L018LEONARD MATINV FIG. 7. Recording of OH, Ov, and OT during and immediately following a voluntary shift of fixation of 15 min in OH (at A). The scale of 1.5, 1.7, and 1.9 min for OT, Oil, and Ov, respectively, refer to 1 cm distance on the original recording. Upward deflection of the trace refers to an eye movement to the left in OH, upward in ov, and top of head turned temporally in OT. Each of the two lower traces monitors the total radiant energy reflected from one of the contact lens mirrors and indicates less than 1% variation in this quantity; the displayed measurements of OH, Ov, and OT are, however, corrected for any such variations (see Sec. IIIC). The very light time-lines are separated by 10 msec intervals. The brief interruptions that appear twice in each trace on this record are used in identifying the separate traces. previously reported. 2 The overshoot in OH during the there was no attachment to the eye." 6 The flick in 6 v voluntary fixation shift at A is typically observed for is also frequently observed during voluntary horizontal this magnitude of shift, although it is slightly larger shift; its magnitude and direction relative to the direc- here than usual; such overshoots have been previously tion of OH is markedly variable. reported during larger voluntary shifts of fixation when 16 G. Westheimer, AMA Arch. Opthalmol. 52, 110 (1954). JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 54, NUMBER 8 AUGUST 1964 Color Responses in an Organic Photoconductive Cell BARNETT ROSENBERG, ROBERT J. HECK, AND KAISER Aziz Biophysics Department, Michigan State University, East Lansing, Michtigan (Received 15 February 1964) A photovoltaic effect and photoconductivity, each with its own spectral response curve, occur simul- taneously in a d-carotene cell. The two effects produce currents in opposite directions and with different response times. The combination of these effects imparts to the total response curve a unique shape for each region of the visible spectrum. Blue light produces a monophasic, negative deflection; red light produces a monophasic positive deflection; and intermediate wavelengths produce diphasic responses, which are mixtures of these two. The relative amount of each component depends upon wavelength. The shapes of these response curves are in detailed agreement with the chromatic "S" potentials found in the retinas of some animals. These results suggest a physical basis for these biological responses. The action spectra for these curves are of the same general nature as the chromatic functions of the Hering opponent- processes theory in vision This suggests that two such cells as these (a red-green, and blue-yellow type) in conjunction with a black-white detector, would suffice to account for all color mixing data, and thus form a new basis for objective colorimetry. Mixtures of light of longer and shorter wavelengths simulate the response to any monochromatic light. Broadband light of a given color produces the same character of response as a monochromatic beam of a similar color. The response curves for a given color are fairly stable with a tenfold change of irradiance The balance (or neutral) point at a wavelength where the opposing currents are equal, is a function of the voltage applied to the cell; it shifts to shorter wavelengths as the voltage increases. A primitive type of color matching can be accomplished with a single cell. I. INTRODUCTION depends monotonically upon the absorbtance. Absorp- pHYSICAL and chemical detectors of radiant energy tion occurs over a band of wavelengths, within which 1 operate on a common principle: if the light is absorbed photons of different wavelengths give equiva- absorbed, a response is obtained, and the response lent responses. Therefore, the response is color blind. 1018 Vol. 54
Transcript
L018LEONARD MATINV
FIG. 7. Recording of OH, Ov, and OT during and immediately following a voluntary shift of fixation of 15 min in OH (at A). The scaleof 1.5, 1.7, and 1.9 min for OT, Oil, and Ov, respectively, refer to 1 cm distance on the original recording. Upward deflection of the tracerefers to an eye movement to the left in OH, upward in ov, and top of head turned temporally in OT. Each of the two lower traces monitorsthe total radiant energy reflected from one of the contact lens mirrors and indicates less than 1% variation in this quantity; the displayedmeasurements of OH, Ov, and OT are, however, corrected for any such variations (see Sec. IIIC). The very light time-lines are separatedby 10 msec intervals. The brief interruptions that appear twice in each trace on this record are used in identifying the separate traces.
previously reported.2 The overshoot in OH during the there was no attachment to the eye."6 The flick in 6 vvoluntary fixation shift at A is typically observed for is also frequently observed during voluntary horizontalthis magnitude of shift, although it is slightly larger shift; its magnitude and direction relative to the direc-here than usual; such overshoots have been previously tion of OH is markedly variable.reported during larger voluntary shifts of fixation when 16 G. Westheimer, AMA Arch. Opthalmol. 52, 110 (1954).
