RETINAL EXPOSURE ASSESSMENT - HORIZONTAL OR VERTICAL ALPHA IRRADIANCE OR ILLUMINANCE?

Abstract

There have been a number of methods to measure ambient illumination for the purpose of evaluating both visual and non-visual effects in a given environment. With the discovery of the photosensitive retinal ganglion cells, much of the attention has related to the most relevant spectral functions, and this was also the focus of the recently published CIE Standard, S026:2018, “System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light.” However, S-026 also notes that the field-of-view of the subject observer significantly affects the actual retinal exposure. It is therefore important to review the spatial aspects of retinal exposure. Since the head is normally upright, many have suggested measuring vertical (rather than horizontal) illuminance (lux) or field-averaged luminance (cdꞏm-2). However, perhaps both measures have merit, since the human visual field is actually aimed downward in most settings. Measurements of spectral reflectance within this field can be most revealing.