RU2801836C1 - Fast hyperspectrometer with controlled spectral filter - Google Patents

Abstract

FIELD: optical instrumentation.

SUBSTANCE: invention relates to a high-speed hyperspectrometer with a controlled spectral filter. The hyperspectrometer includes a controlled spectral filter based on a Fabry-Perot interferometer, a lens, a matrix radiation receiver, and an electronic signal processing unit. The hyperspectrometer uses the Felget gain based on the simultaneous registration of several spectral images by one radiation detector and subsequent mathematical processing of the receiver signal to extract information about the intensity distribution in individual channels.

EFFECT: increased signal-to-noise ratio and the speed of the device.

1 cl, 7 dwg

Description

Technical field

The invention relates to one of the most important areas of optical instrumentation - spectrometry.

State of the art

Spectrometers are designed to determine the emission and absorption spectra of various media. A special place among spectrometers for various purposes is occupied by imaging spectrometers - devices that record the spatial distribution of the spectral characteristics of an object over the entire field of view [1]. Such devices are widely used in aviation and space systems for environmental monitoring and the study of atmospheric processes, in technologies for monitoring the condition of crops and finished agricultural products, in medical diagnostic practice, and in other fields of science and technology.

Imaging spectrometers form a cube of information (2d - spatial and 1d - spectral), to obtain which they use sequential scanning over the spectrum within the working spectral range with a band, the width of which is determined by the technical requirements of the spectral resolution.

Hyperspectrometers are called imaging spectrometers, the bandwidth of the spectral channel, or the spectral resolution, which does not exceed 10 nm, and the channels are evenly distributed over the working spectral range with a small step.

To select a channel in hyperspectrometers, either dispersive systems based on prisms or diffraction gratings are used ([2]; RF Patent No. 2635841, IPC G01J3 / 36, G02B27 / 10 publ. 11/20/2017), or special narrow-band filters: interference ([3] ; Patent No. 2518366, IPC G01B 9/02, IPC G01J 3/26, IPC G02B 5/28, publ. 06/10/2014) or acousto-optic ([4]; RF Patent 2258206C1, IPC G01J3/12, publ. 08/10/2005 ) with the ability to move the selected area within the working spectral range.

The main disadvantage of these systems is their low performance, which arises due to the fact that successive scanning by a section of the working spectral range takes time, often many times greater than the time it takes to form one frame of the object under study.

In spectrometry, a technique is known that makes it possible to reduce the spectrum recording time by several times, while maintaining high resolution. It is called the Felget gain and is based on the fact that the spectrometer radiation receiver registers simultaneously the entire operating spectral range of radiation, and information about the intensity distribution within this range is obtained during subsequent mathematical processing of the receiver signal. These spectrometers include Fourier spectrometers and Hadamard spectrometers [5].

The Fourier spectrometer uses a Michelson interferometer with a moving mirror, and the studied radiation spectrum is obtained by Fourier transform of the interferogram - the output signal of the radiation detector.

In Hadamard spectrometers, in the region of spectrum formation, there is a special coding mask, the transmission of which changes according to a certain law. The resulting signal of the radiation receiver is subjected to a special transformation, the Hadamard transformation, which makes it possible to calculate the functional dependence of the radiation intensity on the wavelength.

The disadvantages of these spectrometers include the constructive complexity of their implementation in application to hyperspectral measurements.

The closest to the proposed invention is a hyperspectrometer with a controlled interference filter based on the Fabry-Perot interferometer [6], which selects a narrow spectral band and scans in the working spectral range by tuning the central wavelength of the narrow spectral band when changing the filter transmittance . The latter is described by the function:

Where - interferometer parameter that determines the bandwidth and depends on the reflection coefficient mirrors (Fig. 2):

- parameter determined by distance between mirrors:

Changing the filter transmittance and, as a consequence, the shift of the wavelength corresponding to the maximum transmission is produced by changing the distance between the interferometer mirrors. The latter is achieved by changing the voltage supplied to a piezoceramic washer with mirrors fixed on it.

