Snellen Chart

Discussion in 'Optometry Archives' started by SteveM, Dec 15, 2003.

  1. SteveM

    SteveM Guest

    Hi There,

    Can anybody tell me how to make a snellen eye chart in word, what size
    of letters in mm do they need to be, i am in the uk so dont know if
    the size is different in the US.

    regards

    Steve
     
    SteveM, Dec 15, 2003
    #1
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  2. SteveM

    Dr Judy Guest

    The size depends upon the viewing distance, not your geographic location,
    the further away you are, the larger the letters should be. A proper paper
    chart is relatively inexpensive, you could ask your eye doctor to order one
    for you. Or search the web for downloadable versions. Here are two:

    http://www.mdsupport.org/snellen.html

    http://www.i-see.org/eyecharts.html

    Dr Judy
     
    Dr Judy, Dec 15, 2003
    #2
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  3. SteveM

    Otis Brown Guest

    Dear Steve.

    The 20/20 line is about 3/8 inch high.

    The 20/40 line is about 3/4 inch high. (DMV Standard)

    The 20/70 line is about 1.3 inches high.

    This should get you started.

    www.i-see.org has good eye charts.

    Best,

    Otis
    Engineer
     
    Otis Brown, Dec 15, 2003
    #3
  4. SteveM

    Evaristo Guest

    Normal vision is when you can recognize the height of the letter is
    such that from your eyes it makes 5/60 of a degree angle (between
    the straight lines from your eyes to the top and to the bottom of the
    letter).

    1 / tan(5/60) = d / h

    tan 5/60 = 0,00145

    h = d * 0,00145

    So, if the distance is say d = 6 meters, then the height of the letter
    should be:

    h = 6 * 0,00145 = 0,0087 = about 9 mm

    The other lines are simply letters that you should see with normal
    vision at different distances. (for example at 12 meters, ecc... )

    The smallest detail of a letter (say the width of the horizontal
    lines of a letter E) must form 1/60 of a degree angle.
     
    Evaristo, Dec 15, 2003
    #4
  5. I highly recommend this one, rather than trying to make your own:

    http://www.i-see.org/random_snellen/random_snellen.html

    I use it every day to track trends in my vision, and there is no way
    the results can be biased by your memory of which letters are present
    on which line. I use a large flat-panel LCD monitor though--I'm not
    sure accurate it would be on a standard CRT, and I suspect that how
    you have your brightness and contrast controls set would affect your
    results. It's at least fine for monitoring changes, if your room
    lighting conditions are always the same.

    Here's a question for people: are serif or non-serif characters better
    for testing Snellen acuity? Does it matter?

    Shawn
     
    Lothar of the Hill People, Dec 17, 2003
    #5
  6. SteveM

    Otis Brown Guest

    Dear Shawn,

    An even better question is the "illumination level".

    This has a profound effect on you reading of the eye chart.

    I do not believe there is any standard illumination level
    specified.

    At room illumination you might well read at 20/30.

    Under "dusk" conditions you might read at 20/100.

    That has more effect that the type of font.

    The DMV tests under "room llumination" conditions.

    If you want very sharp vision at night, (including
    "tonic accommodation" you will need a much stronger
    minus lens.

    Best,

    Otis
    Engineer
     
    Otis Brown, Dec 17, 2003
    #6
  7. Thanks for the responses, Mike and Otis. I am more concerned with
    consistency than accuracy (although I would like to be able to compare
    my measurements with the ones my doc takes), and I do try to measure
    my VA in roughly the same lighting conditions. Since my Snellen chart
    is on my computer, the brightness is always exactly the same, but I do
    try to measure it in the same ambient lighting (at least at the same
    time of day), which is unfortunately not easy to reproduce
    consistently in that room.
     
    Lothar of the Hill People, Dec 18, 2003
    #7
  8. SteveM

    Otis Brown Guest

    Dear Lothar,

    It is important to understand what your problem or complaint
    actually is.

    If you have difficulty passing the DMV test, then you
    must print the chart out and place it at 20 feet.
    That is the same test the OD performs to check your
    distant vision.

    If you problem is "presbyopia" (blur at near due to
    age -- everything else being equal) then checking
    your eyes on the CRT would be the correct way to
    do it. In this case the distant is important,
    eye-to-chart.

    The range-of-accommodation (stop-to-stop) reduces with
    age. Typically, a person with 20/20 distant vision will
    begin to lose some "near" vision by age 45. (Except
    for Francine.)

