Guide to UV Measurement
Introduction to Radiometric Instruments and Devices
When selecting radiometric instruments, it is important to select the proper instrument or device, or a combination of both, that is compatible with your process equipment. Another important determination is whether the instrument measures the proper exposure parameter(s). There are many instrument variations and choices. This section provides an overview of the general classification of radiometric instruments and devices.
Radiometers measure irradiance (usually watts/cm²) at a point, but over a uniquely defined wavelength band. Differences in detectors, filters, construction, and principles of operation result in the fact that different narrow-band radiometers give different results when measuring broad-band sources. A radiometer from one manufacturer can report significantly different UV data from another instrument from a different manufacturer. This is because instruments have different responsivity , or wavelength sensitivity . Also, instruments differ in their spatial sensitivity (angle of view), although most have diffusers to give them an approximate cosine response. As a practical matter, many users prefer to compare data from instruments only of the same type.
Profiling or “Mapping" Radiometers Some of the most dramatic adaptations of radiometers for UV processing are sampling radiometers with on-board memory. After a test exposure, the instrument is connected to a device -- either a computer or a dedicated processor -- to display the entire exposure profile. These instruments can also calculate peak irradiance and energy. Single-band and multiple-band instruments are available. Since these record the "history" of a pass under lamps, they can provide data on the irradiance profile of each lamp in rows of lamps. Relating the time scale to distance requires only the knowledge of the precise speed of the measurement.
Spectroradiometers are very narrow-band instruments, essentially responding to spectral irradiance, and are highly wavelength-specific -- some with resolution as fine as ½ nanometer. These instruments -- actually miniature monochromators -- can be valuable when there is a need to evaluate irradiance in a selected wavelength band of interest, but they don't measure time-integrated energy. Recent developments in these instruments include the ability to select a specific wavelength band for easier evaluation of the spectral distribution of a lamp output or spectral irradiance.
On-line Monitors On-line monitoring of the production process can use relatively simple sensors or more sophisticated instruments that record a number of key variables and compare them to pre-set limits. Situated in a fixed location, on-line sensors view the radiant energy from the lamp and/or the reflector, recording any change that may occur. The purpose for these fixed sensors or probes inserted into the same location may range from a simple comparison over time (used to determine, for example, when to change a bulb) to multiple-wavelength detectors and even spectroradiometers, located carefully to detect changes in reflectors or bulbs.
Dosimeters measure accumulated energy at a surface (watt-seconds/cm² or joules/cm²), also over some uniquely defined wavelength band. There are electronic and chemical types. Many electronic integrating radiometers will also calculate energy. Because this is the only measurement that incorporates time of exposure, it tends to be commonly used.
Radiochromic Films (dosimeters) are tabs that attach to a test surface and respond to total time-integrated energy by changing color or by changing optical density. Depending on the chemistry of the detector, it can change permanently or only temporarily. These photochromic detectors typically respond to a wide range of UV wavelengths. Tabs or tapes that are interpreted by eye or by comparison to a printed color chart are considered less accurate and less repeatable than films read by instruments (colorimeters or densitometers).