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Mercury-vapor HID (High-Intensity Discharge) lamps are known primarily for their characteristic light emission, which is concentrated in the blue-green light band. This is due to the specific spectral output of the mercury vapor when it is energized. The emitted light has a distinct spectral distribution that peaks in the blue-green region, making these lamps less favored in applications where color accuracy is essential because they do not provide a full spectrum of white light.

Other characteristics of mercury-vapor lamps, such as color rendition, efficacy, and lamp life, are notable but do not define them as uniquely as their concentration of blue-green light. While these lamps do exhibit some level of color rendering, it is not particularly good compared to other light sources. In terms of efficacy, while they can be efficient, there are more efficient HID and LED options available. Additionally, mercury-vapor lamps can have a relatively long lifespan compared to some other HID lamps, but they don't qualify under "very short" lamp life. Thus, the prominent defining feature among the provided choices is their emission concentrated in the blue-green light band.