Types of UV Radiometers Available
There are several radiometer models on the market that provide various types of information. A radiometer should be chosen to fit the application and the information required. Functions range from simple intensity and simple dosage to sophisticated mapping devices. Which wavelength to measure is also a matter of choice. Over 95% of the UV radiometers EIT sells measure in the UVA spectrum (320nm-390nm). This is because the majority of the chemistry responds to this band of energy. If it is desirable to measure other parts of the UV spectrum, a radiometer is available to do so. Certain chemistry is formulated to respond to different wavelengths of light and for different applications. Radiometers are also configured for use in different applications.
In UV spot curing, a high intensity UV source is channeled through a liquid light guide in order to control a very high intensity light and focus it onto a relatively small area. UV spot curing is used for many adhesive applications for example, attaching hypodermic needles to the plastic injector. It is important to monitor the output from these systems as the lamp, reflector and light guide deteriorate with use. A radiometer, configured like the SpotCure Intensity Meter, measures the UV intensity from the liquid light guide and can be used to determine when the output has dropped below a usable level. The radiometer can also be used to optimize the positioning of the light guide. Each time the light guide is bent or twisted some UV output is lost. By measuring the UV output with a radiometer, light guide position can be adjusted until the maximum intensity is realized from the system).
Most radiometry typically involves measuring UV dosage in mJ/cm2. More sophisticated measurements involve measuring peak UV intensity in addition to dosage. Dosage is only a measurement of total energy, while the intensity at which the energy was delivered has profound effects on the cure characteristics of the finished product. To offer an extreme example, one would get very different cure characteristics by exposing a workpiece to 500mJ/cm2 of UVA light under a 300-W/in. mercury vapor lamp versus laying the workpiece outside in the sun for a period of time which would produce 500mJ/cm2 (about 3 min.). UVA in this example is primarily 365nm wavelength light. In the curing system, the energy equation would be:
| Dosage Energy |
= UV Intensity X Time |
| = 250 mW/cm2X 2 s |
| = 500 mJ/cm2 |
By contrast, the daylight exposure equation might look something like the following:
| Dosage Energy |
= UV Intensity X Time |
| = 2.5 mW/cm2 X 200 s |
| = 500 mJ/cm2 |
In each case the workpiece was exposed to 500 mJ/cm2 of UVA energy, but the cure properties of each workpiece will be substantially different.
A radiometer like the UV Power Puck can measure both intensity and dosage of multiple wavelength bands, UVA, UVB, UVC, and UVV. This information reveals how the UV was delivered and at what wavelength.
Often the user is interested in the spectral content of the UV source being used. Since different chemistry is affected by different UV wavelengths, it is desirable to optimize the curing process by matching UV lamps to the chemistry of the application. Also, the spectral content of the UV source may shift as it ages with the shorter wavelengths moving toward the longer end of the spectrum. The spectral reflectivity of the irradiator often changes over time as well. UV lamp manufacturers and end users alike are interested in this phenomenon.
It is possible to fully analyze the entire curing system using a mapping device called a UVIMAP. By mapping the curing system, you can determine the irradiance of each lamp in the curing system, peak intensity, dosage, focus, and reflector efficiency. Very simple to use, the UVIMAP provides all the vital information necessary to completely characterize a UV curing system for a given spectral bandpass.
The unit is passed through the curing system. It measures and stores the UV intensity and temperature data it encounters inside the curing system. The unit is then attached to a printer included with the unit or loaded into a personal computer. The data is summarized and the unit prints the total dosage, peak intensity, average intensity, peak temperature, sample rate, number of samples, collection mode and internal temperature. It will also print out when the battery level is low. In addition, the printout actually graphs the UV and temperature conditions inside the curing system. By analyzing the graph, the condition of each UV lamp can be determined and compared to other lamps in the system. The shape of the curve is quite revealing.
Typically, a fresh lamp that is properly focused has a very sharp peak with uniform slope on either side. As the lamp ages and the reflector degrades, the shape of the curve changes. The peak is not as sharp and takes on a rounded appearance. The slope of the sides may no longer be symmetrical as the reflector does not always degrade uniformly.
If the curve exhibits a double peak, then the lamp is not in focus. The lamp could be either too high or too low as the trace looks the same. If one side of the double peak is higher than the other, the reflector may be less efficient on one side than the other or the reflector may be tilted off center.
With a standard radiometer that measures only dosage, the detailed condition of the UV source cannot be determined. The dosage may have dropped off but it is not easy to ascertain why it happened. Particularly in a system with multiple lamps, measuring the dosage does not tell you if all lamps degraded equally or if there is a problem with just one lamp. Even a radiometer that gives you peak intensity does not tell you which lamp had the highest output. The only way to get such information is to "map" the curing system.
