Terahertz dielectric characterisation of fibres in a time-domain spectrometer

An innovative measurement setup for the dielectric characterisation of fibres in a terahertz time-domain spectrometer using an HDPE elliptical lens for coupling into the fibres has been built and validated by measurements of several different types of samples. The setup is based on a commercial all fibre-coupled terahertz time-domain spectrometer. Measurements of the effective refractive index have been conducted on polypropylene-based three-dimensional printing filaments, silica glass rods, and a polytetrafluoroethylene cord of lowered density, covering the frequency range of approximately 100 GHz to 1 THz. The theoretical part of the work includes numerical calculations performed via the finite difference eigenmode method and the characteristic equations of a uniform circular dielectric waveguide for a few guided modes, from which it is clear that primarily the fundamental mode propagates along the fibre. Details on model-based phase corrections, crucial to the accurate determination of the effective refractive index of dispersive fibres, have been presented as well

Testing Measurement Invariance in Physical Education and Exercise Science: A Tutorial Using the Well-Being Self-Efficacy Scale

In physical education and exercise science, it is common to examine mean differences between groups or to assess change across time. However, before group differences or change can be confidently examined, measurement invariance can be tested. Measurement invariance tests the equivalence of a construct across groups or across time. If measurement invariance is supported, then differences in latent means can more confidently be attributed to individuals' different standings on a construct. Though an important first step to confidently examine group differences and change across time, this technique is sometimes not used in the field. Thus, the purpose of this manuscript was to provide a didactic review and illustration of measurement invariance within the field. We review a methodological approach to measurement invariance, the sequential steps used in this approach, assessing model-data fit, and testing partial invariance. We provide an illustration of the technique and conclude with practical considerations