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 54, NUMBER 8 AUGUST 1964
Color Responses in an Organic Photoconductive CellBARNETT ROSENBERG, ROBERT J. HECK, AND KAISER Aziz
Biophysics Department, Michigan State University, East Lansing, Michtigan(Received 15 February 1964)
A photovoltaic effect and photoconductivity, each with its own spectral response curve, occur simul-taneously in a d-carotene cell. The two effects produce currents in opposite directions and with differentresponse times. The combination of these effects imparts to the total response curve a unique shape for eachregion of the visible spectrum. Blue light produces a monophasic, negative deflection; red light produces amonophasic positive deflection; and intermediate wavelengths produce diphasic responses, which aremixtures of these two. The relative amount of each component depends upon wavelength.
The shapes of these response curves are in detailed agreement with the chromatic "S" potentials found inthe retinas of some animals. These results suggest a physical basis for these biological responses. The actionspectra for these curves are of the same general nature as the chromatic functions of the Hering opponent-processes theory in vision This suggests that two such cells as these (a red-green, and blue-yellow type)in conjunction with a black-white detector, would suffice to account for all color mixing data, and thus forma new basis for objective colorimetry.
Mixtures of light of longer and shorter wavelengths simulate the response to any monochromatic light.Broadband light of a given color produces the same character of response as a monochromatic beam of asimilar color. The response curves for a given color are fairly stable with a tenfold change of irradiance Thebalance (or neutral) point at a wavelength where the opposing currents are equal, is a function of the voltageapplied to the cell; it shifts to shorter wavelengths as the voltage increases. A primitive type of color matchingcan be accomplished with a single cell.
I. INTRODUCTION depends monotonically upon the absorbtance. Absorp-pHYSICAL and chemical detectors of radiant energy tion occurs over a band of wavelengths, within which1 operate on a common principle: if the light is absorbed photons of different wavelengths give equiva-
absorbed, a response is obtained, and the response lent responses. Therefore, the response is color blind.
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ORGANIC PHOTOCONDUCTIVE CELL
Various arrangements (calorimeters) have been de-veloped, some of which use several detectors, whileothers use a single detector several times, to simulatecolor discrimination.
This article will describe a method of simulatingcolor discrimination with a single detector, operating asingle time. The novelty will be the production ofqualitatively different physical responses. We believethat the introduction of this third system exhausts thepossible ways of utilizing radiation detectors to deter-mine color. If so, calorimeters and color vision mustbe based upon one or more of these three systems, andno other. The model we describe here is one conceivablemethod of realizing this third system.
Until recently, all theories of color vision and allcolorimeters have been based upon the first and secondsystems. One of us has already proposed a possibleexplanation of color vision based upon the thirdsystem.' We will discuss some elements of this theoryin later sections, but will leave a detailed description of thetheory for a further publication. This paper will describeprimarily the physical phenomena, and the data thatwe have so far obtained to verify that this is indeed acolor sensitive system and suitable as a basic unit forcolorimetry.
The two types of responses which are involved in thissystem are the photovoltaic effect and photoconduction.These effects normally occur in many different ma-terials, both inorganic and organic. While we havechosen to work with carotenoid pigments because oftheir peculiar ubiquity in biological photoreceptors,this implies no lack of generality in the conclusions.Any material having the properties described below willfunction as a color detector.
Previous work has indicated that 13-carotene is asensitive photoconductive material.2' 3 The photovoltaiceffect also occurs in these cells.4 We shall show here thatin a 3-carotene cell, irradiated under particular condi-tions, two opposing current processes occur. Shortwavelength light (blue), which is strongly absorbed,generates a photovoltaic current in one directionwhereas long wavelength light (red), which is weaklyabsorbed, photoconductively passes a current in theopposite direction. These phenomena have slightlydifferent time constants. Light of intermediate wave-lengths will produce a combination of both opposingcurrents. These effects give rise to a complex response,the characteristics of which are unique for each spectralregion and also for each color.
We should like to emphasize here that while a singlecell operating in this manner does indeed show differen-tial color responses, it would require at least two suchcells-with different wavelength sensitivities-to oper-
B. Rosenberg, Photochem. Photobiol. 1, 117 (1962).2 B. Rosenberg, J. Chem. Phys. 31, 238 (1959).
B. Rosenberg, J. Chem. Phys. 34, 63 (1961).B. Rosenberg in tlectronic Conductivity in Organic Solids edited
by H. P. Kallmann and Al. Silver (Interscience Publishers, Inc.,New York, 1962), p. 291.
ate as a calorimeter. The responses of these cells arealmost exact physical counterparts of the physiologicalresponses predicated in Hering's opponent-processestheory.' For the opponent-processes theory to fullyaccount for the data of color mixing, two opponent-process units are necessary for color, and another forblack and white.'
