Parallel light ray measuring apparatus

A parallel light ray measuring apparatus for measuring the parallelism of light rays emitted from a light source, said apparatus comprising:

• a first grating;
• a second grating having the same grating period as that of said first grating and disposed so that the grating lines thereof are in parallel with those of said first grating;
• a photoelectric converting means including at least two photoelectric converting elements for receiving light rays emitted from the light source, said rays having passed through both of said gratings;
• a moving means for moving one of said two gratings in a direction perpendicular to the plane of the grating moved by it;
• and
• a means for detecting a phase difference between signals output from two of said at least two photoelectric converting means, wherein the parallelism of the emitted light rays is determined by the equation:
• .theta.err=Arc Tan (.psi..multidot.P/2.pi..multidot.G)
• wherein:
• .theta.err is the deviation angle of the light rays from parallel;
• .psi. is the phase difference between said two signals output by said at least two photoelectric converting means;
• P is the grating period of said gratings;
• and G is the gap between said two gratings.