- Email: [email protected]
Measurements of minute photographic densities
Physica
IX,
no 7
OF MINUTE
Juli 1942
MEASUREMENTS PHOTOGRAPHIC by E. KATZ
DENSITIES
*)
(Communication from the physical laboratory of the University of Utrecht, Netherlands) Summary Accurate compensation measurements were performed, in a way more or less similar to that ‘followed by J u r r i e n s, on the direct-density production of weakly- absorbed wave lengths. The apparatus used is described in 9 2; the results of the measurements are to be found in 9: 3. The radiations mentioned give rise to direct density-time curves with a peculiar S-shape for densities near 0,O 1. A similar shape may be expected in silver-time curves. We investigated the influences of wave length (fig. 4), of light intensity (fig. 5), and ofwater addition on the shape of these curves. The S-shape graduallv disappears for decreasing wave lengths. This supports the view that it is the resultant of two components, one for Ag production, showing a I3 e c q u e r e l-effect; the other one for Ag destruction, related to the H e r s c h e l-effect. Arguments are derived for justifying the extrapolation of the proportionality between direct density and time, as observed by J u r r i e n.s for blue light, down to the latent image region. The effect of water is to sensitize the production of direct density and to desensitize that of developed density in this wave length region. This may be due to adsorption of OH- ions to the sensitivity specks.
§ 1. Introduction t). According to E g g e r t and N o d d a c k (cf. M e i d i n g e r l)), the effect of light on photographic emulsions can be chemically measured for large exposures down to 1Or5q.a./cm* (= quanta absorbed per cm* of plate). A. v a n K r e v e 1 d suggested optical measurements of minute direct (= print out) densities. This method has been developed by *) This investigation was performed mainly under the stimulating direction of the late; Pr0f.I)r.L.S. Ornstein. t) For an extensive description of details and checks of the procedure of measurement see E. I< a t z, thesis Utrecht 1941.
Physica I?(
755 40
756
E.
KATZ
J u r r i e n s “) who performed measurements down to 1Or2 q.a./cm2. Exposures yielding suitable densities after development in practice range from lC* to lOI q.a./cm2, so J u r r i e n s just reached this “latent-image region”. With the exception of development this method is’at present the most sensitive one for measuring photographic effects. J u r r i e n s’ conclusions are briefly : 1) With blue light the direct density d is proportional to the time of exposure t, for exposures between 1Or2 and 1Or4 q.a./cm2. Probably this proportionality also holds in the latent-image region. On the other hand,-small but systematic deviations were observed for the lowest region measured (ca. 1Or2 q.a./cm2). For a larger range of exposures d is related to t by a (1-exponential) saturation formula with the remarkably low, saturation density of ca. 0,l. The observations cover several sorts of plates, and intensitiesbetween 10” and 10” q.a.lcm2.s. 2) The reciprocity law failure of the prmt-out effect decreases with decreasing exposure. The S c h w a r z s c’h i 1 d exponent p was found to range from 0,8 to 1,03. Probably the failure is absent for direct densities of the latent image region. Of course it is then still present for developed densities (measured after development), p ranging from 1 to 1,3. This indicates that reciprocity law failure depends strongly on development and its conditions. 3) With red light,the (d, t) curves show a marked S-shape. The present paper deals with more or less analogous experiments concerning the direct density caused by light of wave lengths near the absorption limit of the plates (yellow, red). Special attention is given to the S-shape of the (d, t) curves, and to the low saturation value mentioned. 5 2. Method.wd Apparatus for the production and measurement of direct density. The density on the plate P is produced and measured
by means of an apparatus, represented schematically in fig. 1. The light source ZS and the lenses L-L2, together with the mirror M, project a homogeneous light spot (,,acting light”) on P. The intensity of the acting light is controlled by a diaphragm D 1 and a shutter S; it is measured with a thermo-element and will be expressed in arbitrary relative units (1 relative unit = 1,3 x 1O3ergicm2.s). The spectral composition of the acting light is controlled by the filters W
MEASUREMENTS
OF MINUTE
PHOTOGRAPHIC
DENSITIES
757
(=2cmwater),Fl (Schott &Gen.3mmBG17+2mmOG2) and F3, which was either 2 mm RG 1 (“red”) or 2 mm OG3 (“orange”) or no filter (“yellow” acting light). For the relative spectral intensities (per A) caused by these filters see fig. 2 *). Their sharp cut-off on the short wave-length side and the steep sensitivity gradient of the plate in that region practically cause only a rather narrow spectral region near this cut-off to be active in the Ag production at the plate, provided that the presence of longer wave lengths in the beam does not produce any H e r s c.h e l-like effects (see 5 4).
