Water droplet sensor and exposure system for hologram

A water sensor comprising:

• a transparent plate
• a light emitting device for emitting sensing light so that the sensing light travels in the transparent plate by internal reflection in the transparent plate;
• a light receiving device for receiving the sensing light from the transparent plate to sense water on the transparent plate;
• and
• a hologram for defining a light path of the sensing light through the transparent plate from the light emitting device to the light receiving device.

An exposing system for producing a hologram, comprising:

• a transparent plate member for defining a reflection surface and a contact surface, and holding a light sensitive film between the reflection surface and the contact surface;
• a first prism having a contact face for contacting with the contact surface of the plate member, a first face for guiding a first reference beam so that the first reference beam travels through a first region of the light sensitive film, and a second face for guiding a laser beam so that the laser beam travels through the first region of the light sensitive film as a first object beam to form a first hologram in the first region of the light sensitive film with the first reference beam;
• and
• a second prism having a contact face for contacting with the contact surface of the plate member, a first face for guiding a second object beam so that the second object beam travels through a second region of the light sensitive film, and a second face for guiding the laser beam reflected internally from the reflection surface of the plate member so that the reflected laser beam travels through the second region of the light sensitive film as a second reference beam to form a second hologram in the second region of the light sensitive film with the second object beam.