Questions tagged [radiometry]
The radiometry tag has no usage guidance.
49
questions
2
votes
0answers
33 views
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
$$
B_\nu = \...
1
vote
0answers
11 views
Irradiance Measurement of Incandescent Lamps
I have two spectroradiometers that use the same detector from two different manufacturers an Oceaan Optics FX and a Gamma Scientific GS1220. They each have their own cal lamps.
After calibrating ...
2
votes
1answer
97 views
How do I transform flux density into temperature?
I have flux density data from Planck mission (in Jy) - measure in a certain frequency - and I would like to transform it to temperature data (Kelvin). I'm not sure if I should use Stefan Boltzmann's ...
4
votes
1answer
48 views
Microwaves and Water Molecules: Radar vs Cooking
I am studying microwave radiometry and confused over this apparent contradiction: Microwaves emitted at 2.45 GHz (~15 cm) are absorbed by water molecules, causing the molecules to rotate under the ...
2
votes
2answers
107 views
If two observers look at the same quantum object, will they see the same thing?
Let's say two observers, one at the North pole and one at the South pole, both observe the Moon. Will they see the same Moon, or a subtly different Moon?
Intuitively it feels like they should see a ...
-1
votes
1answer
24 views
Why are CIE standard illuminant SPDs given as relative power?
Why is CIE standard illuminant data given as relative power distribution, and not absolute? They are always normalized, usually so that the power at 560nm is 100. Seems like an unnecessary loss of ...
1
vote
1answer
26 views
How do I translate between spectral radiometric quantities and electromagnetic ones?
In the study of electricity and magnetism we learn that light is just a series of waves in the electromagnetic field. We learn about the Poynting vector
$$\mathbf{S}=\mathbf{E}\times\mathbf{H},$$
the ...
1
vote
2answers
638 views
What does “intensity of light” mean?
In this lecture from MIT, the professor defines the intensity of a wave as the square of the amplitude of the wave. But, at the same time, the professor defines the intensity of light as the number of ...
0
votes
1answer
353 views
How can I convert spectral irradiance to spectral radiance [closed]
I have information on the spectral irradiance incoming from the sun at the top of the atmosphere in units of $\rm mW\ m^{-2}\ nm^{-1} $.
The photons hit a hypothetical surface on the earth with 0.3 ...
2
votes
1answer
57 views
Why do we need to define both luminous intensity and illuminance?
Background
If a source radiates $\newcommand{\e}{\mathrm e} \Phi_\e$ total energetic flux (in watts $\rm W$), then it is said to radiate luminous flux $\newcommand{\v}{\mathrm v} \Phi_\v$ defined by
...
4
votes
1answer
122 views
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 ...
0
votes
0answers
115 views
How is radiance conserved? - Power/intensity misconception
Before getting to the main question I'd like to clear some pretty basic concepts regarding energy/power/intensity as my physics is pretty rusty.
According to wiki
In radiometry, radiant flux or ...
1
vote
0answers
58 views
Is power received reflected off a surface dependant on both the incident and emitted angles?
I am trying to calculate the incident power into a detector as reflected from a small illuminated Lambertian surface. Basically I am looking for a check of my calculations as I am not confident that ...
-3
votes
2answers
143 views
21cm line of Hydrogen spectrum [closed]
What is the advantage that we have when we study about 21cm line of Hydrogen spectrum over others, in radio-astronomy?
0
votes
0answers
39 views
Radio vs X-ray pulsar timing
The detection and timing of pulsars is a common goal in astronomy (eg.1, 2).
Typically these observations are done in the radio. I appreciate that observations are taken in the X-ray as well, but is ...
0
votes
1answer
977 views
point sources vs extended source irradiance and radiometry question
How can the irradiance of a point source behave proportionally to 1/R^2, where R is the range of the source yet the irradiance of an extended source be independent of range? It doesn't make any sense ...
0
votes
1answer
366 views
Radiance calculation for extended spherical light source
I have a spherical light source of radius R. There is a surface patch of area $\delta A$ located at a distance $d$ from the spherical source. How do I calculate the scene radiance due to surface patch ...
1
vote
0answers
26 views
Does a (tungsten-halogen) calibration lamp keep calibrated if unused over a period of several years?
We have a calibration lamp at work that is NIST traceable. It's a tungsten-halogen source that's used for radiometric measurements, so we have a table of power per wavelength. These lamps typically ...
1
vote
1answer
335 views
Understanding Incident/Exitant Radiance
Reading "Physically Based Rendering", I'm trying to understand what the meaning of the incident and exitant radiance functions.
I understand that radiance $L(p,\omega) =\frac{d^2\phi}{d\omega dA^{\...
6
votes
1answer
330 views
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 ...
1
vote
0answers
135 views
radiance at a grazing angle
I was analyzing radiance equation, i.e. $L = \frac{\partial\Phi}{\partial\Omega \partial A \cos\Theta}$ and judging by the equation it appears that radiance just blows up as the angle $\Theta$ ...
1
vote
1answer
41 views
radiometric quantity: radiant flux
Irradiance is defined as a ratio of differential flux to differential area, that is: $\frac{d\Omega}{dA}$. What I understand from this is that in the limit when $dA -> 0$ we obtain the density of ...
0
votes
2answers
723 views
Is there a relationship between the driving current and the optical power emitted by a light source?
