JP2001147158A - Spectral image capturing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a spectral image capturing device with an excellent wavelength characteristic without requiring any interference filter having an ideal wavelength characteristic. SOLUTION: This spectral image capturing device is provided with a light collecting optical system 1 forming an intermediate image of an objective body, a vision restricting means 2 positioned in the vicinity of a focal point of the light collecting optical system, a collimating optical system 3 converting light from the vision restricting means into parallel light, spectroscopic means 4, 5 dividing the parallel light into a plurality of optical paths with angular differences respectively, an image forming optical system 6 forming an image of light passed via the spectroscopic means for forming an objective body image in each of the optical paths, an image pickup element 7 converting the light intensity distribution of the objective body image into an electrical signal, a data storage means 8 storing a constant decided on the basis of the wavelength characteristic of the spectroscopic means, and a signal processing means 9 computing a spectral image of the objective body from an output signal from the image pickup element and the constant from the data storage means.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spectral image acquisition apparatus used for remote sensing using the wavelength characteristic of an object.

[0002]

2. Description of the Related Art A spectral image is an image obtained by capturing an image in different wavelength bands. For example, an object has a unique wavelength characteristic, such as a green leaf and a brown soil. Spectral images are effective for identifying what an object to be imaged belongs to and for examining the state of the object, and are widely used in the field of remote sensing.

There are various methods for obtaining a spectral image. Fourier spectroscopy is a type of spectral image acquisition method using a two-beam interferometer such as the well-known Michelson interferometer. This is a spectral image acquisition method that can increase the wavelength resolution, but requires a movable mechanism that moves the reflecting mirror with high accuracy. Further, the subject must be fixed during the period in which the reflecting mirror is moved, and is not suitable for acquiring a spectral image of the subject moving at high speed.

As another method of acquiring a spectral image, there is a method using a dispersive element such as a prism or a diffraction grating. This is to change the imaging position according to the wavelength by changing the incident light beam to an angle corresponding to the wavelength by the dispersion element. Since the dispersion direction of the dispersive element is one-dimensional, an image that is two-dimensional information can be obtained by combining a one-dimensional image pickup element and a scanning unit, but is not suitable for obtaining an instantaneous spectral image.

As a method for easily obtaining a spectral image, there is a method for obtaining a spectral image in a desired wavelength band by limiting the wavelength band of incident light with an interference filter or the like. FIG. 8 is a configuration diagram of a spectral image acquisition device using an interference filter. In FIG. 8, reference numeral 31 denotes a condensing optical system that forms an image of a subject to be imaged. Reference numeral 7 denotes an imaging device such as a CCD that is sensitive to a wavelength band in which an image is to be acquired.
The light intensity distribution of the subject image formed by the condensing optical system 31 is converted into an electric signal, and although not shown here, image storage means such as a semiconductor memory or a magneto-optical disk for storing a spectral image, It is output to display means such as a display for displaying.

The reference numeral 32 denotes a plurality of interference filters (32
a, 32b).
For example, multiple interference filters are attached to the wheel,
It has a function of replacing the interference filter placed in the light collecting optical system 31 by rotating.

Next, the operation will be described. Incident light rays of all wavelength bands emitted from the subject enter the spectral image acquisition device. First, an image in the first wavelength band is acquired by the interference filter 32a. Next, the wavelength band limiting unit 32 replaces the interference filter 32b with another interference filter 32b to acquire an image in the second wavelength band. By performing this operation for all the interference filters, a spectral image can be obtained. However, since the spectral image is obtained by exchanging the interference filter, the image of each wavelength band cannot be obtained at the same time,
It is not suitable for a subject moving at high speed.

FIG. 9 is another block diagram of a spectral image acquiring apparatus using an interference filter. In FIG. 9, reference numeral 34 denotes a condensing optical system, and reference numeral 35 denotes a wavelength band limiting unit. In this example, the wavelength band limiting unit 35 spatially divides the incident light beam by the interference filters (35a, 35b) having different wavelength bands so that the wavelength band differs depending on the passing position. After that, an image is formed at a different position for each wavelength band by the condensing optical system 34 composed of a plurality of lenses, and is detected by the image sensor 7.

