Optical scanning unit operating under changing temperature and moisture conditions

Claim 2

An optical scanning unit operated under varying temperature and moisture conditions, comprising: means for generating a light beam; means for converting the generated light beam into a convergent light beam, said converting means including a first plastic lens which has power of one polarity in a main scanning direction, and a second plastic lens which has power of a different polarity from said one polarity in the main scanning direction, said first and second lenses having focal lengths which vary in complementary to each other in response to temperature and moisture changes; means for directing the light beam emerged from the converting means toward an object, so as to scan the object with the light beam; and means for guiding the convergent light beam directed toward the object while processing the convergent light beam to have a cross section of predetermined size and shape.

Claim 3

An optical scanning unit according to claim 1, wherein: said converting means further includes at least one glass lens located on a beam-incident side thereof, said first plastic lens has a lens having negative power in the main scanning direction, and said second plastic lens has a lens having positive power in the main scanning direction and negative power in a sub-scanning direction; and said guiding means includes a third plastic lens.

Claim 4

An optical scanning unit according to claim 2, wherein the light beam which has passed through the second plastic lens of the converting means includes an axial light beam component which travels in the main scanning direction and which has substantially the same converging angle as the axial light beam component emerging from the glass lens.

Claim 5

An optical scanning unit according to claim 2, wherein said glass lens includes an aspheric lens.

Claim 6

An optical scanning unit according to claim 4, wherein said glass lens includes a lens which is made up of: a spherical glass lens portion; and an aspheric-surface layer formed on the spherical glass lens portion and made of a different material from that of the spherical glass lens portion.

Claim 7

An optical scanning unit according to claim 2, wherein the first and second plastic lenses and the third plastic lens have substantially the same coefficient of thermal expansion.

Claim 8

An optical scanning unit according to claim 2, wherein the first and second plastic lenses and the third plastic lens have substantially the same coefficient of moisture absorption.

Claim 9

An optical scanning unit according to claim 1, wherein each of said converting means and said directing means has an optical characteristic which satisfies the relationship below at least in a sub-scanning direction, ##EQU15## where: ν'i is a pseudo-partial dispersion ratio of an i-th optical element from a light beam-incident side (ν'i=∞ in the case of a glass lens); hi is a distance between a central axis of an optical element and a point at which an axial light beam is incident on the optical element; l is a number obtained by adding 1 to the number of lenses incorporated in the converting means; and m is the total number of optical elements, and where a variation Δfi to which the focal length fi of an i-th optical element of the converting means is subject in response to a temperature change Δt and a moisture change Δm is expressed by ##EQU16##

Claim 10

An optical scanning unit operating under varying temperature and moisture conditions, comprising: means for generating a light beam; converting means for converting the generated light beam into a convergent light beam, said converting means including: (a) at least one aspheric lens located on a beam-incident side thereof; (b) a first plastic lens having negative power in a main scanning direction; and (c) a second plastic lens formed of the same material as that of the first plastic lens and having positive power in the main scanning direction and negative power in a subscanning direction; means, including a plurality of reflecting faces, for directing the light beam output from the converting means toward an object in accordance with rotation of the reflecting faces, so as to scan the object with the light beam; and means for directing the convergent light beam onto a desirable portion on the surface of the object, said directing means including means, which has at least one plastic lens formed of the same material as the first plastic lens of said converting means, each of said converting means and said directing means having an optical characteristic which satisfies the relationship below at least in a sub-scanning direction, ##EQU17## where: ν'i is a pseudo-partial dispersion ratio of an i-th optical element from a light beam-incident side (ν'i=∞ in the case of a glass lens); hi is a distance between a central axis of an i-th optical element and a point at which an axial light beam is incident on the optical element; l is a number obtained by adding 1 to the number of lenses incorporated in the converting means; and m is the total number of optical elements, and where a variation Δfi to which the focal length fi of an i-th optical element of the converting means is subject in response to a temperature change Δm is expressed by ##EQU18##

Claim 11

An optical scanning unit operating under varying temperature and moisture conditions, comprising: means for generating a light beam; converting means for converting the generated light beam into a convergent light beam, said converting means including: (a) at least one aspheric lens located on a beam-incident side thereof; (b) a first plastic lens having negative power in a main scanning direction; and (c) a second plastic lens formed of the same material as that of the first plastic lens and having positive power in the main scanning direction and negative power in a subscanning direction; means, including a plurality of reflecting faces, for directing the light beam output from the converting means toward an object in accordance with rotation of the reflecting faces, so as to scan the object with the light beam; and means for directing the convergent light beam onto a desirable portion on the surface of the object, said directing means including means, which has at least one plastic lens formed of the same material as the first plastic lens of said converting means, each of said converting means and said directing means having an optical characteristic which satisfies the relationship below at least in a sub-scanning direction, ##EQU17## where: ν'i is a pseudo-partial dispersion ratio of an i-th optical element from a light beam-incident side (ν'i=∞ in the case of a glass lens); hi is a distance between a central axis of an i-th optical element and a point at which an axial light beam is incident on the optical element; l is a number obtained by adding 1 to the number of lenses incorporated in the converting means; and m is the total number of optical elements, and where a variation Δfi to which the focal length fi of an i-th optical element of the converting means is subject in response to a temperature change Δm is expressed by ##EQU18##