How to Measure?





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A radiometer (and profiling radiometer ) is a device that measures the intensity and or energy of UV/light striking its detector/optical stack.

In UV curing applications a radiometer is used to:

The parameters to be monitored are the irradiance (W/cm²), and radiant energy density (J/cm²) for the bandwidth of the instrument. Profiling radiometers can also provide irradiance profiles, which are a display of the UV as a function of time.

Irradiance is a measure of the UV arriving at a unit area (square centimeter). Radiometers try to capture this measurement at the cure surface of the item (substrate) being cured. The typical measurement unit is watts (or milliwatts) per square centimeter, (m)W/cm² . This measurement provides a comparative number for insuring process control. The actual irradiance value for each process will be determined by the application.

The irradiance is not a measure of the applied electrical power to the system. For some wide systems this could be as high as 20,000 watts. (400 watt per inch system, 50 inches wide.)

The irradiance is not a measure of the total lamp output. A lamp outputs radiation in the UV, visible and IR regions of the electromagnetic spectrum. The total output from a lamp in the UVA region may be over 100W. The amount of UV irradiance arriving at the cure surface varies based on the system type (arc, microwave) and the design (focus, reflector, distance) of the system. Typical irradiance values range from 0.100-10+ W/cm² in the UVA range. Each point on the sample is getting significantly less UV than the total 100W of UVA generated by the entire lamp.

Radiant Energy Density is the total amount of energy that strikes the material being cured. The measurement is expressed in energy per area units, or Joules (or millijoules) per square centimeter, (m)J/cm². The radiant energy density value is dependent upon the total irradiance, shape of the irradiance profile and the duration of exposure. One watt times one second equals one joule.

For exposures made with a constant irradiance (a film or plate exposure system with a non moving part and a shuttered system), the energy density can be estimated from the exposure time and known irradiance value.

Most UV curing takes place with either the product or lamps moving. In this case the substrate sees a low level of UV, followed by a ramp up, peak intensity area and then a ramp down of the UV. The radiometer takes multiple samples and integrates each sample to determine the total energy density for this type of exposure.

If provided by their ink, coating, resin or adhesive formulator, the suggested irradiance and energy density values needed to cure a particular product are a good starting point. Additional tests should be done for your specific equipment and process parameters before moving to production. The additional tests should identify and narrow down the range of process conditions and parameters in which you can operate. This is also known as your process window. Experience with your process and equipment mixed with your radiometer measurements will provide you with information needed to keep your equipment running. Use these numbers to determine if the energy and irradiance are intense enough to penetrate the UV coating and expose the coating on the substrate for sufficient time to be photo chemically changed, or cured.

In a spot curing radiometer, the light is usually transmitted via a light guide (liquid or fiber) to the point of exposure, and the total irradiance, or intensity of the curing system is measured in units of watts (or milliwatts) per square centimeter, (m)W/cm² .

How they measure

A radiometer is placed in the same position as the material that is being cured. The optical components or stack selectively removes, interferes, segregates or ‘filters' all radiation (UV, visible light, IR) arriving at the surface of the instrument and allows only the UV band(s) of choice for the product to strike the detector.

In the following discussion the term sensor refers to the complete assembly of diffuser filter and detector.

In both radiometers and profiling radiometers, the basic components are the same. Instruments are designed to take into account radiation coming from all directions (link to cosine response) within the curing system. A diffuser is most often used for this. The diffuser can be made from many materials-translucent Teflon, frosted quartz, or an integrating sphere. The diffuser is sometimes referred to as a cosine receptor. view a Cosine Diagram

The filter passes only a specific range of UV wavelengths. The type of filter employed is determined by the wavelengths that actually cause the cure. The UV spectrum has been classified into three regions, UVA, UVB, and UVC. For most applications, the activation wavelengths are well defined and specifying UVA, UVB, or UVC when purchasing a radiometer will be sufficient to assure that the radiometer is predominantly measuring the wavelengths of interest. Some manufacturers have bands that are on the border of the UV-visible range. Some instruments also have the ability to look at more that one UV band at the same time. link to Multi-band Instruments

Once the UV is conditioned to the bandwidth(s) of interest, it strikes a detector(s). The detector responds to the UV from the UV source. A photon (light particle) of certain minimum energy strikes it and an electron is freed from bonds in the detector material, entering an electrical circuit. The phenomenon is known as the photoelectric effect. view a Photodiode-diagram Photons with insufficient energy to knock off electrons will not produce an electrical current no matter how intense the UV (light). The energy is indirectly related to the wavelength, i.e. the shorter the wavelength the higher the energy. Once this threshold (minimum) energy level is surpassed, there is a direct relationship between the number of electrons released and the intensity of the UV (light). The more intense the source, the more electrons released and the higher the current that passes through the circuit to the electronics of the instrument.

The electronics may further condition the signal and allow the instrument to display (or transfer) the values in radiometric units to the user.

Just as there are many different applications and ways to move materials in, under and past UV sources, there are many different shapes of radiometers, detectors and sensors. link to Measurement Strategies

Conveyor based configurations 


Web based configuration


Spot curing configurations  


Profiling radiometers


A radiometer has a dynamic range over which its measurements are linearly related to the current. The UV (light) intensity could be so low that it cannot be distinguished from the electronic noise of the meter. The UV could also be too intense for the instrument and surpass the detector's ability to release enough electrons. This is called saturation. It is for that reason that the dynamic range is an important factor when considering a radiometer.

If the maximum signal measurable by a particular radiometer is 2W/cm² and your UV lamp output exceeds that range, you will never know the real output of the system. The radiometer will always read 2W/cm² until the lamp degrades to some value below that. Some manufacturers offer radiometers with different dynamic ranges to match your UV system.

There is a wide range of detector materials and construction methods that interact with light used to produce detectors. For Ucuring applications, detectors are chosen that are useful in the UV range, but can eliminate unwanted wavelengths such as infrared (IR) and visible light. This selection is important in that the UV lamps also put out a great deal of visible light and a significantly higher level of IR.

The correct radiometer configuration is based upon the published sensitivity curves for the detector/filter combination supplied by the radiometer manufacturer, the lamp spectra, and chemistry used in your process. The following is a diagram showing the interrelationship between these parameters.