Fig. 1. Schematical diagram of the measurement of direct density. LS = W = 2 cm.water. The optical part vertical
apparatus for the production and light source; L = lens (f = 16cm); of the figure is a projection on a plane.
Minute changes in absorption of P were measured with a We st o n barrier layer cell Phl , connected in compensation with, a similar cell Ph2 and a galvanometer G (see fig. 1). . For a more simple interpretation of the measurements we made *) The terms red, these compositions.
orange
and yellow
will
be used throughout
tliis paper
to indicate
758
E. KATZ
the colour of t.he “measuring light”, received by the cell Phi , independent of that of the acting light, by inserting at F4 a 2 mm RG2 S c h o t t filter (see fig. 2); both the measuring and the acting light passed through the same part of the plate continuously. The (d, t) curves vtrere obtained by registration of the galvanometer deflection or by frequent reading. If exposed plates were to be developed, this was done with metol borax developer during 6 min at 18°C. After development, fixing, etc. their density was measured with the same apparatus. Ilford Special Rapid plates were used if not stated otherwise. The accuracy of the method was mainly determined by the properties of the photo electric cells and amounted to Ad = = 10-4 - 10B5, according to circumstances.
Wave
lenqth
in
Fig. 2. Relative spectral intensities of the measuring light and of various colours of the acting light. The filters indicated cause the gradients near which their symbols are placed.
FJ3. Exfierimental Resdts. a) R e p r o d u c i b i 1 i t y o f t h e photographic plates for measurements with r e d 1 i g h t. Fig: 3 gives an impression of the reproducibility of the measurements. A rather large dispersion is observed from curve to curve. This is not due to inaccuracies of the method, as may for example be seen from the errors of points of each curve separately. Check experiments showed that this dispersion could neither be ascribed to differences in coating thickness of the plates, nor to differences of temperature during the measurements, nor to changes in humidity (see, however, for the effect of water, below).
MEASUREMENTS
OF
MINUTE
PHOTOGRAPHIC
759
DENSITIES
Probably the disperson is caused by local Variations of plate sensitivity to these wave lengths (cf. 3)), so that (d, t) curves obtained with this method are safe (one spot exposed), but (d, I )curves have to be considered with some reserve (a different spot exposed for each intensity). In order to reduce the uncertainty resulting from this dispersion we always averaged 7 to 9 curves, obtained from one plate, the parts near the edges of the plate being discarded because they showed much larger deviations.
z
0
0
I 5
IO
I 15
20
Time
25 in minutes
I
Fig. 3. Direct density-time curves with equal intensities of red light on neighboring spots of one plate (a, b, c) and on another plate of the same package (d).
b) (~$1) curves for low densities and for vario u s c o 1o u r s. Fig. 4 represents (d, t) curves obtained for low densities. The curve marked ,,blue” is taken from the work of J u rr i e n s for comparison. We notice, that with decreasing wave length the S-shape shrinks towards zero first in the lower and the in then upper part. A remarkably straight part appears for higher exposures, also for those colours that show a pronounced S-shape for low exposures. It is extended towards the origin with decreasing wave length. Its produced part passes above the origin. IJpon closer consideration J u r r i e n s’ straight lines, observed for blue light, show distinct remnants of the
760
E. KATZ
S-shape, as they also pass systematically above the origin (cf. “) fig. 11 and table I). This fact throws some doubt on the correctness of J u r r i e n s’ extrapolated conclusion, that the amount of photo silver is proportional to the exposure in the latent image region (cf. $ 4, a). Since there a direct experimental verification of this proportionality was not yet possible we have studied the S-shape for red irradiation. We shall discuss to what extent the phenomena observed in this case can be expected to appear on a smaller scale for blue irradiation in the latent image region.
