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Old 08-24-2008, 01:34 PM
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TopCatGr58 TopCatGr58 is offline
Join Date: Jul 2002
Location: Portsmouth, Va.
Posts: 12,621
Yes, You/We ARE Invisible!! Part2


Some more background info:

The visual acuity letter sizes were determined over a 100 years ago, calculated on the density and size of the invidual light receptors(rods and cones) and so 20/20 was found to be about a 1/12th of a degree of visual field. Remember, 180 degrees or a semicircle of vision is what we have looking forward with both eyes. The fields of vision overlap with our two eyes, approx. 45 degrees towards the nose for each eye. This gives us ~90 degrees of both eyes vision. This is what also gives us TRUE 3-D stereo vision/depth perception.

Below is an example of this with vertical red lines marking the 45 degress to either side of center of overlapped vision. But note how much to the side is only seen by 1 eye.

Some more examples of reduced peripheral visions.
View of ~100 degrees of periph. vision.

View of ~70 degrees of periph. vision.

MATH: To determine how big of a "BLUR" area would be according to a person's visual acuity at specific distances. These sizes are NOT totally blind areas, but are the size that an object could be and NOT be discernible or able to tell what it is at these distances.
20/40=1/6th deg; 20/70= 1/3rd deg; 20/200=4/5th deg.

I used basic math.
Circle's circumference= Diam*3.14pi=360degrees
1/2 circumference= 1/2 diam or Radius * 3.14=180 degree
Radius 20ft *12"*3.14=~754" 180 deg
1 degree = 4.18" at 20feet.

This same formula was used for the 50ft, 100ft, 200ft, 500ft distances.

Blur Area Size and Distances
Visual      20feet   100feet   200feet   500feet
20/20       0.35in    1.7in     3.36in     8.4in
20/40       0.63in    2.1in     6.3in      15.7in
20/70       1.14in    6.3in     12.6in     31.5in 
20/200      3.25in    16.8in    33.6in     84 in
Other sizes of interest: Not including a fairing!
XS11 with rider appx. 2ft wide x 5ft high = 10 sq. feet area.
Xterra/SUV appx. 5ft wide x 6ft high = 30 sq. feet area.

Most in town residential traffic is 35mph, maybe 45mph for city. A vehicle at that speed covers 50ft a second. A common block length as ~500 feet, or 1/10th a mile, and will be travelled in 10 seconds, quicker if faster of course!

Now a few shots at the very edge of the intersection like someone would do before turning right. The bike again is a FULL BLOCK away...on the sidewalk! Just 10 seconds away at 35mph.

A zoomed in view.

Here it is ONLY 1/2 block away, a mere 5 seconds away at 35mph.

And a zoomed in view.

So, now looking back at the blur sizes for certain visions at certain distances...a person with 20/20 could have trouble seeing something that is ~8.4" big...the size of your round headlight is 7"! A person with the bare minimum 20/70 vision could not discern an object 31.5" big...remember we are about 24" wide!

Oh...but there's more!! The human eye has a Natural Blind Spot in it. It's where the optic nerve plugs into the eye, all of the millions of fibers span out from this location to map out the eye's vision, but this area itself can not see. It's about 10 degrees wide by 15 degrees high, and just 10 degrees towards the ear from center of each eye. We usually never see it because the other eye's retina sees for it. Here's an example looking straight ahead. Notice how big they are in relation to that TRUCK!

These spots' locations are in reference to the actual photo, not to you sitting here looking at your computer screen. HOWEVER, if you have a 19" monitor at 800x600, are sitting ~20" away, you can cover your right eye and look at the RIGHT dot with your Left eye, and the LEFT red dot will disappear! Vice versa for the other eye. You may have to move inward or outward a little depending on your screen size!!

Our population is aging, their eyesight is failing, their bodies and necks are stiff, so they don't often bother turning their heads very far left or right to check down the road!! And so IF they turn their heads just 30 degrees left, their right eyes 45 degrees of nose side vision will NOT cover the full 80-90 degrees left view down the road, and so their natural BLIND spot can occlude a fairly large area and won't be seen by the right eye, and you can have this situation!'s best to ride as though you ARE invisible.

Be safe out there!
T. C. Gresham
81SH "Godzilla" . . .1179cc super-rat.
79SF "The Teacher" . . .basket case!
History shows again and again,
How nature points out the folly of men!