Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo


Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_353



Photodetectors or photosensors are sensors that are used to transform incoming optical energy into electrical signals. A photodetector used in color applications typically covers the visible spectrum range (380–780 nm).


Figure 1 shows a typical photodetector. It includes three stages: (1) to generate electron–hole pairs or electrons in photosensitive material using an incident light; (2) to transform them through a bias provided by a power supply; and (3) to output the electrical signal directly or via an amplifier [1, 2, 3, 4, 5, 6, 7].
Photodetector, Fig. 1

A schematic diagram to show a photodetector (i.e., also a photodiode)

Photodiode and Photomultiplier Tube

Figure 1 shows a photodiode, a circuit including a p–n junction semiconductor and can output electrical signal. It can be seen that the photosensitive material includes a p–n junction, a boundary or interface between n-type and p-type semiconductors, and a depleted semiconductor region with a high electric field that can separate the electron–hole pair.

As incident light on a photocathode (photosensitive material is made of a multi-alkaline material, for example), it generates photoelectrons; next, the generated photoelectrons travel in a vacuum tube and hit the first dynode and generate secondary electrons. Then, the secondary electrons are multiplied by a series of dynodes and reach an anode to produce an electrical signal. As the electrical signal is multiplied many times and the whole process is performed in a vacuum tube to prevent electron absorption by the air, it is called a photomultiplier tube (PMT), as shown in Fig. 2.
Photodetector, Fig. 2

Aschematic diagram to show a photomultiplier tube. The incident light through the window hits a photocathode (photosensitive material is made by a multi-alkaline material, for example). It generates photoelectrons, which transmit in a vacuum, hit on the first dynode, and is then multiplied by a series of dynodes, finally reaching an anode to produce an amplified electrical signal. To prevent the absorption of photoelectrons and generated electrons into the air, a vacuum tube is used. Finally, the output electrical signal is amplified 104 to 105 times and within in a pico-ampere to micro-ampere current range

Photodiode Array and Charge-Coupled Device

As the discrete photodiodes form an array with an integrated circuit board, it is called a photodiode array.

As an integrated circuit contains an array of capacitors that store charge of electron–hole pairs created by photosensitive material, it is called a charge-coupled device (CCD).


In color applications, photodetectors are included in many color measuring instruments, which measure parameters such as luminance, illuminance or chromaticity. Sze [1] divided photodetectors into four categories: photoconductor, photodiode, avalanche photodiode, and phototransistor. The photodiode and the phototransistor are typically used in color applications.

An Si photodiode together with a filter to mimic the luminous efficiency function (i.e., V(λ) filter) becomes a V(λ) photodetector, which is used for measuring the luminance (in cd/m2 units). In a colorimeter, an Si photodiode combined with filters to mimic the color-matching functions of \( \overline{x} \), \( \overline{y} \), and \( \overline{z} \) are used to measure CIE tristimulus values (X, Y, and Z).

A PMT can detect dim light; it usually works with a monochromator. This arrangement can measure spectral information across the spectrum. The measured spectra multiplying the luminous efficiency function V(λ) can obtain luminance.

In color applications, a photodiode array is placed in an array image plane to form an image-based colorimeter. A CCD combined with the desired color filters measures an array of chromaticity values. Recently, in the display and imaging industry, CCD has been replaced by a complementary metal–oxide–semiconductor (CMOS) as an image-type photodetector because of its low cost. Figure 3 shows an image luminance measurement device (ILMD) [5] that applies a commercial color CCD camera or color CMOS camera to measure the image quality of displays. ILMD is also used for measuring luminaires of road and tunnel lighting, indoor measurements (uniformity, contrast, glare, etc.), near-field goniophotometer, etc. [6].
Photodetector, Fig. 3

The schematic diagram of an image luminance measuring device (ILMD). The color image is captured through an ILMD. It is then transformed into a luminance image through the analyzer, and the luminance distribution can be used for many applications, such as to calculate the glare index



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Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.Center for Measurement StandardsIndustrial Technology Research InstituteHsinchuTaiwan