The spectral dependence of the photovoltaic effectresembles the spectral absorption of the carotenoidpigment. The spectral dependence of photoconductivityobtained by irradiating the cell through the positiveelectrode also agrees with these spectra. However,although the spectral dependence of photoconductivityobtained by irradiating the cell through the negativeelectrode depends upon the shape of the absorption spec-trum of the pigment, it does not agree with it. In fact, inthe wavelength region where light is strongly absorbed,there is no photoconductivity at all. Similarly, in theregion where light is not absorbed, there is no photo-conductivity. The peak of its spectral dependence occursat a wavelength where the absorbance of the pigmentcell is about 0.3. This accounts for the red sensitivityof our cells. On the same basis, we may also predictthat another peak of photoconductivity should occuron the ultraviolet side of the absorption band. Thisregion is at present inaccessible to measurement in ourapparatus.
II. APPARATUS AND METHODS
Pure, synthetic crystalline all trans 3-carotene powder(from the Hoffmann-LaRoche Company) is placedbetween two conductive glass plates, which are clampedtogether to form a sandwich. Since the crucial phe-nomenon is the difference in spectral sensitivities forillumination from the positive and negative electrodes,the sandwich configuration is the only feasible one.Along two sides of the sandwich, Teflon strips twomils thick are placed as spacers. The cell is then gentlyheated in a small bunsen flame until the carotene melts.In the melt, isomerization of the carotene occurs; whenit is cooled, it forms a glass. This appears dark red;it is optically isotropic and its x-ray diffraction patternindicates that it is amorphous. The carotene adheressufficiently to the glass plates to cement the cell together.
/3-carotene is an electronic conductor. The majoritycurrent carriers are positive holes rather than electrons.From the rectification ratio we may infer that the con-tribution of positive holes to the current is about 10'times as great as the contribution of electrons. 2 Wetherefore ignore the electrons and consider only themotions of positive holes in our analysis. One furtherpoint necessary for the explanation of the data givenbelow stems from this; only light absorbed in theimmediate vicinity of the positive electrode is effectivein producing photocurrents.
The cell is covered with black tape except over the
I L. M. Hurvich and D. Jameson, Psychol. Rev. 64, 384 (1957)
August 1964 1019
ROSENBERG, HECK, AND AZIZ
T L
FIG. 1. Diagram of apparatus. The dotted section at a rightangle to the main bench, and the half-silvered mirror (M) wereused to superpose a second light beam upon the first. The labeledparts are: projector (P), interference filter (I), broadband filter(F), lens (L), thermopile (T), shutter (S), conduction chamber(C), voltage source (V), electrometer (E), and oscilloscope andcamera (0).
area irradiated to eliminate an effect of scattered light.It is placed in the conductivity chamber and irradiatedby a 750-W projection lamp through the optical trainshown in Fig. 1. A lens produces a roughly parallel beamof light from the projector. This passes through thefilters appropriate for the experiment-either neutraldensity filters, broadband Corning filters, or interfer-ence filters (Baird-Atomic, type B-3)-in conjunctionwith broadband filters to eliminate all sidebands. Asecond lens of short focal length is used to impart moredivergence to the beams so that we can vary theirradiance by varying the distance on the optical bench.The irradiance is measured with a calibrated Eppleythermopile directly in front of the conductivity chamber.The irradiance on the f-carotene cell is 0.7 of that meas-ured by the thermopile. The irradiance and photonnumbers shown in the figures should be multiplied bythis factor.
For measurements with interference filters, theirradiance is adjusted to give a constant number ofphotons incident on the cell. For light of a broadspectral band we give the irradiance in microwatts persquare centimeter.
To effect the superpositon of two beams of light, asecond projection lamp was placed on another opticalbench at right angles to the first, as shown in Fig. 1.The two beams were superposed by means of a half-silvered mirror at 450 to both beams. The irradiance byeach beam was determined separately.
Voltage was supplied to the cell by a small batteryoperating through a helipot, a voltmeter and a reversingswitch.
'T'he photocurrents were determined with a Keithley603 electrometer (used single-ended) with a 108 Q gridresistor. The output of the electrometer was put into aHewlet-Packard oscilloscope and photographed with aPolaroid camera. The limitations of this apparatus were:
comparatively high noise level at the electrometer out-put, and long response time constants due to low irradi-ance of the f-carotene cell.
