Sentence:  A <WSD>red</WSD> filter , Zeiss barrier filter with the code ( Schott ) designation BG 23 , was also used in the ocular lens assembly as it improved the contrast between specific and nonspecific fluorescence . 
Meaning of the word marked with <WSD> tags:  of a color at the end of the color spectrum (next to orange); resembling the color of blood or cherries or tomatoes or rubies