If we have an LED light source which is driven by 0.5 Ampere can we infer what is the optical power (aka radiant flux, radiant power) emitted by that light source?
...
0
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1answer
59 views
Scattering of acrylic glass in ice
For some biological testing reason, I have to place some tubes in a frozen sea ice environment.
I need to choose a proper material. I would like something that leaves the visible light field within ...
1
vote
1answer
75 views
Find the spectral distribution of a metallic reflection given electron configuration
If I have the electron configuration for a metallic element, how do I find the spectral distribution of its specular reflection?
For example, for gold (2,8,18,32,18,1) I should get a greater ...
0
votes
1answer
69 views
A few questions about the definition of the BRDF
Hi everyone (I'm sorry for my bad english)
I was reading the book "Physically-based rendering" and I came across the definition of the BRDF :
$ f(x,\theta, \psi) = dL(x\rightarrow\theta)/dE(x\...
1
vote
0answers
52 views
Geometric and radiometric distribution of light
I'm studying the behavior of the light in a scene, to know how much light arrives at a specific point of a surface and how much of it leaves from the point.
From the documentation I'm reading, it says ...
2
votes
1answer
807 views
How do I convert lux to lumens
Overview...
I'm a novice in the very basics of light physics and calculation between various units. I'm sure this is a very simple problem to those who know the process. And, before we continue, this ...
3
votes
0answers
97 views
Energy dependence of integrating dosimeters
The graph shows the relative response of a dosimeter at different energies, normalized to 1.25 MeV Co-60 gamma rays. Curve A is the graph of the equation
$$
\frac{\bigg(\frac{r}{X}\bigg)_\bar{E}}{\...
2
votes
1answer
854 views
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 ...
0
votes
2answers
165 views
Is shape of a black body much of a concern in physics?
When I read a book on computer vision, I stumble upon the ideal black body model. Using Lambert's cosine law (wiki), one is able to compute the intensity of an area element dA w.r.t. some observer.
...
0
votes
0answers
241 views
Why do some materials follow Lambert's cosine law?
I perfectly understand Lambert's cosine law, so I can apply it, it isn't really that hard (now that I got it :)
$$I_n=I_\theta*\cos\theta$$
What I don't understand is why some materials follow this ...
5
votes
2answers
328 views
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 ...
2
votes
2answers
933 views
How does a radiometric infrared camera estimate an objects temperature?
Say we have an infrared camera which measures some amount of radiation, in a spectral bandwidth which is given, between wavelength $\lambda_1$ and $\lambda_2$ from a perfect black body. How is it ...
1
vote
1answer
203 views
Polonium tea emergency [closed]
Let's assume that I just realised that tea I drunk 30 minutes ago during meeting with a secret agent was doped with radioactive polonium.
What should I do? What the doctors will do? Are there ...
2
votes
1answer
293 views
Why is radiance measured normal to the beam?
Radiance is defined as:
$L=\frac{d \Phi}{\mathop{d \omega} \mathop{d A} \mathop{\cos\theta}}.$
I’m wondering why there is a $\cos \theta$ factor in the denominator of the radiance ($\theta$ is the ...
4
votes
5answers
664 views
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 piece of mind.
I would ...
3
votes
4answers
3k views
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 ...
1
vote
2answers
1k views
Irradiance at surface point due to lighting from point light source
I'm reading through a computer graphics paper and author says that the radiant intensity emitted by an infinitely small surface point $p$ with normal $ n $ into direction $ \omega $ is
$$
I_p(\omega) ...
2
votes
3answers
4k views
Lambert's cosine law
I am learning how to physically render images in computer graphics. I just saw that the area that gets light is given by the Lambert's cosine law.
In my head it makes perfect sense the relation but ...
1
vote
0answers
131 views
Hemisphere irradiance
How do I calculate sky irradiance from radiance (L) from a hemisphere above a surface which is tilted relative to the normal (x=0,y=0,z=1). I have L as a function of zenith (0 to 180deg) and azimuth (...
4
votes
2answers
2k views
Relationship between irradiance and radiance
A question related to radiometry:
Irradiance $E$ at a point $x$ can be written as:
$$E = \int_\Omega L(x, \omega) cos(\theta) d\omega$$
I understand this formula and where it comes from. The ...
11
votes
2answers
24k views
Relation between Radiance and Irradiance
I know that radiance is expressed as
$$[\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 ...
3
votes
1answer
2k views
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 ...
1
vote
1answer
919 views
Photosensitivity of Photodiodes, lower wavelength -> lower sensitivity?
I saw a diagram of the photosensitivity (Current per Power) of a photodiode.
So there is this diagonal stating the 100% quantum efficiency.
I wondered why the sensitivity for bluer light lower than ...
0
votes
1answer
426 views
Implementing a black-body using Planck's Law in a 3D ray tracer
1_ In one of my software projects, I need to implement Planck's Law in terms of wavelength. I can't be sure if it's right because I've seen different variations of it, and all the implementations ...
0
votes
1answer
246 views
Leaching of radiometric material, is it possible?
I've been doing some reading about radiometric dating and I've come across an interesting find. If anybody has any additional information on this, that would be great.
First my question:
In regards ...
-2
votes
2answers
3k views
How reliable is Radiometric dating? Are there limitations? [closed]
Young earth creationists dismiss radiometric dating as unreliable, whats the truth?
5
votes
2answers
343 views
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 ...
