Photometer vs. Spectrometer

PHOTOMETERS

• Lower cost
• Smaller number of calibrations
• Fixed applications
• Photometers are “tuned” for a specific application
• They will not work for other applications that require different wavelengths unless modified
• Simpler chemistries
• Do not require in-house chemometrics (PLS) expertise

Photometers examine several narrow analytical wavelengths relative to a spectrally non-active reference wavelength. They are generally microprocessor-driven, compact analyzers with easy means to enter calibrations and scale the analog outputs to your DCS computer. They are best suited if your company has limited experience with online analyzers, has dedicated chemistries to monitor, with few components to be measured, or has limited in-house calibration expertise. The wavelengths are selected from a spectrophotometer analysis of your samples. A photometer is  “tuned” to your application. Thus, it is better suited to fixed production applications rather than variable R&D work or pilot testing. They are ideal in applications where the chemistry does not change over time, and only 2-3 parameters need to be measured. They have limited spectral processing capability, such as derivatives. They are well-suited for up to two sample points, if a limited number of calibrations are needed at each point. Their cost is typically half of a spectrometer.

SPECTROMETERS

• More expensive, but may be necessary
• Unlimited number of calibrations
• Complete spectral coverage accommodates new applications
• More complex chemistries
• Enhanced spectral processing (e.g. derivatives)
• Generally requires chemometrics (PLS) expertise within the company

Spectrometers are the workhorse of online UV/VIS/NIR process monitoring. They provide greater flexibility for more complex applications. They are operated by a separate computer, are more expensive and require more in-house expertise to calibrate. They can accommodate up to 12 sampling points, perform measurements of many parameters at each point, analyze finer detail in spectrally complex systems, and use more sophisticated spectral processing methods. They are the instrument of choice if the chemistry changes over time requiring calibration modifications.  They are often needed in pilot plants or R&D testing where diverse chemical systems are encountered.  They are the preferred instrument in a situation where photometers would be suitable at each sample point, but a number of different measurements are being made among numerous sample points.  In summary, spectrometers provide the “flexibility” over time, over multiple sample points or with greater complexity at each sample point than photometers.