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Questions tagged [radiometry]

89 questions
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Relation between Radiance and Irradiance

I know that radiance is expressed as $$[\text{radiance}] = \frac{\rm W}{\rm {sr} \cdot m^2}$$ and $$[\text{irradiance}] = \frac{\rm W}{\rm m^2}$$ but what is the relation between these two quantities? Is irradiance commonly used to referring to…
Mattia
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Why does radiance remain constant along rays of light through empty space?

In Radiometry, radiance (L) is defined as flux density per unit area per unit solid angle. If we move further along a ray, away from a point, shouldn't the radiance decrease? I am unable to grasp an intuitive reason for the statement: Radiance…
Ranveer
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What is the radiation field of a black body with temperature $T$ translating at speed $v$?

Suppose I have a spherical black body at temperature $T$ with radius $R$. When stationary in a vacuum at zero Kelvin, it produces an incoherent radiation field according to Planck's law $$ B_\nu = \frac{2 h\nu^3}{c^2}…
Sean E. Lake
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6
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1 answer

Is antenna noise temperature relevant if the physical system temperature is higher?

Assume a lossless antenna connected to a lossy cable at physical temperature 300K. If this antenna is pointed into space, the antenna noise temperature will depend on the brightness temperature of objects in its beam (cosmic background radiation in…
user80551
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Why does lambertian surface reflection result in uniform radiance measured from every direction?

I'm reading through the light measurement handbook and page 28 is confusing me. I knew previously that lambertian diffuse reflections distribute the reflection of incoming intensity such that the surface looks equally bright from every direction…
TravisG
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Why cosine of angle is in the denominator in the irradiance formula?

I am studying radiometric measurement concepts and found out (on lecture notes), that the cosine term (from the irradiance formula) is in the denominator. I could not grasp it, as it would imply that irradiance goes to infinity when $θ_d$ goes to 90…
Gunay Karaoglu
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2 answers

Visibility of moon-based laser from Earth

If someone (or some robot) on the moon were to point a laser at the Earth, how many watts would the laser need, to be easily seen with the un-aided eye from Earth? Obviously this depends on a number of factors. Assume the beam is made divergent…
JustJeff
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What's the meaning of partial derivative for radiance?

The definition of radiance is: $$L\equiv\frac {\partial^2 \Phi}{\partial A\,\partial\omega\,\cos\theta}$$ where: $\Phi$ is the radiant flux $\omega$ is the solid angle $A\cos\theta$ is the projected surface Why are partial derivatives used and not…
Andrei
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Is there no radioactive decay between nuclear fusion and solid material formation?

I'm aware my question might be considered a duplicate of this one: Radio-dating and the age of the earth I read that one and I looked everywhere and I still can't find my peace of mind. I would really like to understand the following points: does…
Eduard Dumitru
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Must Lambert's cosine law be modified in the context of special relativity?

A Lambertian surface has a constant radiance $R$, regardless of the angle from which it is observed, where $A = A_0\cos(\theta)$ is the apparent surface area of an observer at angle $\theta$ normal to the surface. $$ R = \frac{I}{A} =…
asmaier
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Relationship between irradiance and radiance

A question related to radiometry: Irradiance $E$ at a point $x$ can be written as: $$E = \int_\Omega L(x, \omega) cos(\theta) d\omega$$ I understand this formula and where it comes from. The equation for radiance can be written as: $$L = {d^2\Phi…
user18490
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Radiance equation

I am trying to understand the equation of the radiance, but there is one thing i don't understand: $L_r = \frac{d^2\Phi}{d\omega \space dA\cos{\theta}}$ Why is that second exponent there in the numerator? Thanks ahead!
user408141
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Microwaves and Water Molecules: Radar vs Cooking

I am studying microwave radiometry and confused over this apparent contradiction: Microwaves emitted at 2.45 GHz (~15 cm) are absorbed by water molecules, causing the molecules to rotate under the influence of the electric field component so…
PUNGO9ER
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Obtaining photoelectron counts from known radiance

I am attempting to make a rough estimate of the counts seen on a CCD spectrometer assuming I know the spectral radiance of a calibration source. I understand that this is not the same thing as radiometrically calibrating, but I want to make a…
JN3
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If two observers look at the same quantum object, will they see the same thing?

Let's say two observers, one at the North pole and one at the South pole, both observe the Moon. Will they see the same Moon, or a subtly different Moon? Intuitively it feels like they should see a different Moon - this is because each individual…
Allure
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