Non-luminous means not capable of producing light, but can be capable of reflecting light from another source, according to Dictionary.com. An example is the nearly invisible, bluish colored, non-luminous flame that is the hottest part of a full candle flame.
The almost transparent non-luminous starting point of a flame is surrounded by a yellowish-colored flame. The yellow part of the flame is considered luminous since it visibly shines brightly, radiates light and reflects light. It is the common symbol associated with fire. This flame is also called the safety flame as it is never as hot as the non-luminous flame.