What is the meaning of belt density in photography?

Generally speaking, "belt density" means that camera shooting and photography have a specific time period when shooting location. To be exact, it means that the weather is fine. When evening comes, the sky is still bright, but all the lights, including street lamps, are already on. This time is only 5 minutes in winter and 0/5 minutes in summer. In order to grab the "belt density", personnel and equipment must be prepared early. If used well, the picture is still beautiful. Because the spectral color temperature belongs to the intersection of the sun scene and the night scene, the spectral density is small during the day and large at night. For Nissin, as the name implies, it is called "belt density".