Several aspects of vision contribute to a subjective assessment of clarity or visibility of an object.  Though many characteristics of vision are quantifiable, ultimately, subjective assessments often dominate discussions of lighting needs.  The challenge for lighting designers and industrial hygienists is to combine subjective feedback and objective data to extrapolate lighting system requirements.
     In Part 5, objective measures of light were introduced; the discussion involved the source of light (e.g. luminance) and its path (e.g. refraction, reflection).  In this installment, the receiver of light, the human visual system, is once again considered.  Factors contributing to visual capabilities and quantifiable measures are explored.

     Photometric measurements quantify the light incident on a workspace, but more information is needed.  Effective lighting system design requires that qualitative parameters also be considered.  A strict technical definition of photometry limits the term’s applicability to measurements of the intensity of light.  Here, use of the term is expanded to refer to more-comprehensive evaluations of lighting.  That is, for purposes of this series, photometry includes both quantitative and qualitative assessments.
     Understanding the terms and quantities associated with photometry is critical to the assessment of the suitability of a lighting system.  It also supports the ability to predict or compare the relative performance of two or more lighting systems.  Providing information needed to develop assessment skills is the primary objective of this installment.  A brief introduction to measurement equipment is also provided.

     The functions of the eye presented in Part 3 often get the most attention.  This is simply because they are closely associated with the most-salient features of our sensory perceptions.  That is to say that the eye’s role as a component of the visual system is most familiar; the previously discussed functions may even be mistakenly conceived as the entirety of the visual system.  The exploration in this installment demonstrates that this is far from the reality.
     In addition, the eye plays an important role in the non-image-forming system; the term non-visual system is used here for brevity and to maintain an obvious contrast between functional attributes.  The non-visual system affects sleep/wake cycles, alertness, and related circadian rhythms.