This document addresses how the spectral power distribution (SPD) of light sources can be incorporated into the IES Illuminance Determination System for visual tasks that are categorized as P through Y within interior lighting applications. It describes how a light source's SPD affects pupil size, visual acuity, and visual efficiency under interior lighting conditions, and establishes an optional calculation method, Equivalent Visual Efficiency (EVE), for adjusting IES recommenced illuminance targets under limited circumstances when visual tasks require the ability to discern achromatic visual detail to ensure speed and/or accuracy in the performance of the visual task. This Technical Memorandum documents the fundamentals of visual efficiency, defines the EVE calculation method, and addresses practical concerns for implementation.