III. PHOTOVOLTAIC RESPONSES
A. Wavelength DependenceIn the absence of an applied voltage, a current flows
through the circuit when the cell is irradiated. This isthe photovoltaic effect. With a 1010 Q resistor in thecircuit the voltage generated by white light at anirradiance of about 102 /,W/cm 2 is approximately 200mV. This varies slightly from sample to sample. As wedecrease the resistor value in the electrometer circuit,the voltage decreases, until at 108 S1 it is about 80 mV.Some characteristics of the photovoltaic effect havebeen described in a prior article. 4 In the case of alltrans $-carotene films, the spectral sensitivity isclosely similar to the spectral absorptance of the lowestsinglet 7r-7r* transition (400-500 mIu). In the case ofcis isomers, the photovoltaic spectral sensitivity has amaximum in the region of the "cis peak" absorption at360 m,4 and is a broad band extending through thevisible region. A mixture of isomers may show anymixture of these two spectra, depending on the ratio ofisomers present.
We have determined the spectral sensitivity for thepresent cells, by use of interference filters and the pro-
`N'432 >464 A= 496
.=533 :X = 567 X=585
0= 604 Z= 618 '= 633
700 -^A= 733 3- 766
FIG. 2. Photovoltaic response curves of the #-carotene cell.Each picture is for the wavelength in mu (using interferencefilters) shown beneath. The points at which the shutter was openedand closed are obvious. The horizontal sweep speed was 1 sec/div.The electrometer input resistance was 108 U. The vertical sen-sitivity was 2.9 mV/div. The irradiance was 390X1013 photons/sec/cm2 for all wavelengths.
1020 Vol. 54
ORGANIC PIHIOTOCONDUCTIVE CELL L
jection lamp. The responses for each wavelength areshown in Fig. 2. Here we show not just the magnitudeof the response after a steady state has been reached,but also the curve shapes in time. The peak wavelengthof the interference filter is shown beneath each pictuie.The incident photon number was held constan for allwavelengths at the value of 390X 1013 photons/cm 2/sec.We have adopted the convention here that currentswhich are described by positive holes moving in thedirection of the light will be designated 'negative' andshown as a downward deflection in the oscilloscopepictures.
The time constant of the photovoltaic effect is about0.2 sec. The time constant for decay is greater than therisetime constant. The curve shapes for this singleresponse type are the same for all wavelengths: theyare monophasic. The magnitude of the deflection at thetime the shutter was closed is plotted (curve A, opendots, left-hand scale) in Fig. 3 as a function of the wave-length. The curve has a maximum in the region of400-500 mu4, the region of strong absorption. At longerwavelengths, the absorptance decreases and the sampleis more uniformly excited through its depth. This de-creases the magnitude of the photovoltaic effect, whichintrinsically depends upon nonuniform excitation indepth.
When unfiltered white light is used, the magnitudeof the photovoltaic response seems to be proportionalto the logarithm of the irradiance.
B. Time Dependence
The photovoltaic responses were recorded for whitelight. We attempted to find analytical expressions forthese shapes. Expressions requiring one, two, and threeconstants were tried. The only function of time thatfitted the data was hyperbolic.
The rise curve is given by:
(1)
0
a4
12
16
20
50
100
150
200
,.
A / .
B , ...
400 500 600
WAVELENGTH ( Im~limicrons)
700 800
FIG. 3. Spectral responses of photovoltaic effect and of photo-conduction when irradiation was through the positive electrode.Photovoltaic response: curve A, open dots, left-hand scale.Photoconductive effect: curve B, solid dots, right-hand scale,applied voltage, 5 V. Recorded just prior to closing the shutterand plotted against the wavelength of the interference filter used.The electrometer input resistance was 108 Q. The irradiance was390X 10'3 photons/sec/cm 2 for all wavelengths.
.01 .1 I 10TIME (SEC)
FIG. 4. (A) The rise curve of the photovoltaic effect. Y=/V(Vrn-V). Vin=180 mV; A= 6.1, n=l.1 (B) The decay curve ofthe photovoltaic effect. Y= (Vm/V)-1. Vm = 105 mV; A =0.31;n=0.82.
For the decay curve, the equation is:
V(t)= Vmj(1+AtI ), (2)
where Vm is the current (through a 108-Q resistor) afterinfinite excitation time; A and n are constants (n isusually close to 1.0). By use of these equations, thedata were plotted so as to fall on a straight line if theequations were correct. The results shown in Fig. 4verify these equations. Much faster transients mayoccur, which are not detectable because of the highload resistance. At present we are attempting toevaluate the dependence of the constants A and nupon irradiance and wavelength. The response time isinversely proportional to the absorbed flux.
