diff --git a/src/PhpSpreadsheet/Settings.php b/src/PhpSpreadsheet/Settings.php
index 7fc52cb9..90279db9 100644
--- a/src/PhpSpreadsheet/Settings.php
+++ b/src/PhpSpreadsheet/Settings.php
@@ -38,9 +38,9 @@ class Settings
     const CHART_RENDERER_JPGRAPH = 'jpgraph';
 
     /**    Optional PDF Rendering libraries */
-    const PDF_RENDERER_TCPDF  = 'tcPDF';
+    const PDF_RENDERER_TCPDF  = 'TcPDF';
     const PDF_RENDERER_DOMPDF = 'DomPDF';
-    const PDF_RENDERER_MPDF   = 'mPDF';
+    const PDF_RENDERER_MPDF   = 'MPDF';
 
 
     private static $chartRenderers = array(