What is spectrum?
Light behaves both as a wave and a particle.
When light is captured as a wave, the color appearance of an object changes depending on the quality of the wavelength. For example, humans perceive long wavelengths of visible light as red and short wavelengths of visible light as purple. (Right figure)
By analyzing an object through various wavelengths, the object's more subtle qualities and characteristics can be clarified. The difference in the wave period is called a “wavelength”, and the difference in the wavelength distinguishes the type of light it is.
The unit of wavelength can be measured as a “nanometer (nm)". A nm = 0.000000001 (one billionth of a meter). Light which is visible to the human eye has a wavelength range from around 380 nm to 780 nm.
The method of finely analyzing wavelengths is called “spectroscopy”. And by performing spectroscopy, the characteristics and state of an object can be analyzed in greater detail.
The element of light separated through spectroscopy is called “spectrum”, and the spectroscopic analysis is also known as “spectral analysis”.
The Human Eye's Ability to Perceive Light
The human eye recognizes visible light by dividing it into the three colors of red, green, and blue (RGB) using photoreceptor cells that function as sensors called “cone cells”.
For example, humans recognize strawberries as red because strawberries absorb the blue and green wavelengths of visible light and reflect the red wavelengths. Most digital and video cameras acquire color data in the same way as the human eye (by combining these three colors).
The Possibilities of Spectrum
By analyzing the unique spectrum of an object through spectroscopy, it is possible to evaluate the properties and states of substances that are difficult for human eyes to evaluate.
For example, as shown in the figure on the right, it is possible to identify the types of resin, oil, and powder that the human eye cannot distinguish by itself at all. Therefore, this spectrum technology can be utilized in many valuable and beneficial ways regardless of industry or field of study.
What is a hyperspectral camera?
As mentioned above, the human eye and most cameras divide visible light into three colors to capture the light. However, by finely splitting the wavelength, it is possible to recognize differences in physical properties and phenomena that are difficult or impossible for the human eye to evaluate.
In the case of the red paint in the figure on the right, the color image on the left looks completely red, but by spectrally separating the light, it is possible to see any unevenness in the coating that is difficult to evaluate, as is the case with the image on the right. This is an example of the difference in analyzing an object with a hyperspectral imager versus only using the human eye. Notice the light intensity (spectral distribution's) ability to reveal the precise density between the thick and thin parts of the coating, which is too small for the human eye to even recognize.
In this way, a camera that can finely disperse wavelengths and acquire many continuous wavelength elements (about 100 or more) is called a “hyperspectral camera.”
The difference between the data acquired by the human eye or color camera and the data acquired by the hyperspectral camera is shown in the figure on the right. Visual and color images have only three colors in the depth direction, but you can see that the hyperspectral camera can acquire a lot of wavelength information. With this technology, it is possible to more effectively analyze objects and phenomena that were previously difficult to evaluate.
The history of Hyperspectral imager
Hyperspectral technology was originally born from earth observation in the field of remote sensing.
With the invention of the CCD in 1969, technological development for measuring spectral information as images proceeded.
In the 1980s, A.F.H. Goetz (Science 1985) and others from the California Institute of Technology first proposed hyperspectral camera technology, which was first put to practical use in aircraft.
Initially, it was called by names such as Imaging Spectroscopy [1], but since GJ Brelstaff
(Proc. SPIE 1995) of Bristol University used the name “Hyperspectral camera” in the 1990s [2], the more familiar this name is now popular.
In the 2000s, satellite remote sensing was put to practical use, starting with being mounted on the Earth observation satellite EO-1 developed by NASA.
The equipment mounted on aircraft and satellites was large and expensive, and its use was limited. However, due to the rapid progress of image sensors and computer technology in recent years, the development of a compact and portable hyperspectral camera has become possible.
Now, we have succeeded in developing a product that can be used by many customers by adding our own built-in spectroscopic scanning technology [3].
As a result, the creation of the hyper spectrum market has greatly advanced, and we are currently developing the frontier as a leading company in this field.
[ References ]
[1] A.F.H. Goetz, et al.(1985) Imaging Spectrometry for Earth Remote Sensing. Science, 228, 1147-53.
http://science.sciencemag.org/content/228/4704/1147.long
[2] GJ Brelstaff, et al.(1995) Hyperspectral camera system: acquisition and analysis. Proc. SPIE, 2587, 150–159 .
https://www.spiedigitallibrary.org/conference-proceedings-of-spie/2587/0000/Hyperspectral-camera-system-acquisition-and-analysis/10.1117/12.226819.short?SSO=1
[3] Y Takara, et al.(2012) Remote sensing applications with NH hyperspectral portable video camera. Proc. SPIE, 8527, 85271G.
https://www.spiedigitallibrary.org/conference-proceedings-of-spie/8527/85271G/Remote-sensing-applications-with-NH-hyperspectral-portable-video-camera/10.1117/12.982018.short?SSO=1