IV. PHOTOCONDUCTION PROCESS
A. Irradiated through the Positive Electrode
A dc voltage of 5.0 V was applied across the cell andit was irradiated through the positive electrode. Theconditions of irradiation were identical to those usedto obtain the photovoltaic curves. Under these condi-tions the positive charge moves in the direction of thelight and gives a negative response, as did the photo-voltaic effect. With this large voltage, the photovoltaiceffect gave a negligible contribution to the photoconduc-tion current. The results of these measurements areshown in Fig. 5. Again the largest response is in theblue region (strongest absorption in the neighborhoodof the positive electrode) and gradually diminishes asthe wavelength increases. The response time for bothrise and decay is slower than for the photovoltaic effect.The curves are again monophasic. Curve B and thesolid dots in Fig. 3 show (against the right-hand scale)the photoconductive response as a function of wave-length. The curve is almost identical (except for scale)with curve A, for the photovoltaic effect.
August 1964 1021
V (1) = V.[1 - I/ (1 + A I-)].
2
E
I
In
ROSENBERG, HECK, AND AZIZV
).=432 >=464
'>533 X.567
X=496
X:585
a\633
X=700 X=733 >,:766
FIG. 5. Photoconduction response curves of the 13-carotene cellirradiated through the positive electrode. The applied voltagewas 5.0 V. The irradiance was 390X1013 photons/sec/cm 2 . Thehorizontal sweep speed was 1 sec/div. The electrometer inputresistance was 108 El. The vertical scale was 50 mV/div.
The shapes of the rise and the decay curves have beenanalyzed as before (again, using white light excita-tion). They have the same functional dependence upontime as the photovoltaic effect, viz, they are describedby Eqs. (1) and (2), with different values for the con-stant A (i is again close to 1.0).
B. Irradiated through the Negative Electrode
Reversing the direction of the electric field, so thatthe cell is irradiated through the negative electrodechanges the spectral sensitivity of the photoconductionprocess. With an applied voltage of 4.0 V, the responsesare shown in Fig. 6. They are composed of monophasiccurrents of positive holes flowing in the directionopposite to the incident light, and, therefore, causereversed deflections.
In the blue region of the spectrum there is no photo-,conduction current (the light is too strongly absorbed,and does not penetrate the cell to the neighborhood of-the positive electrode), and only a very small photo-voltaic effect occurs. As the wavelength of the irradiat-ing light increases, the magnitude of the responseincreases. For these cells (2 mils thick) the magnitudeis maximal at about 650 mAu. For thinner cells, themaximum occurs at shorter wavelengths, and forthicker cells, at longer wavelengths. The position ofthis peak is independent of the applied voltage over awide range (60 mV to 40 V). If a cell could be arrangedso that the thickness could be varied, the spectral re-sponse curve could be shifted at will over a fairly widerange of wavelengths.
The response time is slower than in the photovoltaiceffect. The responses all show a gradual diminution ofthe peak value as time increases. This is due to thegradual buildup of a space charge polarization in thesample, which decreases the effective electric field inthe cell interior.6 We used high voltages to minimize thecontribution of the photovoltaic effect. To standardizethe conditions, we irradiated the cell with high intensitywhite light for 2 min with both electrodes grounded.This discharged any persistent internal polarization inthe cell. After this, the cell was kept in the dark for 3 minto allow the free-charge concentration to decrease. Thevoltage was then applied for 1 min before the cell wasirradiated with the test light. Alternatively, we allowedthe cell to become fully polarized as the standard condi-tion. Then the responses showed no diminution withtime. However, under these conditions, the effectiveelectric field, and therefore, the resulting photocurrentis much smaller. The contribution of the photovoltaiceffect is then a larger portion of the response and con-fuses the pictures. We emphasize here that, in themeasurements described in Sec. V, the applied voltagewas of the order of tenths of a volt and no noticeablepolarization effect occurred, so that measurementscould be repeated indefinitely without change.
In Fig. 7 we have plotted the magnitude of eachresponse against the wavelength of the irradiatinglight. This curve is almost identical to one we have
FIG. 6. Photoconduction response curves of the 3-carotene cellirradiated through the negative electrode. The applied voltagewas 4.0 V. The irradiance was 390X1013 photons/sec/cm 2 . Thehorizontal sweep speed was 5 sec/div. The electrometer inputresistance was 108 Q. The vertical scale was 2.0 mV/div. Asmall photovoltaic response is detectable in the first four pictures.
I H. P. Kallmann and B. Rosenberg, Phys. Rev. 97, 1596(1955).
