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  • 執筆者の写真Amir Masoud Gharehbaghi

Let There Be (All Kinds of) Light (あらゆる種類の)光あれ

You probably think you know something about light. Yes, it’s what’s in the famous quote “Let there be light” by God. It’s what allows our eyes to see. But light is not just what’s bright and shiny. Light can be dark and invisible to our mortal human eyes.


Light is actually the electromagnetic spectrum, which includes the entire electromagnetic wavelength range from radio waves all the way to gamma rays. Radio waves have longest wavelengths ranging from 1mm to 100km, and gamma rays have shortest wavelengths around 1pm (picometer, 10^-12m). Radio waves, gamma rays, and any electromagnetic wave at any wavelength whatsoever between them, are all light and thus they all travel at the speed of light like nothing can catch them.


The bright, shiny, colorful "light" we are familiar with, is only a fraction of the light spectrum, and is called visible light (because our eyes can see it), ranging from around 400 to 700 nm (nanometer, 10^-9m). Visible light around 400nm looks violet, and around 700nm looks red, with other rainbow colors in between. Light with slightly shorter wavelengths than violet is called ultraviolet (UV), which damages our skin, and light with slightly longer wavelengths than red is infrared (IR), which feels warm.


In Astronomy, different types of light help us unravel the mysteries of the universe in their own ways. To list a few examples, in the microwave domain (with wavelengths between radio waves and infrared light, also used by your microwave oven to heat your food), leftover radiation from the birth of the universe right after the Big Band can be observed (called cosmic microwave background or CMB). Infrared light, which the highly anticipated James Webb Space Telescope is capable of detecting, is used to study cooler objects in the universe like planets. Gamma rays, with shortest wavelengths, are thought to be released during strong explosions such as massive supernovae or merging neutron stars. The choice of instruments depends on the type of light you wish to observe.

天文学においては、さまざまな種類の光が宇宙の謎を解き明かすために使われている。例をいくつか挙げると、ラジオ波と赤外線の間の波長のマイクロ波(電子レンジが食べ物をあたためるのに使う光)では、宇宙マイクロ波背景放射(cosmic microwave background, CMB)というビッグバン直後の宇宙の誕生時の放射の残りを観測することができる。大いに期待されているジェイムズ・ウェッブ宇宙望遠鏡でも観測される赤外線は温度が低めの惑星などの天体を調査するのに役立つ。最も波長の短い電磁波であるガンマ線は、巨大な超新星爆発や中性子星同士の衝突などの大爆発時に放出されると考えられている。それぞれの光の種類に応じて使用する機材を選択する必要がある。

Ordinary earth-made color cameras are only sensitive to visible light, more specifically, only 3 primary colors of light: RGB (R stands for Red, G for Green, B for Blue). R band is centered around 650nm, G around 550nm, and B around 450nm. Any color in any photograph is expressed as a combination of these RGB bands.


Thanks to the tireless efforts for technological advances and innovations, there are other types of cameras available nowadays, such as multispectral cameras and hyperspectral cameras. Multispectral cameras have up to 10 bands and hyperspectral cameras have 100s of narrow bands, which means they can take higher resolution images than standard color cameras. Those multispectral and hyperspectral bands are not limited to just RGB bands or visible light. Some hyperspectral and multispectral cameras can capture light in IR or UV as well as visible range.



In the following Let There Be Light series, the cool and exciting details of multispectral and hyperspectral imaging will be revealed. Stay tuned!





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