Visual Perception Flashcards
Define radiometry.
The science of the measurement of electromagnetic radiation
NOTE: both visible and non-visible light
What are the units of radiometry?
- Radiant flux
- Radiant intensity
- Radiance
- Irradiance
Which unit of radiometry is this describing?
- A measurement of ALL light waves coming off a light source
- Units: Watts (joules/sec)
Radiant Flux
Which unit of radiometry is this describing?
- Total measure of total light (possibly from multiple steradians) on a surface*
- Units Watts/m^2
Irradiance
Which unit of radiometry is this describing?
- Power per solid
- Power output of a (point) light source in a given direction (solid angle)
- Units: Watts/steradian
Radiant Intensity
NOTE: steradian is a 3D, cone-shaped measurement of light, w a spherical cap coming from a point source (a sphere is 4pi steradians
Solid angle is the angle of light coming from the point source that forms the point of the steradian cone
Which unit of radiometry is this describing?
- Radiant intensity per unit projected area of an (extended source)
- Area of light in one steradian incident on a particular area
- Units: Watts/m^2(steradian)
Radiance
What is the science of the measurement of the eye’s response to (or perception of) electromagnetic radiation? I.e. ONLY visible light?
Photometry
What are the units of photometry?
- Luminous flux
- Luminous intensity
- Luminance
- Illuminance
Which unit of Photometry is this describing?
- Luminous intensity per unit projected area of an (extended source)
- Area of the visible light in ONE steradian incident on a particular area
- Unit: cd/m^2 (nit OR ft-lambert)
Luminance
Which unit of Photometry is this describing?
- Power output of a light source
- Measurement of ALL visible light emitted from a source
- Unit: Lumens
Luminous Flux
Which unit of Photometry is this describing?
- Power output of a point light source in a given direction (solid angle)
- A measurement of visible light in ONE steradian
- Unit: Lumens/steradian (Candela, cd)
Luminous intensity
Which unit of Photometry is this describing?
- Luminous flux incident on an object’s surface
- Total measure of total visible light (possibly from multiple steradians) on a surface
- Unit: Lumens/m^2 (lux)
- English: Foot-candle (ft-cd)
Illuminance
Lumens is the unit for what?
Luminous flux
Candela is the unit for what?
Luminous intensity
“a Candle is INTENSE”
NOTE: Luminous intensity is also Lumens/steradian
Candela/m^2 OR nit OR Foot-Lambert (fL) are the units for what?
Luminance
Lux or Lumens/m^2 or Foot-candle are the units for what?
Illuminance
At 555nm how many lumens are there for both scotopic and photopic systems?
683 lumens
On plot: 1 lm/W @ 555 nm
Find the relative luminance efficiency for the given wavelengths on the plot & multiply by 683 lm/w
Multiply by given watts for each wavelength
Add together
What is the equation to convert illuminance to luminous intensity?
Intensity (I) = Distance (D)^2 x Illuminance (E)
I=D^2 x E
Describe Retinal illuminance and the equation
Its the amount of light getting into the eye
Unit:
Trolands (td)
Equation:
T = L x A
Trolands = Luminence (cd/m^2) x Pupil Area (mm^2)
What’s the equation Inverse square law.
E = I/d^2
E = Illuminance (lux; lumens/m^2; ft-cd) I = Intensity (cd) d = distance (m)
Describe the concept of the inverse square law
As the (d) INCREASES, the same amount of light is dispersed over a proportionally larger area …. therefore, DEC intensity of light!
NOTE: smaller point light sources make this law work BETTER
NOTE: only works if there is NO additional reflected light
Describe the Cosine Law of Illuminance
Illuminance DECREASES with the cosine of the angle from normal
I.e. as the surface is TILTED AWAY from being perpendiculat to the light source, the illuminance of the surface DECREASES!!! (basically most illuminance is 90 degrees from surface)
Equation for Cosine Law of Illuminance?
E = (I/d^2) x cos(theta)
What is a special kind of surface whose luminance does NOT vary with the angle at which it is viewed (i.e. cosine law of illuminance does NOT apply)
Lambertian diffuser
EXAMPLES: matte paper, unfinished wood
With perfect reflectance (r=1.0) all light that falls on a surface is reflected
Illuminance at the surface would equal the measurement of luminance equals the measurement of the luminance from the surface (i.e. same amount of light is incident on the surface and reflected off the surface)
Equation for Lambertian Diffusers?
L = E x r L = luminance (cd/m^2) E = illuminance r = reflectance * only works for english system
How does increase distance affect a light source?
With increased distance from light source to object the projected area decreases
How is Illuminance affected by increased distance?
It DECREASES, inverse square law
E=I/d^2
How is Luminous Intensity affected by increased distance?
