iamlucky13 wrote:
The Messenger site says the MDIS instrument covers a range from 395 nm to 1040 nm. Visible light is about 400 to 750 nm, so about half of the filters are probably visible light, and it doesn't go very far into the infrared.
It wouldn't really be considered a thermal image, as thermal radiation is generally considered to start at the long end of that range and go on out to about 14,000 nm, and normally refers to emitted radiation rather than reflected radiation.
I browsed in Pointons Introduction to statistical mechanics and found the solution Max Planck has derived for the energy of a black body:
E(λ) = 8 π h c / [λ^5 {exp(hc/(λkT))-1}] (equation 4.21)
h is the Planck constant (6.64E-34 Js), c is the velocity of light (3E8 m/s), k is the Boltzmann constant (1.38E-23 JK) and λ is the wavelength of light.
To find the maximum wavelength at a specific temperature, differentiate E(λ) with respect to λ and demand that the derivative is 0:
∂E(λ)/∂λ = 8 π h c {5-(hc)/(λkT)}/[λ^6 {exp(hc/(λkT))-1}] = 0
That leaves us with: 5-(hc)/(λ_max kT) = 0, so
λ_max = hc / (5 kT) ≈ 2.89E-3 / T
When we apply this formula for increasing temperatures (one decade per step), we arrive at the table below:
Code: Select all
T[K] λ_max Object
3 960 µm Big Bang remnant
30 96 µm Dark molecular cloud
300 9.6 µm Earth, planet
3000 960 nm Cool star (IR)
30000 96 nm Hot star (UV)
300000 9.6 nm Hot burst (far UV)
3000000 0.96 nm Coronal discharge (X-ray)
Thermal images, as meant in oncological research, are usually focussed in the range of 10 µm. The old geostationary wheather satellites have two distinct IR band sensors: 6.2 µm (water vapour) and 10.8 µm (cloud temperatures). The newer satelites employ a 3.9 µm sensor as well. From the table it becomes clear why they use precisely these wavelengths: it is where the peak in the (thermal) black body radiation is located.