  We have detected Egyptian blue pigment in the paint layer of the "Birch.
Spring" painting by Robert Falk (1907); we have also found this pigment in the
paints of the sketch drawn on the canvas back side. This is probably the first
discovery of Egyptian blue in a 20th century work of art.
  We have analyzed a modern commercial Egyptian blue pigment (Kremer) and found
it to be suitable as a standard for photoluminescence spectral analysis. The
characteristic photoluminescence band of CaCuSi$_4$O$_{10}$ reaches maximum at
the wavelength of about 910 nm. The luminescence is efficiently excited by
incoherent green or blue light.
  The study demonstrates that the photoluminescence spectral micro analysis
using excitation by incoherent light can be efficiently used for the
identification of luminescent pigments in paint layers of artworks.
