Photometry Flashcards
(42 cards)
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
