Q&A: Which is more important – irradiance or exposure?
They are both important, but the relative effectiveness depends on the physical chemistry of an ink, coating, adhesive or paint – as well as on the optical thickness of the UV-curable material and the properties required of the end application.
A higher flux rate, or irradiance, intitiates a higher rate of simultaneous radical generation, and consequently, more short polymer chains – usually characteristic of harder, less flexible and more resistant properties. Noting that exposure* is the time-integral of irradiance, a longer duration of lower irradiance may yield the same degree of exposure, but the resulting longer chains may exhibit different physical properties, such as flexibility or lower resistance. At increased depth within an optically thick material, a higher effective irradiance (greater than a minimum threshold) can affect adhesion.
A high “intensity” or peak irradiance will have a beneficial effect on the depth of cure of most UV-curable materials. The effective irradiance, or photon flux rate, at any depth within the film to be cured follows a definite relationship between irradiance at the surface and the spectral absorbance of the film (at any specific wavelength).
Most UV LEDs will have a “soft” irradiance profile, and the irradiance very close to the face window of the lamp has become an important primary measure of the “output” of the UV-LED products. Figure 3 is the characteristic irradiance profile of many UV-LED arrays.
The peak of irradiance of medium-pressure mercury UV lamps is affected by such factors as power input to the bulb, bulb diameter, lamp focus, type of reflector and distance from the lamp. The higher the peak of focus, the narrower it may be; and the time of exposure at the peak can be short, so the precision of measuring it becomes less important. The general or average irradiance may be important to curing, but most radiometers cannot measure that.