1022 Vtol. 54
�,, = 604 >�, = 618
ORGANiC PHOTOCONDUCTIVE CELL
described in an earlier publication.2 It was proved therethat the curve shape is due to the filtering action of thecell. In fact, we have produced the identical curve bypassing the light through a similar sample of 3-carotenebefore it is incident on a positive front electrode of ourcell. The explanation for this result is that only lightabsorbed in the immediate neighborhood'of the positiveelectrode produces a large photocurrent. This impliesthat electrons either have a very much smaller intrinsicmobility than the positive holes, that they are muchmore readily trapped, or that preponderantly, one signof charge carrier (positive holes) is created. Generalevidence from the measurements of mobilities of chargecarriers in organic photoconductors indicates that elec-trons and holes have approximately equal mobilities.7Therefore, our results probably do not involve thefirst hypothesis. There are at present no valid groundsfor distinguishing between the latter two hypotheses,although our theory of charge generation in organicphotoconductors naturally leads to the idea of mostlyhole carriers being generated in hydrocarbons.'
V. SUPERPOSITION OF PHOTOVOLTAIC ANDPHOTOCONDUCTION EFFECTS
The photovoltaic effect produces a positive holecurrent parallel to the direction of the incident light,with its maximum response in the blue spectral region.Similarly, when the cell is irradiated through the nega-tive electrode, photoconduction produces a positive holecurrent opposite to the direction of the light, withmaximum response in the red spectral region. Thesetwo opposing effects may be combined in a single re-sponse by irradiating the cell through the negativeelectrode, with an applied voltage small enough to makeboth responses of comparable magnitude. Under theseconditions, blue light will generate negative responses;
400 000 600 700
WAVELENGTH ( ,ijimcOs )
FIG. 7. The magnitude of the photoconduction responses ofFig. 6 plotted against the wavelength of the interference filterused. The magnitude is taken as the peak of the response.
7 R. G. Kepler, Phys. Rev. 119, 1226 (1960).8 B. Rosenberg, J. Chem. Phys. 37, 1371 (1962).
S-=432 a464
>,- 533
IkE),=604
\-567
,=7700 \ 733
>, 496
>= 585
>v 633
>.-=76 6
FIG. 8. Response curves resulting from superposition of thephotovoltaic effect and photoconduction in the 3-carotene cellirradiated through the negative electrode. The applied voltagewas 200 mV. The irradiance was 390X 1013 photons/sec/cm 2 . Thehorizontal sweep speed was 2 sec/div. The electrometer inputresistance was 101 Q. The vertical scale was 1.25 mV/div., exceptfor X= 700, 733, and 766 mg, for which it was 2.5 mV/div.
red light will generate positive responses; and for someintermediate wavelengths, the responses will be equaland opposite and produce no net response (except foron and off effects due to the differences in responsetimes).
This prediction is verified in the set of pictures shownin Fig. 8. The conditions under which these responseswere obtained are: an applied voltage of 200 mV; cellirradiated through the negative electrode; constantphoton number of 390X 1013 photons/sec/cm2 incidentthrough each interference filter; 101 Q electrometerinput resistor. The blue light causes a negative deflection(smaller in magnitude than the pure photovoltaic curvebecause of the addition of the opposing photoconductioncurrent) and the red light causes a positive deflection.Yellow-green light gives zero steady current, only anegative deflection at on and a positive deflection at off.Orange light produces a positive response but withon/off peaks still detectable. These peaks disappear inthe red region. The response times in the blue arefaster than the response times in the red region.
This set of curves constitutes the fundamental colorresponse of this cell. They are in detailed agreement withthe chromatic "S" potentials measured in retinalMuller fibers by Svaetichin9 and others by microelec-trode techniques in excised retinas of some animals.
In Fig. 9, the magnitudes of the responses are againplotted against wavelength and show clearly theopposing processes in the blue and red spectral regions.
9 G. Svaetichin, Acta. Physiol. Scand. 39, (Suppl. 134) 17,(1956).
August 1964 1023
12
8
4
0o
ROSENBERG, HECK, AND AIZ IV
S a 8.
0, -
0x
V)
2Z .
24 2
G400 500
WAVELEN(
a long wavelength light with a short wavelength light.The long wavelength light should produce the positiveresponse, the short wavelength light, a negative re-sponse. By mixing these in various combinations weshould be able to generate curves corresponding to allthe curve shapes shown in Fig. 8. To test this we have atfirst chosen *the interference filters which peak at432 m/u (blue) and 700 mAt (red) and superposed themas discussed in Sec. II. We started with only the bluelight. Then we added (by means of a series of neutraldensity filters) red light until each beam had approxi-mately equal photon numbers. This graded series isshown as the first six pictures of Fig. 10. These curves,
l I I o loo,- in comparison to Fig. 8 give responses similar to theITH ( millimicrons )
FIG. 9. Responses of Fig. 8 taken just prior to the closing ofthe shutter, plotted against the wavelength of the interferencefilter used.
It is also clear that it is composed of a superpositionof Figs. 3 and 7. This curve is of the general nature ofthe Hering opponent-processes chromatic function.'
