## 1 Introduction  
Monodisperse latex particles have a curious history of being one of the first products made in space on the Space Shuttle missions [1]. The preparation of polystyrene (PS) microspheres of uniform size was developed over half a century ago, early in the 1950s [2]. However, in recent years, one can still find reports on new synthesis techniques of monodisperse, controlled-size, surface-functionalized, fluorescent PS microspheres [3; 4; 5]. This attention to PS microspheres is, on the one hand, due to their broad scope of applications, starting from well-known applications, namely, as calibration standards, for flow cytometry, for fluorescence microscopy, as support materials for biochemical analysis, and for drug delivery [6; 7], to more recently reported uses in the fabrication of new materials, such as Pickering emulsions [8] and capsules with nonhomogenous shells [9], and in the assembly of 1D granular structures [10]. On the other hand, possessing controllable size and ideal spherical shapes, PS microspheres themselves have been model systems for studying new physical mechanisms of particle-covered drops in electric fields [11], and they have been key experimental objects for many research works in the field of photonics. Interesting optical phenomena have been observed with microspheres, such as micro-lasers [12] and super focusing of light via photonic nanojets [13]. Periodically ordered structures of self-assembled PS microspheres have also attracted much attention. These structures could serve as ideal platforms to study the optical properties of 2D and 3D photonic crystals [3; 14; 15] and as a tool in a process called nanosphere lithography for the controlled manufacture of ordered metal-dielectric hybrid nanostructures [16; 17]. The sizes of the PS microspheres are one of the main tuning parameters in the experiments mentioned above. The diameters of micrometer-sized PS beads can be easily determined by an optical microscope using the array sizing method [18]. However, sophisticated equipment such as a scanning electron microscope (SEM), dynamic laser scattering (DLS) equipment, and nanoparticle tracking analysis (NTA) equipment [19] are needed to measure the diameters of PS sub-microspheres. It is tempting to exploit UV-vis spectroscopy, which is more common in labs and is routinely used to estimate the sizes of colloidal quantum dots (QDs) [20] and metallic nanoparticles [21; 22], for the size determination of PS sub-micrometer beads. In principle, the scattering extinction coefficient as a function of wavelength can be measured using a UV-vis spectrometer, and the corresponding scattering cross section (diameter) of the particles can then be determined. [PERSON] et al. systematically studied the wavelength, size, and angle dependences of light scattering of 20 nm to 85 \(\upmu\)m diameter PS microspheres in water. The experimental results were verified against [PERSON], [PERSON], and [PERSON]'s light scattering theories and were in good agreement with the theoretical predictions [23]. Optical transmission spectra in the 400-800 nm range for samples of sub-monolayers of 0.5 \(\upmu\)m to 1 \(\upmu\)m diameter PS microspheres reported by [PERSON]'s group were also dominated by light scattering and were microsphere-size-dependent [24]. However, it is not easy to retrieve the sizes of the microspheres from these spectra since diameters can only be determined through complicated curve fitting procedures in the framework of light scattering theories. The transmission spectra of 3D and 2D photonic crystals based on microspheres have been thoroughly studied both theoretically and experimentally [14; 25; 26; 27]. On the spectra, new, distinct features related to the photonic bandgap and optical modes appeared. The spectral peak positions of these features strongly depend on the sizes of the microspheres. In this paper, we demonstrate that this property could be employed to determine the dimensions of polystyrene sub-microspheres.