Figure P52

that 0m2 = 1.5° and the minimum useful intensity contrast is Cie = 0.01. Plot the minimum 8x vs. A to give Cie = 0.01. Consider A from 0.15° to 15°. (Note: When A is large, only a small 8x is required; as A decreases, Sx becomes larger until finally 8x = A/2 to get Cie = 0.01.)

5.3. A certain photoreceptor subject to general illumination has an output depolarization, vm, which is log-linear over the range, {I1, I2}. See Figure P5.3. In this range, vm can be modeled by vm = A ln(I/B), A > 0

Assume I .lies in the range, {I1, I2}. Consider a small increase in intensity, AI, above I.

a. Find an algebraic expression for the contrast in vm, defined as Cvm = Avm/ Vm Note that the approximation, ln(1 + x) Y x, x < 1, can be used.

b. Find an expression for Cvm/Q in the range {I1, I2}. _Is it possible for Cvm/Cj to be > 1? What are the conditions? (Q = AI/I).

5.4. An array of three one-dimensional photoreceptors are connected as shown in Figure P5.4: The visual object is an infinite, {0, Io}, spatial square wave with period, A. The receptor center axes are spaced ±b around the x = 0 axis. Each receptor has a Gaussian DSF with half-angle, 0m/2. Thus,

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