The disadvantage of this device, as noted, is the low speed, no more than 20 spectral lines per second, arising from the fact that with a narrow band of the spectral line, in order to obtain a signal that is many times greater than the noise of the radiation receiver, it is necessary to increase its accumulation time by each dedicated line of the operating range of the device.

Disclosure of invention

The objective and technical result of the claimed invention is the creation of a high-speed hyperspectrometer with a high signal-to-noise ratio based on the use of the Felget gain.

To solve the problem and achieve the technical result, a high-speed hyperspectrometer with a controlled spectral filter has been developed, containing a sequentially installed controlled filter based on a Fabry-Perot interferometer, an objective, a matrix radiation receiver and an electronic signal processing unit.

In this case, the Felget gain is used, which is provided by the registration of p spectral lines simultaneously selected by the filter with a spectral resolution , the values of which are related to the design parameters of the filter by the relations:

Where - boundaries of the working spectral range,

- the average value of the wavelength of the operating range,

is the average distance between the mirrors of the Fabry-Perot interferometer,

ρ is the reflection coefficient of the mirrors of the Fabry-Perot interferometer,

mirror movement range , is expressed by the dependence:

Emission spectrum of an object is determined in the electronic signal processing unit from the solution of the system of Fredholm integral equations of the 1st kind:

Where - the number of spectral channels of the radiation receiver,

- output signal -th channel depending on the magnitude of the voltage applied to the piezoelectric washer of the filter,

- the kernel of the integral equation -th channel, determined by the spectral sensitivity -th channel and spectral transmission controlled filter:

The advantage of the claimed invention in comparison with the prototype is greater performance while maintaining a high signal-to-noise ratio, since the use of the Felget gain allows even for spectral bands a few nm wide to increase the signal-to-noise ratio of the device by several times, which leads to a decrease in the required frame accumulation time.

Brief description of the figures

Figure 1 shows the transmission graphs of the controlled filter - Fabry-Perot interferometer at various values - distance between mirrors with transmittance .

Figure 2 shows the design of the controlled filter.

Figure 3 presents a generalized scheme of the hyperspectrometer.

Figure 4 shows the graphs of the sensitivity of the 8-channel matrix radiation receiver .

Figure 5 shows the graphs of the sensitivity of the receivers of the RGB color matrix .

Figure 6 presents a graph of the distribution of the spectral brightness of the object obtained by the invention. Object - spectral doublet:

Figure 7 shows a graph of the distribution of the spectral brightness of the object, the Object is an area with a Gaussian shape of the spectral density of brightness: a) original, b) restored by the method of the present invention.

Implementation of the invention

The essence of the invention is as follows. A controlled interference filter installed in the entrance pupil of the hyperspectrometer singles out several narrow spectral lines simultaneously in the working area of the hyperspectrometer. The number of these lines is defined by the expression:

Where And are the average values of the distance between the mirrors and the wavelength of the working area, respectively,

And - boundaries of the working area of the spectrum.

With an average value of the distance between the lines, described by the relation:

the width of these lines which determines the spectral resolution of the hyperspectrometer depending on the reflection coefficient of the filter mirrors , is expressed as follows:

Figure 1 shows the transmission graphs of the controlled filter - Fabry-Perot interferometer at various values - distance between mirrors with transmittance .

Figure 2 shows the shape and change of the passbands of the Fabry-Perot spectrometer with the value The presence of many transmission lines makes it possible to increase the signal-to-noise ratio of the device p times, which in turn increases its accuracy and dynamic characteristics. We also note that in order to obtain information about the emission spectrum of an object in the entire working region of the spectrum, it is sufficient to provide a shift of the spectral lines within the distance between the stripes. When the voltage changes on the piezoelectric washer there is a change in distance and hence the filter transmittance Output signal radiation receiver is defined as follows:

Where is the spectral density of the object's brightness,

- sensitivity of the radiation receiver.

The above equation belongs to the class of Fredholm integral equations of the 1st kind:

Where is the kernel of the equation.

