The instrument used in ultraviolet-visible spectroscopy is called a UV/Vis spectrophotometer. It measures the intensity of light passing through a sample (I), and compares it to the intensity of light before it passes through the sample (Io). The ratio is called the transmittance, and is usually expressed as a percentage (%T):
The basic components of a spectrophotometer are:
• light source
• sample holder
• diffraction grating (optical component with a regular pattern, which splits light into several beams travelling in different directions) or monochromator (an optics device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input to separate the different wavelengths of light)
The radiation source is often a Tungsten filament (300-2500 nm), though a deuterium arc lamp (low-pressure gas-discharge light source) is often used when a continuous spectrum in the ultraviolet region is needed (190-400 nm). More recently, light emitting diodes (LED) and xenon arc lamps have been used for the visible wavelengths. The detector is typically a photodiode (a type of photodetector capable of converting light into either electric current or voltage, depending upon the mode of operation).
A spectrophotometer can be either single beam or double beam. In a single beam instrument, all of the light passes through the sample cell. In a double-beam instrument, the light is split into two beams before it reaches the sample. One beam is used as the reference; the other beam passes through the sample. A schematic for a double-beam instrument is shown below, followed by an explanation for how it works: