Q&A: Are there different ways to use radiachromic films?

By R.W. Stowe, UV applications engineering consultant, Heraeus Noblelight America LLC

Since discussing the use of radiachromic films in UV+EB Technology,¹ we have received a few questions on various methods of using them.

There are essentially two types of radiachromic films, but the easiest to use are 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 pressure-sensitive adhesive (PSA) on the back, making them easy to apply and remove from flat, linear curing processes as well as dynamic multidimensional processes or static applications. (Films whose composition includes a photochromic component – and are initially nearly transparent – change their optical density with exposure and are more difficult to use, requiring a color transmission densitometer.)

Radiachromic films respond to exposure (or “dose”) (J/cm² or mJ/cm²) only. They cannot “report” irradiance or any information regarding the irradiance profile. Because they can be attached to sheets, webs or solid objects, they can do something that probes and paddle radiometers can’t: record the actual exposure of the process. Reading and interpreting the results is where methods differ.

How are they correlated to exposure? There are essentially two different principles:

1. Correlated to an integrating radiometer measurement, exposed to the same or an identical UV source, using a color densitometer (Figure 1). This yields a quantitative measure (mJ/cm² in a selected UV wavelength band) of exposure with a radiometer of choice. A chart of measurements vs. exposure conditions (Figure 2) provides surprisingly accurate transfer radiometry. Correlations with other bulbs or other radiometers will yield different curves. Correction is the subtraction of the optical density value of the unexposed film.
2. Non-correlated, visually comparing the color density change to a reference. How the reference is created is the issue with this principle.
   Method 1: Visual comparison to reference color charts (actual film or printed color charts). Errors can result from differences in the reference exposure and the exposure of interest. For example, UV spectral distribution (bulb type), power (irradiance), radiometer model and UV band used to create the reference will affect the film differently. Color charts² that correspond to a few selected exposure parameters and radiometers are available.
   Method 2: Visual comparison with successful UV exposure/cure process, including all exposure and configuration parameters, may be applied as an indicator to warn if any of the parameters have changed. This can be useful in a UV system in which lamps may deteriorate, or speed or time may fluctuate.

One of the most effective uses of films is in configuring exposure of 3D or solid object UV curing systems (Figure 3). They can be used at virtually any surface location to compare exposure and identify areas of potential over- or under-exposure, guiding the positioning and orientation of lamps or the motion of parts. They can be used in either correlated or non-correlated mode. This facilitates the optimization of a system or exposure to a complex part before any wet coating is applied.

References

1. UV+EB Technology; Vol. 2, Issue 2, 2015.
2. UV FastCheck Strips and UV Intensity Labels; Manufactured by Spectra Group Ltd., Millbury, OH, [email protected]

Note: “Radiachromic” and “radiochromic” have the same meaning, although the latter is used dominantly relating to x-ray and high energy dosimeters, the former being more attached to UV technology.