GB2111714A - Determining the focal length of a projection lens in a copier - Google Patents

Abstract

In a copier which is capable of copying at a continuously variable magnification, light from a source 1 passes through a microfilm to be copied which is held by a scanning system 2, and is projected by a lens 3 onto a photosensitive drum 6 by way of a movable reflection system 4 and a fixed mirror 5. In order to measure the focal length f of the lens 3, the reflection system 4 is first positioned at a predetermined location and the lens 3 is moved until an image of optimum sharpness is obtained on the drum 6: the magnification M0 of this image is then measured. Following this, the reflection system 4 is moved a measured distance b to a position 4' to alter the optical path length between the microfilm and the drum 6 by DELTA I. The lens 3 is then moved to obtain a new image of optimum sharpness on the drum 6, whereupon the magnification M' of the new image is measured. The focal length f is then calculated by means of the formula: f = DELTA I/(1/M0 - 1/M' + M0 - M'). <IMAGE>

Description

SPECIFICATION Determining the focal length of a projection lens in a copier This invention relates to a method of accurately determining the focal length of a projection lens in a microfilm copier which is capable of copying at a continuously variable magnification.

Most microfilm copiers are not capable of copying at a continuously variable magnification. Of those copiers in which it is possible to vary the magnification, some employ a plurality of projection lenses which can be interchanged on the optical axis, so that the magnification can be varied only in stages. As the number of such stages is increased, it becomes necessary to provide a larger number of projection lenses, which results in the copier becoming expensive and unduly bulky.

Other copiers have an optical system in which a single projection lens is used: the optical system also includes reflection means which can be moved to alter the optical path length between the object and image planes. By moving the reflection means in conjunction with movement of the back focus of the projection lens, a continuously variable magnification can be obtained.

In copiers of the latter type, it is necessary to know the magnification which will be obtained for a given movement of the lens back focus and the reflection means, for which purpose the focal length of the single projection lens must be determined. If the focal length is calculated by conventional techniques, for example the nodal-slide method, the accuracy is limited to the first decimal place, and accordingly it is impossible for the copier to be made with a high resolving power and magnification accuracy. Consequently, conventional copiers of this type have a low resolving power and a low accuracy.

It is an object of the present invention to overcome this particular problem.

According to the present invention, there is provided a method of accurately determining the focal length of a projection lens in a copier which is capable of copying at a continuously variable magnification and which includes an object plane, an image plane and a reflection system which can be moved to alter the length of the optical path between the object and image planes, the method comprising the steps of:: (a) positioning the reflection system at a predetermined location; (b) moving the projection lens until a first image of optimum sharpness is obtained at the image plane; (c) measuring the magnification Mo of the first image; (d) moving the reflection system to alter said optical path length by a measured amount Al; (e) moving the projection lens until a second image of optimum sharpness is obtained at the image plane; (f) measuring the magnification M' of the second image; and (g) calculating the focal length fof the projection lens in accordance with the formula: f = awl/(1 /M0 - 1 /M' + M0 - M').

Preferably, in step (a) the predetermined location is chosen such that a high image magnification results.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagram of the optical system of a copier in which a method according to the invention can be put into effect; Figures 2 and 3 are schematic diagrams of the optical system shown in Fig. 1 set respectively at two different image magnifications; and Figures 4 and 5 show other forms of copier in which the invention can be employed.

Referring first to Fig. 1, the copier shown therein comprises a light source 1 having a lamp, a focussing lens system, etc. Light from the source 1 passes through a microfilm to be copied which is mounted in a scanning system 2, and is then projected by a projection lens 3 onto a position 6' on a photosensitive drum 6 via a reflection system 4 and a fixed mirror 5. An image of the microfilm is thus formed on the periphery of the drum 6, the magnification of this image being continuously variable by on the one hand moving the back focus of the lens 3 and on the other hand moving the reflection system 4 to alter the length of the optical path between the microfilm and the drum 6.In the arrangement shown, the reflection system 4 turns the optical path through 180 , and it will therefore be manifest that moving the system through a distance b will alter the optical path length by Al, wherein b = awl/2.

The copier also includes a moving device for moving back focus of the projection lens 3 onto the optical axis, and a position detecting device 7 for stopping the moving device at a fixed position. A second position detecting device 8 is used for stopping the movement of the reflection system 4 at a predetermined location.

Fig. 2 depicts the situation where the reflection system 4 is positioned at the said predetermined location, and wherein the back focus of the projection lens 3 has been moved to obtain an image of optimum sharpness on the drum 6 (which is illustrated schematically in this Figure). The predetermined location of the reflection system is chosen so that the resultant image magnification Mo is high. Fig. 3 on the other hand shows the situation where the reflection system 4 has been moved by a distance b (i.e. to the position indicated at 4' in Fig. 1) so as to alter the optical path length by Al, and wherein the lens 3 has been moved by a distance a (i.e. to the position indicated at 3' in Fig. 1) to obtain an image of optimum sharpness having an arbitrary magnification M'.In Fig. 3, 11 denotes the back focus length of the lens 3, while 12 denotes the image distance, and AH denotes the distance between the principal points of the lens 3.

