"This requires a lot of trouble-shooting"

(Image credit: Wammes & Partner GmbH)

LEDs are gradually replacing the old mercury UV sources used in curing during optical bonding. However, integrators often overlook the fact that testing equipment, tests, as well as the adhesive application device including irradiation time or intensity, need to be adjusted to the new spectrum of the LEDs. This sometimes very costly mistake is only recognized late – in the field.

"Many recent examples show that when replacing the UV source, it is simply a matter of replacing the old UV source, mercury vapor lamps of various colors, with LEDs. However, a straightforward swap of old for new does not work," explains Klaus Wammes, managing director of Wammes and Partner GmbH, who is currently receiving more and more troubleshooting inquiries regarding this issue.

According to the expert in display technology, switching UV sources to LEDs is logical and correct for several reasons. For example, LEDs are not dangerous to humans or the environment, unlike mercury vapor lamps. However, the replacement must be system-dependent and, above all, professionally planned: UV LEDs have a specific wavelength and no spectral distribution, and therefore do not offer the multitude of emission maxima that their mercury-based predecessors do.

In some cases, the chemical composition of the adhesive simply does not react to the LED light when combined incorrectly and does not cure at all. In other cases, various optical or mechanical artifacts become visible. These can manifest across the entire display or only in local shifts in color temperature, spots of different types, contrast changes, or alterations in the gamma curve. Mechanical issues, for example, appear as anomalies during shock and vibration tests.

Wammes: "The transition from mercury-based UV sources to UV LEDs in UV exposure systems currently creates a significant need for action. Most errors, however, were made many months ago. The noticeable or visible effects only appear much later – that is, at the end customer in the field – and only after many additional steps in the value chain! The latter scenario is particularly critical because it is no longer repairable, causing substantial problems and costs."