Methods for detecting the presence or absence of ice or liquid water on
surfaces are provided. In a first embodiment of the invention, a
reflectance spectrum from a surface to be tested is measured using any
suitable near-infrared optical system. The midpoint wavelength of the
transition or step of the measured reflectance spectrum near 1.4 microns
is calculated, and compared to a decision threshold wavelength. If the
midpoint wavelength is less than the decision threshold wavelength, the
presence of liquid water on the surface is indicated. If the midpoint
wavelength is greater than the decision threshold wavelength, the
presence of ice is indicated on the surface. In a second embodiment of
the invention, at least three reflectance levels are measured at three
wavelengths, and a dimensionless decision function is applied to the
measured reflectance levels. Output of the decision function is compared
to at least two pre-defined ranges. If the output is within a first
range, the presence of liquid water on the surface is indicated. If the
output is within a second range, the presence of ice on the surface is
indicated. If the output is between the pre-defined ranges, the absence
of ice or water on the surface is indicated.