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PVP > Photometry > Flashcards

Photometry Flashcards

1
Q

The human eye is sensitive to wavelengths between…

A

400 and 720nm

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2
Q

The ozone layer protects us from what?

A

lethal shorter wavelengths
- while H2O and CO2 decrease levels of infra-red radiation in the environment

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3
Q

Define radiometry

A
  • measurements concerned with the electromagnetic spectrum
  • including visual wavelengths
  • doesn’t take into account the visual systems response to wavelengths of light
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4
Q

Define photometry

A
  • measurements concerned with how light affects the visual system
  • does take into account the visual systems differential response to wavelengths of light
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5
Q

for each photometric measurement, there is….

A

…an equivalent radiometric measurement

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6
Q

What is the photopic luminosity curve?

A

it indicates how sensitive the retina is in high light levels to different wavelengths of light

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7
Q

what is the unit for photometry

A

lumens which is a measure of luminous power

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8
Q

By convention, how many lumens per watt

A

680 lumens per watt at 555nm (the peak of the luminosity curve)

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9
Q

How to calculate the luminance power of a light source?

A

watts x 680 x eye sensitivity (read off the graph at whatever nm it is asking for)

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10
Q

what is luminous power and units

A
  • total light power provided by a source as measured by the eye
  • units: lumens
  • used to specify the amount of light a source emits regardless of the direction of the emissions
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11
Q

what is luminous intensity and what are the units

A
  • refers to the number of lumens produced in a given direction
  • units - candela (one lumen per steradian)
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12
Q

what is a steradian

A
  • surface area of a sphere in the same way a radian is related to the circumference of a circle
  • a radian cuts out a length of a circle circumference equal to the radius
  • a steradian cuts out an area of the sphere equal to the radius^2
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13
Q

what is luminance and what are the units

A
  • Luminance quantifies the amount of light coming off a surface in a specified direction.
  • The unit for luminance is commonly candelas per square metre (cd/m2)
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14
Q

what is illuminance and what are the units

A
  • illuminance refers to luminous power that falls on a surface
  • units are lumens per metre squared and per square foot
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15
Q

Illuminance formula

A

E = I/d^2
Where:
E = illumination falling on the surface
I = intensity of the light source
d = distance from the light source to the surface

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16
Q

which parts of the eye provides most of the retinas protection from UVC

A

the cornea and lens

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17
Q

Image quality is dependant on:

A
  • refractive error
  • transparency of media
  • amount of light falling on retina
  • how well the image is focussed
18
Q

what are the four stages of perception

A
  1. Detection - is there anything there?
  2. Identification - What is it?
  3. Discrimination - Is it the same as that thing there?
  4. Scale - How similar/different is it to something else?
19
Q

define what is meant by absolute threshold

A
  • the minimum amount of stimulus energy needed to elicit a detection response from an individual
20
Q

What are the 6 ways to measure a threshold?

A
  1. the method of limits
  2. staircase method
  3. the method of constant stimuli
  4. adaptive methods
  5. the method of adjustment
  6. forced choice procedures
21
Q

method of limits

A
  • most simple method
  • stimuli presented in either ascending or descending order
  • observer says whether he can see the stimulus or not
22
Q

the staircase method

A
  • a combination of ascending and descending limits
  • when observer sees the stimulus direction is reversed and visibility reduced until the observer reports it’s disappeared
  • then reversed and intensity is increased until visible.
23
Q

the method of constant stimuli

A
  • fixed set of stimuli chosen before starting
  • presented randomly and for each, observer reports whether they see a stimulus or not
  • each stimulus presented multiple times
  • stimulus intensity plotted against % of seen responses
  • very time consuming
24
Q

adaptive methods

A
  • involves presenting signals based on previous responses
  • three correct responses means intensity decreased by one step
  • one incorrect response means one step increase in intensity
  • session ends when narrow range of stimuli reached
25
Q

adaptive methods: PEST

A

parameter estimation by sequential testing designed to address the problem of step size and starting intensity

26
Q

the method of adjustment

A
  • subject adjusts IV (for example intensity of light using a knob) until the threshold is reached
  • quick but suffers from variation in subjects threshold criterion
27
Q

Forced choice procedures

A
  • subject is forced to choose from a number of alternative choices, one of which contains the stimulus.
  • a two-alternative forced choice describes a subject choosing between two alternatives.
  • there’s is 50% chance of a correct response so 100+50= 150/2= 75%
28
Q

what is the signal detection theory

A
  • theory assumes that within the observers visual system there is randomly fluctuating noise (N).
  • the task is to tell the difference between the internal noise (N) and the signal combined with this internal noise (N+S)
29
Q

no signal present - just noise

A
  • sometimes you will detect a stimulus when there is only noise

correct rejection - when light is off and observer says no light

false alarm - when noise is so extreme it is beyond the decision criteria and you say the light is on.

30
Q

signal present plus noise

A
  • sometimes the noise will obscure the signal and you will not see it
  • hits and misses
31
Q

what is a liberal criterion

A

A liberal criterion (b) results in a high hit rate when a stimulus is present but also a high number of false alarms.

32
Q

what is a conservative criterion

A

A conservative criterion (b) results in a significant number of misses when a stimulus is present but very few false alarms

33
Q

what does the sensitivity of a test mean

A

Sensitivity = Hits / (Hits+Misses)
= True Positives/(True Positives + False Negatives)

34
Q

what does the specificity of a test mean

A

Specificity = Correct rejects / (Correct rejects +False alarms)
= True Negatives/(True Negatives + False Positives)

35
Q

what is just noticeable difference

A

detection of a stimulus against a background

36
Q

what is weber’s law

A

ΔLt/Lb = K

  • ΔLt is the JND
  • Lb is the background luminance
37
Q

Weber’s law

A
  • starts to break down at very high levels of luminance as the photoreceptors start to become saturated
  • also breaks down at very low levels of luminance because sensitivity is limited by internal noise within the retina
38
Q

what are factors that influence visual sensitivity

A
  1. Background Illumination or Light Adaptation
  2. dark adaptation
  3. eccentricity
  4. pupil diameter
  5. spatial summation
  6. temporal summation
  7. optical and neural factors
39
Q

Ricco’s law (spatial summation) is expressed as:

A

L x A = K

  • L is luminance
  • A is the area of stimulus
  • K is constant value
40
Q

Bloch’s law (temporal summation)

A

L x T = K

  • L is luminance
  • T is the time
  • K is constant value
41
Q

What are the 4 aspects of sine wave gratings that can be altered

A
  1. contrast
  2. spatial frequency
  3. orientation
  4. spatial phase
42
Q

how to work out contrast of grating

A

C = (Lmax - Lmin) / (Lmax + Lmin)
C can have a value between 0.0 and 1.0

PVP (12 decks)

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