FIGURE 6.

Determining the relative optical thickness of disks. (A) BAM grayscale versus analyzer angle for the monolayer, single disks, and double disks (stacked or overlapping) in one typical experiment with DPPC/dchol 75/25 (mol/mol). Each measured grayscale represents the mean of a Gaussian fit to a grayscale histogram taken from the monolayer, disks, or double disks in several images. Error bars give the probable error in the mean (Bevington and Robinson, 1992). α_p, the position of p-polarization, here is 3°, and g₀, the grayscale minimum at s-polarization, is 15 arbitrary units. With g₀ and α_p determined from values for the monolayer, Eq. 1 can be solved using measured g(α) to determine g_p, the extended grayscale that would occur at α = α_p. (B) Predicted variation of extended BAM grayscale with thickness. Calculations assumed that collapsed structures had the same refractive index as monolayers, and that the increment in grayscale produced by that particular monolayer corresponded to its reflectivity. Thickness is expressed relative to an individual monolayer. Horizontal dashed lines give extended grayscale values measured for disks and double disks.