The ornamental design for a lighttherapydevice, as shown.
FIG. 1 is a right perspective view of a light therapy device.
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
The broken lines in FIG. 3 depict portions of the light therapy device that form no part of the claimed design; the broken lines form no part of the claimed design.