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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 ...
KPS's user avatar
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0 answers
11 views

Can the luminance be expressed in $\text{cd}\,\text{m}^{-2}\,\mu\text{m}$?

I have a hyperspectral imaging system that acquires spectral channels over the visible domain. The spectral data in these channels are expressed in $\mu\text{W}\,\text{sr}^{-1}\,\text{cm}^{-2}\,\mu\...
гиви's user avatar
1 vote
0 answers
18 views

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,...
Brokie's user avatar
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0 answers
11 views

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 ...
BlenderBender's user avatar
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0 answers
29 views

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 ...
Snav's user avatar
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0 votes
1 answer
74 views

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 ...
Bartolini's user avatar
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0 answers
51 views

Irradiance from balloon onto sensor

I am new to radiometry and some of the concepts are confusing me. I want to calculate the irradiance on a sensor from a balloon which is half illuminated by the sun and half illuminated by diffuse ...
ngc1300's user avatar
  • 274
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0 answers
53 views

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 ...
Dinesh Iyer's user avatar
1 vote
5 answers
658 views

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/...
tempdev nova's user avatar
1 vote
0 answers
35 views

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 ...
T.S's user avatar
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0 votes
1 answer
658 views

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 ...
Lara's user avatar
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1 vote
1 answer
4k views

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,...
zeellos's user avatar
  • 19
2 votes
2 answers
79 views

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}...
Vincent ISOZ's user avatar
0 votes
2 answers
289 views

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 ...
Tia DS's user avatar
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1 answer
203 views

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 ...
kroko's user avatar
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-1 votes
1 answer
44 views

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?
ivan866's user avatar
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1 answer
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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 ...
synecdoche's user avatar
1 vote
1 answer
108 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{...
iMrFelix's user avatar
  • 111
2 votes
2 answers
140 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 ($\mathrm{cd}/\mathrm{m}^2$) : ...
Florian Cabot's user avatar
1 vote
1 answer
403 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 ...
az_'s user avatar
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0 votes
0 answers
42 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 ...
az_'s user avatar
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2 votes
0 answers
98 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 ...
user226375's user avatar
1 vote
1 answer
437 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 ...
Yoav Zack's user avatar
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0 votes
1 answer
63 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 ...
ole's user avatar
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1 answer
203 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 ...
user avatar
2 votes
1 answer
242 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 ...
gen-ℤ ready to perish's user avatar
0 votes
1 answer
1k 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. ...
Sri vishnu Bharat's user avatar
-2 votes
1 answer
4k 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?
Mitra Walidain's user avatar
0 votes
1 answer
779 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 ...
PBJ's user avatar
  • 103
1 vote
0 answers
325 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 ...
binaryfunt's user avatar
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21 votes
3 answers
14k 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 ...
Physther's user avatar
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0 votes
2 answers
1k 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? ...
Kristof Pal's user avatar
1 vote
0 answers
144 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 ...
Sanket Doshi's user avatar
2 votes
2 answers
304 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, ...
Samuel's user avatar
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1 vote
1 answer
457 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 ...
Remco Hekker's user avatar
1 vote
1 answer
123 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, ...
ShanZhengYang's user avatar
2 votes
1 answer
533 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 ...
Abanob Ebrahim's user avatar
1 vote
1 answer
108 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 ...
ravi's user avatar
  • 111
5 votes
1 answer
8k 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 ...
ShanZhengYang's user avatar
3 votes
1 answer
1k 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+...
Andy's user avatar
  • 393
2 votes
0 answers
82 views

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 ...
honestann's user avatar
  • 135
6 votes
1 answer
2k 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 ...
J L's user avatar
  • 2,877
2 votes
1 answer
234 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-...
LearnOPhile's user avatar
0 votes
2 answers
216 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 ...
KarSho's user avatar
  • 103
2 votes
2 answers
8k 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 ...
Evan Krall's user avatar
0 votes
1 answer
535 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 ...
Arlen's user avatar
  • 3,907
3 votes
1 answer
517 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 ...
user avatar
2 votes
1 answer
54 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 ...
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