Characterized profiles of BLR-disk (σ₀ + σ₁) with one-arm structure (in blue) and differential phase curves (in orange). The panels are for different orientation and azimuthal angles. Dashed-lines represent emission from an homogeneous disk, and the solid lines from the disk with spiral-arm. In the calculations, we take the perturbations of surface density (σ₁/σ₀)_max = 0.2. The differences between the solid and dashed lines are provided as residuals with the corresponding colors. The differential phases can be measured by GRAVITY, and the residual to the homogenous disk can be also detected for the BLR with the present parameters. Less massive black holes can be measured by GRAVITY+ (the next generation of GRAVITY).