Visual acuity measurements in monochromatic and low-blue illumination

It is well known that visual acuity increases with light intensity. We report on the possibility to further increase visual acuity by changing the spectrum of the light while keeping light intensity fixed, or alternatively, provide the same visual acuity at lower light intensity. For a broad-spectrum (polychromatic) light source, there is only one wavelength in focus on the retina because of chromatic aberration of the eye optics, the so-called emmetropic wavelength. All other wavelengths lead to retinal blur. By concentrating the spectral power around the emmetropic wavelength, the effect of chromatic aberration in the eye optics is minimized, leading to sharper vision. To examine the impact of stimulus spectra on visual acuity, we conducted two experiments. In the first, we used lasers to study visual acuity using monochromatic light. Our results confirm what is predicted from theory, namely a reduction of visual acuity for the short wavelengths for which the eye has the largest refractive error. In the second experiment, we measured visual acuity of a standard 4000 K white light spectrum and of a spectrum with reduced blue content. Visual acuity was higher for the low-blue spectrum. To reach a visual acuity of 1, the low-blue spectrum required less than half the illuminance level of the 4000 K spectrum. The gain in visual acuity by the spectral change is accompanied by a fundamental reduction in color rendering, which makes it less suited for general white light applications with high color rendering requirements. However, when color rendering is of lesser importance, like in an evening home or outdoor environment, applications to support sharper vision (e.g., a reading lamp) seem opportune.