During the past 25 years, China and South Korea have become much brighter on the DMSP images due to intense urbanization, while the radiance of Japan has stayed roughly the same. Already observed by Croft in the 1970s, the illuminated areas in the Sea of Japan are created by fishing vessels that use powerful lights to attract squid and octopus.
In terms of absolute radiance, China is unmatched by any other country in the region. If we adjust for country size, we get a different picture. Compared to Japan, Korea or Taiwan, China has a lower percentage of illuminated areas. The radiance levels, uncalibrated therefore dimensionless, are subject to noise due to weather and different satellite generations.