Before light sources with considerable coherence length were available (think lasers), opticians formed fringe patterns using a test plate, also known as an optical flat. Spherical test plates also exist, to check the surface of lenses, or curved mirrors. These test plates could be placed in close proximity to each other, so common light sources could be used.
Read MoreAn interferometer is a device to make precise measurements using light. Modern interferometers often use laser light because the light is generally a discrete wavelength and the light beam can readily be collimated or expanded. The name is derived from utilizing the characteristic property of the wave-like nature of light to combine a light beam with another, or often with itself, to cause subtraction or addition to the intensity at a point (interference) measured (metered) by the instrument.
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