The contrast of faculae near the solar limb

We have measured the contrast of solar faculae near the limb on direct digital video images made with the 65 cm vacuum reflector at the Big Bear Solar Observatory. We used six broad band filters with different wavelengths from red to violet. The range of heliocentric angle covered in our measurements is 0.05 < μ = cos θ < 0.4 (θ = 87°–66°). About 300 images were measured from observations made during the summers of 1983 and 1985. Over 20000 faculae were measured. By averaging the contrasts of faculae and plotting them vs heliocentric angle, we found that contrast increases monotonically towards the limb for the shorter wavelengths; for longer wavelengths, contrast has a tendency to peak around μ = 0.15, and then decrease towards the extreme limb. The contrast increases as wavelength decreases