Analyzing optical dual-wavelength-band cameras operating in short-wave and medium-wave infrared spectral regions

Abstract

This study analyzed the optical and laser spot acquisition capabilities of a newly developed dual-wavelength-band camera. The camera performance was evaluated using a 3.0 m × 1.8 m Styrofoam target and a 70-mJ laser target designator; mid-infrared images were acquired based on the target distance, and a laser beam was irradiated onto the target to detect laser spots. The dual-wavelength-band camera demonstrated target recognition and spot detection ranges of 3 and 2 km, respectively. The results demonstrated that laser spot images could be obtained with a laser reception power of 65 μW or higher.

1. Introduction

Missile guidance methods employ various seekers for precise target tracking. The seekers include pulse-modulated infrared seekers, which monitor infrared signals emitted by the target flames; infrared image seekers, which capture the infrared images of their targets for tracking; and semiactive laser seekers, which follow the laser beams reflected from their targets [[1], [2], [3], [4]]. Unlike an infrared seeker, a semiactive laser seeker requires the operator to designate the target from either a ground or an airborne platform. Subsequently, a coded laser beam with a wavelength of 1064 nm is emitted toward the target using a laser irradiator [[5], [6], [7], [8]]. The semiactive laser seeker detects and tracks laser signals reflected from the target during the terminal guidance phase. Aviation platforms typically employ laser irradiators mounted on electro-optical/infrared (EO/IR) equipment, such as targeting pods, to precisely direct the laser beam toward the line of sight (LOS) of the target via gimbal control. A laser output energy of at least 200 mJ is essential for operations spanning tens of kilometers. Using a tripod-mounted laser irradiator positioned on the ground and the reticle center of a ground laser target designator, an operator can manually adjust the LOS to direct the laser beam toward the target. A laser output energy of at least 70 mJ is necessary for short-range operations spanning several kilometers [9,10].

Operating a ground laser irradiator requires specific target recognition and laser beam detection equipment. A telescope is employed for target recognition during daylight hours, while a mid-wave infrared (MWIR) camera is used at night. A short-wave infrared (SWIR) camera is required to determine whether the laser beam effectively illuminates the target surface. Generally, cameras that can capture both SWIR and MWIR images are employed. The laser irradiators and camera equipment used in ground-based operations must be compact and lightweight to be portable. Because the operator has to carry the equipment in a backpack, it should weigh only several kilograms. Hence, a dual-band infrared detector capable of simultaneously detecting infrared radiation in both the SWIR and MWIR wavelength bands is essential. Indium arsenide antimonide (InAsSb) materials, which are III–V compounds, are suitable for use as sensing layers in dual-band image detectors because their bandgaps align with those of SWIR and MWIR wavelengths. The operator can effectively recognize targets, such as tanks and buildings, and detect laser beams falling on target surfaces during nighttime operations using a camera equipped with a video graphics array format InAsSb focal plane array.

When developing a dual-wavelength-band infrared camera, the acquisition of thermal images of targets and reception of laser beams reflected from those targets must be considered. When designing a dual-wavelength-band camera, the entrance pupil diameter of the optical system must be increased to increase the target recognition distance and the amount of light received from the laser beam. The focal length of the optical system must be reduced to improve the signal-to-noise ratio of the laser signals, which represents the ratio of the laser signals to detector noise. This reduction is important because the detection distance of the laser sensor can be increased only after minimizing the size of the laser spot on the detection surface, which results in an enhanced laser spot density. Furthermore, the optical system of the camera should exhibit high transmittance for the dual-band wavelengths. Specifically, a narrow bandwidth coating with a full width at half-maximum (FWHM) of several tens of nanometers, along with an antireflection coating with a thickness of 3–5 μm, is required for acquiring MWIR images provided that the spectral width of the 1064-nm Nd:YAG laser does not exceed 1 nm [11]. The resolution of the built-in display should be at least equal to the detector resolution because operators must detect, recognize, and identify target images using handheld cameras. Devices capable of identifying MWIR images and laser spots include the Mini See Spot (MSS) from Leonardo DRS. The MSS is equipped with a microcooler MWIR detector with a spectral response of 3.7–5 μm and a narrow bandwidth of 1064 nm. Additionally, it employs a focal plane array comprising 640 × 480 pixels, with a field of view (FOV) of 4.1° × 3.2° [12]. The newly developed dual-wavelength-band camera with optical and laser spot acquisition capabilities operates in both SWIR and MWIR bands. It improves target recognition and laser spot detection. The study includes a comprehensive theoretical and experimental analysis of the optical system design, atmospheric penetration evaluation, and performance evaluation of the developed camera using simulation software.

