10.1117/2.1201105.003651

Terahertz imaging for detectionor diagnosisIwao Hosako and Naoki Oda

A camera that operates at video frame rate offers a solution to stand-off

imaging for search and rescue in fire disasters and label-free biomaterial

detection.

Terahertz (THz) imaging is a powerful technique that exploits a

nonionizing (i.e., safe) portion of the electromagnetic spectrum

located between microwaves and the IR. Directed toward an

appropriate sample, such as wood or ceramics, THz light passes

through and is detected by a camera. Because of its nondestruc-

tive nature, THz imaging is applicable to many areas. For exam-

ple, it has advantages over x-rays for diagnosis of skin cancer1

and can detect sketches underlying paintings.2 The technique is

also potentially advantageous in conditions of hot, black smoke,

where conventional imaging is unsuitable. Even ordinary smoke

is difficult to see through at visible wavelengths, and hot, black

smoke saturates and scatters IR radiation. THz wavelengths

are absorbed by smoke less strongly and transmitted more

readily through it, compared with the IR. However, although

well-placed for (bio)materials analysis, THz imaging was until

recently considered ill-adapted for hot conditions,3 limiting its

utility in search and rescue missions.

We report a THz camera consisting of a quantum-cascade

laser light source4, 5 and a microbolometer focal-plane array (an

IR detector common in thermal cameras). The camera is based

on one we previously designed,3 but with improved power and

sensitivity. We describe two applications of our imaging system:

stand-off imaging for search and rescue in a fire disaster, and

label-free biomaterial detection.

The camera requires high-intensity illumination. A blackbody

is normally used as a reference source to calibrate imaging in

the mid-IR (thermal IR) region. Yet its relatively weak radiative

power makes it unsuitable for THz calibration and imaging (see

Figure 1). Here, strong THz sources, such as quantum cascade

lasers, are indispensable. The average power output of our liq-

uid nitrogen-cooled laser system—which operates for several

hours—is several microwatts, sufficient for the improved

Figure 1. Top: Blackbody spectra at various temperatures. The smaller

spectral radiance in the terahertz (THz) range, relative to the mid-

IR, is insufficient for imaging. Bottom: Photograph of stand-off THz-

imaging experiments. LN2: Liquid nitrogen. QCL: Quantum-cascade

laser. FPA: Focal-plane array.

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10.1117/2.1201105.003651 Page 2/3

Figure 2. Demonstration of the label-free biomaterial detection system

at a 2009 exhibition in Kyoto. A 1200K blackbody was used as a light

source because venue regulations prohibited the use of liquid nitrogen.

Inset: Schematic of the detection principles. PVDF: Polyvinylidene di-

fluoride.

microbolometer array (320�240 elements). Thus, the quantum

cascade laser acts as an external source.

We improved that camera’s sensitivity by changing the sheet

(electrical) resistance of an absorption (light-gathering) layer,

developing THz-transparent materials, and adding long-pass

metal mesh filters to block IR radiation.4, 5 We included frame-

and pixel-integration functions to increase the signal-to-noise

ratio (SNR). However, simple frame integration did not work as

expected because the microbolometer generates low-frequency

noise. Consequently, we incorporated lock-in imaging, which

reduces low-frequency noise and increases SNR as a function of

the square root of the frame-integration number.

With these technical improvements in place, we performed

stand-off (�5m distance) THz-imaging experiments under sim-

ulated fire conditions (see Figure 1). We obtained images in hot,

black smoke (with an SNR of�140 vs.�6000 with no smoke) that

blocked visible light and and induced signal saturation in the

long-wavelength IR (LWIR) region. These results clearly show

an advantage of THz compared with LWIR or visible imaging.6

Our camera is particularly adapted to search and rescue

missions due to its 60Hz (real-time) image-acquisition rate.

In pharmaceutical drug discovery, molecular interactions are

commonly tracked using chemical ‘labels’ that are costly and

prone to error. We applied our THz camera to label-free detec-

tion of small-molecule reactions with proteins. The THz waves

are readily absorbed and enable sensing of very small changes

Figure 3. Expected future applications of the THz-imaging system.

�: Optical wavelength.

in biomaterials (see Figure 2). The detection sensitivity of a

label-free biotin-streptavidin reaction (routinely used in biotech-

nology), for example, was nearly the same as that of conven-

tional methods, and the THz system has the advantage of high

throughput and low cost. We note that for real life-science appli-

cations, our detection system requires a very large biomaterial

spectral database in the THz range.

In summary, we have used a THz camera to acquire images

through hot, black smoke and to detect a biomolecular reaction.

In the future, we plan to improve the performance of the imaging

system. In terms of the light source, we expect soon to be produc-

ing milliwatt-class laser output, which represents much higher

power than the several microwatts currently possible. The sensi-

tivity of the microbolometer array can be enhanced one order of

magnitude by optimizing the bolometer structure to THz waves

and using better materials to fabricate the device. Ultimately, as

suggested in Figure 3, we expect such an imaging system to be

useful in a range of applications from preventive health care to

quality control, surgery, and nondestructive evaluation.

Author Information

Iwao Hosako

National Institute of Information and

Communications Technology

Koganei, Japan

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10.1117/2.1201105.003651 Page 3/3

Iwao Hosako is a leader of the THz project. His research in-

terests include semiconductor emitters, detectors, and optical

thin films. He serves on several research committees for THz

technologies.

Naoki Oda

Guidance and Electro-Optics Division

NEC Corporation

Fuchu, Japan

References

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c 2011 SPIE