286
KENNETH V. THIMANN AND GEORGE M. CURRY
Two simple cases will serve to illustrate these relationships. First,
suppose that a is 0.1 (approx. 10% absorption) while m is 0.5 (approx.
40% absorption). Direct substitution in the equation for Q A will show
that A then absorbs 7.5% of the incident light when both materials are
present. If, now, m increases by 20% (e.g., when the wavelength is
changed), Q A falls to 7.2% of i 0 . Hence, for a and m in this range a 20%
rise in M's absorption spectrum will cause only a 4% fall in the action
spectrum (assuming A's absorption spectrum is flat in this region).
Second, assume that a is 0.0001 and m 5.0 (approx. 99% absorption by
m). In this case a 20% rise in m will cause a 20% decrease in Q A . At such
concentrations the basic absorption laws may not be strictly applicable,
but the effect of M on A cannot be greater than this.
These relationships give some basis for Reinert's results. However,
since he used concentrations of riboflavin comparable with those of
carotene, the masking effect to be expected would be more like the
former example above, i.e., large changes in masking cause only small
changes in action spectrum. This being the case, it is difficult to see
from a quantitative point of view exactly how his final curve resulted,
although it is not qualitatively unreasonable.
Similar "negative" masking would occur in a nonhomogeneous system where the masking pigment overlies the "active" absorber. Suppose
we have two units, the first containing M and the second containing A.
In the first unit the intensity, I 0 , is attenuated to I 0 e~
m and in the second
unit the fraction (1 — e
_a ) of this is absorbed, so that Q A = I 0 em
(1 — ea ). As m increases Q A decreases, which is "negative" masking
again.
An example of this type is the erythemal spectrum (action spectrum
for sunburn), which has a sharp minimum near 280 τημ. To quote
Blum (133), "The strong absorption by the corneum in the region of
0.28 μ probably accounts to a considerable extent for the minimum of
effectiveness of these wavelengths in producing erythema. . . . With a
spectrally selective absorbing layer (the corneum) superficial to that in
which the photochemical reaction occurs (the Malpighian layer) the
measured erythemal spectrum does not reflect directly the nature of
the absorbing substance which is concerned in that reaction."
As contrast to the above, let us consider a case where a gradient of
light action is involved. Suppose we have a container three units thick,
the first unit containing A, the second (center unit) M, and the third
(back side) A. In unit 1:
Q A>1 = 7o(l - e-«)
In unit 2:
QA.2 = 0
KENNETH V. THIMANN AND GEORGE M. CURRY
Two simple cases will serve to illustrate these relationships. First,
suppose that a is 0.1 (approx. 10% absorption) while m is 0.5 (approx.
40% absorption). Direct substitution in the equation for Q A will show
that A then absorbs 7.5% of the incident light when both materials are
present. If, now, m increases by 20% (e.g., when the wavelength is
changed), Q A falls to 7.2% of i 0 . Hence, for a and m in this range a 20%
rise in M's absorption spectrum will cause only a 4% fall in the action
spectrum (assuming A's absorption spectrum is flat in this region).
Second, assume that a is 0.0001 and m 5.0 (approx. 99% absorption by
m). In this case a 20% rise in m will cause a 20% decrease in Q A . At such
concentrations the basic absorption laws may not be strictly applicable,
but the effect of M on A cannot be greater than this.
These relationships give some basis for Reinert's results. However,
since he used concentrations of riboflavin comparable with those of
carotene, the masking effect to be expected would be more like the
former example above, i.e., large changes in masking cause only small
changes in action spectrum. This being the case, it is difficult to see
from a quantitative point of view exactly how his final curve resulted,
although it is not qualitatively unreasonable.
Similar "negative" masking would occur in a nonhomogeneous system where the masking pigment overlies the "active" absorber. Suppose
we have two units, the first containing M and the second containing A.
In the first unit the intensity, I 0 , is attenuated to I 0 e~
m and in the second
unit the fraction (1 — e
_a ) of this is absorbed, so that Q A = I 0 em
(1 — ea ). As m increases Q A decreases, which is "negative" masking
again.
An example of this type is the erythemal spectrum (action spectrum
for sunburn), which has a sharp minimum near 280 τημ. To quote
Blum (133), "The strong absorption by the corneum in the region of
0.28 μ probably accounts to a considerable extent for the minimum of
effectiveness of these wavelengths in producing erythema. . . . With a
spectrally selective absorbing layer (the corneum) superficial to that in
which the photochemical reaction occurs (the Malpighian layer) the
measured erythemal spectrum does not reflect directly the nature of
the absorbing substance which is concerned in that reaction."
As contrast to the above, let us consider a case where a gradient of
light action is involved. Suppose we have a container three units thick,
the first unit containing A, the second (center unit) M, and the third
(back side) A. In unit 1:
Q A>1 = 7o(l - e-«)
In unit 2:
QA.2 = 0