This paper outlines the design of the sensor unit of a dual-wavelength-band camera. The optical performance of the developed prototype of the dual-wavelength-band camera was assessed through experiments, and the laser spot acquisition performance of the dual-wavelength-band camera was evaluated through experimental measurements and theoretical analysis using a laser target designator.

2. Optical system design

Fig. 1 shows the detector configuration used in the dual-wavelength-band camera, specifically the SCD Sparrow See Spot (02S). The detector, which uses an InAsSb sensing layer, exhibits a 10-μm array pitch in both vertical and horizontal directions with a detector array format comprising 640 (horizontal) × 512 (vertical) elements. Signal processing was applied to convert analog signals into digital signals. The detector was cooled using a Dewar cooler connected to the engine. The dual-wavelength-band image detector contained a coated sapphire cold filter, providing a spectral window at 1.064 μm, along with a bandpass filter for MWIR. The response wavelength range of the detector was within the mid-infrared band of 3.6–4.2 μm. The coated filter exhibited an average transmission of 85 % in the MWIR spectral range of 3.7–4.05 μm, with an FWHM not exceeding 37 nm and a transmission not less than 60 % at 1.064 μm. An F/3.6 internal cold shield with a height of 12 mm was incorporated into the detector. The environmental robustness of the detector at high and low temperatures complied with the MIL-STD-810E standard, while the mechanical shock and altitude specifications of the detector complied with the MIL-STD-810D standard.

Fig. 1.

Sparrow See Spot detector and its supporting electronics.

Fig. 2 shows the design of the objective optical system of the dual-wavelength-band camera. The design process used commercial software Zemax. The horizontal and vertical lengths of the image field were 6.4 and 5.1 mm, respectively, based on the 640-pixel (horizontal) and 512-pixel (vertical) resolution of the detector. The objective optical system comprised four lenses (Fig. 2), all constructed using a material that transmitted both MWIR and SWIR wavelength bands. The F-number of the optical system was F/3.6 based on the detector cold stop, and the effective measured focal length was 85.9 mm. The horizontal and vertical FOVs were 4.2° and 3.4°, respectively.

Fig. 2.

Design of the objective optical system.

Fig. 3 shows the design of the eyepiece optical system of the dual-wavelength-band camera. The eyepiece optical system comprised five lenses (Fig. 3) constructed using a material that transmitted visible light. The measured eyespot distance of the optical system was 25 mm with an effective focal length of 21.1 mm. The designed FOV was 32°.

Fig. 3.

Design of the eyepiece optical system.

3. Analysis of optical performance

Fig. 4 shows the variations in atmospheric transmission in the MWIR band at different target distances. The atmospheric transmission spectrum was 3.6–4.2 μm, which aligned with the receiving wavelength of the dual-wavelength-band camera. The visibility distance of 15 km during midlatitude winter. The atmospheric transmission was analyzed on the ground under horizontal atmospheric conditions using Modtran, a commercial software package [13]. The results indicated a transmissivity of ∼79 % at a distance of 2 km. The analysis of the variation in the atmospheric transmission with the target distance helped evaluate the optical performance of the dual-wavelength-band camera.

Fig. 4.

Atmospheric transmission in the mid-wave infrared band with the target distance.