In this method, since images in all wavelength bands can be acquired at the same time, this method is a spectral image acquisition method suitable for a fast-moving subject. However, since the incident light beam is subdivided, the signal amount in each wavelength band is reduced. Originally, in spectral image acquisition in which the signal amount is reduced by limiting the wavelength band, this is a major problem.

In order to solve the above problems,
For example, there is a spectral image acquisition device disclosed in US Pat. No. 5,926,283. FIG. 10 shows U.S. Pat. No. 5,926,28.
FIG. 4 is a configuration diagram of the spectral image acquisition device shown in No. 3; FIG.
At 0, reference numeral 1 denotes a condensing optical system that forms an intermediate image of a subject to be imaged. Reference numeral 2 denotes a field-of-view limiting unit placed in the vicinity of the image forming position of the light-collecting optical system 1, and determines a range in which the light-collecting optical system 1 performs imaging. Reference numeral 3 denotes a collimating optical system that converts light from the visual field limiting unit 2 into parallel light.

Reference numerals 4 and 5 denote spectral means, each of which has a different reflection angle depending on the wavelength band. Reference numeral 6 denotes an image forming optical system, which forms an image of a subject for each wavelength band by forming an image of light having passed through the spectral units 4 and 5. 7 is an image pickup means,
The light intensity distribution of the subject image formed by the imaging optical system 6 is converted into an electric signal. The converted electric signal is output to an image storage unit or a display unit (not shown).

Here, the spectroscopic means 4 comprises an interference filter 4a and a reflecting mirror 4b. The interference filter 4a transmits light in a predetermined wavelength band and reflects light in other wavelength bands. The reflecting mirror 4b is arranged at a different angle with respect to the interference filter 4a. Similarly, the spectroscopy unit 5 includes an interference filter 5a and a reflecting mirror 5b.

FIG. 11 shows the transmission characteristics of the interference filters 4a and 5a. The interference filter 4a has a characteristic of reflecting light of wavelengths λ1 and λ2 and transmitting light of wavelengths λ3 and λ4. Similarly, the interference filter 5a has wavelengths λ1 and λ
4 has the characteristic of reflecting light of wavelengths 4 and transmitting light of wavelengths λ2 and λ3.

The light incident on the spectroscopic means 4 is reflected by the interference filter 4a, and only the light of the wavelengths λ1 and λ2 is reflected and incident on the spectroscopic means 5. On the other hand, the lights of wavelengths λ3 and λ4 pass through the interference filter 4a and are reflected by the reflecting mirror 4b. Since the reflecting mirror 4b is arranged at an angle different from that of the interference filter 4a, the light is reflected at an angle different from that at which the light enters the reflecting mirror 4b. The lights of the wavelengths λ3 and λ4 reflected by the reflecting mirror 4b pass through the interference filter 4a again and enter the spectral means 5.

As described above, the light having the wavelengths λ1 and λ2 reflected by the interference filter 4a and
The light of wavelengths λ3 and λ4 reflected at b can have an angle difference. Similarly, the light having the wavelengths λ1 and λ4 reflected by the interference filter 5a and the light having the wavelengths λ2 and λ3 reflected by the reflection mirror 4b can have an angle difference by the spectroscopic means 5.

The angle difference generated by the spectroscopic means 4 and the spectroscopic means 5
By appropriately setting the angle difference generated in
The traveling directions of light of 1, λ2, λ3, and λ4 can be divided. That is, since the lights of the wavelengths λ1, λ2, λ3, and λ4 enter the imaging optical system 6 at different angles, even at the same position of the subject, images are formed at different positions for each wavelength band due to a difference in optical path. be able to.

Since two light paths can be set by each spectroscopic means, four light paths can be set for convenience. FIG.
FIG. 4 is a diagram showing transmission characteristics for each optical path. Interference filter 4a
The optical path reflected at 5a and 5a is referred to as an RR path. Similarly, an optical path reflected by the interference filter 4a and transmitted through the interference filter 5a is an RT path, an optical path transmitted through the interference filter 4a and reflected by the interference filter 5a is a TR path,
The optical path transmitting through a and 5a is named TT path. Thus, different wavelength bands can be set depending on the difference in the optical path.

