1. Field of the Invention
This invention relates to an optical scanning type image pickup apparatus wherein main and sub scannings of a light beam are effected over a scanning surface by reciprocally traversing both the scanning surface and an optical means for irradiating the light beam over the scanning surface relative to each other, and where a light beam reflected from the region of the scanning surface on exposure to a beam of light is detected by a photodetector.
Further, this invention relates to an optical scanning type microscope and, particularly to, a scanning type microscope wherein an optical means for irradiating a light beam to a sample is supported on a tuning fork, and the tuning fork is rendered to vibrate such that the sample may be scanned with the light beam.
2. Description of the Prior Art
Optical type scanning microscopes have heretofore been used. With the scanning microscope a light beam is converged to a small spot on a sample, and the sample is two-dimensionally scanned with the light spot. The light beam which has passed through the sample during the scanning, the light which has been reflected from the sample during the scanning, or a fluorescence which is produced by the sample during the scanning, is detected by a photodetector. An enlarged image of the sample is thereby obtained in the form of an electrical signal. An example of the scanning microscope is disclosed in Japanese Unexamined Patent Publication No. 62(1987)-217218.
In the conventional optical type scanning microscope, a mechanism which two-dimensionally deflects a light beam by a light deflector has primarily been employed as the scanning mechanism.
However, the scanning mechanism described above has the drawback in that an expensive light deflector, such as a galvanometry mirror or an acousto-optic light deflector (AOD), is necessary. Also, the foregoing mechanism, wherein the light beam is deflected by the light deflector, induces momentarily a change of the incident angle of a deflected light beam upon an objective lens of the light projecting optical means, resulting in an aberration. Therefore, the scanning mechanism has also been deemed as disadvantageous in that the elimination of such an aberration restricts designing of the objective lens. Particularly, in the case where the ACD is utilized, astigmatism occurs in the light beam irradiated out of the AOD as well as from the objective lens. In such a case, a special correction lens is required, which renders the optical means more complicated.
In order to overcome the drawbacks in the prior art being set forth in the above, there has been proposed a scanning mechanism in which a sample is scanned with the light spot of the light beam without deflecting the beam of light. For instance, Japanese Unexamined Patent Publication No. 1(1990)-248946 filed by the same inventor of this application and assigned to the same assignor describes that the light projecting optical means is shifted relative to a sample supporting member to effect scanning of the light spot of the light beam.
As one specific exemplification of such a scanning mechanism wherein the optical means is traversed with respect to the sample supporting member, as disclosed in Japanese Unexamined Patent Publication No. 2(1991)-198550 filed by the same inventor of this application, there has been proposed a scanning mechanism comprised of a tuning fork at one end thereof supporting the optical means, and an electromagnet for applying a driving force, the strength of which varies periodically, to the tuning fork to cause resonation of the tuning fork. According to another specific exemplification, as disclosed also in the Japanese Unexamined Patent Publication No. 2(1991)-198550, a scanning mechanism is proposed wherein a piezoelectric element is mounted on the above-mentioned tuning fork, and the resonance of the tuning fork is carried out by providing a periodic stress to the element under the control of a voltage applied to the piezoelectric element.
This scanning mechanism is advantageous in securing a wider image pick-up area of the microscope because the relative width of tracing of the optical means, or the width of scanning of the light beam (determined by the amplitude of the tuning fork), can be set to a large value when compared with, for instance, scanning mechanisms using piezoelectric elements, ultrasonic vibrators and the like.
However, small amounts of displacements of the image pickup area in a direction of scanning, which is generated by the vibration of the tuning fork, may arise because of the use of a light beam scanning mechanism composed of a combination of a tuning fork and an excitation means for effecting the resonance of the tuning fork. In such cases, the size of an output image will be dependent upon the way a signal output from the foregoing photodetector is processed, or the way a magnifying power may differ even under a constant image size.
In the meantime, in many of the conventional scanning type microscopes, digital image data per each main scanning line is produced by feeding a series of outputs from the photodetector, for detecting a light beam reflected from the sample, and by quantizing and sampling this output signal.
With such a configuration, if the light beam scanning mechanism set forth in the above is employed, an image may be displaced entirely along a direction of main scanning of the light beam when the amplitude of the tuning fork has been changed for zooming when capturing an image with the microscope, for instance. Namely, the phase of a driving voltage to vary the amplitude of the tuning fork and the phase of the tuning fork are not always coincident with each other, and if the driving conditions of the electromagnet for changing the amplitude of the tuning fork, that is, a voltage, a duty ratio, and the like are changed, the phase of the tuning fork will also be varied. Accordingly, in the case where a sampling initiation timing of image data, one every main scanning line, is defined on the basis of the phase of the driving voltage of the electromagnet, as has been practiced in the prior art, the sampling initiation timing is changed in response to the driving conditions, and consequently the captured image will be displaced along the direction of main scanning.
Although the above description has been given of the case where the light beam scanning mechanism is constituted of the tuning fork and the electromagnet in combinations, similar problems can arise when beam of light scanning mechanisms other than the above are employed, if the phase of a signal to drive the scanning mechanism and the phase of a sample to be traversed will be out of phase with each other.
In addition to the drawbacks in the existing scanning type microscope having been set forth in the above, there have been generally known various types of optical scanning type image pickup apparatus wherein main and sub scannings of a light beam are effected across a scanning surface by reciprocally traversing both the scanning surface and an optical means for irradiating the light beam to the scanning surface relative to each other, and where a light beam reflected from the target scanning surface is detected by a photodetector. In the same manner as the scanning type microscope, the optical scanning type image pickup scanning may suffer from drawbacks such as the fluctuation of the image pickup area and the displacement of the captured image along the direction of main scanning.