## 1 Introduction
High energy efficiency, instant lamp activation/de-activation and elimination of lamp warm-up times generate electricity savings of 45-60%. The lifetime of LED lamps is predicted to be 10,000-20,000 h, with a 5% drop in performance over this time. Compared with conventional mercury lamps, the operating time is around 500-2000 h, and the power reduction at the end of life is around 50% [17; 21; 33; 34]. UV LED technology's undoubted disadvantage is the coating's lower degree of cross-linking [35]. Finished product manufacturers seeking to increase production efficiency increasingly reduce curing times by increasing line speeds. However, this raises some problems with the cross-linking of paint products. Therefore, combined technologies are used in combination with conventional lamps. Lower line speeds were confirmed to provide a higher degree of curing [36]. Due to the efforts to maximize line efficiency, researchers are looking for suitable acrylate amine oligomers that allow the desired coating cross-linking to be achieved [37]. Intensive work is also underway on LED lamps with higher intensity and in the UV-B and UV-C spectrum [38]. The use of LED lamps involves the risk of failure of individual diodes. This, in turn, carries the risk of insufficient curing control. If the issue is not identified during manufacturing, the customer is exposed to residual monomers, photoinitiators and other harmful ingredients. The development of Industry 4.0 requires the definition of precise process requirements [39]. Thanks to the integrated sensors in the illuminator (e.g., bulb temperature, air temperature, UV intensity, humidity, air pressure), the Advanced Intelligent Monitoring System (AIMS) can transfer data to a cloud-based environment to co-create Industry 4.0. An intelligent IoT-enabled UV system provides real-time process performance data and allows analysis for predictive diagnostics [40]. One important aspect that determines the attractiveness of the finished products is their optical properties, such as colour and gloss. Many researchers have described the issue of obtaining the right gloss. After many years of the popularity of high-gloss coatings, malt surfaces are attracting increasing interest [41]. UV-cured coatings exhibit higher gloss values than other technologies [42]. New varnish formulations are being developed to provide a higher matt finish. Achieving good matting is about finding the right amount of matting agent, reactive ingredients and application and curing parameters. It has been proven that a high photoinitiator level, a high line speed and a high dose of UV light reduce the matting effect [41]. Roller application also produces a higher gloss than spray application [43]. Low substrate roughness, coating thickness and number of layers are all factors that affect the coatings' optical properties, and UV coatings turned yellow in aging tests [44,45]. Repeated reports in research about the lack of adequate coating curing with LED lamps and issues with optical properties at high line speeds provided the impetus to address the topic. Despite increasing pressure to implement LED-based UV curing technology, it still lacks clear coating curing procedures [16,46]. The challenge is determining the impact of technological parameter combinations that ensure adequate coating cross-linking and high final quality. The analysis of the available literature includes studies, only fragmentarily connected with the signalled subject matter. This is mainly due to the fact that achievements of this kind are covered by the highest confidentiality clause. Therefore, it was decided to study the optical properties of varnish coatings obtained in industrial conditions to control the parameters of the technological process of surface finishing. This paper aims to determine the impact of fibreboard (HDF) density, grinding parameters, the amount of basecoat and topcoat applied, the number of coats and different lamp powers on a combined industrial line using LED and mercury lamps at 50 m/min.