Therefore, since the conventional spectral image acquisition apparatus is configured as described above, four images having different wavelength bands can be simultaneously detected and imaged by the image sensor.
In the above description, the number of spectral units is two, but the number of wavelength bands can be increased by increasing the number of spectral units.

[0019]

Since the conventional spectral image acquisition apparatus is configured as described above, an extremely ideal interference filter having excellent wavelength selectivity is required. If the characteristics of the interference filter differ from the ideal characteristics, light other than the wavelength band to be imaged is included, and the wavelength characteristic of the spectral image is degraded.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a spectral image acquisition apparatus having excellent wavelength characteristics without requiring an interference filter having ideal wavelength characteristics. Aim.

[0021]

According to the present invention, there is provided a spectral image acquiring apparatus, comprising: a condensing optical system for forming an intermediate image of a subject;
Field-of-view limiting means placed near the focal point of the focusing optics,
A collimating optical system that converts light from the visual field limiting unit into parallel light, a spectral unit that divides the parallel light into a plurality of optical paths, each having an angle difference, and an optical path that forms an image of light that has passed through the spectral unit. An imaging optical system for forming a subject image for each, an image sensor for converting a light intensity distribution of the subject image into an electric signal, and a data storage unit storing a constant determined based on a wavelength characteristic of the spectral unit. Signal processing means for calculating a spectral image of a subject image from an output signal from the image sensor and a constant from the data storage means.

Further, the spectroscopic means is characterized by comprising a parallel plate interference filter and a reflecting mirror arranged at an angle to the interference filter.

Further, the spectroscopic means is characterized by comprising an etalon and a reflecting mirror arranged at an angle to the etalon.

Further, the spectroscopic means is a wedge-shaped component having an interference filter on one side and a reflection film on the other side.

The spectroscopic means is characterized by comprising a plurality of parallel plate interference filters, and reflecting mirrors arranged at different angles from the interference filters.

Further, the spectroscopic means is characterized by comprising a wedge-shaped component having an interference filter on one surface and another interference filter on the other surface, and a reflecting mirror arranged at an angle to the wedge-shaped component. It is assumed that.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. 1 is a configuration diagram showing a spectral image acquiring apparatus according to Embodiment 1 of the present invention. 1, the same parts as those of the conventional example shown in FIG. 10 are denoted by the same reference numerals. Reference numeral 1 denotes a condensing optical system that forms an intermediate image of a subject, 2 denotes a field-of-view limiting unit placed near the focal point of the condensing optical system 1, and 3 denotes a collimating optical system that converts light from the field-of-view limiting unit 2 into parallel light. Reference numerals 4 and 5 denote spectroscopy means for dividing the parallel light into a plurality of optical paths so as to have an angle difference between them.
It is composed of parallel plate interference filters (4a, 5a) and reflection mirrors (4b, 5b) arranged at an angle to the interference filters. Reference numeral 6 denotes an image forming optical system that forms an object image for each optical path by forming an image of the light that has passed through the spectral units 4 and 5, and 7 denotes an image sensor that converts the light intensity distribution of the object image into an electric signal.

As a new code, reference numeral 8 denotes the spectroscopic means 4
And 5 are data storage means storing constants determined based on the wavelength characteristics. Reference numeral 9 denotes a signal processing unit which calculates a spectral image of a subject image from an output signal from the image sensor 7 and a constant from the data storage unit 8. The output signal of the signal processing means 9 is sent to an image storage means or a display means (not shown).

FIG. 2 shows an example of transmission characteristics of the interference filters 4a and 5a. The transmittance of the interference filter 4a is 0.4, 0.8, 0... In the order of wavelengths λ1, λ2, λ3, λ4.
6, 0.2. Similarly, the transmittance of the interference filter 5a is set to 0.2, 0.2, 0.8, 0.8.
Each of the interference filters does not need to have an ideal wavelength characteristic unlike the interference filter used in the conventional spectral image acquisition apparatus as shown in FIG.
The wavelength characteristics shown in FIG.

