Questions tagged [photometry]

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1answer
191 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,...
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0answers
25 views

Calculating illumination at a point by light created a artificial light source

I am studying about lighting design. I find a question regarding artificial light source. Its' luminous intensity is given by, $250(1+2cos({\alpha}))$. $\alpha$ is measured angle to a point from ...
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0answers
18 views

Angle of a light source relative to illuminance readings

Given only: two illuminance readings $E_A$ and $E_B$ and the distance between those two sensors can we find the angle (x) at which the light point source is tilted? (I only need the angle and not the ...
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2answers
61 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}...
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2answers
130 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 ...
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1answer
45 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 ...
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1answer
33 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?
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1answer
35 views

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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1answer
60 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
90 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
172 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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0answers
35 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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0answers
89 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
213 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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1answer
39 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
129 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
145 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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1answer
940 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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1answer
2k 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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1answer
211 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 ...
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0answers
226 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
10k 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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2answers
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? ...
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0answers
130 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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2answers
212 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
289 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
115 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, ...
2
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1answer
276 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
104 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 ...
3
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1answer
6k 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 ...
3
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1answer
884 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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0answers
74 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 ...
5
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1answer
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 ...
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1answer
226 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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2answers
208 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 ...
2
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2answers
7k 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
465 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 ...
3
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1answer
485 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 ...
2
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1answer
53 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 ...