We use a surface treatment allowing to obtain planar anchoring, azimuthally degenerated, by deposition of a silane coupling agent (GLYMO) on an isotropic substrate (bare glass plate). This technique of "passivation" prevents the liquid crystal adsorption on the substrate and avoids memory effects. We present a method of measure of the spontaneous geometric parameters for chiral mesophases by simple optical observations, using a cell with variable thickness and azimuthally degenerated planar anchoring on one of the plates. The cholesteric and the TGB phases were studied for two: 12F(2)BTFO(1)M(7)(TGB(c)) and 10F(2)BTFO(1)M(7) (TGB(A)). The cell allows to observe the slabs, even for the TGB(A) phase, and to measure independently their thickness and twist. Our results are in agreement with previous X-ray measurements.