As described above, since the wavelength characteristics of the interference filters 4a and 5a are not ideal, the wavelength characteristics of each optical path also change. For example, if an optical path transmitting through the interference filters 4a and 5a is a TT path, the transmission characteristics shown in FIG. 3 are obtained. As described above, in the conventional spectral image acquisition apparatus, a spectral image including light other than the captured wavelength band is obtained under the influence of the wavelength characteristic of the interference filter as shown in FIG. 3, and the wavelength characteristic is deteriorated.

Next, the operation according to the first embodiment of the present invention will be described using mathematical expressions. First, the subscript representing the wavelength is i
And Accordingly, the transmission characteristic of the interference filter 4a is T4 (i), and the transmission characteristic of the interference filter 5a is T5 (i).
And The subscript representing the optical path is j, j = 1 for the TT path, j = 2 for the TR path, j = 3 for the RT path, and RR.
Let the path be j = 4. Let T (i, j) be the transmission characteristic of each optical path.
Since the absorption by the interference filter and the reflecting mirror is very small, if this is ignored, it is expressed by the following equation (1).

[0032]

(Equation 1)

Here, the spectral spectrum of the object is represented by I
(I), assuming that the output of the image sensor is O (j), I (i)
And O (j) are as shown in equations (2) and (3).

[0034]

(Equation 2)

[0035]

(Equation 3)

Therefore, even when the interference filter is not ideal, the spectrum of the subject can be obtained by the above calculation. The elements of the matrix A in the above equation (2) are constants determined based on the wavelength characteristics of the spectroscopic means 4 and 5, and the elements of the inverse matrix in the above equation (3) are obviously the wavelengths of the spectroscopic means 4 and 5. This is a constant determined based on the characteristic, and is stored in the data storage unit 8. The signal processing means 9 obtains the spectrum of the subject based on the constant and the output O (j) obtained from the output of the image sensor 7 based on the method of the above equation (3).

Next, the flow of processing will be described with reference to FIG. First, the output of the image sensor 7 in which a plurality of subject images corresponding to different optical paths are captured is divided and rearranged for each optical path. Next, a pixel output corresponding to the same part of the subject is extracted from the image for each divided optical path. This output O
(J) is used to calculate the spectrum of the subject from equation (3) using a constant stored in the data storage means 8. By performing this processing for each pixel of the image sensor 7, a spectral image can be obtained.

Therefore, since the spectral image acquiring apparatus according to the first embodiment is configured as described above, it is not necessary to provide the interference filters 4a and 5a with ideal wavelength characteristics. Can be obtained at the same time.

In the above description, the spectral means (4, 5) is
As described above, in the present invention, the number of spectral units may be further increased. By increasing the number of spectral units, it is possible to increase the number of optical paths, that is, to increase the number of divisions of the wavelength band, and to obtain a spectral image with higher wavelength resolution. For example, if the number of spectral images is three, the number of optical paths is eight, and if the number of spectral images is four, the number of optical paths is sixteen.

The interference filters 4a and 5a include:
A stack of thin films may be used, or a stack utilizing multi-beam interference such as an etalon may be used. Expression (3) shows one of the methods for obtaining I (i) from the output of the image sensor 7 as O (j) and the spectral spectrum of the subject as I (i) from Expression (2). Although the method using the inverse matrix has been described, since the present invention solves the equation (2) as an inverse problem, it is difficult to obtain I (i) using another solution. Needless to say.

Embodiment 2 FIG. 5 shows the configuration of the second embodiment of the spectral means used in the spectral acquisition apparatus according to the present invention. In FIG. 5, reference numeral 10 denotes spectroscopic means,
It has a wedge shape. An interference filter 10a having wavelength selectivity is formed on one side, and a reflection film 10b is formed on the other side.
Are formed. At the interference filter 10a, a part of the light is reflected and the remaining light is transmitted. Since the spectroscopic means 10 has a wedge shape, the interference filter 10a and the reflection film 10b are not parallel but formed with an angle difference. Therefore, light reflected by the interference filter 10a and
There is an angle difference between the light transmitted through the interference filter 10a, reflected by the reflection film 10b, and transmitted again through the interference filter 10a.

