COLOR VISION: CHAPTER 9

Transcription

1 Clr Visin 1 Perceptin (EXP 4204) Christine L. Ruva, Ph.D. COLOR VISION: CHAPTER 9 What Are Sme Functins Of Clr Visin? What Physical Attributes Are Assciated With Clr? We see bjects because f the frm them and that light in the visible spectrum has wavelengths that are assciated with. Qute frm yu text Clrs are created by ur perceptual system, and althugh specific clrs are related t specific wavelengths, the cnnectins between wavelength and the experience we call clr is an arbitrary ne. There is nthing intrinsically blue abut shrt wavelengths r red abut lng wavelengths. In fact, the light rays are simply energy that has n clr at all. Lking at it this way, clr is nt a prperty f wavelength but is the brain s way f letting us knw what wavelengths are present. Reflectance Curve: What Physical Attributes Are Assciated With Clr? Achrmatic Clrs: light reflectin is acrss the spectrum cntain n hues. cnsists f a cmbinatin f different clr lights with similar/equivalent reflectins. : reflecting almst n light Chrmatic Clrs: sme wavelengths are than thers. Chrmatic clr is als referred t as and is the psychlgical reactin t wavelengths ranging frm abut 400 nm (seen as vilet) t abut 700 nm (seen as red).

2 Clr Visin 2 Selective Reflectin: Example: Yur jeans lk blue because their surface is absrbing mst f the and reflecting t yur eyes primarily light frm the blue prtin f the spectrum. Example: Glaciers inside d nt reflect all wavelengths equally. Instead, the ice selectively reflects and this causes the ice t appear deep blue. HOW CAN WE DESCRIBE COLOR EXPERIENCE? Basic Clrs: Clr Wheel was prpsed by Newtn in 1704 as a way t rganize clrs which matches the rder f the clrs in the visible spectrum. Peple can discriminate amng abut acrss the visible spectrum. Saturatin: Mre clrs can be created by changing a clr s saturatin e.g., Pink = desaturated red. Intensity = making a clr brighter r dimmer. Munsell Bk f Clrs cntained 1,225 clr samples. Additive Clr Mixing Mixing f different wavelengths All wavelengths are available fr the bserver t see Superimpsing blue and yellw lights leads t percept If we were t mix paints f the same clr the perceptin wuld be. Subtractive Clr Mixing Mixing paints with different pigments Additinal pigments (clrs) reflect wavelengths Mixing blue and yellw leads t green

3 Clr Visin 3 TRICHROMATIC THEORY OF COLOR VISION Yung (1802) & vn Helmhltz (1852) bth prpsed that the eyes have 3 types f clr receptrs that detect 3 primary clrs. All ther clrs can be derived by cmbining these three. Clr Matching Experiments: Ss were asked t adjust the amunts f f light mixed tgether in a cmparisn field until the clr f this mixture matched the clr f a single wavelength in a test field. Peple with nrmal clr visin needed t use at least wavelengths t make a match. Peple with clr deficiencies nly needed t mix wavelengths t make the match. Physilgical Evidence fr the Trichrmatic Thery Three Cne Visual Pigments have been identified and are named fr the wavelength that they are mst sensitive t: Shrt (419 nm): These were the last t be identified by scientist. They differ frm the ther cnes because their abslute sensitivity is lwer, they are mre vulnerable t disease, and they are almst cmpletely absent in the center f the fvea. Medium (531 nm) Lng (558 nm) The S-, M-, and L- cnes ccur in a rati f 1:5:10 in the retina. Respnse f Cnes and Clr Perceptin Clr perceptin is based n the respnse f the. Respnses vary depending n the wavelengths available. Cmbinatins f the respnses acrss all three cne types lead t perceptin f all clrs. Clr matching experiments shw that clrs that are perceptually similar ( ) can be caused by different physical wavelengths.

4 Clr Visin 4 Are Three Receptr Mechanisms Necessary fr Clr Visin? Dichrmats have just 2 types f cne pigment and Recent evidence suggests that sme humans have mre than three cne pigments. These different cne systems cause a shift in sensitivity resulting in different clr perceptins than peple with 3 cne systems. OPPONENT PROCESS THEORY Trichmatic thery culd nt explain all clr perceptins. OPPONENT-PROCESS THEORY OF COLOR VISION Ewald Hering (1818, 1820): Clr visin is caused by ppsing respnses generated by and by. Supprt fr this thery came frm: Clr afterimages and simultaneus cntrasts. Types f clr blindness are red/green and blue/yellw. Trichmatic Thery culd nt explain these. AFTER IMAGE DEMO SUPPORT FOR THE OPPONENT-PROCESS THEORY Oppsing Afterimages: a field generates a after image (paired) a field generates a after image (paired) Simultaneus clr cntrast: Surrunding an area with a clr changes the appearance f the surrunded area. In the case f the flag the green afterimage surrunds a white area and causes it t appear red. Visualizing clrs: it is easy t visualize 2 clrs that are nt ppsed (e.g., reddish-yellw) than 2 clrs that are ppsed (e.g., reddish-green). Peple wh are clrblind t are als clr-blind t and peple wh can t see als cannt see.