If there is a matrix radiation detector in the image plane, for example, a color RGB matrix or another multichannel radiation detector, the desired spectral brightness of the object is determined from the solution of the system of integral equations:

Where - the number of spectral channels of the radiation receiver,

- the kernel of the i- th channel equation:

Where - spectral sensitivity of the radiation receiver channel

The solution of these equations is well known, at present it is included in various mathematical packages of computer-aided design, for example, Matlab, and is easily implemented without any technical difficulties.

As an example of a specific implementation in Fig.3 presents a generalized scheme of the claimed invention. The device works as follows: the OB lens directs the radiation from the object of study to the matrix radiation receiver, the signal from which enters the electronic signal processing unit, which performs further calculations. A filter F is placed in the entrance pupil of the lens, the transmission of which controlled by alternating voltage applied to the piezoceramic washer. The design of the controlled filter is shown in Fig.2. Working spectral range of the device - ( nm, ). The filter simultaneously highlights spectral lines with a width In this case, interferometer mirrors are used with a reflection coefficient Average distance between mirrors , the range of movement of the mirrors for shifting one band within . The device uses an 8-channel matrix radiation receiver , the spectral sensitivities of the receiver pixels are shown in Fig.4.

Figure 5 shows the graphs of the sensitivity of the receivers of the RGB color matrix .

As examples illustrating the possibilities of the claimed invention, Fig. 6 shows a graph of the function demonstrating the spectral resolution of the instrument. The figure shows that the device reliably resolves 2 spectral lines, the distance between which is Figure 7 shows the result of modeling the measurement (b) of the spectral brightness of an object with a Gaussian shape of the spectral brightness (a).

Bibliography

1. Imaging Spectrometry / ed. Meer F.D. vander, Jong S.M. de. Dordrecht: Springer Netherlands, 2002. Vol. 4.

2. Feng L. et al. Hyperspectrometer based on curved prism fabrication for space application // https://doi.org/10.1117/12.2317851. SPIE, 2018. Vol. 10692. P. 247-251.

3. Carmo J.P. et al. A review of visible-range Fabry-Perot microspectrometers in silicon for the industry // Opt Laser Technol. Elsevier, 2012. Vol. 44, No. 7. P. 2312-2320.

4. Korablev O.I. et al. Acousto-optic tunable filter spectrometers in space missions [Invited] // Appl Opt. Apple Opt, 2018. Vol. 57, No. 10. P. C103.

5. Zaidel A.V., Ostrovskaya G.V., Ostrovsky Yu.I. Technique and practice of spectroscopy. M. Science. 1976. 375 p.

6. Monarch II Camera - Unispectral [Electronic resource]. URL: https://www.unispectral.com/product/monarch-ii-camera/ (accessed 09/25/2022).

Claims (14)

A high-speed hyperspectrometer with a controlled spectral filter, containing a sequentially installed controlled filter based on a Fabry-Perot interferometer, an objective, a matrix radiation receiver and an electronic signal processing unit, characterized in that the Felget gain is used, provided by the registration of p spectral lines simultaneously separated by the filter with a spectral resolution of δλ , the values of which are related to the design parameters of the filter by the relations: where λ ₁ , λ ₂ are the boundaries of the working spectral range, λ _cf - the average value of the wavelength of the operating range, h _cf is the average distance between the mirrors of the Fabry-Perot interferometer, ρ is the reflection coefficient of the mirrors of the Fabry-Perot interferometer, in this case, the range of movement of the mirrors Δh is expressed by the dependence: and the radiation spectrum of the object L(λ) is determined in the electronic signal processing unit from the solution of the system of Fredholm integral equations of the 1st kind: where m is the number of spectral channels of the radiation receiver, I _i (u) is the output signal of the i-th channel, depending on the magnitude of the voltage u applied to the piezoelectric filter washer, K _i (u, λ) is the kernel of the integral equation of the i-th channel, determined by the spectral sensitivity S _i (λ) of the i-th channel and the spectral transmission τ(u, λ) of the controlled filter: