pupil Flashcards
Does the iris have optical power?
no
iris anatomy
- Thin, muscular piece of tissue
- Sits right in front of the lens, touching the first surface
- Bulges forward when the eye accommodates
- Opaque tissue due to the melanin granules in the epithelium and sometimes stroma.
role of iris
controls light going into the eye
Important optical roles of the iris
1. Defines pupil of the eye by blocking undesirable light (pigmented tissue) 2. Regulates retinal illuminance 3. Controls depth-of-field 4. Aides in controlling retinal blur (blur circle, aberrations, diffraction)
_______ iris contains melanin granules in stroma and epithelium.
brown
_____________ iris contain granules only in epithelium.
blue and green
Unique pattern makes
it ideal for ____________.
biometric identification
What type of light is scattered more than other wavelengths?
blue
Transparency of tissue is governed by the tissue’s ability to do what to light?
- absorb
2. scatter
What color iris scatter (Rayleight) light, which inhibits transparency?
blue
What color iris absorbs light?
brown
A condition in which the two irises of an individual have different colors
heterochromia
cause of heterochromia
Most cases of heterochromia are hereditary, caused by a disease or syndrome, or due to an injury. Sometimes one eye may change color following certain diseases or injuries.
How does the pupil size affect the brightness of the retinal image?
Image brightness is proportional to the pupil area, not pupil diameter.
The human visual system operates over what amount of log range?
10-log
How much does the changing pupil size contribute to this?
The 16x change in luminance (1.2-log) helps, but is relatively small compared to the 10,000,000,000x change (10-log) of the entire visual system.
Pupil diameter as a function of age
As we age, our pupil decreases.
*inverse relationship
Maximum size of acceptable blur at the retina determines what?
depth of filed and depth of focus
Range of object or image plane over which the image appears to be in focus.
Depends on the idea that a small amount of blur is allowable.
Depth of field & depth of focus:
Depth of field of the eye ultimately sets the:
(1) precision to which the refractive state of the eye may be measured,
(2) The precision required in focusing optical instruments, and
(3) The accuracy of the accommodation system of the eye
range of object space
depth of field
range of image space
depth of focus
A decrease in pupil size increases what?
depth of field
how does KAMRA inlay help presbyopic pts?
artificially decreases pupil size, so that pt has larger depth of field
What is the typical depth of field for the eye?
0.5-1 D
Depth of field can be increased by increasing what? And who is this beneficial for?
ambient light level
presbyopes
What is the downside of small pupils?
vision is more sensitive to nuclear cataracts
Accurate measurement of
accommodative amplitude and subjective refraction is compromised by what?
small pupils
Pinholes are an efficient way to screen for ___________ without measuring for them.
refractive errors
Does pinhole correct refractive error? T/F
false
A decrease in pupil diameter leads to a decrease ______________ at the retina
geometric blur size
If pupil size increases, what happens to blur?
increases too
*direct relationship
What is the relationship for aberrations and diffraction in the eye?
B=PK defines a linear relationship b/w the retina blur and pupil diameter when in the presence of a refractive error
an optical defect in a lens or mirror, causing image blur.
aberration
Whats the relationship of pupil size to aberration?
increase pupil size, increase aberration
*direct relationship
The deviation from rectilinear propagation that occurs when light advances beyond an obstruction
Phenomenon of diffraction
Can we use Snell’s law to predict diffraction?
No, Diffraction is accurately predicted by Wave Optics
How does diffraction affect the retinal image?
- For practical reasons, imaging systems are unable to collect the entire wavefront emitted from an object. For the eye, only a section passes through the pupil with the iris representing an obstruction that deviates the light.
- This deviation blurs the image.
- The image blur size for a point object = 1.22 λ f / d
large slit generates _____ deviation of waves
little
*inverse relationship
small lit generates ______ deviation of waves
large
pt is best corrected and under dark environment (big pupil), whats the final limit for VA?
anatomy of neural system imposes upper limit on supernormal vision
defines the bundle of light rays that pass from object to retinal image. And as we have just discussed, this bundle of rays influences retinal illuminance, depth of focus, and retinal blur size.
real pupil
is the real pupil what we see when we look into someone’s eye, as for example when measuring this patient’s pupil with a pupillometer?
no, we are looking at image of pt’s pupil (entrance pupil)
image of the real pupil formed by the cornea. All light rays that pass through the real pupil also pass through the entrance pupil. It is an erect, virtual image that resides 0.5 mm in front of the real pupil.
entrance pupil
image of the real pupil formed by the crystalline lens. All light rays that pass through the real pupil also pass through the exit pupil. It is an erect, virtual image that resides 0.1 mm behind the real pupil.
exit pupil
