In remote sensing field, passive sensors are used to measure emitted radiation which is converted to temperature via the Placks Law of black body radiation. In natural conditions the reflected portion of radiation is negligible given the relatively low radiation revieced from the sun in that spectral region. However, some laboratory active sensors allow to measure reflected energy in that spectral region (e.g., 4100 ExoScan FTIR). The nature of such radiation is different from that of emited radiation, so I wonder how can they be related.
الأشعة تحت الحمراء (أو إشعاع تحت الأحمر) هو الإشعاع الكهرومغناطيسي مع الطول الموجي بين 0.7 و300 ميكرومتر، وهو ما يعادل تقريبا نطاق الترددات بين 1 و400 تيراهيرتز.[1][2][3]
صورة بالأشعة تحت حمراء لكلب
طول موجته أطول (وتردده أدنى) من الضوء المرئي، ولكن طول موجي أقصر (والتردد العالي) من تلك الموجات من الإشعاع التراهرتز. ضوء الشمس الساطع يوفر من حوالي 1 كيلو وات لكل متر مربع عند مستوى سطح البحر، ومن هذه الطاقة، 527 واط هو ضوء الأشعة تحت الحمراء، و445 واط من الضوء المرئي، و32 واط من الأشعة فوق البنفسجية.
- First of all, Planck's law of radiation describes the spectral density of EM radiation emitted by a black body, and a black body is an idealized physical body that absorbs all energy. It doesn't reflect or transmit energy (absorptivity=emissivity=1).
- So, if you want to know the reflected radiation, then the body you're dealing with a real or non-black body. thus, for a non-black body we can generally say that the reflectivity is not zero, and the total quantity of radiation that leaves a non-black surface is the combination of emitted radiation and reflected radiation.
- Therefore, the total rate at which radiant energy leaves the surface is: J=(emissivity)*(Eb)+(reflectivity)*(G)
where:
- J= Radiosity, total radiation that leaves a surface (by way of emission or reflection) per unit area per unit time (W/m^2)
- G=Irradiation, total radiation (from all sources) the hits a surface per unit area per unit time (W/m^2)
- (emissivity)*(Eb)=emissive power of the body
As you can see for a black body (reflectivity=0) we have J=Eb.
sources:
Heat Transfer Principles and Applications by Dutta
https://en.wikipedia.org/wiki/Black_body
https://en.wikipedia.org/wiki/Planck%27s_law
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