Optical Calibration Standards

Optical calibration standards are used to calibrate the optical properties of optical detectors and optical instruments. Photometric and radiometric light measurement applications use optical calibration standards for values such as illuminance or irradiance, luminance or radiance, luminous flux or radiant power, and luminous intensity or radiant intensity. Examples of optical calibration standards include light sources and lamps, light detectors, and reflectance plates. In the United States, certification laboratories such as the National Institute of Standards and Technology (NIST) define the primary optical calibration standards. Transfer standards are used to compare or transfer these primary values to secondary calibration facilities. Reference standards or working standards are then used to calibrate, test, and inspect optical components and instruments.

There are many different types of optical calibration standards. Categories include lamp calibration standards, spectroscopy standards, and photometer standards. There are two basic types of lamp calibration standards: calibrated irradiance sources and spectral calibration lamps. Calibrated irradiance sources are used to irradiate a sample with a known power density. Spectral calibration lamps produce narrow spectral lines and are used to calibrate monochromators, spectrographs, spectral radiometers, and other optical instruments. There are many types of spectroscopy standards. Examples include mass spectroscopy standards, alpha spectroscopy standards, and X-ray spectroscopy standards. Atomic absorption spectroscopy standards are available for elements such as aluminum, barium and cobalt in various sources (matrices) and sizes.

Selecting lamp calibration standards requires an analysis of product specifications such as lamp type, lamp wattage, lamp current, approximate luminous intensity, and radiating area. Choices for lamp type include cesium (Cs), helium (He), mercury-cadmium (Hg-Cd), sodium (Na), neon (Ne), thallium (TI), zinc (Zn), and mercury (Hg). Additional parameters include mounting style and line power supply. Other types of optical calibration standards carry different specifications.

Some suppliers offer polymer or polymer-based optical calibration standards. Polymer standards were designed originally for use with turbidimeters in water quality and water treatment applications; however, they are now used to calibrate both light-scattering and light-absorbing optical instruments. Small bead sizes and Brownian motion enable submicron polymer spheres in ultra-pure water to remain in a homogeneous suspension. When used with turbimeters, polymer-based optical calibration standards may be suitable for biomedical, pharmaceutical, petrochemical, semiconductor, food and beverage, and pulp and paper processing applications.

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