Remains CONSTANT
Smaller proportion is actually hitting the surface
Constant/proportional ratio of candelas (intensity) to projected surface area
Recall:
Luminous Intensity = Lumens/steradian
True or False. Luminance remains constant with increase in distance because the power of the light source does NOT change.
True
What is the definition of threshold?
The minimum value of a stimulus required to elicit a perceptual response or an altered perceptual response
NOTE: the stimulus level (or value) above which a response is elicited and below which a response is NOT elicited
What is absolute threshold?
The lowest level of stimulus for which a response can be elicited (can you see it or not?)
What is the difference/increment threshold?
The minimum difference in stimuls levels btwn 2 stimuli that can be detected
What is the most efficient way to measure threshold?
- Fewest # of stimulus presentations
- Task is easy to understand
- Few opportunities for cheating or checking for correctness
Define detection.
Do you see the stimulus or not?
Define discrimination/resolution.
Can you tell the difference? Are there one or two lines?
Define recognition.
Identification/making a cognitive choice; what shape or letter do you see?
What are some reasons why threshold may vary?
- Stimulus
- Neural activity
- Attention
- Psychological bias: older pts don’t want to be wrong, children want to prove themselves
What is the best psychological method?
a. Method of constant stimuli
b. Method of limits
c. Staircase method
d. Bias guessing
a. Method of constant stimuli
This is when you choose several stimuli levels to present in random order
Pt does not know when/what they will see, so they need to pay attention
Does the method of constant stimuli get rid of bias?
NO
Typically gives a sigmoid psychometric function
What are catch trials?
They check for guessing, bias, compliance, etc
The pt is shown something they should never see or something they should definitely see and check their response
Alternative/interval forced choice tasks/methods aid in this effort (2AFC, 3AFC, etc)
How do we determine threshold?
Guessing rate + 50% of the chance of guessing incorrectly
Ex. threshold for a 4AFC is 62.5% bc if guessing, the subject will be correct 1/4 or 25% of the time, leaving a 3/4 or 75% chance that they will guess incorrectly; half of 75% is 37.5% and this is added to the guessing rate is 62.5%
What is formed from plots of frequency of “noise” responses and frequency of
noise + signal” responses with increasing stimulus?
ROC curve
the area of overlap can indicate region of confusion
What is on x and y axis of ROC curves
x-axis: Hit Rate (TP)
y-axis: False Alarm Rate (FP)
What responses act as catch trials in order to determine the reliability of the test?
Fixation errors
False negatives
What is magnitude of sensations?
a form of response where the pt must rate the test stimulus given a reference stimulus
Have a magnitude, but no obvious scale or units
Describe ratio production.
Show a light, tell subject to make/set a second light that is 2/ or 1/2x for example as bright
What is is ratio estimate?
Show a light
Ask the subject to tell how much brighter or dimmer a second test-light is
Sensory and stimulus magnitude are proportional is describe by which law?
Steven’s Power Law
Brightness: sensation perception is limited at high levels; adaptation
Length: fairly accurate
Shock/pain: increases rapidly after initial shock; fear
What does a shutter in an optical system used for?
To control FLASH duration
A long wavelenged dim red light is used for what in the dark adaptation test?
Excites cones at fixation and does NOT affect rod performation
L x A = C
m = 1
This describes which law?
Ricco’s Law
L x A^1/2 = C
m = 1/2
This describes which law?
Piper’s Law
“PIPE down”
NOTE: beyond critical area; partial summation (occurs at slower rate)
As background intensity DEC/contrast INC, how much luminance is needed for a given flash area (deg^2)?
LESS luminance is needed
L x t = C
Describes which law?
Bloch’s Law
Critical duration = 100 msec
NOTE: threshold amount of quant (light units) must be presented within the critical duration to be perceived
What is threshold magnitude related to and how?
Threshold mag is Inversely proportional to sensitivity
Meaning higher threshold magnitude means lower sensitivity. Lower threshold magnitude means high sensitivity!
How many quanta get to the rods?
4% absorbed by the cornea –> 96%
50% –> retina
20% –> absorbed by rods!
So, 20%…
One rod can be stimulated by one quantum, but you need at least how many stimulated rods to perceive a signal/light?
10 rods
One cone can be stimulated by how many quant? At least how many cones need to be stimulated to perceive a signal/light?
5 quanta
2 cones
What is absolute threshold for light defined as?
The LEAST amount of light needed for light perception
What is the absolute threshold for humans?
~10 photons minimum on the retina is needed to detect light
Where is the greatest density of rods in the retina?
~20 degrees eccentrically from the fovea at all major meridians
What was the fixation wavelength for the HSP experiment?
Long wavelength or DIM red light
NOTE: any stray photons entering eye could mess up threshold, therefore fixation must be done by cones & NOT rods. Any photons from dim red light (~650 nm) that might reach the rods would have 0% relative luminous efficiency & would NOT affect threshold reading
In low light, is there a receptive-field surround?
