Photometry Flashcards
The human eye is sensitive to wavelengths between…
400 and 720nm
The ozone layer protects us from what?
lethal shorter wavelengths
- while H2O and CO2 decrease levels of infra-red radiation in the environment
Define radiometry
- measurements concerned with the electromagnetic spectrum
- including visual wavelengths
- doesn’t take into account the visual systems response to wavelengths of light
Define photometry
- measurements concerned with how light affects the visual system
- does take into account the visual systems differential response to wavelengths of light
for each photometric measurement, there is….
…an equivalent radiometric measurement
What is the photopic luminosity curve?
it indicates how sensitive the retina is in high light levels to different wavelengths of light
what is the unit for photometry
lumens which is a measure of luminous power
By convention, how many lumens per watt
680 lumens per watt at 555nm (the peak of the luminosity curve)
How to calculate the luminance power of a light source?
watts x 680 x eye sensitivity (read off the graph at whatever nm it is asking for)
what is luminous power and units
- 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
what is luminous intensity and what are the units
- refers to the number of lumens produced in a given direction
- units - candela (one lumen per steradian)
what is a steradian
- 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
what is luminance and what are the units
- 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)
what is illuminance and what are the units
- illuminance refers to luminous power that falls on a surface
- units are lumens per metre squared and per square foot
Illuminance formula
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
which parts of the eye provides most of the retinas protection from UVC
the cornea and lens
Image quality is dependant on:
- refractive error
- transparency of media
- amount of light falling on retina
- how well the image is focussed
what are the four stages of perception
- Detection - is there anything there?
- Identification - What is it?
- Discrimination - Is it the same as that thing there?
- Scale - How similar/different is it to something else?
define what is meant by absolute threshold
- the minimum amount of stimulus energy needed to elicit a detection response from an individual
What are the 6 ways to measure a threshold?
- the method of limits
- staircase method
- the method of constant stimuli
- adaptive methods
- the method of adjustment
- forced choice procedures
method of limits
- most simple method
- stimuli presented in either ascending or descending order
- observer says whether he can see the stimulus or not
the staircase method
- 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.
the method of constant stimuli
- 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
adaptive methods
- 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
adaptive methods: PEST
parameter estimation by sequential testing designed to address the problem of step size and starting intensity
the method of adjustment
- 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
Forced choice procedures
- 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%
what is the signal detection theory
- 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)
no signal present - just noise
- 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.
signal present plus noise
- sometimes the noise will obscure the signal and you will not see it
- hits and misses
what is a liberal criterion
A liberal criterion (b) results in a high hit rate when a stimulus is present but also a high number of false alarms.
what is a conservative criterion
A conservative criterion (b) results in a significant number of misses when a stimulus is present but very few false alarms
what does the sensitivity of a test mean
Sensitivity = Hits / (Hits+Misses)
= True Positives/(True Positives + False Negatives)
what does the specificity of a test mean
Specificity = Correct rejects / (Correct rejects +False alarms)
= True Negatives/(True Negatives + False Positives)
what is just noticeable difference
detection of a stimulus against a background
what is weber’s law
ΔLt/Lb = K
- ΔLt is the JND
- Lb is the background luminance
Weber’s law
- 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
what are factors that influence visual sensitivity
- Background Illumination or Light Adaptation
- dark adaptation
- eccentricity
- pupil diameter
- spatial summation
- temporal summation
- optical and neural factors
Ricco’s law (spatial summation) is expressed as:
L x A = K
- L is luminance
- A is the area of stimulus
- K is constant value
Bloch’s law (temporal summation)
L x T = K
- L is luminance
- T is the time
- K is constant value
What are the 4 aspects of sine wave gratings that can be altered
- contrast
- spatial frequency
- orientation
- spatial phase
how to work out contrast of grating
C = (Lmax - Lmin) / (Lmax + Lmin)
C can have a value between 0.0 and 1.0
