Figure 2. Optical characteristics of the D-Light system (Karl Storz, Tuttlingen, Germany) for fluorescence detection. Blue excitation (380–440 nm) light (I) is blocked by an observation filter in the scope eyepiece (II). A strictly defined amount of blue light (approximately 2%) passes the filter (Δ). This allows color contrast and orientation in the fluorescence negative areas. The protoporphyrin IX spectrum is shown in (III) with the typical peak at 635 nm and a lower secondary emission peak in the far red around 700 nm.