    Best,

    Otis
    Engineer


    Hope this clarifies.
     
    Otis Brown, Dec 19, 2003
    #8
  9. SteveM

    Otis Brown Guest

    I was just pulling your leg. You said you cured your
    presbyopia -- who am I to doubt you.
    Fine -- you continue to be the leader in this area. Keep
    up the good work.
    We are born with about 13 diopters of accommodation (so called
    "amplitude of accommodation"). The movement (stop-to-stop)
    gradually decreases with age. Therefore no "sharp" lines
    can be drawn to say "presbyopia".

    But typically it is noticed in dark restaurants at between
    age 38 to 50. This is one place where a slight touch
    of "nearsightedness" is helpful. If you are at -1/2 diopter
    (20/40), and your amplitude of accommodation is 2 diopter,
    with a depth-of-field of +/- 0.6 diopters, you could
    probably get along without plus lenses for reading
    until age 60 or so.
    I was attempted to figure out why this man was reading
    the Snellen eye chart on the CRT. Again you
    totally mis-read what I actually said -- and why
    I said it.
     
    Otis Brown, Dec 20, 2003
    #9
  10. I think that someone should write a song in a Gilbert and Sulivan vein about
    Francine's eyesight. The "Ruler of the Queen's Navy" song, (or is it
    Nighvee) would be a good starting place.

    Bill
     
    Repeating Rifle, Dec 20, 2003
    #10
  11. I have neither a problem nor a complaint. I am using my Snellen chart
    simply for evaluative purposes to track any potential changes in my
    vision as my eyes adapt to extended-wear with my Focus Night & Day
    lenses. I am myopic, in case that's what you meant.

    In case anybody is curious, I have not noted any changes whatsoever in
    my vision over the 2 months that I have been wearing this brand. My
    Snellen measurements are not always exactly consistent every morning,
    either from eye-to-eye, or from day-to-day in the same eye, but they
    do average out to exactly the same over a span of days or weeks. From
    what I have heard here and elsewhere, it is very common for visual
    acuity (or lens power required) to change after switching to these
    lenses, and from conversations I have had with people I suspect that
    is due to a "healing" of the cornea (e.g., long-term edema induced by
    hypoxic conditions with non-silicone lenses). I rarely used to wear
    my last lenses overnight though, so perhaps there was no healing to be
    done.
    I don't understand that statement. For one thing, I don't think a
    Snellen chart needs to be placed at 20 feet (it certainly isn't in my
    doctor's office) to be accurate, as long as you correct for the
    reading distance and the size of the letters, and for another, I can't
    see what reading a Snellen chart at home has to do with trying to pass
    a DMV test. You'll either pass it or not, right?

    I am not able to place my home Snellen chart 20 feet away from me, due
    to the fact that my computer monitor is in a room that is too small
    for that. I do correct for the distance though by reading another
    line on the chart as 20/20 or adjusting my reading distance (which is
    more like 15 feet) after measuring the letter heights. I may or may
    not be actually getting the correct Snellen calculations, but at least
    I know I am as consistent as I can be from measurement to measurement.

    Lothar
     
    Lothar of the Hill People, Dec 20, 2003
    #11
  12. Ah, okay, if that's where Otis was heading with that statement (to try
    to therapeutically improve vision in order to pass a DMV test, for
    example), then I understand his comment now. I will reserve judgment
    on the merit of such methods because I've never tried them and have no
    basis to evaluate them--and I know it is a matter of heated debate
    which I don't care to get involved in! :)

    Thanks for the comment, Francine.

    Lothar
     
    Lothar of the Hill People, Dec 20, 2003
    #12
  13. SteveM

    Otis Brown Guest

    Dear Lother,

    Thanks for your commentary -- and the reasons for reading
    the Snellen eye chart.
    Correct. But if you used a different distance you must recalculate
    the size of the letters (assuming the required 5 minute-of-angle
    size.

    In the metric system you can multiply the distance
    by 0.00146 to calculate the size of the letter.

    i.e., 6 meters * 0.00146 = .00876 meters


    (it certainly isn't in my
    You'll either pass it or not, right?

    If you read it at home, you will know if you pass or
    not BEFORE you go to the DMV.

    If you purpose was to IMPROVE your vision (with a plus
    or Bates) then it would be wise to check at home. That
    way you will be certain of any improvement you might
    achieve.

    After all this is how Francine improved her vision.
    What you "see" is what you get.


    Again, thanks for the info and method of
    checking your distant vision.

