The invention relates to a system for measuring light transmission and/or light reflection properties of a transparent sample sheet (S), the system comprising a detection assembly and a control unit (10), wherein the detection assembly comprises an integrating sphere (1) having a sample port (2), an illumination port, a detection port, an internal light source (4) positioned at the illumination port, and a photodetector (3) coupled to a spectrometer and positioned at the detection port; means to detect radiation coming either directly from the sample port or from the wall of the integrating sphere or both directly from the sample port and from the wall of the integrating sphere, e.g. movable baffle (8); an external light source (5) or a transmittance detector axially aligned with the sample port; means to illuminate with the internal light source or with the external light source if present or with no light source; a reference standard (9), and means to position it at and from the sample port. This system is relatively compact, and can advantageously be used at existing sheet production lines for process and quality control. The invention also relates to a method for measuring light transmission and/or light reflection properties of a transparent sample sheet that applies said system; and to processes of making a sheet, especially an AR-coated glass sheet, comprising said method.