UV QAs: LEDs and Radiometers; Non-Reciprocity and Design or QC; Radiachromic Films

By R.W. Stowe, Director of Applications Engineering, Heraeus Noblelight America LLC

Question: I plan to experiment with LEDs. Will I need a different radiometer from what I am using?

Answer: Yes. Medium-pressure (MP) mercury lamps emit UV over a very wide region of the UV spectrum. There are few instruments that can capture the emitted energy over the entire UV range.

TABLE 1. Radiometer response bands, with X representing the nm wavelength and Y representing the normalized response. UVC is red, UVB is green, UVA is purple and UVV is dark blue. Courtesy of EIT Instrument Markets, Sterling, VA

A radiometer uses a detector and filter combination to cover only a comparatively narrow band. These are typically designed to be sensitive within the UVC, UVB or UVA bands. (For UV curing, an additional long-wavelength band, UVV, is often added to the traditional UV bands). An instrument may include more than one detector-filter set in order to better cover the wavelength spread of medium-pressure lamps. Radiometer response in the UVC (red), UVB (green), UVA (purple) and UVV (dark blue) is illustrated in Table 1.

Today’s UV LEDs emit in narrow wavelength bands, typically centered at 365 nm, 385 nm, 395 nm or 405 nm. They are nearly monochromatic, having a wavelength spread within about 10 nm of their center wavelength. Comparing these wavelengths (with the exception of 365 nm) to the traditional radiometer bands, it is apparent that they fall between the traditional sensing bands. In fact, attempting to characterize LEDs with the same radiometer bands used for MP lamps can result in measurements that are very wrong.

Since “UVA” or “UVV” fails to describe the LED range, a designation, “UVA2” or “UVA2,” was invented. Again, different “LED” radiometers may have different upper and lower wavelength sensitivity limits. So, it is important to know the spectral response band of a radiometer and its relation to the emission wavelengths of the LED of interest.

Question: In your last column, you mentioned “non-reciprocity” in UV curing. How does that relate to design or QC of a system?

Answer: Reciprocity is a term often used in a photographic context. It refers to the inverse relationship between the intensity and duration of light that determines the reaction of light-sensitive material. This “inverse relationship” suggests a result will be the same if the intensity is increased (or decreased) in the same proportion that duration is decreased (or increased). When this principle is explored in UV curing, we quickly find that most UV-curable materials do not behave this way, hence the “non-reciprocity of UV-curable materials.”

Exposure (“dose”) in UV curing is the time-integral of irradiance. Irradiance and time (or speed) are independent variables and exposure is a combination of the two. Exposure, in J/cm² or mJ/cm², is commonly presented as a design specification of a curable material. This implies “reciprocity” or the expectation that higher speed combined with higher power will give the same result as lower speed and lower power. There are several factors that “interfere” with the simple reciprocity assumption:

  1. The complex character of the irradiance profile;
  2. The spectral opacity and absorbance of the curable film; and
  3. The effect of time of exposure on temperature.

Clearly something is missing – usually it is either the irradiance required (W/cm² or mW/cm²) or the irradiance profile delivered. Either of these is easily determined and used for design specification and a QC measure.

TABLE 2. Non-reciprocity when UV curing a typical coating, illustrated here as a heavy wood coating and the exposure required to achieve the same chemical resistance (MEK rubs) cured at different peak irradiance levels. The result is more likely to be a consequence of UVC – although proportional to UVA.

See Table 2, which is an illustration of non-reciprocity in UV curing a typical coating. This example of a heavy wood coating shows the exposure required to achieve the same chemical resistance (MEK rubs) cured at different peak irradiance levels. The result is more likely to be a consequence of UVC (although proportional to UVA). Many materials will show a difference in the total energy required to cure when exposed to higher or lower irradiance. The internal photochemical mechanisms can be complex, but the net result can be very clear. From opaque thick films to clear thin films, the relationship can vary from negative reciprocity to positive. Bench testing can reveal this.

Question: I can’t fit a radiometer through my UV system. Can I use radiachromic films, and what’s the best way to use them?

Answer: Yes, there are essentially two types of radiachromic films: Films or tabs whose surface is coated with a photochromic coating. Most commercial films of this type exhibit a change of hue with exposure, changing their optical density in a specific color range. Many of these have a PSA on the back. Films whose composition includes a photochromic component. These films are initially nearly transparent and change their transmission color or optical density with exposure.

Radiachromic films respond to exposure (J/cm² or mJ/cm²) only. They cannot “report” irradiance or any information on the irradiance profile. Because they can be attached to sheets or webs, they can do something that probes and paddle radiometers can’t – record the actual exposure of the process.

One method uses them in a strictly comparative way, to compare the color or density change to that of a film previously passed through the same (or identical) system. Be cautious of preprinted scales or values, because those were most likely produced by exposure to lamps of different spectra, irradiance and speed.

TABLE 3. Correlate the response of the film to the actual exposure of the subject UV system, using any integrating radiometer of choice – illustrated here as transfer radiometry.

Another effective method is to correlate the response of the film to actual exposure of the subject UV system, using any integrating radiometer of choice. Offline exposures of the film and integrating radiometer are made with the same lamp(s), typically on a conveyor at a succession of speeds. The change in color density is read before and after exposure with a reflection color densitometer – or a transmission densitometer for transparent films. A simple chart, illustrated in Table 3, provides interpretation of exposure in what we call “transfer radiometry.”

R.W. Stowe is director of applications engineering for Heraeus Noblelight America LLC. Do you have a question that you’d like to see addressed in this column? Submit your idea for consideration to [email protected].