"The Snellen chart is an effective tool for the measurement of visual
acuity, but has some limitations."

There, that and ten pages should get you an A.

Dear Marc,

Subject:  Who invented the Visual-Acuity chart -- and why.

Prior to 1860 there was no systematic or "invented" method
of measuring visual acuity.

The analysis of the eye, and the recognition of nearsightedness
as an "optical" issue was described by Johan Kepler ca 1600.

It had been recognize (from astronomy) that the human
eye could resolve stars separated by 1 minute-of-arc (approximately).

Dr. Snellen took this concept, and developed block letters
that had widths of 1 minute-of-arc, for a letter-size of
5 minutes-of-arc.

A distance of 20 feet (or 6 meters) was established
as reasonable "optical infinity".

Thus this very-high resolution became the ability
to read 3/8 inch (0.9 cm) letters at 20 feet.

Since most people could be "corrected" to this
value with a proper-strength lens, it was then
decleared that retinas that could resolve these
letters were 20/20, or "normal".

Hope this helps start your research.

Best,

Otis

Also, Marc, it would be a good idea to provide a "computer"
Snellen and test your visual acuity

http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html

All you need to do is click on this site, and then "Display",
and a chart for 20 feet will apear on your screen.

The chart is good.  It has random letters and
pictures if needed.

Excellent support for your school project.

Best,

Otis

Snellen is about vision quantity and is only marginally about vision
quality. Research contrast sensitivity measurement and wavefront
diagnostic to see some of the techniques that more accurately measure
vision quality. Research vision quality under different lighting
conditions.

Glenn Hagele
Executive Director
USAEyes.org
Patient Advocacy Surgeon Certification

"Consider and Choose With Confidence"

Email to glenn dot hagele at usaeyes dot org

http://www.USAEyes.org

Lasik Bulletin Board
http://www.USAEyes.org/Ask-Lasik-Expert/

I am not a doctor.

Copyright 2006
All Rights Reserved

Also, Marc, it would be a good idea to provide a "computer"
Snellen and test your visual acuity

http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html

All you need to do is click on this site, and then "Display",
and a chart for 20 feet will apear on your screen.

The chart is good.  It has random letters and
pictures if needed.

Excellent support for your school project.

Best,

Otis

 You should make the connection to the LogMAR charts as to how the
Snellen charts should have been incremented in their step sizes. Those
charts are a consistent 5 letter across regardless of the size.

 Then you should asked the question as to why the resolution at the
upper end (i.e., that is 20/150 to 20/200, etc) have such large
increments, when in this area one would like to finer steps to see
"improvements" from treatment.

 I also think the entire line should be read correctly to establish
the 20/over figure, without any fudging of 20/over plus/minus
something.  Therefor that line should be infinitely variable in size.
No jumps.

 Go to PubMed (Google will link) and in PubMed just type in "Snellen".

 Good luck on the other 19 pages.

Don W.

snellen is a useful quick test, but it is a limited contrast
sensitivity test. a more complete contrast sensitivity test can be more
descriptive of a person's state of vision
the choice of optotypes used can also influence the score on a snellen
test imo.