Therefore, according to the spectroscopic means 10 according to the second embodiment, it functions similarly as the spectroscopic means as in the first embodiment, and a spectral image can be obtained. Since the spectral unit used in the spectral image acquisition device of the second embodiment is configured as described above, the configuration of the device can be simplified.

Embodiment 3 FIG. 6 shows the configuration of the third embodiment of the spectral means used in the spectral acquisition apparatus according to the present invention. In FIG. 6, reference numeral 11 denotes spectroscopic means,
Parallel-plate interference filters 11a and 11b and reflector 1
1c. The interference filter 11a, the interference filter 11b, and the reflecting mirror 11c are arranged at different angles. In the case of the third embodiment, the interference filter 11
The optical path can be divided into three optical paths: an optical path reflected by a, an optical path reflected by the interference filter 11b, and an optical path reflected by the reflecting mirror 11c.

Therefore, according to the spectral unit 11 according to the third embodiment, three optical paths can be generated by one spectral unit, so that more optical paths can be set. Thereby, the number of divisions of the wavelength band can be increased, and a spectral image with higher wavelength resolution can be obtained. Here, the interference filter is 2
Although the embodiment using one or more interference filters has been described, the same effect can be obtained even when three or more interference filters are used.

Embodiment 4 FIG. 7 shows the configuration of another embodiment of the spectral means used in the spectral acquisition apparatus according to the present invention. In FIG. 7, reference numeral 12 denotes spectroscopic means,
It comprises an interference filter 12a having wavelength selectivity formed on one surface of a wedge-shaped substrate, an interference filter 12b formed on the other surface of the wedge-shaped substrate, and a reflecting mirror 12c. The reflecting mirror 12c is arranged so that the optical path reflected by the interference filter 12a or 12b and the optical path reflected by the reflecting mirror 12c have an angle difference. In the case of this embodiment, an optical path reflected by the interference filter 12a,
The optical path can be divided into three optical paths, an optical path reflected by the interference filter 12b and an optical path reflected by the reflecting mirror 12c.

Therefore, according to the spectral unit 12 according to the fourth embodiment, three optical paths can be generated by one spectral unit, so that more optical paths can be set. Thereby, the number of divisions of the wavelength band can be increased, and a spectral image with higher wavelength resolution can be obtained. Of course, the number of wedge-shaped interference filters may be increased to two or more, or a parallel plate interference filter may be added.

[0047]

As described above, according to the present invention, a light-collecting optical system for forming an intermediate image of a subject, a field-of-view limiting means placed near the focal point of the light-collecting optical system, and the field-of-view limiting means A collimating optical system that converts light from the light into parallel light, a spectroscopic unit that divides the parallel light into a plurality of optical paths, and has an angle difference between each of the collimated optical paths, An image forming optical system, an image sensor for converting a light intensity distribution of the subject image into an electric signal, a data storage unit storing a constant determined based on a wavelength characteristic of the spectral unit, and the image sensor. Signal processing means for calculating a spectral image of a subject image from an output signal from the apparatus and a constant from the data storage means, so that an interference filter having an ideal wavelength characteristic is not required, and the wavelength characteristic is excellent. Obtained spectral image acquisition device It is possible.

Further, since the spectroscopic means is composed of a parallel plate interference filter and a reflecting mirror arranged at an angle to the interference filter, the interference filter does not need an ideal wavelength characteristic, and the wavelength band can be reduced. Different images can be obtained at the same time.

Further, since the spectroscopic means is composed of the etalon and the reflecting mirror arranged at an angle to the etalon, it is possible to simultaneously obtain images having different wavelength bands using multi-beam interference.

Further, since the spectroscopic means is constituted by a wedge-shaped part having an interference filter on one side and a reflection film on the other side, the structure of the apparatus is simplified, and an ideal wavelength characteristic is required for the interference filter. Instead, images with different wavelength bands can be obtained at the same time.

