I'd like to put a big oar in here, based on personal experience over the
past year. I have enjoyed unaided perfect vision by all standards up to
the age of ~50 when presbyopia set in, requiring progressively stronger
reading glasses with the years, as is usual. I ended up with +2.5 for
reading and still nothing for distant vision up to the age of 76. In
January of last year, I was finally diagnosed with Giant Cell Arteritis
(GCA), after a few months of increasing symptoms. This is an auto-immune
disease which can affect the eyes in several ways as secondary effects,
even possibly causing total blindness in one or both eyes. As a result,
the progress of this rare disease was followed by an ophthalmologist and
I learnt a bit about visual acuity. I was lucky in that my eyes remained
good.
The treatment for GCA are high doses of corticosteroids (prednisone)
which, itself, can cause glaucoma as a side-effect, just to add to the
fun. When I started treatment, I considered my eyesight to be still
perfect, in fact improved, as I mysteriously found that my presbyopia
had improved and I could read comfortably with +1.0-+1,5 glasses. My
right eye, in particular, was thus affected but my distant vision
required -1.0 in it, apparent mild myopia. However, nature being what it
is, the better eye took over whatever I was doing, the left eye for
distant and the right eye for close. Driving, I could focus both on the
road and the instruments unaided!!!!
The ophthalmologist explained why this was so, after examination. My
"perfect" vision, as I believed, was marred by cataracts, caused by the
GCA, with ~30% light transmission in the right and ~70% in the left
eyes. These caused the lens to deform hence the change in focus.
However, I still found the corrected acuity to be fine.
Shortly after, glaucoma set in from the steroids and the increased
eyeball pressure distorted vision and, for the first time, I had poor
acuity with difficulty reading, even corrected. This was quickly
corrected with eyedrops and acuity returned to normal or so I thought.
Another problem caused by the GCA was diplopia (lack of the eye muscles
to turn the eyeballs in a coordinated way so that the focus of each eye
was on the same object). This started and was a random effect sometimes
worse than others.
On the advice of my ophthalmologist, I had operations for the cataracts
last autumn. This consisted of removing the natural lenses by breaking
them up with ultrasonic phacoemulsification and sucking out the debris,
followed by the insertion of flexible plastic lenses with a dioptrage to
suit the lens-to-retina distance. When the bandages were removed the
following day, I was astounded. Not only was the acuity much better (I
had not even realised that it had, in fact, badly deteriorated), but
colours appeared to be so much brighter (I remarked that I was now
seeing in glorious technicolour).
Today, the distant vision is better than 20/20 unaided, but I need +2.5
glasses for reading, as the plastic lens cannot change focus and the
optical accommodation from corneal pressure is very small. I am now
coming to the point of this long diatribe. Each eye is quasi-perfect
with excellent acuity (aided for close work) but the depth of field for
good focus is very limited, even in fairly good light (iris closed). I
have to hold my book at a given distance ±3 cm to be able to read normal
print comfortably. Furthermore, especially if I'm tired, the diplopia
often makes words unrecognisable as letters may get superimposed. If I
look at a 35 mm filmstrip, it can cause adjacent perforations to
superimpose and the contents of the frames appear to overlap. The point
is that ordinary eye tests may not reveal optical faults that could be
fatal for inspection. For example, I'm quite sure that I could no longer
inspect rows of identical solder joints because they would unwittingly
appear superimposed so that faults could be masked.
The bottom line: IMHO, based on my recent experiences, the only valid
eye test for inspectors is practical inspection of real boards with
known faults and expect, say, at least 90% correct fault detection.
Conventional eye testing is OK to determine required correction, but may
be useless for inspection purposes.
Visually yours,
Brian
On 01/06/2011 21:25, Don McFarland wrote:
> Does anyone have a standard for an inspector's ability to see? We have required them to pass color-blind testing in the past, but have recently mulled over a visual acuity requirement. I wondered if this is more normal than not, because it seems like we would want to ensure that they see lines with definition rather than blurry masses.
>
> I humbly look forward to any input that may be offered.
>
> Don
>
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