Perbandingan Hasil Pemeriksaan Pattern Erg (PERG) Pada Primary Open Angle Glaucoma (POAG) Derajat Early, Moderate, Dan Severe Dan Korelasinya Dengan Ketebalan Retinal Nerve Fiber Layer (RNFL) Dan Ganglion Cell – Inner Plexiform Layer (GCIPL) = Comparison of Pattern ERG (PERG) Result in Early, Moderate, and Severe Primary Open Angle Glaucoma (POAG) and The Correlation with Retinal Nerve Fiber Layer (RNFL) and Ganglion Cell - Inner Plexiform Layer (GCIPL) Thickness