Fig. 4. Direct
density as a function of time for various colours.
of exposure
c) Experiments with’various sorts of plates. A few qualitative experiments were performed with Ilford plates with orthochromatic and panchromatic sensitization. The results are similar to those described above. The S-shape appears at slightly longer wave lengths. The longer wave lengths were obtained with the aid of S c h o t t filters RG 2,5,8, 10 with cut-off wave lengths 6300,6750,7000,7800 A respectively. d) Experiments at various intensities of red a n d o r a n g e i r r a d i a t i-o n. Notwithstanding the dispersion mentioned, a comparison of (d, t) curves for various intensities was
MEASUREMENTS
OF
MINUTE
PHOTOGRAPHIC
DENSITIES
761
deemed worth while. Fig. 5 represents sets of@, t) curves for various intensities of orange and red light. All the curves show an S-shape but cannot be made to coincide, neither by multiplication in the d nor in the t direction. For high intensities their difference becomes relatively small. In order to characterize the9shape of the curves two parameters may be introduced, namely the steepest gradient y,,, and the gradient in the straight part above in the S-shape y. OOE
aos-
qrange
OG3
red
RGl
I
00-l-
/
IS
Fig. 5. Direct density as a function of time of exposure for various intensities. The mean error, as determined from the deviations of 7 or 9 individual observations from t$heir average amounts to about Ad = f 0,0007. Orange OG3 curve intensity n 6. I b
13.4
c
19.7
d
25.3
Red curve a b c d ; Lz
RG I intensity 11.4 16.8 21.7 24.0 32.6 39.5 49.9
The dependence of y,,, and y on the light intensity I is shown in fig. 6. The fact that y,,, and y are only proportional to I for lowvalues of the intensity indicates a definite reciprocity failure. The
762
E. KATZ
S c h w a r z s c h i 1 d exponent fi amounts for our intensity region to about 1,4. We have not checked p for these intensities and developed density. General evidence indicates that our intensities were below the optimum, so p < 1 for the developed density. 250 t
200
0
/
O.
-
Fig. 6. Slope of the inflection tangent y,,, and of the straight part y of direct density-exposure curves as a function of intensity of orange and of red light.
e) Experiments on the course of (d,t) curves for higher densities. A few long-exposure experiments were performed in order to trace (d, t) curves for higher densities and eventual saturation. Fig. 7 shows a curve taken with yellow light and two curves with red light, of high and of low intensity. A maximum density at some low value of d, as assumed by J u r r i e n s, is not present, but a low rate of d-production sets in at the region near d = 0,2 to’6,3. The first part of the curve can be represented as the difference between a slowly ascending linear function and an exponential one. f) Experiments on the infl.uence of water. We remarked already that changes in humidity are not responsible for the lack of reproducibility mentioned in section a). Experiments performed in this connection yielded somewhat unexpected results to be reported herebelow.
MEASUREMENTS
OF MINUTE
PHOTOGRAPHIC
DENSITIES
763
We compared the (d, t) curves of plates whiCh were soaked in water and those of ordinary plates for red light, the soaked plates still being wet during the exposure, and the gelatin still being swollen. The water changed the colour of the plates from yellow to greenish, so that the absorption of the wet plates was larger for the red light. In accordance herewith the (d, t) curves rise steeper, almost proportionally by a factor 2. The same effect was observed if the time of soaking was varied from 2 to 25 minutes. After such plates had dried again, the greenish colour and the increased sensitivity for direct density production by red light remained. Even after drying for 4 days in a CaCl, exsiccator the results were the same, so the effect is difficult to reverse. Incidentally we remark that the density at which the rate of density production becomes small, as exposed in section e), is not altered.
II Time
Fig.
7. Direct
density
as a function
Of exposure
I” hours
of (long)
times
of exposure.
However, this water treatment caused a strong desensitization for developed-density production by the red light. The direct density and the developed one are, apparently, influenced by water in opposite ways. fj 4. Discussion.
The experiments
presented
do not allow
a defi-
764
E.
KATZ
nite interpretation. A few tentative remarks, from the standpoint of the G u r n e y-M o t t theory of latent-image formation, will be advanced 4) (cf. also W e b b “)). u) The S-shape of (d,t) curves for red light. The threshold-like start of the (d, t) curves for developed density and for direct density, caused by red light, have no common basis of interpretation since they occur at exposure values differing by several orders of magnitude “). We shall assume that the S-shape in the (d, t) curves correspods to a similar S-shape in (Ag, t) curves. The following arguments support this assumption : 1) The proportionality of d and t for blue light suggests that any anomalies in the relation between d and the amount of Ag do not occur in this region of densities. 2) An estimate of the size of our Ag-specks (cf. “)) shows that these are smallcompared with the wave length of light (of the order of a few atoms to a few hundred ones), so that proportionality of the absorption and the amount of Ag is probable for low densities. Since the H e r s c h e l-effect *) is known to occur in our wavelength region we shall discuss in how far this effect maybe related to the appearance of the S-shape. We have checked (by development) that the red light yielded appreciable developed densities after a few seconds of exposure, so that the H e r s c h e l-effect may occur here as a decrease of the rate or efficiency of Ag production but not as an effect of latent image erasure. The red and infrared exposure in our experiments was 1O’e--1 019 quanta/cm2 per 5 min, which is about 10” times as much as the lowest latent image exposures with blue light, so that at most a small Ag destroying action may be expected. Then d’, the time derivative of d (see fig. 8) must he the difference between two functions, one for Ag production (I) and one for Ag destruction (II) (see fig. 9). The G u r n e y-M o t t theory permits one to predict the shape of (II) qualitatively. It is postulated that the effect is due to a photo*) We shall understand by “H e r s c h e I”-effect the destruction of photo silver iu the grains by light. We shall not require that the rate of this destruction shall surpass that of production of Ag by the same (or simultaneously acting) light; the latter situation obtains in what is usually called the H e 1‘ s c h P I-effect (latent image rrasurr by red or infrared light).
