diff --git a/css-color-hdr/Overview.bs b/css-color-hdr/Overview.bs
index 2358be1c38d..971743b4929 100644
--- a/css-color-hdr/Overview.bs
+++ b/css-color-hdr/Overview.bs
@@ -126,6 +126,9 @@ Predefined colorspaces for HDR: {#predefined-HDR}
 
 	The Perceptual Quantizer (PQ) electro-optical transfer function is used
 	[[SMPTE-ST-2084]],[[!Rec.2100]].
+	PQ assumes a reference viewing condition
+	where the scren surround
+	is at 5  cd/m².
 
 	ITU Reference 2100 is used for HDR 4k and 8k television.