W17 Limits of Vision - Absolute Luminance Detection

what is absolute detection of luminance

ability of a visual system to detect and respond to light intensity at its lowest possible level

how does light behave

as a particle and a wave

what is a quanta

packet of energy

name the group of people that investigated 'how many photons it takes to trigger a rod photoreceptor'

Hecht, Shlaer & Pirenne

describe hecht's experiment

Observer sat in the dark for 30 mins to dark adapt

Light presented 20° to the left from point of fixation

Observer asked to respond yes or no to whether they saw the flash, dimmest light detected 60% of the time determined

describe the light presented in Hecht's experiment in regards to

colour

size

duration presented

green (wavelength of 510 nm)

small (10 mins of arc)

brief (1 millisecond in duration)

Results of Hecht's experiment:

what was the min number of photons detected at the cornea

54-148 photons

Results of Hecht's experiment:

how many of those photons were actually absorbed by rods

5-14

10%

what was the conclusion of Hecht's experiment

one rod detects one photon of light

name the group of people that investigated 'how many photons it takes to trigger a cone photoreceptor'

Koeing & Hofer

Results of Koeing's experiment:

how many photons of light were detected at the cornea in this experiment

203

Results of Koeing's experiment:

how many of these photons were absorbed by the cones

22-71

2-5 per cone

what was the conclusion of Koeing's experiment

cones require multiple photons to respond

why do cones require multiple photons to respond to light

they have a higher activation threshold compared to cones

what did Tinsley et al want to investigate

rods can respond to an individual photon, can humans?

describe Tinsely et al's method

light source created which generates a single photon

ptp given 2AFC (was the photon in the 1st or 2nd interval?)

Results of Tinsley et al's experiment

over 2420 trials, 51.6% answered correctly

what is maximum retinal sensibility

the highest sensitivity of the retina to light, which is primarily determined by rod photoreceptors in scotopic (low-light) conditions.

what are the 6 factors that affect maximum retinal sensibility

state of the eye

stimulus location

stimulus wavelength

stimulus size

stimulus duration

experimenter criterion

how can the state of the eye affect max retinal sensibility

(in relation to Hecht's experiment)

Observer sits in the dark for 30 mins to dark adapt

human vision can cope with 9 magnitude of lighting levels due to the 2 photoreceptor classes:

rods (low/scotopic) & cones (high/photopic)

how many mins after darkness does it 10 for the rod system to take over

10 mins

how many mins after darkness is max rod sensitivity reached

30 mins

how can stimulus location affect max retinal sensibility

(in relation to Hecht's experiment)

Light presented 20° left of fixation

Rod sensitivity peaks at eccentricity of ~20°

no rods at the centre of the fovea

how can stimulus wavelength affect max retinal sensibility

(in relation to Hecht's experiment)

Light greenish w wavelength of 510 nm

This maximises rod response because it corresponds to the peak sensitivity of rhodopsin pigment

how can stimulus size affect max retinal sensibility

(in relation to Hecht's experiment)

Light was small and subtended at angle of 10 minutes of arc

Ricos law

photos arriving in this area will be summed completely

what is Rico's law

for small areas of the retina the absolute detection threshold is reached if luminance x area reaches a constant

100 photons that fall on one rod are as effective as one photon falling simultaneously on 100 rods

creates complete summation

how can stimulus duration affect max retinal sensibility

(in relation to Hecht's experiment)

The light was brief (1 millisecond in duration)

Blochs law

made sure all photons are summed completely

what is Bloch's law

for brief stimuli absolute detection is reached if luminance x duration reaches a constant

eg: a dim light shown for a longer time can be just as visible as a brighter light shown for a shorter time, as long as the total energy remains above the detection threshold.

what is critical duration

maximum time over which temporal summation occurs in the visual system

how long is critical duration

10-100ms

how can experimentor criterion affect max retinal sensibility

(in relation to Hecht's experiment)

Dimmest light detected 60% of the time was determined

Tinsely et al's findings (51.6%)

Therefore sometimes we can detect 1 photon, but unreasonable to claim we can detect