VI. SUPERPOSITION OF LONG ANDSHORT WAVELENGTHS
A second prediction can be made from the resultsof Secs. III and IV. This concerns the effect of mixing
432= 120 4:32= 120 432= 120
700 0 700=33 700= 50
I a~S- E || HIMiMMMM
432= 120 432= 120 432= 120
700= 66 700= 106 700= 133
IEE11. III M1101011432=43 432=23 432= 16
700= 133 700= 133 700=133*UPEM MEEUMq
flEMEEE mW 1
432=5 432=4 432=0
700=133 700=133 700=133
F;IG. 10. Response curves resulting from su1)erposition of along wavelength mnnnchrnmatic light (700 ma) and a short N ave-length monochromatic light (432 m~o) on the negative electrodeof the /3-carotene cell. The applied voltage wvas 200 mV. The hori-zontal sweep speed wvas 5 sec/div. The electrometer input resist-ance was 108 t2. The vertical scale wvas 1.25 mV/div. The irradianceof each of the two beams is shown beneath each picture in unitsof 1013 photons/sec/cm 3 .
FIG. 11. Response curves in the /3-carotene cell for differentmonochromatic lights incident on the negative electrode for threedifferent applied voltages; 170, 200, and 300 mV. The irradiancewas 390X10's photons/sec/cm 2 . The horizontal sweep speed was5 sec/div. The electrometer input resistance was 108 U. The verticalscale was 0.5 mV/div.
blue, green, and yellow monochromatic responses. Wethen diminished the irradiance by blue light, keepingthe irradiance with red light constant. This series gaveus pictures 7-12 in Fig. 10. Again, comparison with themonochromatic responses of Fig. 8 shows that this rangegave responses similar to the orange and red mono-chromatic responses. Thus, the series of purples canfairly well mimic the entire set of monochromaticresponses in this single cell and verifies the prediction.These results seem similar in principle to the Landcolor coordinate system,10 although we are working
10 E. H. Land, Proc. Nat. Acad. Sci. U. S. 45, 636 (1959).
+
;~ 1024 Vol. 54
ORGANIC PHOTOCONDUCTIVE CELL
with a single opponent-process receptor whereas Landinvolved the two chromatic opponent-process unitsof the human eye.
VII. DEPENDENCE OF THE BALANCE POINTON THE APPLIED VOLTAGE
The balance (or neutral) point occurs at a wavelengththat stimulates photovoltaic and photoconductioncurrents of equal magnitude. The former effect isindependent of the applied voltage, whereas the latter isdirectly proportional to it. Thus the balance point mustbe a function of the applied voltage. In fact, we cansay that the balance point will shift to shorter wave-lengths with an increase in the applied voltage. In theprevious measurements we have arbitrarily chosen anapplied voltage of 200 mV which produced a balancepoint in the vicinity of 585 mA as shown on the pictures.If we had extended the exposure for a longer period oftime, the true balance point would have occurred at567 m,4 (see Fig. 11).
In Fig. 11 we portray the results of a series of meas-urements with four interference filters covering thespectral region of interest, and with three differentapplied voltages, 170, 200, and 300 mV. Reading thepictures across the figure, we see that increasing thevoltage does indeed increase the contribution of thephotoconduction current and makes the response curveresemble that corresponding to a slightly longer wave-length. If we take the difference between the dark cur-rent value and the value just prior to turning off thelight as the magnitude of the response, these may thenbe plotted against wavelength. The results shown inFig. 12 clearly indicate the shift of response withvoltage. The straight lines have been drawn to connectpoints of constant voltage. The complete responsecurve is of course nonlinear; however, the small regionaround the balance point is approximately linear.
We have previously stated, and we repeat it here foremphasis, that while the balance point is a function
BALANCE POINT SHIFT
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FIG. 13. Response curves of the 0-carotene cell irradiatedthrough the negative electrode with broadband green light. Theapplied voltage was 200 mV. The irradiance is shown beneatheach picture in units of 10(2 MW/cm2 . The horizontal sweep speedwas 5 sec/div. The electrometer input resistance was 108 U. Thevertical scale was 1.25 mV/div.
of voltage, the wavelength regions where the peaks occurin the blue and red region are independent of voltage,and depend only on the pigment absorption curve andthe thickness of the cell respectively.
VIII. STABILITY OF COLOR RESPONSETO INTENSITY CHANGES
It is generally accepted, and we have verified it forthe case of our /3-carotene cells, that the photovoltaiceffect increases as the logarithm of the irradiance of thelight absorbed, for strong absorption. No direct deter-mination has been made of the dependence on irradi-ance for wavelengths which are weakly absorbed. Wehave also reported that the photoconduction currentis directly proportional to the irradiance of the lightabsorbed. Therefore, it appears that a green responsefor a wavelength at the balance point would occur onlyfor a given irradiance. If the irradiance changes, thensince the photovoltaic and photoconduction currentshave different dependencies upon irradiance, the bal-ance condition should no longer hold. This effectwould of course limit the potential value of this colorresponse system in the green spectral region.