MEASUREMENTS
OF MINUTE
PHOTOGRAPHIC
765
DENSITIES
electric ejection of electrons from the photo4lver into AgBr, with subsequent neutralization of the photosilver by the expulsion of Ag+ ions into interlattice positions of the AgBr crystal. Repetition of this process makes the Ag specks vanish. The theory makes essentially use of the smallness of the latent image Ag specks (dimensions of a few atoms) ; for larger Ag aggregates it is improbable that Ag+ ions should be regenerated in this way. In accordance herewith the H e rs c h e l-effect has not been observed for direct densities in the region of chemical detection (see 1) p. 322).
t3:y0vI 5”IO I---& 15
Fig.
8.
Direct
20
Time
in minutes
density d as function of time of exposure for orange (I = 13,4) (a) and its time derivative d’ (b).
OG3
Therefore Cl1 is proportional to d (i.e. to the number of absorptions in the Ag) in the latent image region, whereas the relative rate dj,/d approaches zero in’ the region of chemical detection. Between these two extremes, so in our experiments, the shape of (d’, d) will, therefore, resemble curve 11 fig. 9. The total rate of Ag production is then shown in curve I. Its shape is simpler than that of the original (d, t) curves, but still shows a superproportional start. Possibly its shape is connected with changes of the absorption spectrum of AgBr during the exposure (B e c q u e r e l-effect) *). On the basis of the analysis given the wave-length dependence of *) It is known that the adsorption of Agf ions to AgBr causes a large shift of absorption towards the infrared, but we do not know of similar experiments the effect of adsorbed Ag atoms upon the absorption of AgBr.
of the limit concerning
766
E. KATZ
the S-shape can qualitatively be understood. For shorter wave lengths, owing to the large increase in AgBr absorption, the B e cq u e r e l-effect will soon become indistinguishable, which causes shrinking of the quadratic start of the (d, t) curves. For still shorter wave lengths the relative importance of the H e r s c h e l-effect also decreases for the same reasons, so that the second bend of the (d, t) curve is removed, yielding a straight line.
3
Fig. 9. Schematical representation of constructive (I) and destructive components of the rate of direct-density production.
(II)
The S-shape of the (a!, t) curves for red light is, therefore, due to complications, superimposed on the normal shape of the curves, observed by J u r r i e n s for blue light (H e r s c h e l-effect and B e c q u e r e l-effect). Even the remnants of non-linearity as observed by that author (see 5 1) with blue acting light and red measuring light are now comprehensible as complications arising from the latter Under ideal experimental conditions the extrapolation of the proportional start down to the latent image region appears to be justified. b) The e f f e c t of w a t e r. It is known ‘) p. 93, that adsorption of OH-ions shifts the absorption limit of AgBr towards the infrared. It is plausible that the change in colour of the emulsion and
MEASUREMENTS
OF
MINUTE
PHOTOGRAPHIC
767
DENSITIES
its increased sensitivity for direct blackening after the water treatment is due to such an adsorption of OH- ions. The reported decrease in sensitivity for developed-density formation possibly means that the OH- ions are adsorbed mainly in the neighborhood of (superficial) sensitivity specks. Owing to their negative charge’ photo electrons will then preferably settle at specks in the interior of the grain, which are inaccessible for the developer. Received
June
3rd,
1942.
REFERENCES I) 2) 3) 41 5)
W. hl e i d i II g e r, Handb. wiss. angew. Photogr. V. H. J. J II rr i e n s, Thesis Utrecht 1938. E. K a t z, Thesis Utrecht 1941. R. W. G II r n e y and N. F. Mot t, Proc. roy. Sot. London J. H. W c I) b, J. appl. Phys. 11, 18, 1940.
