Question

Explain how the following detectors of visible radiation work: (a) photomultiplier tube, (b) photodiode array, (c) charge coupled device, and (d) image intensified charge coupled device.

Short answer

(a)Photomultiplier tube is a photosensitive electron tube

(b)A photodiode is a p-n junction.

(c)In charge coupled device the image is projected onto the sensor by a lens and falls on the photoactive part composed of a network of photodiodes behind which are the capacitors.

(d) Image intensified charge coupled device amplifies incoming photons by prior to detection

Step-by-step solution

Step: 1 Definition of visible radiation:

Visual radiation, also called visible light or “light,” is a form of electromagnetic radiation.

Step: 2 The photomultiplier tube:

(a) Photomultiplier tube is a photosensitive electron tube in which light by an external photoelectric effect, as in a photocell, expels electrons from the photosensitive cathode between the cathode and the anode there is a series of electrodes, the so-called dynodes, each at an ever-increasing positive potential. Accelerated primary electrons hit the dynodes and each erupts several secondary electrons, so the electron current is multiplied many times over.

Step: 3 The photodiode array:

(b)

A photodiode is a p-n junction. When a photon of sufficient energy strikes a diode, it creates an electron-hole pair. This mechanism is also known as the internal photoelectric effect. If absorption occurs at the junctions of the depletion region or one diffuse length thereof, these carriers are removed from the junction by the embedded electric field of the depletion region. Thus the holes move towards the anode and also the electrons towards the cathode and a photocurrent is made. The full current through the photodiode is that the total of the dark current and also the photocurrent, therefore the dark current should be reduced to extend the sensitivity of the device.

Step: 4 The charge coupled device:

In charge coupled device the image is projected onto the sensor by a lens and falls on the photoactive part composed of a network of photodiodes behind which are the capacitors. The more light falls on the photodiode, the more it is charged accumulates in the accompanying capacitor. After the entire net was exposed to light, the control circuit causes each capacitor to transfer its charge content to its neighbor. The last capacitor in the series switches the charges to an amplifier that converts them into voltage. By repeating this process, the control circuit converts the contents of the entire network in a series of voltages.

Step: 5 The image intensified charge coupled device:

Image intensified charge coupled device amplifies incoming photons by prior to detection. Photons enters the exhausted device through a window at the left and strikes a cathode that's optimized for ultraviolet, visible, or infrared wavelengths. Once the cathode is irradiated by light, it emits electrons. Electrons from the cathode strike the microchannel plate. The potential becomes a lot of positive from left to right in every channel within the enlargement. A single electron hanging a channel causes many secondary electrons to be emitted. Emitted electrons are accelerated by the a lot of positive potential deeper into the channel. Every secondary electron strikes the wall additional into the channel and emits many a lot of electrons. A microchannel behaves sort of a photomultiplier tube with continuous walls rather than separate plates. Electrons rising from the microchannel plates strike a fluorescent screen that emits photons that are carried through optical fibers onto the pixels of the charge coupled device.