Fig. 2

Mean comparisons of \(D_{{{\text{HbO}}_{2} }}\)(a), \(D_{{{\text{Hb}}}}\)(b), \(ND{\text{HbO}}_{2} I\) (c), \(ND{\text{Hb}}I\) (d), \(t{\text{Hb}}\)(e), \(D_{{{\text{water}}}}\) (f) for every periodontal disease severity level. Analysis of variance of each spectral index was conducted. \(ND{\text{HbO}}_{2} I\), normalized difference index of oxyhemoglobin; \(ND{\text{Hb}}I\), normalized difference index of deoxyhemoglobin; \(t{\text{Hb}}\), \(D_{{{\text{Hb}}}}\), \(D_{{{\text{HbO}}_{2} }}\), \(D_{{{\text{water}}}}\) represent spectral absorption depths of total hemoglobin, deoxyhemoglobin, oxyhemoglobin and tissue water, respectively. “*” indicates significant differences from the healthy group at P < 0.05. Error bars indicate standard deviation