A 164,
151,
1938.
IX,
no 7
OF MINUTE
Juli 1942
MEASUREMENTS PHOTOGRAPHIC by E. KATZ
DENSITIES
*)
(Communication from the physical laboratory of the University of Utrecht, Netherlands) Summary Accurate compensation measurements were performed, in a way more or less similar to that ‘followed by J u r r i e n s, on the direct-density production of weakly- absorbed wave lengths. The apparatus used is described in 9 2; the results of the measurements are to be found in 9: 3. The radiations mentioned give rise to direct density-time curves with a peculiar S-shape for densities near 0,O 1. A similar shape may be expected in silver-time curves. We investigated the influences of wave length (fig. 4), of light intensity (fig. 5), and ofwater addition on the shape of these curves. The S-shape graduallv disappears for decreasing wave lengths. This supports the view that it is the resultant of two components, one for Ag production, showing a I3 e c q u e r e l-effect; the other one for Ag destruction, related to the H e r s c h e l-effect. Arguments are derived for justifying the extrapolation of the proportionality between direct density and time, as observed by J u r r i e n.s for blue light, down to the latent image region. The effect of water is to sensitize the production of direct density and to desensitize that of developed density in this wave length region. This may be due to adsorption of OH- ions to the sensitivity specks.
§ 1. Introduction t). According to E g g e r t and N o d d a c k (cf. M e i d i n g e r l)), the effect of light on photographic emulsions can be chemically measured for large exposures down to 1Or5q.a./cm* (= quanta absorbed per cm* of plate). A. v a n K r e v e 1 d suggested optical measurements of minute direct (= print out) densities. This method has been developed by *) This investigation was performed mainly under the stimulating direction of the late; Pr0f.I)r.L.S. Ornstein. t) For an extensive description of details and checks of the procedure of measurement see E. I< a t z, thesis Utrecht 1941.
Physica I?(
755 40
756
E.
KATZ
J u r r i e n s “) who performed measurements down to 1Or2 q.a./cm2. Exposures yielding suitable densities after development in practice range from lC* to lOI q.a./cm2, so J u r r i e n s just reached this “latent-image region”. With the exception of development this method is’at present the most sensitive one for measuring photographic effects. J u r r i e n s’ conclusions are briefly : 1) With blue light the direct density d is proportional to the time of exposure t, for exposures between 1Or2 and 1Or4 q.a./cm2. Probably this proportionality also holds in the latent-image region. On the other hand,-small but systematic deviations were observed for the lowest region measured (ca. 1Or2 q.a./cm2). For a larger range of exposures d is related to t by a (1-exponential) saturation formula with the remarkably low, saturation density of ca. 0,l. The observations cover several sorts of plates, and intensitiesbetween 10” and 10” q.a.lcm2.s. 2) The reciprocity law failure of the prmt-out effect decreases with decreasing exposure. The S c h w a r z s c’h i 1 d exponent p was found to range from 0,8 to 1,03. Probably the failure is absent for direct densities of the latent image region. Of course it is then still present for developed densities (measured after development), p ranging from 1 to 1,3. This indicates that reciprocity law failure depends strongly on development and its conditions. 3) With red light,the (d, t) curves show a marked S-shape. The present paper deals with more or less analogous experiments concerning the direct density caused by light of wave lengths near the absorption limit of the plates (yellow, red). Special attention is given to the S-shape of the (d, t) curves, and to the low saturation value mentioned. 5 2. Method.wd Apparatus for the production and measurement of direct density. The density on the plate P is produced and measured
by means of an apparatus, represented schematically in fig. 1. The light source ZS and the lenses L-L2, together with the mirror M, project a homogeneous light spot (,,acting light”) on P. The intensity of the acting light is controlled by a diaphragm D 1 and a shutter S; it is measured with a thermo-element and will be expressed in arbitrary relative units (1 relative unit = 1,3 x 1O3ergicm2.s). The spectral composition of the acting light is controlled by the filters W
MEASUREMENTS
OF MINUTE
PHOTOGRAPHIC
DENSITIES
757
(=2cmwater),Fl (Schott &Gen.3mmBG17+2mmOG2) and F3, which was either 2 mm RG 1 (“red”) or 2 mm OG3 (“orange”) or no filter (“yellow” acting light). For the relative spectral intensities (per A) caused by these filters see fig. 2 *). Their sharp cut-off on the short wave-length side and the steep sensitivity gradient of the plate in that region practically cause only a rather narrow spectral region near this cut-off to be active in the Ag production at the plate, provided that the presence of longer wave lengths in the beam does not produce any H e r s c.h e l-like effects (see 5 4).