NO
Rather, there is a LARGE receptive field center
NOTE: spatial summation occurs over a receptive-field center
In the dark, is there a purpose to have a receptive field surround?
NO
NOTE: spatial summation occurs over a receptive-field center
What happens to spatial summation are you INC retinal eccentricity?
Spatial Summation INCREASES!
More rods
HSP chose what size stimulus?
10 min arc
RECALL: 4-7 degrees eccentric = 20-30 min arc which is too large and would stimulate too many receptive fields and alter threshold for light detection
What is ricco’s law and its equation?
Describes reciprocal relationship btwn THRESHOLD LUMINANCE & STIMULUS AREA
L x A = C
L = luminance
A= area
C = # quanta at threshold
m= -1.0 (1:1)
Ricco’s law only holds for stimulus areas ____ than the critical area.
LESS than critical area
NOTE: size of “critical area” varies w retinal location
What is the critical size for ricco’s law? What happens when you are outside of the critical size?
21’
You need to use PIPER’s LAW when you are outside
What is the equation for Piper’s Law and what is it?
L x A^1/2 = C
m = -0.5 (1:2) –> 1 log unit INC in L offset by 2 log units DEC in A
Piper’s law is a PARTIAL SUMMATION function & applies outside the critical area of ricco’s law
This happens bc the retina can no longer sum light perfectly
Spactial summation still occurs, but at slower rates (i.e. smaller slop)
True or False. Once you hit a flash size BEYOND piper’s critical point, you no longer summate
TRUE
What is temporal summation
The adding together of events that occur at slightly diff time
NOTE: in the eye, temporal summation occurs w/in a limited time period of approx 10-100 ms
True or false. Any combination of quanta w/in the temporal summation interval (10-100 ms) will result in a subject reporting ONLY a single flash
TRUE
Bloch’s law equation and definition?
L x t = C
Describes a reciprocal relationship btwn threshold luminance & stimulus duration
Change in L can be offset solely by a change in t (or vice versa)
m= -1.9
True or False. Bloch’s law is a total summation function.
True
The critical point of 100 ms for Bloch’s law applies only for which retinal cells?
RODS
The critical point for cones is much shorter
What is the square root law?
an unnamed square root law is a PARTIAL SUMMATION function & applies outside the crical area of Bloch’s law
After upper limit of square root law is passed –> straight line (vertical!)
96% of quanta get past the cornea and only 50% reach the retina…. what happened to the rest of the quanta?
- Internal reflection
- Scattering
- Absorption
Of the 50% of the quanta that reach the retina how many get absorbed by the rods?
20%
Loss is due to:
- Absorption by cones & RPE cells
- Passing through outer segment w/o isomerizing rhodopsin
How much worse are cones than rods at catching quanta?
~2X worse
Need ~200 photons at the cornea to reach threshold of light perception
NOTE: 5 quanta need to excite a single cone but summation over 2 cones are required to reach threshold
When we dark adapt is sensitivity increased or decreased?
INCREASED
NOTE: inc sense is DEC threshold
What is a cone branch
Time period in which the test flash is detected by the cones
What is a rod-cone break?
Point during dark adaptation at which rods become more sensitive than cones
NOTE: divides the portion of the dark adaptation curve where the subject detected the test flash w cones from the portion where rods were used to detect the test flash
What is the rod branch?
Time period in which the test flash is detected by the rods
What is the Dowling-Rushton Equation?
log (I threshold/I dark adapted threshold intensity) =P (fraction of photopigment bleached)
What 4 factors allow our eyes to operate over such a wide range of light levels?
- Photoreceptors
- Pupil Size (~1 log unit)
- Changes in Photopigment Concentration (9 log units)
- Neural reponsiveness (1-2 log units)
What is the Duplicity theory under photoreceptors describe?
Rods are used in low-luminance (scotopic) conditions & cones are used in high-luminance (photopic) conditions
How much can our pupil account for light levels at the absolute extreme?
25X difference (~1.4 log units)
The PR/neural adaptation accounts for the other 10-11 log units
What is the equation for pupil size area?
A = pi * (r)^2
1 mm –> 5mm is a 25X difference
If the eye is exposed to 2 log unit INC in luminance, pupil constriction CANNOT fully compensate to keep the retinal illuminance constant
What happens when all photopigments are isomerized?
You are at THRESHOLD & can’t perceive any more light
How long does it take to regenerate rods?
5 minutes, slower than Cones
How long does it take for cones to regenerate?
2 minutes
When does the rod cone break occur?
When 10% of the rods still bleached on a standard dark adaptation curve
NOTE: 10% bleach —> 2 log units above rod threshold
50% bleach —> 8 log units above rod threshold
Does it how much you are bleached with light in order to reach threshold?
NO final threshold will always be the same ~1 log unit
As you dark adapt how will blue light appear vs red light?
