achromatic interval

n. 1. Eyes are able to see light at a lower level than they can see the color of light. The achromatic interval is the difference between the brightness a light of a particular wavelength needs to have to be seen and the brightness it needs for a subject to be able to recognize the color of the light. 2. In hearing it is the difference between the minimal loudness at which sound can be detected and the loudness at which the tone of the sound can be recognized.