We have directly tested for this effect, and found asurprising stability of the response curve for green overa fairly wide irradiance range. The effect is there, butit seems to be moderated. The exciting light wasproduced by passing white light through a combination
1025August 1964
ROSENBERG, HECK, AND AZIZ
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FIG. 14. The response curves of the 0-carotene cell irradiatedthrough the negative electrode with white light plus 533 mygreen light. The irradiance of each of the superposed beams isshown beneath each picture in units of 102 uW/cm2 . The appliedvoltage was 200 mV. The horizontal sweep speed was 5 sec/div.The electrometer input resistance was 108 Q. The vertical scalewas 1.25 mV/div.
of Corning filters 3385 and 5031: this produced a highirradiance with green light, with a peak in the wave-length band at 510 m,4 and a bandwidth, at half-peak,of 64 m~i. The irradiance was varied by interposingvarious neutral density filters in the beam. The resultsare shown in Fig. 13. The irradiance (in units of 102MW/cm2 ) is shown beneath each picture. The curveshape, typical of a monochromatic green response,shows both "on" and "off" peaks, and no net current atthe steady state. This is a good approximation forirradiances from 2.4 up to 28.3, where a small positiveresponse becomes apparent. Thus, over a range of irradi-ance of about 10 :1, the curve shape is stable. At higherirradiance the positive response increases in magnitudealmost linearly. This would imply, according to themonochromatic shapes shown in Fig. 8, that the re-sponses are becoming typical of longer wavelengthresponses as the irradiance increases. The cell responseto the green light at high irradiance appears more likethe yellow-green light response. This result is phe-nomenologically quite reminiscent of a characteristicof color vision (the well-known Bezold-BrUcke effect 1).
IX. A "COLOR MATCHING" EXPERIMENT
In Fig. 13, we have presented the color responsecurves of the 13-carotene cell to a broadband green lightwhich has a peak in the wavelength distribution at
11 Y. Lecyrand(l, Lighit, Culoitr aniid Visim (John Wiley & Sons,New York 1957), p. 213.
510 mbt. In order to verify that these responses aretruly color responses and not wavelength or photonenergy discrimination in such a cell, it is desired to showthat light of the same color (to the human eye) willproduce the same response regardless of the nature of theenergy spectrum composing that light. We havealready shown that broadband blue light produces thesame negative deflection as does monochromatic blue;that broadband red light produces the same positivedeflection as does monochromatic red; and that broad-band green light produces only on/off peaks and no netcurrent, as does monochromatic green. We have alsoshown that almost any monochromatic response can besimulated by a superposition of a long and short wave-length light. Here, we should like to extend this proof onestep further, and show that it is possible in a primitivemanner to match a broadband green of a certain irradi-ance with white light plus a monochromatic light of533 mn, of suitable irradiance. We were limited in thismeasurement by the small irradiance of the mono-chromatic light we could obtain. Therefore, we haveused this maximum irradiance and added to it (bymeans of the second projector and a half-silveredmirror) various amounts of white light. The firstpicture in Fig. 14 is the response to the 5 33 -m,4 filteralone. The last picture is the response to white lightalone. The intervening pictures are due to mixtures;the irradiance of each of the two components is shownbeneath each picture.
Unfiltered white light of high irradiance shows aresponse that cannot be matched by any of the mono-chromatic responses. It is similar but not identicalto the broadband green light at highest irradiance (com-pare the last picture in Fig. 14 with the first picture ofFig. 13). The 53 3 -m4 light shows only a small negativeresponse and an off peak. This cannot match any curveof Fig. 13. However, the middle three pictures ofFig. 14 are almost identical to the first three picturesof Fig. 13. The matches can be expressed as: 19 unitsof 533 mA+10.6 units of white =47.1 units of broad-band green; 19 units of 533 mM+2 2 units of white
52.8 units of broadband green; 19 units of 533mi,+34 units of white ==83.9 units of broadband green.Considering the primitive nature of the method, andthat we are using only one opponent-process chromaticunit rather than the two required by color mixture data,we are satisfied that such matches can be made andthat this is truly a color response of the 3-carotene cell.
ACKNOWLEDGMENTS
This investigation was supported in whole by PublicHealth Service Research Grant NB-04145 from theNational Institute of Neurological Diseases and1Blindness.
