Questions tagged [photometry]
49 questions
21
votes
3 answers
Why do materials change color when exposed to the sun for a long time?
I have a lot of solid objects that were exposed to the sun for many years and "obviously" they changed their color. I write obviously because I know it empirically and from other people, but what is actually happening in the material? Are the…
Physther
- 564
6
votes
1 answer
Relation between brightness and magnitude
Let say we measure the superficial brightness of an object in the $B$ band, $I_{B}$ measured in $\displaystyle\frac{L_{*}}{\mbox{pc}^2}$ where $L_{*}$ is solar luminosity and $\mbox{pc}$ is parsec. I have to show that, in magnitudes per arc…
J L
- 2,947
5
votes
1 answer
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…
5
votes
1 answer
Photometric surveys vs. Spectroscopic surveys
I consistently read about certain astronomical surveys which are either described as "photometric surveys" or "redshift surveys". I'm still unclear as to how these two methods differ.
Photometry uses the total light (or flux or brightness) received…
ShanZhengYang
- 907
4
votes
1 answer
Do we use transit photometry to look for a black hole star binary systems?
What would a light curve look like for a black hole transiting a star? Initially I thought it would bend all light essentially blacking out a star but we would probably still detect some however the curve should look substantially different from a…
Joe
- 1,354
3
votes
1 answer
How does the Luminosity Function relate to the tristimulus sensitivity function?
In colorimetry, the irradiance spectrum is weighted with the luminosity function to obtain the perceived luminosity, and the tristimulus sensitivity functions to obtain the perceived RGB-representation.
How do these functions (just one scalar for…
LearnOPhile
- 149
3
votes
1 answer
Error analysis of flux from telescope-CCD setup (photometric calibration)
I am doing photometric calibration of a telescope and am trying to work out the errors on the flux that I measure. I have attempted to work out the errors due to the process of removing bias, dark frames, etc. using Poisson statistics, but this…
Tan
3
votes
1 answer
Distance from redshift
I am looking for a exact derivation of a relation between redshift $z$ and distance $d$.
What I know is the definition
$$z=\frac{\lambda_{\text{observed}}}{\lambda_{\text{unshifted}}}-1=\sqrt{\frac{1+\frac{v}{c}}{1-\frac{v}{c}}}-1$$
and that the…
Andy
- 393
2
votes
1 answer
Planck's law in terms of flux
The Stefan-Boltzmann law describes blackbody radiation in terms of radiant flux per unit area i.e. how much energy passes through a 2D surface per unit time. I find this very simple and I understand how to do calculations with it.
Planck's law…
Volbla
- 21
2
votes
2 answers
Planck distribution integration
Does anyone know how to compute analytically or numerically the following integral (for $T=10^4$K)?:
$$n_\gamma=\frac{1}{\hbar^3\pi^2c^3}\int\limits_{2.1789\cdot 10^{-18}}^{+\infty}\dfrac{E^2\mathrm{d}E}{e^{\frac{E}{kT}}-1}$$
I tried with R, MATLAB,…
Vincent ISOZ
- 247
2
votes
2 answers
Why is human eye dynamic range expressed in terms of luminance?
I am learning about photometry to do Physically Based Rendering. As I checked the human eye dynamic range, I was surprised to see it was expressed in terms of luminance ($\mathrm{cd}/\mathrm{m}^2$) : from $10^{-6}\,\mathrm{cd} / \mathrm{m}^2$ to…
2
votes
0 answers
Zebra and Donkey Watching
Suppose I have a white donkey in front of me.
Say i paint black stripes on the objective of my camera lens and click a photo of it.
The context of this question is that when we cover a lens horizontally and view an object with it, there is no…
user226375
- 267
- 3
- 8
2
votes
2 answers
Why is the candela dimension J, not W?
According to the table at the bottom of the Wikipedia page for the candela, the dimension for candelas is J (joules). Why is this not W (watts)?
The luminous intensity for light of a particular wavelength λ is given
by
$I_v(\lambda) =…
Evan Krall
- 131
2
votes
1 answer
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
$$\Phi_\v = K_{\rm cd} \int_{0\ \rm m}^\infty…
2
votes
1 answer
Existing photometry data of the open star cluster Cepheus OB3?
I am doing research with my teacher on photometry of the open star cluster Cepheus OB3, and we are trying to find research papers so we can target stars in the photos we took.
Are there any existing resources available on this matter?
Joshua