Lens unit for multibeam scanning device

A lens unit for a scanning device, comprising:

• a frame having a hollow cylindrical shape, the frame being defined with a lens contact portion therein;
• a lens accommodated in the frame contacting the lens contact portion defined in the frame;
• and
• a retainer accommodated in the frame to retain the lens in position, the retainer having a hollow cylindrical shape, one end side face of the retainer contacting a peripheral portion of the lens received by the frame, another end portion of the retainer being secured to the frame so that the retainer presses the lens toward the lens contact portion of the frame to fix the lens to the frame,
• wherein deformation of the frame, lens and retainer due to the load generated as the retainer presses the lens absorbs deformation of the frame, lens and retainer due to temperature change at least within a predetermined temperature range so that a fixed status of the lens with respect to the frame is not released due to the temperature change within the predetermined temperature range,
• wherein the lens has a linear expansion coefficient ρ1, a longitudinal elastic modulus E1, and a cross-sectional area S1 orthogonal to an optical axis direction,
• wherein the frame has a linear expansion coefficient ρ2, a longitudinal elastic modulus E2, and a cross-sectional area S2 orthogonal to the optical axis direction, in which the optical system is installed,
• wherein the retainer has a linear expansion coefficient ρ3, a longitudinal elastic modulus E3, and a cross-sectional area S3 orthogonal to the optical axis direction, the retainer applying the lens with a load P,
• wherein the lens unit being configured to satisfy following condition:
• Δ
• ⁢
• ⁢
• t
• ⁢
• {
• ρ
• 2
• ⁢
• L
• 2
• -
• (
• ρ
• 1
• ⁢
• L
• 1
• +
• ρ
• 3
• ⁢
• L
• 3
• )
• }
• <
• P
• ⁡
• (
• L
• 1
• E
• 1
• ⁢
• S
• 1
• +
• L
• 2
• E
• 2
• ⁢
• S
• 2
• +
• L
• 3
• E
• 3
• ⁢
• S
• 3
• )
• ,
• wherein,
• L1 represents a length of the lens from a contact point of the lens and the lens contact portion of the frame to a contact point of the lens and the retainer in the optical axis direction at a predetermined temperature t0,
• L2 represents a length of the frame from the contact point of the lens and the lens contact portion of the frame to a lens side end of the other end portion of the retainer at a predetermined temperature t0,
• L3 represents a length of the retainer from the contact point of the lens and the retainer to the lens side end of the other end portion of the retainer at a predetermined temperature t0,
• wherein L2=L1+L3, and
• wherein Δt represents a change of temperature with respect to the predetermined temperature.