Further, since the spectroscopic means is composed of a plurality of parallel plate interference filters and reflecting mirrors arranged at different angles from the interference filters, many optical paths can be set, and the wavelength can be set. The number of band divisions can be increased, and a spectral image with higher wavelength resolution can be obtained.

Further, the spectroscopic means is composed of a wedge-shaped component having an interference filter on one surface and another interference filter on the other surface, and a reflecting mirror arranged at an angle to the wedge-shaped component. The optical path can be set, the number of divisions of the wavelength band can be increased, and a spectral image with higher wavelength resolution can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a spectral image acquiring apparatus according to Embodiment 1 of the present invention.

FIG. 2 is an explanatory diagram showing an example of transmission characteristics of the interference filters 4a and 5a in FIG.

FIG. 3 is an explanatory diagram of transmission characteristics taking an optical path transmitting through the interference filters 4a and 5a in FIG. 1 as an example.

FIG. 4 is an explanatory diagram illustrating a processing flow in the spectral image acquisition device according to the first embodiment of the present invention.

FIG. 5 does not show a configuration of a spectral unit used in the spectral acquisition device according to the second embodiment of the present invention.

FIG. 6 does not show a configuration of a spectral unit used in the spectral acquisition apparatus according to Embodiment 3 of the present invention.

FIG. 7 does not show a configuration of a spectral unit used in a spectral acquisition apparatus according to Embodiment 4 of the present invention.

FIG. 8 is a configuration diagram illustrating a conventional spectral image acquisition device using an interference filter.

FIG. 9 is a configuration diagram showing another conventional spectral image acquisition device using an interference filter.

FIG. 10 is a configuration diagram of a spectral image acquisition device disclosed in US Pat. No. 5,926,283.

FIG. 11 shows interference filters 4a and 5 in FIG.
It is explanatory drawing which shows the transmission characteristic of a.

FIG. 12 is an explanatory diagram showing transmission characteristics for each optical path in FIG.

[Explanation of symbols]

1 condensing optical system, 2 field limiting means, 3 collimating optical system, 4,5 spectral means, 4a, 5a interference filter,
4b, 5b reflecting mirror, 6 imaging optical system, 7 image sensor,
8 data storage means, 9 signal processing means, 10 spectral means, 10a interference filter, 10b reflecting mirror, 11 spectral means, 11a, 11b interference filter, 11c reflecting mirror, 12 spectral means, 12a, 12b interference filter,
12c reflector.

Claims (6)

[Claims] A focusing optical system for forming an intermediate image of a subject; a visual field limiting means disposed near a focal point of the optical focusing system; and a collimating optical system for converting light from the visual field limiting means into parallel light. A splitting unit that splits the parallel light into a plurality of optical paths, each of which has an angle difference, an imaging optical system that forms light through the splitting unit to form a subject image for each optical path, and the subject An image sensor that converts a light intensity distribution of an image into an electric signal; a data storage unit that stores a constant determined based on a wavelength characteristic of the spectroscopic unit; an output signal from the image sensor and a signal from the data storage unit. A signal processing unit for calculating a spectral image of the subject image from the constants. 2. The spectral image acquiring apparatus according to claim 1, wherein said spectral means comprises a parallel plate interference filter, and a reflecting mirror arranged at an angle to said interference filter. Spectral image acquisition device. 3. The spectral image acquiring apparatus according to claim 1, wherein said spectral means comprises an etalon and a reflecting mirror arranged at an angle to said etalon. 4. The spectral image acquiring apparatus according to claim 1, wherein said spectral means comprises a wedge-shaped component having an interference filter on one side and a reflection film on the other side. 5. The spectral image acquiring apparatus according to claim 1, wherein the spectral unit includes a plurality of parallel plate interference filters, and reflecting mirrors arranged at different angles from the interference filters. A spectral image acquisition device, characterized in that: 6. The spectral image acquisition device according to claim 1, wherein the spectral unit is provided with a wedge-shaped component provided with an interference filter on one surface and another interference filter on the other surface, and arranged at an angle to the wedge-shaped component. A spectroscopic image acquisition apparatus, comprising: