  The critical behavior at the transition from uniform nematic to twist-bend
modulated nematic phase is revealed and shown to be well explained by the mean
field approximation. The study was performed on a group of materials that
exhibit an unusually broad temperature range of the nematic phase above the
twist-bend modulated nematic phase, so the critical range in which the order
parameter fluctuations are strong and can be experimentally observed is wide.
The formation of instantaneous helices is observed already several degrees
above the transition temperature, strongly influencing the birefringence of the
system. The analysis of a critical part of the birefringence changes provides a
set of critical exponents that are consistent with the mean field
approximation, indicating a large correlation length of helical fluctuations in
the nematic phase.
