**10. Detector**

Whilst a radiation is passed via a pattern cellular, part of its miles being absorbed by means of the pattern solution and rest is being transmitted. The transmitted radiation falls on the detector and the intensity of absorbed radiation can be decided.

#### **10.1 Barrier layer or photovoltaic cell**

The barrier mobile includes a semiconductor, consisting of Selenium that is deposited on a sturdy steel base, inclusive of iron. A completely skinny sheet of silvery or aurelia is stammer ended the surface of the semiconductor to behave as collector electrode. The emission falling at the floor yield electron at the selenium silver interfaces. A barrier exists between the selenium and iron, which rule out the electrons against streaming into iron. The electrons are collected on the silver surfaces. The buildup of electrons on the silver surfaces produces an electric voltage distinction between the silver surfaces and the base of the mobile.

If the peripheral circuit secures a low resistance, a photocurrent will glide, that is precisely equivalent to the intensity of the incident radiation beam. It is holed directly to micrometre or galvanometer to read its output (**Figure 13**) [12].

**Figure 13.** *Barrier layer or photovoltaic cell.*

#### **10.2 Phototubes or photoemissive cell**

It consists of a high-sensitive cathode in the form of a half-cylinder of metal is contained in an evacuated tube. The inside surface of the photocell is coated with a light touchy layer. While the mild is incident upon a photocell, the floor coating emits electron. Those are attracted and amassed by an anode. The modern-day, that is created among the cathode and anode, is seemed as a measure of radiation falling at the detector. A phototubes is greater touchy than photovoltaic cellular due to the fact excessive diploma of amplification can be used (**Figure 14**) [13].

#### **10.3 Photomultiplier tubes**

A photomultiplier tube is a combination of a photodiode and an electronmultiplying amplifier. A photomultiplier tube consists of an evacuated tube that contains one photo-cathode and 9–16 electrodes referred to as dynodes. The surface of each dynode is Be-Cu, Cs-Sb.

While radiation falls on a metallic floor of a photocathode, it emits electrons. The electrons are attracted towards the primary dynode that is kept of a fine voltage. While the electron strikes the primary dynode which is saved at a wonderful voltage. Whilst the electron strikes the first dynode, extra electrons are emitted with the aid of the floor of the dynode; these emitted electrons are then attracted via a second dynode, wherein comparable sort of electron emission take area. The technique is repeated over all of the dynodes gift in the photomultiplier tube until a bath of electrons reaches the collector. The range of electrons accomplishing the collector is the degree of the depth of light falling at the detector (**Figure 15**).

#### **11. Recording system**

The signal from the detector is finally by the recording system. The recording is done by recorder pen.

**Figure 14.** *Phototubes or photoemissive cell.*

*UV-Visible Spectroscopy for Colorimetric Applications DOI: http://dx.doi.org/10.5772/intechopen.101165*

**Figure 15.** *Photomultiplier tubes.*
