Re: How much light does a human eye need to see an object clearly at 12m?

Steve Horner:

     I apologize for the delay in answering your question.  I'll try to 
answer your question as thoroughly as I can without being too scientific.

     I am assuming that the officer has not only 20/20 vision, but normal 
retinal functioning in which I mean not only good central vision (20/20) 
but normal night vision.  Night vision depends primarily on "rod" 
photoreceptors in the retinal layer of the eye.  If there is a deficiency 
in rod functioning, night vision also decreases.  During the day, we depend 
more on "cones" for vision which accounts for all color vision and good 
central vision.  

     Unless you desire a mathematically based answer as to how much 
illuminance of the object is needed, I will just give you some basic ideas 
and theories.  The kind of object the officer is trying to see is important 
because a "complex" object with various patterns and structure, like a 
pistol, will perhaps be more difficult to recognize than a "simple" object 
like a club.  It takes at least 20 minutes(in the dark) for an average 
human to become completely dark-adapted, in which case one should be able to 
see a club just using luminance from a full moon.  If you are not dark-
adapted, however, you need more light to accurately identify the object 
against the dark background. Furthermore, a stationary object 
will be easier to recognize than a moving one.  Incidently, there is an 
inversely proportional relationship between "sharpness" or acuity of vision 
and dark adaptation.  Increased dark adaptation means decreased acuity or 
sharpness of vision. Also, some people have, if you will, an inate visual 
sense in which they are better able to discriminate small differences in 
shapes of objects, thus more correctly identifying an object.  This 
probably has to do with visual processing in the cortex, which differs from 
human to human.   
     I hope this gives you some kind of idea to answer your question. By 
the way, luminance is usually measured in foot-Lamberts.  Best visual 
acuity in humans is attained at about 100 foot-Lamberts. Let me know if you 
need clarification.

Kenton McWilliams