visual acuity. visual acuity: is the smallest visual angle that a person can see clearly. we will...

VISUAL ACUITY

Visual Acuity:

Is the smallest visual angle that a person can see clearly.

We will talk about four very different tasks…

Yet all of these are considered a measure of acuity.

Detection Acuity

LocalizationAcuity

ResolutionAcuity

Letter/optotypeAcuity

The smallest spot or thinnest line that can be

seen against a brighter or darker background

the smallest visible position shift of a dot or

line compared to another dot or line

the smallest gap between spots or lines that can be

seem. *if it is a set of parallel lines it is called Grating

Resolution

The smallest letter or other object that can be

correctly identified. Closely related to resolution acuity.

Is there a spot or not?Vernier: are the two lines

aligned?Bisection: is the middle line left or right of the

center?

Two dots or just one? Two lines or one?

Grating or just gray square?

Name the letter or identify the orientation of the

letter.

1. Bright line or spot2. Single dark line3. Single dark spot

1. spatial interval2. Vernier lines

1. Two bright lines/spots2. Two dark lines/spots3. gratings

1. Letters or numerals

1. ca. 0 sec arc2. 0.5-1.0 sec arc3. 15-20 sec arc

1. 2-4 sec arc2. 3-6 sec arc

1. 40-60 sec arc2. 30-40 sec arc3. 30-40 sec arc

1. 30-40 sec arc

FACTORS AFFECTING VISUAL RESOLUTIONI. Image Quality

-luminance of the image

-contrast of the image

II. Point Spread Function of the Eye

- diffraction: pupil size and the Airy Disk- Optical quality of the eye:

- Sphere- Cylinder- Other optical aberrations

III. Sampling Resolution

-density of cones in the central fovea, expressed as cone spacing in arc minutes

-density of ganglion cells in the periphery, where many cones (or rods) are pooled into one ganglion cell

The retinal image of a bright point is described as the point spread function (PSF) of the eye

The smaller the PSF, the better your image resolution.

As the size of your pupil changes, the PSF of an image in your eye also changes.

Smaller pupils : limit the PSF by diffractionLarger pupils : limit the PSF by aberrations

For most people, the narrowest PSF occurs when the pupil is about 2.5 mm in diameter.

At this pupil size, the PSF is about 1 arc min wide.

In this figure:

Solid curve : the typical PSF of the eye for each pupil size

Dashed curve : the ideal, diffraction limited point spread.

The difference between the two is due to optical aberrations in the eye.

Notice how the difference changes with pupil size.

More about Detection Acuity

…. Remember this is detecting a spot against a background

So..you can have

1. bright spots on a dark backgroundon a light background

2. or dark spots on a light background

- A bright spot on a dark background always has nearly 100% contrast on the retina.

- So even though some of the light will get scattered by the eye’s optics, the contrast will only get reduced very slightly.

- This means that this spot is still detectable after it gets spread out.

A bright target on a dark background is detected by your retina when the number of photons it is emitting

(i.e., the amount of energy)

exceeds the retinal threshold for light.

Rule : the detection “acuity” for a bright point or line does not depend on its size, but on the number of photons in the target’s image.

Rule : the detection “acuity” for a bright point or line does not depend on its size, but on the number of photons in the target’s image.

As long as the PSF of the target’s image is not bigger than the summation area (Ricco’s area)

then the slight blurring of the spot does not change detection threshold.

So “acuity” for a single bright spot is infinitesimal.

So a star, Aldebaran, the “eye” of Taurus the Bull, subtends 0.02 seconds of arc, but we can still detect it!

However, if the background is bright, then the detection is limited by its contrast against the background.

This is why we can’t see starts in the daytime.

For Dark Lines against Bright Backgrounds, the contrast does get reduced when the image gets spread out.

This means that for these targets, the smallest we can see does depend on the size of the PSF.

Moving on to Localization Acuity…

Now for Resolution Acuity …

Optotypes!

FORMULAS! Woohoo!

So how would we relate Snellen fractions to grating acuity?

Q: What’s so special about 20/20?

NOTHING!

Just kidding. It means you see ok.

But…..

For the next figure, let’s learn some definitions….

Translation: one eye is worse than the other.

Translation: you have an eye turn.

Translation: usually, when a child’s vision is worse than 20/40 and glasses can’t immediately fix it.

(you had this in Grand Rounds, right?)

In clinic, find the lowest thing the patient can see.No letters? finger counting motion perception light perceptionConfrontation fields test periphery. No static confrontations? test moving CVFs

Quick question: why do our eyes constantly move?

Bc when we stare at something, the image will fade.

According to the previous slide, though, these micro-movements don’t affect acuity.