January 22, 2002Psyc , Term 2, Copyright Jason Harrison Perception of Brightness The physics and psychophysics…

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Perception of brightness psychophysics: relate psychological measures to physical ones perception of brightness is one of the simplest aspects of vision to study what is the relationship between –psychological perception of brightness –physical parameters of light intensity

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Units:watts Property: intensity = (amplitude) 2 = energy/sec = power Physical measures 1. Light Ray

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Units: watts, lumens (1 lumen ≈ 1mW) Property: radiance = total energy / sec (all light produced) Physical measures 2. Light Source 120 volt, 100 watt "standard" bulb produces ~1750 lumens

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Units:watts/m 2, lumens/m 2 Property: irradiance = power / area (total falling on surface from all directions) Physical measures 3. Light Received

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Units: fraction (between 0 and 1) - 0 = total absorption (black) - 1 = total reflection (white) Property: reflectance (albedo) = outgoing light incoming light Physical measures 4. Light Reflected

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Psychological measures Brightness subjective estimate of… –intensity? –radiance? –irradiance? –reflectance? of light from a light source or from a reflecting surface

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Psychological measures Lightness subjective estimate of the whiteness of a surface: –intensity? –radiance? –irradiance? –reflectance?

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Relating physical and psychological measures 1.brightness versus intensity 2.brightness versus wavelength 3.brightness depends on surroundings 4.lightness depends on illumination

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Relating physical and psychological measures 1.brightness versus intensity 2.brightness versus wavelength 3.brightness depends on surroundings 4.lightness depends on illumination

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Brightness versus intensity standard light at fixed intensity test light with adjustable intensity adjust power of test until just begins to differ just noticeable difference: JND

January 22, 2002Psyc , Term 2, Copyright Jason Harrison StandardTest A just noticeable difference (JND) at 11W 1 W above standard Brightness versus intensity

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Forced-choice Response A bit more rigorous Q: brighter light on left or right?

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Forced-choice Response A bit more rigorous Q: brighter light on left or right?

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Forced-choice Response A bit more rigorous Q: brighter light on left or right?

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Forced-choice Response A bit more rigorous Q: brighter light on left or right?

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Forced-choice Response A bit more rigorous Q: brighter light on left or right?

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Forced-choice Response A bit more rigorous Q: brighter light on left or right?

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Forced-choice Response A bit more rigorous Q: brighter light on left or right? Analyse accuracy of response versus intensity of lights

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Psychometric function accuracy (probability correct) as a function of intensity of test light Probability of correct response

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Psychometric function accuracy (probability correct) as a function of intensity of test light

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Psychometric function accuracy (probability correct) as a function of intensity of test light

January 22, 2002Psyc , Term 2, Copyright Jason Harrison For I = 10 W, jnd  I = 0.8W For I = 30 W, jnd  I = 2.5W For I = 50 W, jnd  I = 4.0W  I / I =.08 Weber’s Law for a large range of intensities:  I / I = constant

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Weber’s Law for a large range of intensities:  I / I = k k = Weber constant different value of k for different senses –light intensity: k = 0.08 –sound intensity: k = 0.05

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Relating physical and psychological measures 1.brightness versus intensity 2.brightness versus wavelength 3.brightness depends on surroundings

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Brightness depends on wavelength Light 1: at one wavelength Light 2: at different wavelength Adjust power of second light until its brightness is the same as the first

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Brightness depends on wavelength Light 1: at one wavelength Light 2: at different wavelength Adjust power of second light until its brightness is the same as the first

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Brightness depends on wavelength Light 1: at one wavelength Light 2: at different wavelength Adjust power of second light until its brightness is the same as the first

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Brightness depends on wavelength Light 1: at one wavelength Light 2: at different wavelength Adjust power of second light until its brightness is the same as the first

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Result: Luminosity Curve different wavelengths have different luminosities some wavelengths are brighter than others

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Luminosity Curve wavelength (nm) Luminosity Sensitivity in daylight

January 22, 2002Psyc , Term 2, Copyright Jason Harrison wavelength (nm) Luminosity Sensitivity in daylight Sensitivity at night Luminosity Curve

January 22, 2002Psyc , Term 2, Copyright Jason Harrison wavelength (nm) Luminosity Sensitivity in daylight Sensitivity at night Luminosity Curve Photopic vision (cones)

January 22, 2002Psyc , Term 2, Copyright Jason Harrison wavelength (nm) Luminosity Sensitivity in daylight Sensitivity at night Purkinje shift Luminosity Curve Photopic vision (cones)

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Pulfrich Effect

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Relating physical and psychological measures 1.brightness versus intensity 2.brightness versus wavelength 3.brightness depends on surroundings 4.lightness depends on illumination

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Simultaneous brightness contrast: two squares of the same intensity

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Simultaneous brightness contrast: left one looks brighter

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Simultaneous brightness contrast: pattern increases difference

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Recall structuralism perception of each patch = atom atoms are independent perception of patch should not be influenced by surroundings but it is… Visual system always takes context into account –visual intelligence

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Is this lateral inhibition? reduction of signal when neighboring areas are active

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Context matters! simultaneous brightness contrast –affected by immediate surrounding and –context further away the right context can make this effect even stronger…

January 22, 2002Psyc , Term 2, Copyright Jason Harrison The snake illusion (Adelson)

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Relating physical and psychological measures 1.brightness versus intensity 2.brightness versus wavelength 3.brightness depends on surroundings 4.lightness depends on illumination

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Lightness depends on illumination one patch (on a simple background) estimate lightness of patch when different illumination is used

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Result? Lightness unaffected by illumination –lightness constancy Visual system computes lightness as a ratio = intensity of light from square intensity of light from background

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Lightness is invariant under illumination level Thus, obtain an estimate that is a property of the world (object surface) –doesn’t change with lighting rather than the image (light on retina) –does change with lighting

January 22, 2002Psyc , Term 2, Copyright Jason Harrison Finally What was the purpose of this presentation? Which question remains unanswered?