Where LEDs with high output power are used, specific heat management techniques such as water cooling or strobing should be considered to avoid temperature problems.
Compared to LEDs, gas vapour discharge illumination such as fluorescent lights are generally affected by the AC current used to drive them. This drastically affects any measurements that have exposure times equal to or shorter than the cycle time of the light, and flickering will occur at a rate proportional to the AC and the driving ballast. Although high frequency ballasts and tubes can reduce this, fluorescent lights are becoming less common for machine vision applications.
Unit life: As the vast majority of vision applications are designed for continuous 24 hour use, the lifetime of an illumination quickly becomes a major factor. Due to their size, cost and high intensity, fluorescent tubes are still used in selected applications such as large web inspection, but only have a lifetime of roughly 1000 hours compared with around 50,000 hours normally associated with LED technology. The long-term savings offered by LED illumination can be significant, even without considering the many other inherent benefits of this technology, despite the higher initial cost.
The future of illumination: LED lighting is still a developing technology with ever increasing intensities, improving efficiency and becoming cheaper. Ultraviolet LED lighting is such an expanding market, making applications both affordable as well as possible. The other technologies still have their place, but LED illumination is gaining much more importance.
To produce an image that best suits machine vision software, an illumination solution needs to be selected that reveals the particular defects or features for the camera. In the following section we group these features under five headings that will be explained in detail: