SPEKTRA Volume 4 issue 3 contains 5 papers: 1) Analysis of Structure and Antimicrobial Activity of CeO₂ and Nd₂O₃ Nanoparticles; 2) Utilization of Coal Bottom Ash as Briquette Material; 3) Internet of Things of Electronic Seal Base on GPRS Short Message Service and Thingspeak; 4) Buried Waveguide Polymethylmethacrylate Modeling for Refractive Index Sensor Application using Finite Element Method; 5) The Application of Shift Method using Moving Laser on Cancer Cases in Head Area using 3DCRT and VMAT Technology.