heliogram
Historical term for a message sent by reflecting sunlight, typically using a mirror or heliograph. Rare in modern contexts.
Heliograms were used in 19th-century military communication.
ヘリオグラフ is the device itself (heliograph), while 日光反射信号 refers to the message or signaling method.
Compound of 日光 (sunlight), 反射 (reflection), and 信号 (signal). Likely a calque from English 'heliogram' or related terms.