  Silicon direct bonding offers flexibility in the design and development of Si
optics by allowing manufacturers to combine subcomponents with a potentially
lossless and mechanically stable interface. The bonding process presents
challenges in meeting the requirements for optical performance because air gaps
at the Si interface cause large Fresnel reflections. Even small (35 nm) gaps
reduce transmission through a direct bonded Si compound optic by 4% at $\lambda
= 1.25 \; \mu$m at normal incidence. We describe a bond inspection method that
makes use of precision slit spectroscopy to detect and measure gaps as small as
14 nm. Our method compares low finesse Fabry-P\'{e}rot models to high precision
measurements of transmission as a function of wavelength. We demonstrate the
validity of the approach by measuring bond gaps of known depths produced by
microlithography.
