" back to the cathode. Reason: When an electron exits the cathode, the cathode is thereby charged positive. Opposite electric charge attract each other.
- At higher voltages, the photo current increases up to a limit value, it is called saturation. Then, all the electrons are removed by suction from the anode, which are released by the light from the photocathode. Upon further increase in the applied voltage above approximately 100 V, the current does not increase further. This setting is selected, for example, the photomultiplier, if you will prove extremely low light intensities and may be lost no electron.
Operation with reverse voltage
If no power source is connected to the photo cell and light enough to high frequency ( and therefore energy ) is illuminated, is formed between the anode and the cathode at a low exposure hardly reliable voltage of about one volt from. The photocell operates as a current source because some of the electrons from the photocathode caused to land on the anode, and can not return to the cathode. Therefore, the anode, the cathode loads on negative, positive. This photovoltage increases with the frequency of the incident light.
This mode can only be selected when the External Photoelectric Effect is to be presented, received the Nobel Prize for his explanation Albert Einstein.