The effects of pigments contained in N-isopropylacrylamide (NIPAM) gels on their volume-change properties were investigated. All the NIPAM gel particles, containing various kinds and concentrations of pigments, showed a volume phase transition at 34 degrees C. No pigment affected the volume-phase-transition temperature of the NIPAM gels. As the concentration of the pigment in the NIPAM gels was increased, the amount of the volume change of the NIPAM gels was reduced. The water absorptivity of the NIPAM gels in the swollen state decreased as the pigment concentration increased, whereas the water absorptivity in the shrunken state was almost constant. Reducing the initial monomer concentration of the polymerization of the NIPAM gel increased the water absorptivity in the swollen state. With an increase in the water absorptivity, the volume changes of the NIPAM gels containing pigments were increased. Prototype light modulators in which the NIPAM gel particles containing pigment were dispersed between glass plates were fabricated. The light modulator using the gel particles with improved diameter change (d/d(0) = 2.3, where d and do are the equilibrium diameter and the diameter of the fully shrunken state at 50 degrees C, respectively) exhibited a larger transmittance change from 8 to 79% than that using the gel particles before the improvement (d/d(0) = 1.7; from 38 to 79%) according to temperature changes. (c) 2006 Wiley Periodicals, Inc.