The present invention provides methods, apparatus, and systems of sensing a substrate in a chamber. The invention includes emitting radiation of at least two different wavelengths; directing the emitted radiation of a first wavelength through a view port of a chamber at an interior of the chamber; directing the emitted radiation of a second wavelength through the view port of the chamber at a location of a hole in a blade of a substrate carrier; detecting any of the emitted radiation reflected by the blade, the interior of the chamber, or a substrate on the blade; and determining if a substrate is present on the blade based on the reflected radiation detected. Numerous additional aspects are disclosed.