Dark-lanes as a diagnostic for bar length measurement. Panel a: face-on surface brightness distribution at t = 9 Gyr. The solid black lines denote the contours of constant surface brightness. Here, a conversion of 1 arcsec = 1 kpc and a magnitude zero-point (m₀) of 22.5 mag arcsec⁻² are used to create the surface brightness map from the intrinsic particle distribution. The red and blue lines denote the bar’s major and minor axis, respectively. Panel b: corresponding light profile along the bar major and minor axis (red and blue dashed lines, respectively). The radial location where the light deficit around the bar reaches its maximum is indicated by the maroon arrow. The vertical magenta line denote the bar length, R_bar, 1. A solid square represents a single pixel of the face-on surface brightness map. Panel c: distribution of the maximum light deficit around the bar with the bar length, R_bar, 1 (obtained from the constancy of the m = 2 phase-angle). A straight line of the form Y = AX + B is fitted separately (cyan and magenta dashed lines) to points, before and after t = 5 Gyr, and the corresponding best-fit values are quoted. For details, see the text. The Pearson correlations are computed, and shown in the top left corner. Panel d: temporal evolution of R_bar, 1 (bar length) and R_{dark − lane} for the fiducial bar model. For further details, see the text.