Focus detecting device for an optical apparatus

Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems

Reexamination Certificate

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C250S201400, C359S383000

Reexamination Certificate

active

06649893

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a focus detecting device used in an optical apparatus in which the observation, measurement, and examination of an object are carried out through an optical system.
2. Description of Related Art
In an optical apparatus in which the observation, measurement, and examination of an object are carried out through an optical system, for example, in an optical microscope, an observer must perform focusing, in order to observe a sharp image of the object, by moving the stage of the microscope vertically to adjust a distance between an objective lens and the object. In this case, if the objective lens has a high magnification, a depth of focus is small, and thus a focusing position cannot be found when the stage is widely moved. Hence, the observer must move the stage little by little and needs much time to find the focusing position. On the other hand, if the objective lens has a low magnification, the depth of focus is larger, and thus, sometimes, it becomes difficult that the observer determines an optimum position of the stage where the object is brought to a focus.
In order to solve such a problem, a focus detecting device has recently become combined with the optical apparatus. Various systems are available in focus detecting devices. One of them is an active system focus detecting device in which light is radiated toward an object and reflected light from the object is detected by a photodetector to determine an in-focus or out-of-focus state in accordance with the reflected light.
The arrangement of the active system focus detecting device is shown in FIG.
1
. In this figure, reference numeral
4
represents a light source;
5
, a collimator lens;
7
, a light-blocking plate;
8
, a polarization beam splitter;
11
, a quarter-wave plate;
12
, dichroic mirror;
3
, an objective lens;
13
, an imaging lens; and
14
, a photodetector. Reference symbol S represents a sample which is an object.
The light source
4
is a semiconductor laser, which emits laser light in an infrared wavelength region. This light is linearly polarized. The laser light is changed by the collimator lens
5
into a parallel beam, which is incident on the polarization beam splitter
8
. In this case, by the light-blocking plate
7
interposed between the collimator lens
5
and the polarization beam splitter
8
, a half of the light beam is blocked. The polarization beam splitter
8
has the characteristics of reflecting the linearly polarized light of p polarization and of transmitting the linearly polarized light of s polarization. Thus, when the semiconductor laser is previously placed so that the orientation of polarization of the laser light coincides with that of the p polarization, all the laser light incident on the polarization beam splitter
8
is reflected by the reflecting surface of the polarization beam splitter
8
, and hence the intensity (amount) of light is not lost.
The laser light reflected by the reflecting surface of the polarization beam splitter
8
is incident on the quarter-wave plate
11
. The quarter-wave plate
11
is placed so that the linearly polarized light incident thereon is changed to circularly polarized light, which emerges therefrom. The laser light emerging from the quarter-wave plate
11
is reflected by the dichroic mirror
12
and is incident on the objective lens
3
. The objective lens
3
converges the laser light on the sample S.
The laser light reflected by the sample S passes again through the objective lens
3
. At this time, the laser light does not follow the same optical path as in the case of incidence, but takes an optical path on the opposite side with respect to the optical axis. The laser light is reflected by the dichroic mirror
12
and enters the quarter-wave plate
11
. Here, the laser light which is the circularly polarized light is changed to the linearly polarized light and emerges therefrom. However, since the orientation of the linearly polarized light becomes identical with that of the s polarization, all the laser light incident on the polarization beam splitter
8
passes through the polarization beam splitter
8
and enters the imaging lens
13
. The imaging lens
13
condenses the laser light incident thereon. The photodetector
14
is located at a position where the laser light is condensed, and produces an electric signal in accordance with the intensity of the laser light. The photodetector
14
is such that two independent light-receiving sections A and B are arranged closely adjacent to each other. For example, a binary photodiode could be used.
In the arrangement shown in
FIG. 1
, the laser light collected on the sample S through the objective lens
3
is practically circular in shape and assumes a convergent point having an extremely small area (which is hereinafter referred to as spot light or a spot beam). The spot light has one spot. Such a construction is hereinafter termed a single-spot projection system.
In the single-spot projection system, how in-focus and out-of-focus states are decided (detected) is explained below with reference to FIG.
2
A and
FIGS. 3A-5B
.
FIG. 2A
shows a state where the spot light is radiated on a convex surface of a sample of irregular shape and is brought to a focus on the convex surface. Specifically, a position where the size of the spot light collected by the objective lens
3
is minimized (which is hereinafter referred to as the focal position of the objective lens) coincides with the convex surface of the sample S. In this case, the spot light reflected by the convex surface, as shown in
FIG. 4A
, is converged at the center of the photodetector
14
. For reference, the intensity distribution of a convergent beam (the spot light) is shown on the right side of the photodetector
14
.
The photodetector
14
is constructed with the two light-receiving sections A and B which are identical in shape. A slight space (simply indicated by a solid line in the figure) is provided between the light-receiving sections A and B and coincides with the optical axis.
As seen from
FIG. 4A
, in the in-focus state, reflected light from the sample S is collected on the optical axis, and thus the spot light formed on the photodetector
14
has an intensity distribution of bilateral symmetry with respect to the optical axis. Specifically, since half of the spot light is formed on the light-receiving section A and the remaining half is formed on the light-receiving section B, the areas (intensities) of the spot light formed on the light-receiving sections A and B are equal. Hence, in the in-focus state, electric signals produced from the two light-receiving sections A and B are also equal.
In out-of-focus states, there are cases where the sample S is located at a distance away from the objective lens
3
with respect to the focal position and at a distance closer to the objective lens
3
. Here, the former case is called a rear focus state and the latter is called a front focus state. In the rear focus state, as shown in
FIG. 3A
, the laser light reflected from the sample S is collected in front of the photodetector
14
, and thus a light beam of a larger diameter than in
FIG. 4A
is formed on the photodetector
14
. Furthermore, the light beam formed over the two light-receiving sections A and B has no spot of bilateral symmetry, and a larger part of the light beam is formed on one light-receiving section, namely the light-receiving section B. In the rear focus state, therefore, the electric signal generated in the light-receiving section A is smaller than in the light-receiving section B. Conversely, in the front focus state, as shown in
FIG. 5A
, a larger part of the light beam is formed on the light-receiving section A, and thus the electric signal generated in the light-receiving section A is larger than in the light-receiving section B.
As mentioned above, since the magnitude of the electric signal changes with the space between the objective lens
3
and the sample S, it is possible to decide whether the in-focus state

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