| 摘要 |
A lens including a diffraction grating formed on the inner side area and the outer peripheral side area of the refraction face in a concentrically circular shape around its optical axis such that a plurality of light beams with different wavelengths are focused at different positions. The diffraction grating of both the inner side area and the outer peripheral side area is formed so as to have the same blaze height "h", which is set to be coincided with or approximated to both the values "h<SUB>1</SUB>" and "h<SUB>2</SUB>", which are obtained by the following conditional expressions: <?in-line-formulae description="In-line Formulae" end="lead"?>h<SUB>1</SUB>=(m<SUB>1</SUB>xlambda<SUB>1</SUB>)/(n-1) the conditional expression (1): <?in-line-formulae description="In-line Formulae" end="tail"?> <?in-line-formulae description="In-line Formulae" end="lead"?>h<SUB>2</SUB>=(m<SUB>2</SUB>xlambda<SUB>2</SUB>)/(n2-1) the conditional expression (2): <?in-line-formulae description="In-line Formulae" end="tail"?> wherein "m<SUB>1</SUB>" is an integer number of 1 or more, "m<SUB>2</SUB>" is an integer number more than "m<SUB>1</SUB>", "lambda<SUB>1</SUB>" is a wavelength of the first light beam, "lambda<SUB>2</SUB>" is a wavelength of the second light beam, "n<SUB>1</SUB>" is a refractive index of the refraction face to the first light beam, and "n<SUB>2</SUB>" is a refractive index of the refraction face to the second light beam.
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