Questions tagged [photometry]
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47 questions
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Planck's law in terms of flux [duplicate]
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 ...
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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 ...
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Definition of Candela and Lumen
I have been trying to understand the significance of the number $685$ in the definition of lumen: 1 lumen is the luminous flux emitted by a $1/685$ Watt monochromatic source of $555$ nm. I have read ...
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CIE198: Simplification of SMCF (spectral mismatch correction factor)
The CIE198:2011 suggests to simplify the spectral mismatch correction factor (SMCF) using the relation of the color temperatures of the Planckian distributions:
$$
F(T_d,T_A) = \frac{\int P(\lambda,...
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What are the values of the (photopic) CIE 1924 luminous efficiency function?
Apparently, the luminous flux and therefore indirectly the lumen (lm), one of the 7 basic SI units, is defined in terms of "the" 1924 luminous effiency function $\overline{y}(\lambda)$ of ...
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How do you find the extinction coefficient for Gaia photometry?
I have Gaia photometry g, bp, and rp. My final goal is to find the photometric temperature but first I need to account for reddening by finding the extinction coefficient. I’m really stumped on how to ...
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Lambertian surface and the luminous intensity
I am currently studying the basics of photometry to better understand the rendering equation of Kajiya. One thing I'm currently struggling with is Lambert's cosine law.
Let's go over the premises:
A ...
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What do the CIE Standard Illuminants SPDs represent?
The CIE publishes the relative SPD for standard illuminants. I was curious what photometric quantity it represents i.e. radiance or irradiance or something else.
I was reading the "Intro the ...
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Are radiance and luminance really independent of distance to observer?
Recently, I learned that apparently both radiance and luminance are independent of the distance between the light emitting object and the observer.
The reasoning was that although the radiant/...
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Total brightness from emission spectra?
Some background: I'm looking to analyze stars through a telescope; I'd like get both the visual spectrum and the brightness of each target. I'd also like to have to build/carry as few instruments as I ...
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How do I convert a .FITS into a numpy array (or a file that can be used in Python)? [closed]
I'm trying to train an AI algorithm to determine photometric redshifts of galaxies, and to do so I have a .FITS file which contains the training data. I need to convert this .FITS file into a format ...
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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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2
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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}...
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Angle of minimum deviation and prism [duplicate]
Does angle of minimum deviation depend on angle of prism?
I know that angle of deviation does but I don't feel like the particular behavior is relevant for the angle of deviation as well? Please ...
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Photometry and seemingly two different definitions for foot-lamberts, why?
It is more on theoretical-math-physics stuff side, it is not crucial for practical application (the practical context of this question is "video projection"), just curiosity and liked to ...
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Convert cps units to candelas / sq. meter
Spectra photometer was reported to measure luminance at given cps rate (counts per second). Is it enough data to know which luminance value it measured, given model and vendor are known?
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What does the "area" represent in the luminance of the sky?
Wikipedia and plenty of other sources give values for the luminance of the sky, of e.g. $\mathrm{2\ kcd/m^2}$. What area is the denominator representing here? How can something like the sky, that ...
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How two light sources with identaical illuminance can have different luminance?
My question is regarding a slide from a lecture we had about photometric units. Namely, after being introduced to the luminous flux ($lm$), illuminance ($lx := \frac{lm}{m^2}$) and luminance ($\frac{...
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Why is human eye dynamic range expressed in terms of luminance?
I am learning about photometry to do Physically Based Rendering. As I checked the human eye dynamic range, I was surprised to see it was expressed in terms of luminance ($\mathrm{cd}/\mathrm{m}^2$) : ...
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483
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Calculate Retinal irradiance (W/cm² to log photons/cm²/s)
I have been trying to calculate what retinal irradiance value I get with a 1 blue LED system.
Since the manufacturer didn´t give the spectral distribution information, I will approximate LED as a ...
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Photometry - Photopic and Scotopic vision
I would like to measure the eye sensitivity to a blue LED when a person is placed in a dark room. I need to convert the luminous flux (lumen) to optical power (W).
Can someone tell me if I should use ...
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Zebra and Donkey Watching [closed]
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 ...
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473
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What is BVI Photometry and how is it used?
I searched for data about Cepheids light curves (often used as standard candles for measuring distances), and found that all of the data comes in a form of "BVI Photometry" or "BVI Photoelectric ...
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How does an IR camera determine the temperature?
The IR cameras detect light in the IR where low temperature bodies (-20 - 40 deg) have their peaks of the black body radiance.
How the detectors associate temperature to intensity measurement on a ...
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How to evaluate LED illuminance based on luminous flux and directivity / radiation pattern?
I am building an LED-light and am trying to figure out how many LEDs I need to be on it. I know how much illuminance I need as well as the distance to the subject and the luminous flux of one of the ...
user202748
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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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Converting illumination to irradiance
I need get intensity of an LED light source, but I cannot convert lux to $W/M^2$
How does one convert illumination to irradiance? And what is the irradiance formula?
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847
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Illuminance at a given distance from a diffuse emitting light box
Therapeutic light boxes purport to deliver 10,000 lux to a user, typically sitting ~24" from a box. I was wondering how many lumens are necessary to achieve this illuminance, e.g. what lamps ...
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What is meant by the Lyman alpha color temperature?
I understand the color temperature of radiation to be the temperature of a blackbody that has a similar spectrum as the radiation. I am puzzled by references in astrophysics to the Lyman-alpha color ...
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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 ...
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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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Stretch Factor for SNIa
How do I compute the stretch factor for a Supernova Ia ?
Data I have : Redshift, Points on the light curve, Rise time (∆m=2), Fall time (∆m=2), Time from the peak day for an inflection point in the ...
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Synthetic Photometry - Calculating a colour index
I have a theoretical black body spectrum as described by plancks law. I also have the bandpass sensitivity function for various filters. I would like to calculate a colour index from this information, ...
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Best way for calculating Lux from candela vs from lumen?
Researching this topic, I came across two methods that seem to contradict each other. Need to decide which to use.
I am a novice in this field so please be gentle.
Also, I would have loved to use ...
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How to begin using SDSS data?
SDSS has gone through so many updates, it's difficult to find out how to access the data today in 2015.
How can I use SQL and Python to access SDSS data, e.g. the photometry and spectra of quasars, ...
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If the Sun were smaller but had the same surface temperature, would it still have the same luminance?
Let's assume we have two stars that have the same surface temperature but very different size. I understand how luminosity depends on surface area so the two stars will have different luminosity, but ...
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During transition of the electron from high to low energy state, is the photon released always of same energy?
Is it not possible that during transition of an electron from higher to lower energy state, it can release multiple photons of low energy instead of a single photon of the exact energy difference ...
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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 ...
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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+...
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need data-point: count rate of APD (avalanche photo-diode) for specific aperture and stellar magnitude
I hope lab / experimental physics is fair game for this web-site. If not, sorry!
I'm designing a sensor system to perform specialized [astronomy and space-sciences] experiments, and need a "reality ...
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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 ...
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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-...
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Can I see the light from 10 million km away?
If light is switched ON, only for a second, and the distance between the observer and the light source is 10 million kilometers, can I still see the light spark?
For example, let's assume that the ...
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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 ...
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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 ...
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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 ...
Tan
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1
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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 ...
Joshua