The present invention provides a backlight module structure. The sleeve is screwed on the fixing portion to compress the binding ring to reduce an inner diameter thereof, so as to grab and hold the bearing. Thus, dots located adjacent to each other in the auxiliary scanning direction are not formed by same scanning, and a quantity of ink spreading on a recording medium without penetration of the ink through the latter can be reduced. The transmitter within the transmitter housing constantly emits a signal detectable by the accompanying receiver located in the receiver housing. The angled portion has an angle greater than zero and includes a third hole near the end of the angled portion that is opposite the linear portion.