Radially resolved ¹³CO J = 6−5 to ¹³CO J = 2−1 line ratio in the LkCal5 (left; (combined) high and intermediate resolution data) and HD 169142 disk (right). The rings in the continuum and the radius where the ¹³CO J = 2−1 transition peaks are indicated with the vertical black lines. The horizontal dotted black line indicates the theoretical maximum line ratio if both lines are optically thick and emit from the same disk region. The beam is indicated with the horizontal bar in the top-right corner. The gray shaded region shows the 10 % absolute calibration error. The line ratio is not shown for radii where the ¹³CO J = 6−5 or the J = 2−1 integrated intensity is below 1.5σ. Furthermore, the intensity outside the MRS of the observations is shown in a lighter shade of the corresponding colors.