Fig. 1. Schematical diagram of the measurement of direct density. LS = W = 2 cm.water. The optical part vertical
apparatus for the production and light source; L = lens (f = 16cm); of the figure is a projection on a plane.
Minute changes in absorption of P were measured with a We st o n barrier layer cell Phl , connected in compensation with, a similar cell Ph2 and a galvanometer G (see fig. 1). . For a more simple interpretation of the measurements we made *) The terms red, these compositions.
orange
and yellow
will
be used throughout
tliis paper
to indicate
758
E. KATZ
the colour of t.he “measuring light”, received by the cell Phi , independent of that of the acting light, by inserting at F4 a 2 mm RG2 S c h o t t filter (see fig. 2); both the measuring and the acting light passed through the same part of the plate continuously. The (d, t) curves vtrere obtained by registration of the galvanometer deflection or by frequent reading. If exposed plates were to be developed, this was done with metol borax developer during 6 min at 18°C. After development, fixing, etc. their density was measured with the same apparatus. Ilford Special Rapid plates were used if not stated otherwise. The accuracy of the method was mainly determined by the properties of the photo electric cells and amounted to Ad = = 10-4 - 10B5, according to circumstances.
Wave
lenqth
in
Fig. 2. Relative spectral intensities of the measuring light and of various colours of the acting light. The filters indicated cause the gradients near which their symbols are placed.
FJ3. Exfierimental Resdts. a) R e p r o d u c i b i 1 i t y o f t h e photographic plates for measurements with r e d 1 i g h t. Fig: 3 gives an impression of the reproducibility of the measurements. A rather large dispersion is observed from curve to curve. This is not due to inaccuracies of the method, as may for example be seen from the errors of points of each curve separately. Check experiments showed that this dispersion could neither be ascribed to differences in coating thickness of the plates, nor to differences of temperature during the measurements, nor to changes in humidity (see, however, for the effect of water, below).
MEASUREMENTS
OF
MINUTE
PHOTOGRAPHIC
759
DENSITIES
Probably the disperson is caused by local Variations of plate sensitivity to these wave lengths (cf. 3)), so that (d, t) curves obtained with this method are safe (one spot exposed), but (d, I )curves have to be considered with some reserve (a different spot exposed for each intensity). In order to reduce the uncertainty resulting from this dispersion we always averaged 7 to 9 curves, obtained from one plate, the parts near the edges of the plate being discarded because they showed much larger deviations.
z
0
0
I 5
IO
I 15
20
Time
25 in minutes
I
Fig. 3. Direct density-time curves with equal intensities of red light on neighboring spots of one plate (a, b, c) and on another plate of the same package (d).
b) (~$1) curves for low densities and for vario u s c o 1o u r s. Fig. 4 represents (d, t) curves obtained for low densities. The curve marked ,,blue” is taken from the work of J u rr i e n s for comparison. We notice, that with decreasing wave length the S-shape shrinks towards zero first in the lower and the in then upper part. A remarkably straight part appears for higher exposures, also for those colours that show a pronounced S-shape for low exposures. It is extended towards the origin with decreasing wave length. Its produced part passes above the origin. IJpon closer consideration J u r r i e n s’ straight lines, observed for blue light, show distinct remnants of the
760
E. KATZ
S-shape, as they also pass systematically above the origin (cf. “) fig. 11 and table I). This fact throws some doubt on the correctness of J u r r i e n s’ extrapolated conclusion, that the amount of photo silver is proportional to the exposure in the latent image region (cf. $ 4, a). Since there a direct experimental verification of this proportionality was not yet possible we have studied the S-shape for red irradiation. We shall discuss to what extent the phenomena observed in this case can be expected to appear on a smaller scale for blue irradiation in the latent image region.
Fig. 4. Direct
density as a function of time for various colours.
