Post-Objective Type Scanning Device

1. Field of the Invention

This invention relates to a photo-scanning device, and particularly to a photo-scanning device of post-objective type with a curvature of field reduced.

2. Description of the Prior Art

In optical photo-scanners used for laser printers, laser displays, etc., there are known a post-objective type in which an optical deflector is arranged after a beam has been formed into a condenser light flux by a focusing lens and a pre-objective type in which a focusing lens is arranged after the deflector. The former is subjected to focusing action prior to deflection and therefore the focusing lens is required to resolve on-axis image formation alone, thus enabling to make the lens structure simple, but the focusing point is generally formed along a curved surface. On the other hand, in the pre-objective type, the beam is incident on the focusing lens after deflection, and therefore, a focusing lens needs to be designed as a wide angle lens sufficient to cover the deflection angle, thus complicating the structure of the lens but enabling the focusing point to be formed along more of a planar image surface. At present, this type is principally used.

This invention is to provide a highly practical post-objective type scanning device by finding an optical arrangement in which the curvature of the resulting image surface of the focusing point is reduced.

FIG. 1 is a view for explaining the principle of the post-objective type scanning device to which this invention pertains. In a deflector, a spherical or cylindrical mirror M having a center C rotates about an axis O.

The scanning light flux is incident so as to be focused on a point S, and reflects at the mirror M to be image-formed at a point S'. In the figure, S'.sub.o denotes an image forming point when a deflection angle .theta. is zero (0).

Let L.sub.o be the length between S'.sub.o and O, and let L be the length to the foot of the perpendicular line extended down from the point S', then

where R is the radius of curvature of the reflecting mirror and k'R is the length from the foot of the perpendicular line which extends down from S' to the line connecting O and C. Let k.sub.o R be the length between O and S, and let kR be the length between the foot of the perpendicular line extended down from the line OC to the point S, then k'=k.sub.o cos.sup.2 .theta./cos.sup.2 .theta.-2k) and after all, L is given by

In the condition where the mirror M rotates about O, the length L is equal to the length L.sub.o when .theta.=0 is given by

and at this time, the following is given,

On the other hand,

then,

Therefore,

Using the above relations, the invention provides an improved post-objective scanning device including a focusing cylindrical lens in the vicinity of the scanning surface in order to reduce irregularities in and the curvature of the resulting image surface.

FIG. 1 is an explanatory view of the principle of of a post-objective type scanning device;

FIGS. 2(a) and (b) illustrate an optical arrangement of one embodiment of the scanning device in accordance with this invention;

FIG. 3 is a reference view of another light source; and

FIGS. 4 and 5 are graphic representations showing the degree of an improvement in the curvature of the resulting image surface in the scanning device in accordance with the present invention.

FIGS. 2(a) and (b) shows a printer optical system using a semi-conductor laser as one embodiment of a scanner which satisfies the conditions of this invention.

A reference numeral 1 designates a semi-conductor laser serving as a light source, 2 a coupling lens for focusing laser light fluxes to focus them on S.sub.o, 3 a first cylindrical lens having a refractive power within the surface perpendicular to the scanning line, 4 an optical reflector, 5 a second cylindrical lens having a refractive power within the aforesaid surface, and 6 a photosensitive member. Light fluxes from the laser light source 1 are condensed so as to focus on the point S.sub.o through the coupling lens 2 to be incident on the first cylindrical lens 3, and within the surface having the refractive power, the light fluxes are focused on the deflector 4. On the other hand, within the surface including a scanning line, the light fluxes are focused towards the point S.sub.o without any change. The deflecting mirror 4 rotates about the axis O to deflect the incident laser light fluxes, which are focused on the photosensitive member 6 by the second cylindrical lens 5 for scanning.

The cylindrical lens 5 is provided so that the deflecting point 4' and the scanning surface on the photosensitive member 6 are arranged to provide a geometric-optically conjugate perpendicular to the scanning line so as to correct irregularities in pitch of the scanning line and a bend of the scanning line and to reduce a bend of a sagittal image surface (an image surface in a direction along the scanning line).

The deviation .DELTA.L.sub.s of the sagittal image surface resulting from the curvature of the image surface, where the second cylindrical lens 5 is used, is expressed by ##EQU1## where s.sub.o =(1-l.sub.o)L.sub.o is the length from the second cylindrical lens to the scanning surface, and n.sub.c is the refractive index of the second cylindrical lens. From Snell's law,

In the following, the numeric examples of the optical system in this embodiment and the prior art examples corresponding thereto are given. .DELTA.LM indicates the deviation of the meridional image surface resulting from the curvature of the image surface.

If an angle of rotation .theta.=.+-.10.degree. (deflection angle .+-.20.degree.) is present, and L.sub..theta. =L.sub.o =300 mm:

From Equation (2), R=153.480 mm

In the second cylindrical lens,

f.sub.c =27 mm

n.sub.c =1.5

Therefore, in Equation (3),

s.sub.o =30 mm

l.sub.o =0.9 mm. Compared to the above example of the invention, the prior art example corresponding thereto is as follows:

If an angle of rotation .theta.=.+-.5.degree. (deflection angle .+-.10.degree.) is present, and L.sub..theta. =L.sub.o =600 mm:

From Equation (2), R=301.720 mm

In the second cylindrical lens,

f.sub.c =28.5 mm

n.sub.c =1.5

Therefore, in Equation (3),

s.sub.o =30 mm

l.sub.o =0.95 mm

In comparison, the prior art example corresponding thereto is as follows:

The degree of decrease in image surface curve of Example 1 is shown in FIG. 4 and Example 2 is shown in FIG. 5. The degree of decrease in displacement of the image surface is as given in the following Table.

While in the above-described embodiment, a semi-conductor laser has been used as a light source, it will be noted that a gas laser 1' and a modulation optical system 2' can be used as shown in FIG. 3. The modulation optical system 2' comprises a photo-modulation element 2'-2, a stop-down lens 2'-1 and a focusing lens 2'-3. When used for a laser display, a reflection or transmission type screen can be installed at the scanning surface position of the photosensitive member.