From Figs. 2 and 3, the following relationships can be established: a=(1/M'-1/M0)f... ) Al = 2b = (1 /M0 - 1 /M' + M0 - (2) where a, b, Al, Mo and M' are as defined above, and fis the focal length of the lens 3.

Since fand Mo are fixed constants, the distances a and b can be computed to control the amount of movement of the lens 3 and the reflection system 4, making continuously variable magnification possible in theory.

The depth A of the projection lens 3 on the object side is given roughly by the expression: A=.F.(a=1/M)... (3) wherein e is the reciprocal of the resolving power, F is the aperture value of the lens, and M is the resulting magnification. The value of F will in practice be within substantially fixed limits; for example F may be between 4 and 10.

The magnification M' may be expressed as follows: M'=l2/l1..

It will therefore be evident that the accuracy of the resolving power of an image to be copied and the accuracy of magmification both depend upon the accuracy with which the back focus of the lens 3 and the reflection system 4 are positioned. This will in turn depend upon the measurement accuracy of the constants Mo and f used in expressions (1) and (2) above.

It is comparatively easy to measure Mo accurately, using a reference subject and a reference measure on the drum 6. However, if fis measured using conventional techniques, for example the nodal-slide method, then the accuracy of the measurement will be limited to the first decimal place, which is not precise enough to obtain high accuracy in the resolving power and in the magnification.

In order to overcome this problem, the present invention provides the following method of determining the value of f. Firstly, the reflection system 4 is moved to its aforementioned predetermined location, and the position of the projection lens 3 is adjusted until an image of optimum sharpness has been obtained on the drum. The magnification Mo of this image is then accurately measured.

Following this, the reflection system is moved through a distance b (which is accurately measured) so as to alter the optical path length by Awl( = 2b). The lens 3 is then moved until the new image on the drum 6 reaches optimum sharpness, whereupon the magnification M' of the image is accurately measured. The focal length of the lens 3 can then be calculated by means of the following formula, which is derived from expression (2) above: f = AI/(1 /M0 - 1 /Mt + M0-M').. . ..15) In this way, the value of fcan be determined with a high degree of accuracy, enabling the values of a and b in expressions (1) and (2) above to be accurately determined for any desired magnification.

Fig. 4 shows an alternative type of copier with which the invention can be used. In this copier, light from the light source 1, after passing through the microfilm mounted in the scanning system 2, is projected onto the drum 6 by way of a fixed mirror 9, the reflection system 4 and the fixed mirror 5. In other words, the copier is substantially the same as that shown in Fig. 1, save for the provision of the additional fixed mirror 9.

Fig. 5 illustrates a further type of copier to which the present invention can be applied.

This copier differs from that shown in Fig. 1 in that the reflection system 4 turns the optical path through 90 rather than 180 . In this case, it will be apparent that the distance b moved by the reflection system 4 will be equal to the resultant change Al in the length of the optical path.

The above-described method of determining the focal length of the projection lens 3 enables the copier to have a high resolving power and a magnified copy of high accuracy to be obtained. In addition, the method is simple to perform and, since the determination is carried out after the optical system has been assembled, repeated tests are not necessary. Furthermore, the optical system comprises two separate parts, namely the lens 3 and the reflection system 4, which facilitates its installation in a copier or its removal for repair.

Claims (8)

1. A method of accurately determining the focal length of a projection lens in a copier which is capable of copying at a continuously variable magnification and which includes an object plane, an image plane and a reflection system which can be moved to alter the length of the optical path between the object and image planes, the method comprising the steps of:: (a) positioning the reflection system at a predetermined location; (b) moving the projection lens until a first image of optimum sharpness is obtained at the image plane; (c) measuring the magnification Mo of the first image; (d) moving the reflection system to alter said optical path length by a measured amount Al; (e) moving the projection lens until a second image of optimum sharpness is obtained at the image plane; (f) measuring the magnification M' of the second image; and (g) calculating the focal length fof the projection lens in accordance with the formula: f = he/(1 /M0 - 1 /M' + M0 - M').

2. A method as claimed in Claim 1, wherein the reflection system turns said optical path through substantially 180 , and during step (d) the reflection system is moved through a measured distance b = Al/2.

3. A method as claimed in Claim 1 or 2 wherein the copier includes a single projection lens.

4. A method as claimed in Claim 1, 2 or 3, wherein the copier includes a moving device for moving the back focus of the projection lens onto the optical axis, and a stopping device operative to stop the moving device at a fixed position.

5. A method as claimed in any preceding claim, wherein the image plane is formed by the peripheral surface of a photosensitive drum.

6. A method as claimed in any preceding claim, wherein the copier includes a position detecting device which is operative to stop movement of the reflection system.

7. A method as claimed in any preceding claim, wherein a high image magnification is obtained when the reflection system is positioned at said predetermined location.

8. A method of accurately determining the focal length of a projection lens in a copier, which is capable of copying at a continuously variable magnification, substantially as hereinbefore described with reference to Figs. 2 and 3 and any one of Figs. 1, 4 and 5 of the accompanying drawings.