Absorbance is measured by spectrophotometer. There are ultraviolet visible spectrophotometer and visible spectrophotometer. There are many kinds and types. According to the structure, spectrophotometer can be divided into single beam and double beam.

A spectrophotometer usually consists of the following five basic components:
The light source, monochromator, sample cell, and detector signal display system
The common home-made 722N type visible spectrophotometer adopts grating self collimation dispersion system and single beam structure light path.
The function and performance of each component of spectrophotometer are briefly introduced.

1. Light Source
The role of light source is to emit a continuous spectrum at a specific wavelength range. The light tungsten lamp is usually used in the visible range of 6 ~ 12V. In the near ultraviolet region, hydrogen lamp or deuterium lamp is used as the light source, and its wavelength range is from 180 to 375nm. The light source should be stable, that is, the supply voltage remains stable. To do this, the power regulator is usually configured at once.

2. Monochromator
The monochromator is the decomposition of the continuous spectrum emitted by the light source device for monochromatic light, it is the core component of spectrophotometer. The dispersion ability of monochromator is stronger, the higher the resolution, the more pure monochromatic light. The monochromator dispersion element and an accessory. The common dispersion elements are prisms and gratings.
Prisms are usually made of glass or quartz materials, and different wavelengths of light have different refractive indices when passing through prisms. Thus, the composite light is decomposed into monochromatic light in wavelength order. The purity of monochromatic light depends on the dispersion ratio of the prism and the width of the exit slit. The glass prism has a larger chromatic dispersion of 400 to 1000nm wavelength and is suitable for visible spectrophotometer. The quartz prism is available in both the UV and visible range.
Gratings are commonly used in transmission gratings and reflection gratings, while reflection gratings are used more widely. According to the composite grating light dispersion for monochromatic light with different wavelengths of light interference and diffraction principle, and then the desired wavelength of light through the slit exposure to the absorption pool. As the advantages of grating monochromator for wide wavelength range, uniform dispersion, high resolution, convenient preservation; disadvantage is that there will be overlap at all levels of spectral interference, need to choose appropriate filter to remove other spectrum level.

3. Sample Pool
The sample pool, also known as the sample room, holds various types of absorption chambers (also known as cuvette) and corresponding pool attachments. There are two kinds of absorption cell: Quartz tank and glass tank. The quartz pool should be adopted in the ultraviolet region, and the glass is usually used in the visible area. Each instrument is usually equipped with a thickness of 0.5cm, 1.0cm, 2.0cm, 3.0cm and other specifications of the absorption tank for selection. Between the light absorption cell of the same specification of each other"s error rate should be less than 0.5%, in order to reduce the loss of incident light and reflection caused by optical path difference, should pay attention to the placement of the absorption cell, the transparent surface perpendicular to the beam direction. Fingerprints, oils, or other deposits on the vessel affect their transmission characteristics.

4. Detector
The function of the detector is to accept the transmitted light from the cuvette and convert it into electrical signals for measurement. When measuring absorbance, it is measured by converting the intensity of light into an electric current. This photoelectric converter is called a photoelectric detector. A general visible spectrophotometer usually uses a selenium photocell or a photoelectric tube as a detector. In addition, common detectors include photomultiplier tubes and photodiode array detectors. The photocell is a vacuum (or a small amount of inert gas) diode consisting of an anode and a photosensitive cathode. The cathode surface is coated with light-sensitive materials such as alkali metal or alkali metal oxide. When it is irradiated by photon with enough energy, it can emit electrons. When the two electrode asks for a potential difference, the emitted electrons flow to the cathode and produce an electric current. The magnitude of the current depends on the intensity of the irradiated light, about 2~25 mu A. As the photocell has very high internal resistance, the current produced is very easy to magnify. At present, the domestic photoelectric tube: UV sensitive phototube, cesium cathode, suitable for 200 ~ 625nm; red sensitive phototube, silver cesium cathode, for the wavelength of 625 to 1000nm.
The principle of a photomultiplier tube is similar to that of a photoelectric tube. It is modified by a photoelectric tube. There are several (usually 9) additional electrodes in the tube called the multiplying pole. Thus, a light excited current can be amplified, and a photon produces about 106~107 electrons. The applicable wavelength range is from 160 to 700nm. Photomultiplier tubes are widely used in modern spectrophotometer, and their sensitivity is 200 times higher than that of photoelectric tubes.
In recent years, optical multichannel detectors, such as photodiode array detectors, have been assembled into spectrophotometer. The photodiode array is a series of photodiodes arranged closely on crystalline silicon, such as the HP8452A diode array, consisting of 316 diodes in the range of 190 to 820nm. When light passes through crystalline silicon, the intensity of the electric signal produced by the diode is proportional to the intensity of the light. Each diode corresponds to the exit slit of a monochromator. The wavelength of the center distance of the two diodes is called the sampling interval. Therefore, in the diode array spectrophotometer, the higher the number of diodes, the higher the resolution. The HP8452A diode array, each diode in 0.1s were determined at 2nm intervals once, and at the same time using parallel data acquisition method, can be simultaneously measured 316 0.1s data, in a very short period of time, can obtain the optical spectra. The general spectrophotometer, once measured every 2nm, to obtain all optical spectrum within the range of 190 ~ 820nm, needs to be measured 316 times. If 1s is needed every time, 316s is needed to obtain the full spectrum. Therefore, fast spectrum acquisition by diode array instruments is a technical feature.

5. Signal Display System
The function of a signal display system is to display or record an amplified signal in the form of absorbance, A, or transmittance T.
A simple spectrophotometric meter display device is commonly used galvanometer, microammeters, digital display recorder. The modern spectrophotometer detection device, the photomultiplier tube output current signal conversion by A / D, collected by computer digital signal processing, get the absorbance A or transmittance T, and display the results on the screen.