5 Clr Visin 5 OPPONENT-PROCESS THEORY Three Mechanisms f Oppnent Prcess Thery, each f which respnds in ppsite ways t different intensities r wavelengths f light: Black (-) & White (+) Red (+) & Green (-) Blue (-) & Yellw (+) These respnses were believed t be the result f chemical reactins in the retina Physilgy f Oppnent-Prcess Thery Oppnent Neurns were fund in the They respnd with an respnse t light frm ne end f the spectrum and with an respnse t light frm the ther end. The B+Y- cell The G+R- cell The Y+B- & R+G- cell Oppnent Prcess & Trichrmatic Theries Cmbined Bth theries are supprted by physilgical evidence. Each thery describes physilgical activity at in the visual system. Prcessing Of Clr Infrmatin Takes Place in 2 Stages First, the cnes in the retina respnd with different patterns t different wavelengths. Secnd, neurns in the LGN integrate the inhibitry and excitatry signals frm the receptrs. Clr visin als invlves a number f different, but mre research is needed t examine hw clr is cded in the crtex. COLOR DEFICIENCY

6 Clr Visin 6 COLOR DEFICIENCY TESTING Clr Visin Tests called Ishihara Plates: clr deficient peple either perceive different numbers than thse with nrmal clr visin r n numbers at all. COLOR DEFICIENCY Clr Matching Tests determine the minimum number f wavelengths needed t match anther wavelength in the spectrum. This prcedure has revealed 3 types f clr deficiencies: Mnchrmat: Need nly wavelength Dichrmat: Need wavelengths Anmalus Trichrmat: Needs wavelengths Mixes wavelengths in different prprtins than trichrmats and will nt be as gd at discriminating between wavelengths that are very clse tgether. Mnchrmatism = clr blind They see everything in shades f lightness rare genetic disrder that ccurs in. Only and n functining.. Ability t see nly. Very sensitive t. Gene Disrders & Dichrmatism Sex r X linked disrders ccur when a allele is carried n the X chrmsme. are mre likely t be affected, since they nly have chrmsme and therefre d nt have a crrespnding dminant gene t verride the recessive gene. Red-green clr blindness fr sn t be clr-blind, what must mther be? fr daughter t be clr blind what must parents be?

7 Clr Visin 7 COLOR DEFICIENCY 3 Frms f Dichrmatism: Prtanpia: sex-linked recessive disrder inherited thrugh a gene n the X chrmsme. Affects 1 % f males &.02 % f females. Deuteranpia: sex-linked recessive disrder inherited thrugh a gene n the X chrmsme. Affects 1 % f males &.01 % f females. Tritanpia: very rare, affecting nly.002% f males &.001 % f females. COLOR DEFICIENCY Crtical Clr Blindness: The disrders just described are clr deficiencies resulting frm receptr prblems, but clr deficiency can be caused by crtical prblems resulting frm injury r strke. Cerebral Achrmatpsia: clr visin lss due t brain injury and peple with this cnditin typically see the wrld withut clr. The cne systems are nt affected in these individuals. Therefre, prcessing infrmatin abut wavelength is nt the same as perceptin f clr. Crtical areas are needed t change this stimulatin int smething meaningful. What Des a Persn with Clr Visin Deficiencies Perceive When He/She Views Varius Clrs? T answer this questin we must use a Unilateral Dichrmat wh is a persn with trichrmatic visin in ne eye and dichrmatic visin in the ther. This persn can lk at a clr with his/her dichrmatic eye and then determine which clr it crrespnds t with his/her trichrmatic eye. COLOR CONSTANCY

8 Clr Visin 8 Perceiving Clrs Under Changing Illuminatin Clr cnstancy - perceptin f clrs as relatively cnstant in spite f changing light surces. Sunlight has apprximately equal amunts f energy at all visible wavelengths ( ) Tungsten lighting has mre energy in the lng-wavelengths ( ) Objects frm these tw surces Why des clr cnstancy ccur? Chrmatic Adaptatin: prlnged expsure t chrmatic clr such as red light yur lng-wavelength cne pigment, which decreases yur t red light. When in a tungsten lit rm yur eyes becme adapted t the yellw/lngwavelength-rich tungsten light, which will decrease yur eyes sensitivity t lng wavelengths. This decreased sensitivity causes the lng-wavelength light reflected frm bjects t have less effect than befre adaptatin and this cmpensates fr the greater wavelengths in the tungsten light. The effect f the surrundings: clr cnstancy wrks best when the bject is surrunded by bjects f different clrs and wrks less well if the surrundings are masked. LAB ASSIGNMENT COLOR VISION CHAPTER 9