An objective lens assembly includes a first lens group configured to have a positive refractive power, the first lens group being positioned to receive visible light along an optical path extending therethrough. The objective lens assembly further includes a second lens group configured to have a negative refractive power, the second lens group being positioned along the optical path to receive the visible light from the first lens group. The objective lens assembly further includes a center lens disposed between the first lens group and the second lens group and an aperture stop centered along the optical path and positioned in front of the first lens group to direct visible light from a scene to the first lens group.

A method of in situ monitoring a thin film deposition process on a substrate, the method including a) defining a desired spectrum, the desired spectrum being a transmission or a reflection spectrum; b1) illuminating the substrate by means of a light source emitting light within the desired spectrum, b2) receiving light reflected from the substrate or transmitted through the substrate, b3) determining a transmission or a reflection spectrum out of the received light; c) defining a spectrum or a combination of spectra in dependency of the spectrum determined in step b3) to be a current spectrum; d) determining a weight spectrum as a function of the current spectrum; e) calculating a real number as a function of the current spectrum, of the desired spectrum and of the weight spectrum; f) exploiting the real number as indication for a deviation of the current spectrum from the desired spectrum.