limits of vision: luminance detection Flashcards
how does electromagnetic radiation (light) behave ?
. light behaves as wave ( e.g. in diffraction )
. lights also behaves as if it was a stream of discrete particles that we call photons
what is the quantal nature of light ?
. a 60-watt lightbulb emits 1.8x10^20 photons per second
. the sun emits 10^45 visible photons ( i.e. photons in the visible light spectrum ) per second
.not all photons reach our eyes
. for a bright star in a clear sky, around 10^5 photons reach your eye
what was the Hecht, Schlaer and Pirenne study about?
. how many photons does it take to trigger (hyperpolarise ) a rod photoreceptor ?
what was the Hecht experiment performed ?
. participants sat in a dark room , bitting on a bite bar, staring at a fixation cross , pressing a button when they see light over the course of several years
what was the method of Hecht experiment ?
1 . the observers sat in the dark for 30 minutes ( to dark adapt )
2 . then a light was presented 20 degrees to the left of the point of fixation
3 . the lights was
- greenish ( wavelength of 510nm +/- 10nm)
- small ( subtended an angle of 10 minutes of arc )
- brief ( 1 millisecond in duration )
4 . the observer was asked to respond yes or no to say whether they saw the flash
5 . the dimmest light detected 60% of the time was determined
6 . the light was varied in intensity ( method of constant stimuli ) , a psychometric function ( frequency of seeing curve ) was fitted to the data and 60% correct threshold computed
what was the minimum detectable photons according to Hecht experiment ?
. 54 to 148 photons
. observer SS managed to detect 54 photons with 60% reliability though on some days he needed as much 104 photons
what happens to the photons that are not absorbed by rod?
. not all photons will be absorbed by a rod
. some (50%) will be reflected off the cornea and ocular media and never reach the retina
. some will reach the rod but will not be absorbed by rhodopsin
how many photons are absorbed by rods according to the Hecht experiment ?
. they concluded that only 10% of incoming photons are absorbed by rods
. so we are actually able to detect 5 to 14 photons
how many rods are the 5 to 14 photons spread over ?
. given the stimulus size , those 5 to 14 photons are spread over 500 rods
. so a single rod is able rod is able to respond to ( signal the presence ) of a single photons of light
what is the conclusion of Hecht experiment ?
. rods can respond to a single photon
what was the Baylor and co experiment ?
. empirical follow-up to Hecht theoretical work
. an electrode was attached to individual toad rods
. they directly observed a response ( change in membrane current ) on around on third of trials
rod summary ?
. rods can respond to a single photon
. effect has been replicated several times since
what was the Koeing and Hofer experiment ?
1 . replication of Hecht but with cones
2 . with rods able to detect 50+/-16 photons at cornea
3 . with cones able to detect 203+/- 38 photons at the cornea
- this corresponds to 22-71 absorbed photons by a cone
- given the size of stimulus and and 199,000 cones per mm square
- this implies 22-71 photons falling across 14 cones
- this means 2-5 photons per cone
what was the Tinsley experiment ?
. the questions was if rods can respond to an individual photon. But can humans ?
1 . Tinsley and co created a light source capable of generating a single photon
2 . gave observers a 2 alternative forced choice
3 . was the photons in the 1st or 2nd interval
4. over 2,420 trials, observers responded correctly 51.6% of the time
5 . a very small but statistically significant difference
6 . not much worse than ideal , since on >90% of trials the photons would not be expected to be absorbed
what is the dimmest light we can see?
- a rod can detect a single photon ( Hecht )
- a person can detect 50 photons at the cornea ( Hecht )
- a person can sometimes can detect a single photon ( Tinsley )
- it depends ??
what are the factors affecting maximum retinal sensibility ?
- state of the observer’s eye ( dark adaptation )
- stimulus location
- stimulus wavelength
- stimulus size
- stimulus duration ( Bloch’s law )
- experimenter criterion
( target threshold)
how does the state of observer’s eye ( dark adaptation ) affect maximum retinal sensibility ?
1 . after 10 mins of darkness rod system takes over as the more sensitivity system
2 . after 30 mins max rod sensitivity reached
how does human vision cope with 9 orders of magnitude of lighting levels ?
. this is achieved by maintaining 2 photoreceptor systems: rods, optimised for low ( scoptopic/mesopic ) lighting and cones, optimized for high ( photopic ) lighting
how does stimulus location affect maximum retinal sensibility ?
. rod density peaks at an eccentricity of approximately 20
. no rods in the centre of fovea ( L and M cones only )
how does stimulus wavelength affect maximum retinal sensibility ?
. wavelength of green light is 510nm +/- 10 nm
. this was intended to maximise rod response
. it was few nm higher than ideal
what is Ricco’s law ?
. it states that for small areas of the retina , absolute detection threshold is reached if luminance x area reaches a constant
. this means that if 100 photons falling on a single rod are as effective as one photon falling simultaneously on 100 rods ( complete summation )
. this is because all nearby photoreceptors ultimately feed into a single ganglion or other high-level neurons
. beyond some critical area , Ricco’s law no longer holds: detection threshold improves , but more gradually
( incomplete summation )
how does stimulus size affect maximum retinal sensibility ?
this is explained by Ricco’s law which suggest that for small areas of the retina, absolute detection threshold is reached if luminance x area reaches a constant
how large is the critical area and how small do we need to make stimulus to ensure there is perfect summation between rods ?
. it varies with eccentricity
. generally thought to be around 30 arcmin in the parafovea
. increasing to around 120 arcmin in the periphery
why did Hecht chose an area of 10 arcmin diameter ?
. since any photons arriving within this area would be summed completely
( maximum sensitivity )
what is Bloch’s law ?
. it is the temporal equivalent of Ricco’s law , and states that for brief stimuli , absolute detection threshold is reached if luminance x duration reaches a constant
. the critical duration is 10-100ms
how does stimulus duration affect maximum retinal sensibility ?
. this is due to Bloch’s law which states that for brief stimuli absolute detection threshold is reached if luminance x duration reaches a constant
. Hecht chose a brief stimulus to ensure that all photons would be summed completely ( maximising sensitivity )
how does experimenter criterion affect maximum retinal sensibility ?
. is Hecht experiment the dimmest light detected 60% of the time was determined
. there is no objectively right or wrong threshold level when measuring vision
. but it is likely that an ordinary unpractised observer would require many hundreds or thousands of photons to perform at a reasonable level
what are the circumstances that would yield the maximum retinal sensibility according to Hecht and co?
. they involve dark adaptation , peripheral vision , small test fields , short exposures and selected portions of the spectrum
