Sentence:  The composite filter system includes <WSD>topside</WSD> and backside light path with different filter[s]. The <WSD>topside</WSD> image of [the] weld pool is formed by the illumination from arc emission in the spectral window of 100-200 nm. 
Meaning of the word marked with <WSD> tags:  The side or part of something that is at the top.