What is a detector?

Detectors are one of the most important parts of spectrophotometers. Here you can learn more about which detectors are used in spectrophotometry and how do they work.

The role of the detector is to convert light into a proportional electrical signal to which the spectrophotometer can respond.


CCD camera

One of the most popular detectors used in spectrophotometers is a CCD camera (charge coupled device). It is an integrated circuit composed of an array of coupled (photo) capacitors that allow the conversion of incoming photons into an electric charge. 

The capacitors comprise of millions of detector elements also called pixels.

The mechanism of the device is based on internal photoelectric effect, where photons with adequate energies hit the material which cause the transfer of charge that is then carried between the energy bands of the material. When the surface of the CCD is illuminated, the carriers are released and accumulated in capacitors. The carriers are displaced in the circuit which enables the measurement of the intensity of incoming light. 

CCD image sensors provide high-quality image data with low noise and high sensitivity. 


Photomultiplier Tube

The Photomultiplier Tube is also a very popular type of detection used especially within UV-Vis spectrophotometers. 

It is extremely sensitive to light in the UV, Vis and near-infrared ranges. 

The main part of the detector is a vacuum tube which consists of a photocathode and an anode. Incident photons with adequate energy strike the photocathode material and excite the electrons on its surface. Emitted electrons move towards the anode, which creates a current flown within the vacuum tube. Additionally, between the cathode and the anode an extra dynode is placed which works as an electron multiplier. Such an arrangement is able to amplify the tiny current emitted by the photocathode typically by a factor of one million.

These kind of detectors might get damaged when exposed to high intensity light. 


Diode Array Detector (Photodiode Array)

The Diode Array Detector (DAD), also know as Photodiode Array (PDA), is a multichannel detector capable of simultaneous acquisition across a wide range of wavelengths. Typically, DAD covers wavelength from 190 nm up to 800 nm.

DAD comprises of an array of silicon photodiodes on a single silicon chip. Usually around 500-1000 diodes are used for the detection. 

The white light is coming through the flow cell onto the diffraction grating, which splits the light into a single wavelength. The dispersed light is then detected by the multichannel array. 

The diode array detector is less sensitive than the photomultiplier tube but it enables a  simultaneous measurement of different wavelengths. 


Besides the main three detectors described above there are also the silicon photovoltaic, the single solid state silicon photodiode, the two photo diode detectors, the phototube and many more which operating principles and functionalities are based on similar mechanisms. 


1) https://www.chromatographyonline.com/view/how-does-it-work-part-iv-ultraviolet-detectors

2) https://lab-training.com/2015/06/20/characteristics-of-uv-vis-spectrophotometric-detectors/

3) https://en.wikipedia.org/wiki/Charge-coupled_device