Questions tagged [photometry]
The photometry tag has no usage guidance.
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Why are luminous intensity, luminous flux and illuminance physical quantities if they are based on the sensitivity of the human eye?
If luminous intensity and other derived quantities are based on the luminosity function, which is based on the sensitivity of the human eye, why are they physical quantities? Why is luminous intensity ...
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29 views
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 ($\frac{...
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2answers
64 views
Why is human eye dynamic range expressed in terms of luminance?
I am learning about photometry to do Physically Based Rendering. As I checked the human eye dynamic range, I was surprised to see it was expressed in terms of luminance ($\mathrm{cd}/\mathrm{m}^2$) : ...
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1answer
68 views
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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29 views
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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82 views
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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1answer
98 views
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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30 views
Observed Data for Asteroid Lightcurve
I am looking to find out more about writing statistical analysis programs in python for astronomy problems and was recommended that I could analyse asteroid light curves for an introduction into the ...
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1answer
38 views
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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1answer
91 views
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 ...
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1answer
92 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
...
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552 views
Effective wavelength
While measuring light from sky or anyother things we use photometer. If current flowing in photometer is proportional to intensity and frequency why there is something called effective wavelengths. ...
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665 views
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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110 views
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 would be ...
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168 views
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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3answers
8k views
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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870 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?
...
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121 views
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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1answer
163 views
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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1answer
263 views
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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1answer
107 views
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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1answer
131 views
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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1answer
97 views
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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108 views
Does Lambert's formula give correct results of luminous flux density?
A uniform point-source S, having luminous intensity, $I$, is located at a point at a distance $r(=OS)$ from the center O of a plane having area $A$ such that $\theta$ is the angle between the line ...
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1answer
4k views
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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1answer
769 views
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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1answer
1k views
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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1answer
214 views
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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203 views
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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2answers
6k views
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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1answer
439 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 ...
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1answer
460 views
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 ...
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1answer
52 views
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 ...