Fig. 5 shows the optical performance of the dual-wavelength-band camera. The analysis of the optical performance was conducted using NVTherm, an infrared camera system analysis software. Table 1 summarizes the parameters and analysis conditions of the optical system, atmosphere, and target. The resolution criteria were based on 50 % probabilities, with one, three, and six cycles required for the detection, recognition, and identification of the target, respectively [14]. The optical performance of the dual-wavelength-band camera was thoroughly analyzed. The results indicated that the detection, recognition, and identification distances for a 3.0 m × 1.8 m target were 6.7, 2.3, and 1.2 km, respectively, for a target contrast of 2 °C.

Fig. 5.

Optical performance of the dual-wavelength-band camera.

Table 1.

Optical performance analysis parameters and conditions of dual-wavelength-band camera.

Parameter	Value
FOV (H × V)	4.2° × 3.4°
Effective focal length	85.9 mm
F-number	3.6
Detector resolution (H х V)	640 × 512
Pixel pitch	10 μm
Wavelength	3.6–4.2 μm
Atmospheric conditions	MidLatitude Winter
Target size (H х V)	3.0 m × 1.8 m
Target contrast	2 °C
Resolution criteria detection	1 cycles on target
recognition	3 cycles on target
identification	6 cycles on target

4. Dual-wavelength-band camera

Fig. 6 shows the block diagram outlining the interface of the dual-wavelength-band camera. The main circuit board comprised components essential for a camera, such as a video processor for image processing, a microcontroller unit (MCU) for camera control, and a power regulator to distribute the input power efficiently. Several key components, namely an MWIR detector for image acquisition, a digital magnetic compass (DMC) for directional information acquisition, a display for image visualization, a key switch for camera function control, a battery supplying power to the camera, and an external connector for communication via an RS-422 interface, were connected to the main circuit board. The operational software of the camera was embedded within the MCU, ensuring seamless functionality.

Fig. 6.

Block diagram of the dual-wavelength-band camera interface.

Fig. 7 shows both the internal design and external shape of the dual-wavelength-band camera. As shown in Fig. 7(a), the proposed dual-wavelength-band camera included various components, such as an objective lens to capture MWIR images and laser beams, a detector to convert optical information into electrical signals, a display to present cropped images at a resolution of 512× 512 with a 1:1 ratio, an eyepiece through which the operator could identify images during daylight hours and at night, and a battery pack comprising three lithium batteries capable of continuing camera operations for at least 4 h. The total weight of the dual-wavelength-band camera, with dimensions of 263 mm (length) × 64 mm (width) × 93 mm (height), was below 1.3 kg. The optical performance specifications of the camera included a FOV of 3.4° (horizontal) × 3.4° (vertical), with minimum target recognition and laser detection distances of 2 km. The camera exhibited 2 × and 4 × electronic zoom capabilities, image processing functionalities, such as nonuniformity correction, bad pixel replacement, auto gain control, and a built-in test function for hardware anomaly detection. The dual-wavelength-band camera had a DMC for azimuth display, an RS-422 interface for external communication, and a power protection mechanism. The environmental durability of the system complied with the MIL-STD-810G standard and was validated through tests covering operating temperatures in the range from −32 °C to +43 °C, storage temperatures from −33 °C to +63 °C, humidity, transportation vibration, and transportation drop. Fig. 7(b) shows the external design of the proposed dual-wavelength-band camera.

Fig. 7.

Dual-wavelength band camera. (a) Internal design and (b) external shape.

5. Experimental equipment

Fig. 8 shows the configuration of the experimental outdoor setup. The dual-wavelength-band camera was mounted on the Picatinny rail of the laser target designator (Fig. 8(a)). The laser target designator, identified as Type 163 from Leonardo, UK, emitted a laser with a wavelength of 1.064 μm, an output energy not less than 70 mJ, a pulse width of 18 ± 7 ns, a beam divergence that did not typically exceed 200 μrad, and a repetition rate of 20 Hz. The tripod-mounted Type 163 laser target designator was equipped with direct view optics (DVO) to enable target verification by the operator, along with a laser rangefinder capable of measuring distances from 100 m to 9.999 km. The portable Type 163 laser target designator, with a total weight of 2.43 kg enabled handheld operation. As shown in Fig. 8(b), the target was mounted at the side of a truck. A piece of Styrofoam measuring 3.0 m (width) × 1.8 m (height) served as the target for acquiring target images and detecting laser beams from a distance of 3 km. Styrofoam was selected as the laser reflection target based on its Lambertian reflectance properties of Styrofoam. Fig. 8(c) shows the configuration of the SWIR camera installed near the target to identify the laser beam irradiating the target. The sensor used in the SWIR camera was a dual-mode InGaAs sensor (NIT; WIDY SenS 640V-STP) with a SWIR wavelength band lens (Edmund Optics; #83–170) with a 100-mm focal length.