of exposure
c) Experiments with’various sorts of plates. A few qualitative experiments were performed with Ilford plates with orthochromatic and panchromatic sensitization. The results are similar to those described above. The S-shape appears at slightly longer wave lengths. The longer wave lengths were obtained with the aid of S c h o t t filters RG 2,5,8, 10 with cut-off wave lengths 6300,6750,7000,7800 A respectively. d) Experiments at various intensities of red a n d o r a n g e i r r a d i a t i-o n. Notwithstanding the dispersion mentioned, a comparison of (d, t) curves for various intensities was
MEASUREMENTS
OF
MINUTE
PHOTOGRAPHIC
DENSITIES
761
deemed worth while. Fig. 5 represents sets of@, t) curves for various intensities of orange and red light. All the curves show an S-shape but cannot be made to coincide, neither by multiplication in the d nor in the t direction. For high intensities their difference becomes relatively small. In order to characterize the9shape of the curves two parameters may be introduced, namely the steepest gradient y,,, and the gradient in the straight part above in the S-shape y. OOE
aos-
qrange
OG3
red
RGl
I
00-l-
/
IS
Fig. 5. Direct density as a function of time of exposure for various intensities. The mean error, as determined from the deviations of 7 or 9 individual observations from t$heir average amounts to about Ad = f 0,0007. Orange OG3 curve intensity n 6. I b
13.4
c
19.7
d
25.3
Red curve a b c d ; Lz
RG I intensity 11.4 16.8 21.7 24.0 32.6 39.5 49.9
The dependence of y,,, and y on the light intensity I is shown in fig. 6. The fact that y,,, and y are only proportional to I for lowvalues of the intensity indicates a definite reciprocity failure. The
762
E. KATZ
S c h w a r z s c h i 1 d exponent fi amounts for our intensity region to about 1,4. We have not checked p for these intensities and developed density. General evidence indicates that our intensities were below the optimum, so p < 1 for the developed density. 250 t
200
0
/
O.
-
Fig. 6. Slope of the inflection tangent y,,, and of the straight part y of direct density-exposure curves as a function of intensity of orange and of red light.
e) Experiments on the course of (d,t) curves for higher densities. A few long-exposure experiments were performed in order to trace (d, t) curves for higher densities and eventual saturation. Fig. 7 shows a curve taken with yellow light and two curves with red light, of high and of low intensity. A maximum density at some low value of d, as assumed by J u r r i e n s, is not present, but a low rate of d-production sets in at the region near d = 0,2 to’6,3. The first part of the curve can be represented as the difference between a slowly ascending linear function and an exponential one. f) Experiments on the infl.uence of water. We remarked already that changes in humidity are not responsible for the lack of reproducibility mentioned in section a). Experiments performed in this connection yielded somewhat unexpected results to be reported herebelow.
MEASUREMENTS
OF MINUTE
PHOTOGRAPHIC
DENSITIES
763
We compared the (d, t) curves of plates whiCh were soaked in water and those of ordinary plates for red light, the soaked plates still being wet during the exposure, and the gelatin still being swollen. The water changed the colour of the plates from yellow to greenish, so that the absorption of the wet plates was larger for the red light. In accordance herewith the (d, t) curves rise steeper, almost proportionally by a factor 2. The same effect was observed if the time of soaking was varied from 2 to 25 minutes. After such plates had dried again, the greenish colour and the increased sensitivity for direct density production by red light remained. Even after drying for 4 days in a CaCl, exsiccator the results were the same, so the effect is difficult to reverse. Incidentally we remark that the density at which the rate of density production becomes small, as exposed in section e), is not altered.
II Time
Fig.
7. Direct
density
as a function
Of exposure
I” hours
of (long)
times
of exposure.
However, this water treatment caused a strong desensitization for developed-density production by the red light. The direct density and the developed one are, apparently, influenced by water in opposite ways. fj 4. Discussion.
The experiments
presented
do not allow
a defi-
764
E.
KATZ
nite interpretation. A few tentative remarks, from the standpoint of the G u r n e y-M o t t theory of latent-image formation, will be advanced 4) (cf. also W e b b “)). u) The S-shape of (d,t) curves for red light. The threshold-like start of the (d, t) curves for developed density and for direct density, caused by red light, have no common basis of interpretation since they occur at exposure values differing by several orders of magnitude “). We shall assume that the S-shape in the (d, t) curves correspods to a similar S-shape in (Ag, t) curves. The following arguments support this assumption : 1) The proportionality of d and t for blue light suggests that any anomalies in the relation between d and the amount of Ag do not occur in this region of densities. 2) An estimate of the size of our Ag-specks (cf. “)) shows that these are smallcompared with the wave length of light (of the order of a few atoms to a few hundred ones), so that proportionality of the absorption and the amount of Ag is probable for low densities. Since the H e r s c h e l-effect *) is known to occur in our wavelength region we shall discuss in how far this effect maybe related to the appearance of the S-shape. We have checked (by development) that the red light yielded appreciable developed densities after a few seconds of exposure, so that the H e r s c h e l-effect may occur here as a decrease of the rate or efficiency of Ag production but not as an effect of latent image erasure. The red and infrared exposure in our experiments was 1O’e--1 019 quanta/cm2 per 5 min, which is about 10” times as much as the lowest latent image exposures with blue light, so that at most a small Ag destroying action may be expected. Then d’, the time derivative of d (see fig. 8) must he the difference between two functions, one for Ag production (I) and one for Ag destruction (II) (see fig. 9). The G u r n e y-M o t t theory permits one to predict the shape of (II) qualitatively. It is postulated that the effect is due to a photo*) We shall understand by “H e r s c h e I”-effect the destruction of photo silver iu the grains by light. We shall not require that the rate of this destruction shall surpass that of production of Ag by the same (or simultaneously acting) light; the latter situation obtains in what is usually called the H e 1‘ s c h P I-effect (latent image rrasurr by red or infrared light).
