Perception of brightness. Perception of Brightness. Physical measures 1. Light Ray. Physical measures 2. Light Source
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1 Perception of Brightness The physics and psychophysics 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, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison Physical measures 1. Light Ray Physical measures 2. Light Source Amp Property: intensity = (amplitude) 2 = energy/sec Property: radiance = total energy / sec (all light produced) l = power Units: watts Units: watts, lumens (1 lumen 1mW) 120 volt, 100 watt "standard" bulb produces ~1750 lumens January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison
2 Physical measures 3. Light Received Property: irradiance = power / area (total falling on surface from all directions) Units: watts/m 2, lumens/m 2 Physical measures 4. Light Reflected Property: reflectance (albedo) = outgoing light incoming light Units: fraction (between 0 and 1) - 0 = total absorption (black) - 1 = total reflection (white) January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , 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 Psychological measures Lightness subjective estimate of the whiteness of a surface: intensity? radiance? irradiance? reflectance? January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison
3 Relating physical and psychological measures 1. brightness versus intensity 2. brightness versus wavelength 3. brightness depends on surroundings 4. lightness depends on illumination 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, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , 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 Brightness versus intensity Intensity = 10 W Intensity = 12 W Light 1 Light 2 Standard Test A just noticeable difference (JND) at 11W 1 W above standard January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison
4 Forced-choice Response Forced-choice Response January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison Forced-choice Response Forced-choice Response Intensity = 13 W Intensity = 10 W January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison
5 Forced-choice Response Forced-choice Response January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison Forced-choice Response Psychometric function accuracy (probability correct) as a function of intensity of test light Standard = 10W Analyse accuracy of response versus intensity of lights Probability of correct response 100% Probability of rrect response 50% Intensity of test light January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison
6 Psychometric function accuracy (probability correct) as a function of intensity of test light Psychometric function accuracy (probability correct) as a function of intensity of test light January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison Weber s Law for a large range of intensities: DI / I = constant For I = 10 W, jnd DI = 0.8W For I = 30 W, jnd DI = 2.5W For I = 50 W, jnd DI = 4.0W DI / I =.08 DI / I =.08 DI / I =.08 Weber s Law for a large range of intensities: DI / I = k k = Weber constant different value of k for different senses light intensity: k = 0.08 sound intensity: k = 0.05 January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison
7 Relating physical and psychological measures 1. brightness versus intensity 2. brightness versus wavelength 3. brightness depends on surroundings 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, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison Brightness depends on wavelength Light 1: at one wavelength Light 2: at different wavelength 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 Adjust power of second light until its brightness is the same as the first January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison
8 Brightness depends on wavelength Light 1: at one wavelength Light 2: at different wavelength Result: Luminosity Curve different wavelengths have different luminosities some wavelengths are brighter than others Power = 10 W Power = 3 W Light 1 Light 2 Adjust power of second light until its brightness is the same as the first January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison Luminosity Curve Luminosity Curve in daylight at night in daylight Luminosity Luminosity wavelength (nm) wavelength (nm) January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison
9 Luminosity Curve Luminosity Curve Purkinje shift at night in daylight at night in daylight Luminosity Scotopic vision (rods) Photopic vision (cones) Luminosity 0.6 Luminosity 0.4 Scotopic vision (rods) Photopic vision (cones) wavelength (nm) wavelength (nm) January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison Pulfrich Effect 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, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison
10 Simultaneous brightness contrast: two squares of the same intensity Simultaneous brightness contrast: left one looks brighter January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison Simultaneous brightness contrast: pattern increases difference 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, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison
11 Is this lateral inhibition? reduction of signal when neighboring areas are active Context matters! simultaneous brightness contrast affected by immediate surrounding and context further away the right context can make this effect even stronger January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison The snake illusion (Adelson) 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, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison
12 Lightness depends on illumination one patch (on a simple background) estimate lightness of patch when different illumination is used 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, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , 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 Finally What was the purpose of this presentation? Which question remains unanswered? rather than the image (light on retina) does change with lighting January 22, 2002 Psyc , Term 2, Copyright Jason Harrison January 22, 2002 Psyc , Term 2, Copyright Jason Harrison
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