Fig. 8.

Outdoor equipment: (a) dual-wavelength-band camera mounted on a Type 163 laser target designator, (b) target, and (c) SWIR camera.

6. Experimental results

Fig. 9 shows the DVO image captured using the Type 163 laser target designator. A DVO system uses an optical telescope with 10 × magnification, a 2° FOV, and an open cross reticle. The operator can use the DVO system to identify the target and adjust the pitch and yaw axes of the system to position it at the center of the unobscured central region of the open cross reticle. Fig. 9 shows the images obtained by attaching a camera with a complementary metal–oxide–semiconductor sensor to the DVO system with the target indicated by a yellow arrow. Fig. 9(a) shows an image of a target positioned at a distance of 1.5 km from the laser target designator and within the unobscured central region of the DVO system. Fig. 9(b) shows an image of a target at a distance of 3039 m from the laser target designator and measured using the Type 163 laser target designator, as displayed on the man–machine interface display of the laser target designator. In this experiment, the laser was focused on a precise target.

Fig. 9.

DVO images of a Type 163 laser target designator with target distances of (a) 1.5 km and (b) 3.0 km.

Fig. 10 shows the images of laser beams irradiating a target captured by a closely positioned SWIR camera. The SWIR sensor, made of InGaAs, had a 640 × 512 pixels resolution and a pixel size of 15 μm × 15 μm. The sensor spectral response ranged from 0.9 to 1.7 μm, enabling detection at a laser wavelength of 1.064 μm. Fig. 10(a) shows the image of a target at a distance of 2.0 km, with the laser beam highlighted using a red arrow. Fig. 10(b) shows the image of a target at a target distance of 3.0 km. Considering the divergence angle of the Type 163 laser target designator, the laser beam size at a target distance of 3.0 km was presumed to be between 200 and 300 mm, which was larger than its size at a target distance of 2.0 km. The Type 163 laser target designator was a Q-switched Nd:YAG laser with an adjustable pulse repetition rate. When capturing a video of the pulsed laser beam irradiating the target using a SWIR camera, the image shown in Fig. 10 could only be acquired when the SWIR camera and the laser pulse were synchronized.

Fig. 10.

Laser beam images obtained using SWIR camera at target distances of (a) 2.0 km and (b) 3.0 km.

Fig. 11 shows the MWIR images of a target acquired using a dual-wavelength-band camera. Fig. 11(a) shows an MWIR image obtained at a target distance of 2.0 km. In this figure, the target is positioned to the left of the crosshair at the center of the image. Fig. 11(b) displays an MWIR image acquired at a target distance of 3.0 km, with the target positioned to the right of the crosshair at the center of the image. Fig. 11(c) and (d) show images obtained by digitally zooming the dual-wavelength-band camera to 2 × and 4 × magnification, respectively, under the conditions indicated in Fig. 11(b). The target was recognizable in both images. As shown in Fig. 5, the results of the optical performance analysis indicate that the target recognition distance was 2.3 km for a target area of 5.4 m² and a contrast of 2 °C. However, the target could be identified based on the outcomes (Fig. 11). Because the experiment was conducted during winter when the background temperatures were low, the target contrast exceeded the analysis results. Thus, the experimental results had surpassed the analysis predictions. Moreover, the analysis of the target recognition distance based on target temperature, which was performed using the NVTherm, revealed that for a target measuring 3.0 m × 1.8 m, the recognition distances for target contrasts of 2 °C and 10 °C were 2.3 and 2.9 km, respectively, which aligned with the experimental results.

