In plane spherical foil bearings, the effects of bearing penetration on the spacing distribution and the minimum spacing are experimentally investigated to clarify the air-lubrication phenomenon in rotary head recording devices. A technique has been developed to measure the spacing distribution with an accuracy 0.013 μm by converting monochromatic interference light intensity to spacing. The minimum spacings and spacing patterns measured by this technique are compared with the results obtained by numerical analysis. Good agreement is obtained when bearing penetration is less than 140 μm.