MEASUREMENTS
OF MINUTE
PHOTOGRAPHIC
765
DENSITIES
electric ejection of electrons from the photo4lver into AgBr, with subsequent neutralization of the photosilver by the expulsion of Ag+ ions into interlattice positions of the AgBr crystal. Repetition of this process makes the Ag specks vanish. The theory makes essentially use of the smallness of the latent image Ag specks (dimensions of a few atoms) ; for larger Ag aggregates it is improbable that Ag+ ions should be regenerated in this way. In accordance herewith the H e rs c h e l-effect has not been observed for direct densities in the region of chemical detection (see 1) p. 322).
t3:y0vI 5”IO I---& 15
Fig.
8.
Direct
20
Time
in minutes
density d as function of time of exposure for orange (I = 13,4) (a) and its time derivative d’ (b).
OG3
Therefore Cl1 is proportional to d (i.e. to the number of absorptions in the Ag) in the latent image region, whereas the relative rate dj,/d approaches zero in’ the region of chemical detection. Between these two extremes, so in our experiments, the shape of (d’, d) will, therefore, resemble curve 11 fig. 9. The total rate of Ag production is then shown in curve I. Its shape is simpler than that of the original (d, t) curves, but still shows a superproportional start. Possibly its shape is connected with changes of the absorption spectrum of AgBr during the exposure (B e c q u e r e l-effect) *). On the basis of the analysis given the wave-length dependence of *) It is known that the adsorption of Agf ions to AgBr causes a large shift of absorption towards the infrared, but we do not know of similar experiments the effect of adsorbed Ag atoms upon the absorption of AgBr.
of the limit concerning
766
E. KATZ
the S-shape can qualitatively be understood. For shorter wave lengths, owing to the large increase in AgBr absorption, the B e cq u e r e l-effect will soon become indistinguishable, which causes shrinking of the quadratic start of the (d, t) curves. For still shorter wave lengths the relative importance of the H e r s c h e l-effect also decreases for the same reasons, so that the second bend of the (d, t) curve is removed, yielding a straight line.
3
Fig. 9. Schematical representation of constructive (I) and destructive components of the rate of direct-density production.
(II)
The S-shape of the (a!, t) curves for red light is, therefore, due to complications, superimposed on the normal shape of the curves, observed by J u r r i e n s for blue light (H e r s c h e l-effect and B e c q u e r e l-effect). Even the remnants of non-linearity as observed by that author (see 5 1) with blue acting light and red measuring light are now comprehensible as complications arising from the latter Under ideal experimental conditions the extrapolation of the proportional start down to the latent image region appears to be justified. b) The e f f e c t of w a t e r. It is known ‘) p. 93, that adsorption of OH-ions shifts the absorption limit of AgBr towards the infrared. It is plausible that the change in colour of the emulsion and
MEASUREMENTS
OF
MINUTE
PHOTOGRAPHIC
767
DENSITIES
its increased sensitivity for direct blackening after the water treatment is due to such an adsorption of OH- ions. The reported decrease in sensitivity for developed-density formation possibly means that the OH- ions are adsorbed mainly in the neighborhood of (superficial) sensitivity specks. Owing to their negative charge’ photo electrons will then preferably settle at specks in the interior of the grain, which are inaccessible for the developer. Received
June
3rd,
1942.
REFERENCES I) 2) 3) 41 5)
W. hl e i d i II g e r, Handb. wiss. angew. Photogr. V. H. J. J II rr i e n s, Thesis Utrecht 1938. E. K a t z, Thesis Utrecht 1941. R. W. G II r n e y and N. F. Mot t, Proc. roy. Sot. London J. H. W c I) b, J. appl. Phys. 11, 18, 1940.
A 164,
151,
1938.