Fig. 11.

MWIR images of a target obtained using the dual-wavelength-band camera. at a target distance of (a) 2.0 km, (b) 3.0 km, (c) 3.0 km at 2 × magnification, and (d) 3.0 km at × 4 magnification.

Fig. 12 shows a laser spot image captured by the dual-wavelength-band camera at a target distance of 2.0 km. This inverted image, derived from the black and white images shown in Fig. 11(a), revealed the white laser beam irradiated at the center of the target, highlighted by the red arrow. The blinking laser spot was discernible when observed using the dual-wavelength-band camera because of the pulsed operation of the Type 163 laser target designator. Although the laser spot could be detected at target distances less than 2.0 km, no laser spot could be detected at 2.5 or 3.0 km. The detection of the laser spot depended upon several factors, including the divergence angle and output energy of the laser beam, atmospheric influence on laser beam transmission, diameter of the entrance pupil, transmissivity of the dual-wavelength-band camera, and detector sensitivity.

Fig. 12.

Laser spot image of a target at a target distance of 2.0 km obtained from the dual-wavelength-band camera.

7. Analysis of laser experiment results

The analysis of whether the laser spot was detected using a dual-wavelength-band camera when the laser beam of the Type 163 laser target designator irradiated onto a target was conducted using the following formula [15]:

$$P_{Rx}=\left(\frac{E_{Pulse}}{\Delta t}{\tau }_{Tx}\right)\left(e^{-\alpha R}\right)\rho {\int }_{-\frac{H}{2}}^{\frac{H}{2}}{\int }_{-\frac{W}{2}}^{\frac{W}{2}}\frac{e^{-\left(x^2/2R^2\delta +y^2/2R^2\delta \right)}}{2\pi R\sqrt{\delta }}dxdy\left(e^{-\alpha R}\right)\left(\frac{\pi {\left(D_{Rx}/2\right)}^2}{\pi R^2}\right){\tau }_{Rx}\text{,}$$ (1)

where ${\mathrm{E}}_{\text{Pulse}}$ denotes the laser output energy per pulse, $\Delta \mathrm{t}$ is the laser pulse width, ${\mathrm{\tau }}_{\text{Tx}}$ represents the transmissivity of the laser target designator, $\mathrm{\alpha }$ is the atmospheric extinction coefficient between the camera and target, $\mathrm{R}$ is the distance between the dual-wavelength-band camera mounted on the laser target designator and target, $\mathrm{\rho }$ is the target reflectivity, $\mathrm{H}$ represents the height of the laser footprint, $\mathrm{W}$ denotes the width of the laser footprint, ${\mathrm{D}}_{\text{Rx}}$ represents the diameter of the entrance pupil of the dual-wavelength-band camera, and ${\mathrm{\tau }}_{\text{Rx}}$ is the transmissivity of the dual-wavelength-band camera optical system. The jitter of the laser target designator can be expressed as $\delta ={\delta }_b^2+{\delta }_j^2+{\delta }_i^2$ where ${\mathrm{\delta }}_{\mathrm{b}}$ is the laser beam divergence, ${\delta }_i=3.98\sqrt{{\left(1/\lambda \right)}^{2/5}{\left(C_n^2\right)}^{6/5}{R}^{16/5}}/R$ is the stabilization divergence of the transmitter, $\mathrm{\lambda }$ is the laser wavelength, and ${\mathrm{C}}_{\mathrm{n}}^2$ is the refractive index structure function. In the calculation of the amount of laser light received by the dual-wavelength-band camera, the atmosphere was considered the refractive index structure function, while clear atmospheric air was considered the extinction coefficient [16,14]. Table 2 summarizes the parameters and values used in Eq. (1).

Table 2.

Parameters for calculating the power received by the camera.