Blue=BRIGHTER
Red is the same…
What is a photochromatic interval?
Difference brown the final rod threshold and cone final threshold
When the target luminance is > background luminance, what kind of contrast is this ?
Positive contrast
OR
Negative polarity
NOTE: target luminance is what the target letter is
When the target luminance is < background luminance, what kind of contrast is this ?
Negative contrast
OR
Positive polarity
NOTE: this is what the standard snellen chart is
What is the Weber’s contrast equation?
Cw = [(L target - L background)]/L background
NOTE: Neg contrast (darker letter) —> range: -1.0 to 0 Pos contrast (darker background) —> range: 0 to infinity
What’s the Michaelson Contrast equation?
C m = (Lmax - Lmin)/(Lmax + Lmin)
NOTE:
Negative Contrast range = 0 to 1
Positive Contrast range = 0 to 1
What is deVris Rose Law?
Theoretical minimum for deltaL based on quantum fluctuations alone
m = 0.5
For every 0.5 log increase in background, the deltaL increases by 1 log unit
When does the deVries-Rose law hold true for?
Rod mediated vision under scotopic conditions
NOTE: this law is also known as the square root law
Describe the Weber’s law.
Constant proportional relationship bran the increment threshold deltaL
m = 1
For every 1 log unit increase in background, the deltaL also increases by 1 log unit
When does the Weber’s law hold true?
Higher light levels (~3.18 cd/m^2)
Holds true for BOTH rod-mediated and cone-mediated vision
A line/circle connecting retinal regions with similar increment thresholds (or sensitivity) is what?
Isopter
Inside –> above threshold
Outside –> test light not seen (below threshold)
An optic nerve or coloboma will show what kind of scotoma on VF testing?
Absolute
Visual sensitivity = 0 and the increment threshold is infinity!
A 10 dB change in sensitivity indicates how many log unit change in sensitivity?
1 log unit OR 10X change in sensitivity to a given stimulus
What is veiling luminance/glare? How does it affect contrast?
Scattered light causing a veiling luminance over the retinal image that is proportional to the square of the angle of displacement of a glare source from a directly viewed light
NOTE: extra light in the eye can undergo SCATTER (more w more media opacities), causing a reduced contrast in the retinal image (spreading of the point spread function)
How does veiling luminance affect Weber and Michelson contrast?
It REDUCES both bc it adds together with the background luminance
Threshold acuity depends primarily on what?
CONTRAST/LUMINANCE
NOT on the angle of subtends…… for example a star subtends 0.018 degrees & is primarily detected bc of the contrast/luminance (how bright the star is) & not the small angle
Which type of acuity is the angular size of the smallest visible target (i.e. diameter of the smallest spot of light, width of the thinnest line)?
Detection acuity
Which type of acuity is the smallest spatial offset or difference in location btwn targets that can be disriminated?
Localization acuity
AKA hyper-acuity, spatial-interval acuity, and vernier acuity (5 years old adult like)
What type of acuity is the smallest spatial separation btwn 2 nearby points or lines that can be discriminate?
Resolution acuity
AKA MAR, clinically implemented as GRATING acuity
- Pediatrics (teller cards)
- Disease (aka inferometry)
What type of acuity is an acuity that measures the subject’s MAR by determining their ability to identify a letter, number, or character?
Recognition acuity
AKA VA
True or false. Threshold acuity is dependent on PR spacing
FALSE
It is NOT dependent on PR spacing - it is ONLY dependent on LIGHT
NOTE: a just-detectable spot is wider than the just-detectable line bc the visual system has less opportunity to spatially summate contrast info for a spot, as it can along the length of a line
What is the threshold for a single black line?
0.5”
What is the threshold for a single black dot?
15”
What is the threshold for spatial interval acuity?
2-6”
NOTE: this is the same for vernier as well
Where are the neural mechanisms for localization acuity occur at?
Visual Cortex!
NOT THE RETINA
How is resolution set?
From the trough to one single peak, which is formed btwn 2 photoreceptors (aka center-to center distance of 2 PR)
What is it called when the numerator is 1 min of arc times the test distance and the denominator is the tested observer’s MAR times the test distance?
Snelle fracion
What is the critical detail always?
1/5 of the 5x5 grid OR 1’ (1/5 of total letter height)
To maintain MAR, what must happen to the dimensions of the letter as you move it FORWARD and BACKWARDS?
Forward: decrease
Backwards: increase
What is the equation for VAR?
VAR = 100 - (50 x logMAR)
Each letter is worth how many logMARs?
0.02 logMAR
therefore, 5 letters per line
How much is a one line improvement change a 1.0 logMAR acuity?
0.9 log MAR now
NOTE: every line is a 0.10 logMAR change
In this case 20/200 -> 20/160. 20/20 is 0.0 logMAR
