This information is based on the following published article:
Welsh J A, Horak P, Wilkinson J S, Ford V, Jones J C, Smith D C, Holloway J A, Englyst N A, FCMPASS software aids extracellular vesicle light scatter standardisation., Cytometry Part A, doi: 10.1002/cyto.a.23782, pdf.
Collection optics and light scatter
A core reason for needing side scatter calibration (read more here) is to normalize the variability in flow cytometer side scatter collection angles. One of the biggest factors leading to variability between flow cytometer collection angles is the flow cell geometry, and the numerical aperture of the collection lens, shown below.
The limiting system collection angle from these components can either be due to the internal flow cell dimensions, the external flow cell dimensions, or the collection lens numerical aperture. While some systems are limited by their internal flow cell dimensions, most system are limited by their external flow cell dimensions. These can be seen in the instrument preset examples below.