    Otis
    Engineer
     
    Otis Brown, Dec 20, 2003
    #13
  14. Okay, I understand your comment now, Otis. Sorry for the confusion!

    Lothar
     
    Lothar of the Hill People, Dec 20, 2003
    #14
  15. Okay, I understand your comment now, Otis. Sorry for the confusion!

    Lothar
     
    Lothar of the Hill People, Dec 20, 2003
    #15
  16. Thanks for the interesting commentary, Francine, and you are certainly
    right. I can't imagine using a Snellen chart for any useful tracking
    purpose without my contacts in, as all the letters are blurry (even if
    I can technically read some of them) and I don't think I could track
    any subtle changes in acuity over time. However, with my contacts in,
    I am *very* aware of minor changes in my vision from day to day on my
    Snellen chart. I surmise that a Snellen chart is probably more useful
    for tracking changes in corrected vision than in uncorrected vision
    for somebody with myopia, but that's just my sense.

    To illustrate this point, without my lenses in, I can only estimate
    that my vision is probably somewhere between 20/100 and 20/200 (a huge
    range), With my lenses in, I can easily distinguish a difference
    between 20/15 one day and, say, 20/17 (if I interpolate between lines
    on the chart) the next day.

    Incidentally, I don't think my vision is actually changing from day to
    day when I notice differences on the chart. I suspect it has more to
    do with how moist my contacts are and if there are any remnants of
    overnight oily lipid deposits on them. I can sometimes improve my
    acuity just by massaging my eye gently through my eyelid and checking
    it immediately again. So it's hard for me to say what my "true"
    Snellen acuity is (or even how accurate those charts are for notating
    the numbers), as it's rare that I get the same measurements on any 2
    successive days for both eyes.

    Lothar
     
    Lothar of the Hill People, Dec 20, 2003
    #16
  17. For me, I strongly believe that it is primarily my tear fiim. Why
    else would me VA often change markedly by simply rubbing my eyes? I
    suspect that the volume of tears under my contact lenses has a big
    deal to do with the power of my lenses, and I expect that this varies
    greatly over the course of a day, or from day to day. I always
    measure my vision on my Snellen chart shortly after awakening, when I
    am well-rested.

    Lothar
     
    Lothar of the Hill People, Dec 21, 2003
    #17
  18. SteveM

    Otis Brown Guest

    Dear Fran,

    Why do you so consistently miss the point -- with all due respect.

    You DO NOT improve your vision by looking at an eye chart.

    It is like losing weight. If I weigh 190, and must get
    down to 180 or less to pass a flight physical -- then
    what am I going to do. Not get on the scales, to determine
    my weight?

    No, I am going to check. You said you improved your vision -- but
    you never checked your "near" snellen chart. So you don't
    know one way or the other.

    If you wish anyone to believe you, you should report your
    vision (say 20/60) before you start your "exercise",
    and your vision as it gradually improves by LOOKING AT YOUR
    CHART. That is the only way you will ever know.

    The ODs and MDs have an excellent point. People report
    "improvement" but the is a "gut feel" thing. The people
    who report this "improvement" never look at an eye
    chart.

    Sure, I believe them -- but they are indeed subjective.

    At least the pilots who work to clear their vision pay
    close attention to what they are seeing on their
    eye chart.

    But this is a friendly chat -- and we all do things
    are own way. Most people lack the motivation to
    do any of this work in any event.

    By all means, all of us should have our eyes checked
    by an MD or OD. There are a number of medical reasons
    for loss of distant vision. Once the medical reasons
    have been elliminated (and the ODs and MDs are very good
    at this -- that is their profession) and you wish
    to clear your vision from 20/50 to 20/30, then you should
    certainly make the effort using Bates or other techniques
    that you might choose.

    As an engineer, I like to hear the numbers you report
    on reading an eye chart -- at least that is quantative.
    But that is why I suggest checking your own eye chart.
    There is no law against it.

    Best,

    Otis
    Engineer
     
    Otis Brown, Dec 21, 2003
    #18
  19. I'm not sure if you were pointing me to one particular abstract in
    that list, Francine, but I saw several interesting ones there. Thanks
    for posting it!

    Lothar
     
    Lothar of the Hill People, Dec 21, 2003
    #19
  20. Yeah, that abstract was one of the ones I found particularly
    interesting! I would be particularly curious to know how/if daily
    variations in intraocular pressure correlate with variations in visual
    acuity.

    Lothar
     
    Lothar of the Hill People, Dec 21, 2003
    #20
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