Questions tagged [radiometry]

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The Event Horizon Telescope collaboration will announce groundbreaking observations of our galaxy at 12 May 2022, what this announcement could be? [closed]

"Live webcasting for the ESO-hosted EHT 2022 event On 12 May 2022 at 15:00 CEST, the European Southern Observatory (ESO) will host a press conference on groundbreaking new Milky Way results from ...
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Visualizing Radiance

I am learning about the basics of radiometry and I am having some trouble visualizing radiance. Firstly radiance is defined as irradiance with respect to solid angles. More generally we gave: $$L(p,\...
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How can I find the light power of a polychromatic LED based on the minimum sensitivity of a photodiode array?

I'm building an optical sensor that scans a newborn baby's finger for diseases. I'll be using a polychromatic light (ie, white light) that will pass through the finger. There will be a diffraction ...
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How to find the radiance value on any point of the surface of a finite emissive-reflective plane facing an exact equivalent one?

The problem consists of two parallel square plates that are entirely equivalent. Their length size is $h$, they are distant by $h$, and both are emissive. They are emitting an exitance of $E$ in all ...
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Compute UVC Irradiance On A Surface

I would like to be able to get a ballpark approximation of the Irradiance on a surface at a distance away from a UV source. The datasheet gives the following: Radiant Power Lambertian Emitter Graph ...
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Lambert's cosine law in 3D, why $\cos\theta$ instead of $\cos^2\theta$?

I'm reading the book An Invitation to 3-D Vision and I don't understand a part of the explanation regarding the "basic radiometric image formation model". Looking at the Wikipedia page of ...
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Power captured by a Lens from a Lambertian Source

I am trying to understand how much optical power of a Lambertian source can be captured by an optical system with this derivation: https://de.coursera.org/lecture/optical-efficiency-and-resolution/...
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How can radioactive-dating techniques, such as uranium-thorium analysis, tell you when a human fashioned a tool out of a rock or coral?

This is an archaeology or anthropology question.... Scientists in a recent 'Nature' said they compared recent genetic analyses of theirs, concerning when Polynesian islands were first settled, with ...
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How to measure luminance of a monitor screen (NITS) with a lux meter (LUX)?

Before you scold me out for using a Lux meter to measure NITs in my screen, let me explain to you the procedure I tried. I basically just need confirmation. I have a Lenovo Thinkvision P32u-10 monitor,...
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Calculate radiance of Lambertian emitters: Is the solid angle $\pi$ or $2 \pi$?

Given a Lambertian emitter (or reflector) of area $A$ that is emitting a total power (resp. flux) of $\Phi$ (units $W$). To calculate the radiance $L$ (units $\frac{W}{m^2 sr}$) the solid angle $\...
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Difference between radiant power and radiant flux

In Wikipedia the definition of radiant flux is: radiant energy per unit of time. (deriving w.r.t area you get the radiant exitance or radiant emittance). So this means that the radiant flux is the ...
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Radiance definition

I have been trying very hard to get my head around how the radiance definition is illustrated on the web. Every website puts the apex of the solid angle on the surface, but the definition said the ...
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Brightness Temperature layered atmosphere

I'm trying to figure out how to apply the radiative transfer model to calculate the brightness temperature Tb in microwave (Rayleigh–Jeans approximation) considering atmosphere and no scattering. If I ...
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How does Tx Power affect the Free Space Path Loss model?

On this website one can use the FSPL model to calculate the Path Loss. This is typically interpreted online as the theoretical signal strength (in dB) that one may receive (e.g. from some WiFi). ...
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How is VLBI delay calculated, practically?

I’m working on a research project wherein we are trying to solve a problem very similar to VLBI delay. We have two radio receivers, and we know their locations. We also know at what time one receiver ...
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What is the “single pulse radiometer equation”?

This document https://arxiv.org/abs/1601.03547 refers to the “single pulse radiometer equation”. Apparently, it’s used to calculate the flux of radio transients. What is the “single pulse radiometer ...
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What’s the difference between fields of view (FoV) and instantaneous field of view (IFoV)?

I read that radio telescopes have “huge fields of view (FoV)”, but are unable to precisely localized objects due to their “small instantaneous field of view (IFoV)”. Apparently, somehow the size of ...
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1 answer
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Why do high frequency radio telescope observations require precise localization?

Why do high frequency radio observations, particularly of transient phenomenon such as Fast Radio Burst and Pulsars, require increasingly precise localization with increasingly high frequency?
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2 answers
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How is interferometry used to measure distances?

I understand how a basic interferometer works, and how Michaelson used an interferometer to more-or-less disprove the “luminiferous anther”, but, analytically, how would one go about using an ...
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Is my illuminance calculation correct?

I have been reading about illuminance, illuminous flux, lumens, lux, irradiance, etc on wikipedia. I made this toy problem to test my knowledge. I am posting it here to test if I'm correct. (note, ...
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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 = \...
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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 ...
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2 votes
1 answer
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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 ...
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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 ...
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3 votes
2 answers
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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 ...
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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 ...
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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 ...
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2 votes
2 answers
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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 ...
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1 answer
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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 ...
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2 votes
1 answer
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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 ...
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4 votes
1 answer
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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 ...
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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 ...
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1 vote
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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 ...
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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?
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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 ...
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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 ...
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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 ...
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1 vote
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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 ...
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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^{\...
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6 votes
1 answer
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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 ...
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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 $\Theta$ ...
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1 vote
1 answer
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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 ...
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2 answers
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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? ...
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1 answer
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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 ...
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1 vote
1 answer
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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 ...
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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\...
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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 ...
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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 ...
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3 votes
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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}}{\...
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6 votes
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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 ...
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