Description	Symbol	Value
Laser energy per pulse	${\mathrm{E}}_{\text{Pulse}}$	70 mJ
Laser pulse width	$\Delta \mathrm{t}$	18 ns
Laser efficiency	${\mathrm{\tau }}_{\text{Tx}}$	1.0
Laser wavelength	$\mathrm{\lambda }$	1064 nm
Extinction coefficient	$\mathrm{\alpha }$	0.105 km−1
Target hemispherical reflectivity	$\mathrm{\rho }$	0.9
Stabilization divergence	${\mathrm{\delta }}_{\mathrm{j}}$	50 μrad
Refractive index structure constant	${\mathrm{C}}_{\mathrm{n}}^2$	${1⨯10}^{-14}$ m−2/3
Receiver efficiency	${\mathrm{\tau }}_{\text{Rx}}$	0.95

Fig. 13 shows the variation in the amount of laser light received by the dual-wavelength-band camera with increasing the target distance. The amount of laser light received by the camera was determined for a moving target using the values provided in Eq. (1) and Table 2, assuming that both the laser target designator and dual-wavelength-band camera were positioned at the same location. The amount of laser light received by the dual-wavelength-band camera progressively decreased from 1.7 mW to 381.9, 145.6, and 65.8 μW as the target distance increased from 0.5 km to 1.0, 1.5, and 2.0 km, respectively. According to Fig. 12, a laser peak power ≥65 μW must capture a laser spot image through a dual-wavelength-band camera. The camera performance was dependent on the atmospheric conditions. The visibility distance of the camera was set to 15 km during its design and analysis. In the outdoor experiments, target recognition and spot detection distances improved with increasing visibility distance. The effects of atmospheric turbulence on the laser beam can be mitigated using adaptive optics tool, such as fast steering mirrors, deformable mirrors, and wavefront sensors. The dual-wavelength-band camera must be small and lightweight. Because it measures short distances extending only up to few kilometers, the camera is unsuitable for use in adaptive optics applications. However, it would be suitable for use in long-distance surveillance optical systems.

Fig. 13.

Variation in the power received by the dual-wavelength-band camera with increasing target distance.

8. Conclusions

Herein, a dual-wavelength-band camera was developed, followed by an extensive evaluation of its optical and laser spot acquisition performance. A detector employing an InAsSb sensing layer complemented by a coated sapphire cold filter with a spectral window at 1.064 μm and a bandwidth optimized for MWIR was integrated into the camera to facilitate image acquisition across the dual-wavelength bands. The optical system had an effective focal length of 85.9 mm and horizontal and vertical FOVs of 4.2° and 3.4°, respectively. The laser target designator, Leonardo's Type 163, provided a laser beam with a wavelength of 1.064 μm and a guaranteed output energy of no less than 70 mJ. The experimental measurements revealed that the developed dual-wavelength-band camera could handle a target recognition distance of 3 km for a target measuring 3.0 m × 1.8 m, along with a laser spot detection distance of 2 km. Subsequent calculations indicated that laser spot images could be reliably obtained at a laser receiving power of ≥65 μW. Thus, the developed dual-wavelength-band camera will capture the image of a 1.064-μm laser spot irradiated on a target within 2 km from the camera and recognize a vehicle-sized target within a range of 3 km from the camera. Exploring other parameters, such as laser pulse width, peak power, and pulse repetition rate, is imperative for comprehensive quantitative analysis of the acquisition of laser spot images using a dual-wavelength-band camera. Additionally, the camera requires a comparative analysis using MSS, which uses a microcooled MWIR image sensor with 640 × 480 pixels to produce mid-infrared images with a 1.064-nm laser spot and a FOV of 4.1° × 3.2°. In future research, it will be necessary to analyze target image recognition and spot detection distances by increasing the entrance pupil diameter of the dual-wavelength-band camera.

Data availability statement

All data supporting the conclusions have been either provided or are otherwise publicly available.

CRediT authorship contribution statement

Duck-Lae Kim: Writing – review & editing, Writing – original draft, Visualization, Validation, Investigation, Formal analysis, Data curation, Conceptualization. Hyun-Woo Park: Writing – review & editing, Writing – original draft, Visualization, Validation, Resources, Investigation, Formal analysis, Data curation, Conceptualization. Nam-Koo Ha: Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Data curation, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.