What is Radiance: Definition and 32 Discussions

In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area. Spectral radiance is the radiance of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. These are directional quantities. The SI unit of radiance is the watt per steradian per square metre (W·sr−1·m−2), while that of spectral radiance in frequency is the watt per steradian per square metre per hertz (W·sr−1·m−2·Hz−1) and that of spectral radiance in wavelength is the watt per steradian per square metre per metre (W·sr−1·m−3)—commonly the watt per steradian per square metre per nanometre (W·sr−1·m−2·nm−1). The microflick is also used to measure spectral radiance in some fields. Radiance is used to characterize diffuse emission and reflection of electromagnetic radiation, or to quantify emission of neutrinos and other particles. Historically, radiance is called "intensity" and spectral radiance is called "specific intensity". Many fields still use this nomenclature. It is especially dominant in heat transfer, astrophysics and astronomy. "Intensity" has many other meanings in physics, with the most common being power per unit area.

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  1. Tone L

    Optical Measurements of the Sun's radiance and ray tracing

    Summary: This is a hobbyist project I am working on. I am building an instrument to measure radiance from the sun at 500 nm, using optically filtered photodetector with bandwidth ± 10 nm. This type of science can be called Sun Photometry, so let's get to it! Geometry for Optics: The sun has an...
  2. J

    I Relation between spectral radiance and density of cavity

    Hi, my teacher showed us how we can derive de relation between spectral radiance and density of cavity (of a black hole), but I have a doubt. This is the equation of the energy that are coming from definited directions by the intervals of angles θ and Φ with frequency in a determined interval...
  3. A

    I Radiance and energy density of a black body

    How can I find the relation between the radiance and the energy density of a black body? According to Planck's law, the energy density inside a blackbody cavity for modes with frequency ##\nu \in [\nu, \nu + \mathrm{d}\nu]## is given by $$ \rho(\nu, T)\mathrm{d}\nu =...
  4. H

    Solar Panel - Voltage and Current from Spectral Radiance

    Homework Statement i)A beam of light at wavelength 600nm, with spectral width of 4nm and spectral radiance of 1300 Wm-2μm-1, illuminates an area of 10cm2 . What is the voltage this beam could generate if converted to energy? ii) What current could this beam produce? iii) How would the current...
  5. E

    How to calculate blackbody temperature from radiance band?

    I know how to calculate blackbody radiance in a spectral band from a temperature by integrating (or summing) planks function. How can I do the reverse? For example, I have a Radiance of 1000 W/m^2/sr in the 3-5um band, I'd like to be able to calculate 666.6K.
  6. E

    How does Radiance change with a parabolic reflector?

    Lets say I have a coil that is 1cmx1cm in area that gets heat up. I am told that I get 2W/cm^2/sr output. So if I had 100 of them in an array, the total radiant intensity would be (2W/cm^2/sr x 100 x 1 sqcm =) 200W/sr. Now, let's say I add a parabolic reflector around each coil. How would I...
  7. E

    A Question about Radiance, Solar Flux and Reflectance

    I stumbled upon this equation for a Lambertian uniform target: ##\rho^{*} = \frac{\pi L}{\mu_s E_s}##, where ##\rho^{*}## is the reflectance, ##L## is the measured radiance, ##E_s## is the solar flux at the top of the atmosphere, and ##\mu_s## is the cosine of the sun zenith angle...
  8. A

    Path radiance in atmosphere

    It is common knowledge that the scattering of blue photons by the atmosphere is more significant than for other colors hence aerial images of Earth look bluesh. The graph below obtained from MODTRAN 'allegedly' supports this fact. While trying to quantify this effect, I've preformed several...
  9. G

    IR LED irradiance, radiant exposure and radiance

    Hi! I'm trying to design a device to detect eye blinking while a person is sleeping. I have tried to use TCRT5000 sensor (http://www.vishay.com/docs/83760/tcrt5000.pdf) in the prototype and it works like charm ! However, since the TCRT5000 sensor has an IR emmitter I have some safety...
  10. H

    Emission radiance in participating media

    This link discusses radiation in participating media. Eq. 9.13 gives a prediction of the changing power of a ray along the path length as: I_\eta(x)=I_\eta(0)\exp(-\tau_\eta)+I_{b\eta}[1-\exp(-\tau_\eta)] where \tau_\eta is the absorption coefficient times the length. So, the first term...
  11. V

    Violation in diffraction? Lagrange (Optical) invariant

    It says you can not change with lenses the value L - radiance. Below I have an example where it proves that you can or where am I wrong? (I made L for 2D case, in 3D case everything the same - L2>L1)
  12. G

    Radiance change with light distance

    I am trying to solve the rendering equation with monte carlo integration. I gather all of the incoming radiance from light sources and plug it into the rendering equation. A reference I am using said that radiance does not change with distance. Does this mean that a point light source will...
  13. H

    Creating a Laser with Different Types of Radiance

    Hello, Can we make a laser with any type of radiance (IR,UV,visible,microwave,..) ? If we consider a three level system for example. In such a system, how can we avoid absorption from level 1 to level 2 ? Is it by choosing atoms/molecules in which the level 2 is greater than the fundamental ...
  14. B

    Why is radiance defined per projected area normal to the beam direction?

    Radiance is defined as radiant flux per solid angle per projected area normal to the beam direction: ##L = \frac{d^2 \Phi}{d \vec\omega \cdot d A_\perp}## where ##A_\perp = A \cos \theta## and ##\theta## is the angle between the beam direction ##\vec\omega## and the surface normal. I kind of...
  15. S

    My quest to understand radiance

    Hi all, been a while since I've posted here, but figured this would be a great place to ask this question. I've gone over the first chapter of McCluney's Introduction to Radiometry a number of times, and more recently have discovered this great resource, but I am still confused about something...
  16. B

    Spectral radiance of the sun for 400nm

    Homework Statement Using Planck's law, calculate the spectral radiance of 400nm sunlight arriving at earth. Assume 1/3600 steradians of view and 5800 K of temperature. Homework Equations Planck's law The Attempt at a Solution I am not the best at typing formulas. Lets...
  17. N

    Can You Calculate Radiance from Irradiance Outputs Using a Spectroradiometer?

    Hi, When you measure irradiance using a spectroradiometer, can you calculate radiance based on the irradiance outputs? How does one measure the angle in cosθ and solid angle Ω in the radiance formula? Thanks!
  18. mnb96

    Radiance of a Lambertian emitter

    Hello, I am considering an infinitesimal surface dA that receives a total radiant flux d\Phi, so basically we know the irradiance, that is given by dE = \frac{d\Phi}{dA}. If I assume that this surface reflects 100% of the light, and it is Lambertian, how am I supposed to calculate the...
  19. W

    The Blushing Face of Saturn: Radiance, Luminosity & Habitable Zone

    Okay, reeling from a defeat in the stellar ring with a self gravitating mass of gas that imploded at a massive temperature when all I wanted it to do was hang around nice and calmly while I put gravitating bodies in it...now I move on to a slightly more conventional model. I do plan to do most...
  20. D

    About radiance equation in Radiometry

    Homework Statement According to some pages on web, differential formula for calculating radiance L is : L = d2P / ( dA*cos(a)*dw) L : radiance P : power flux A*cos(a) : projected area w : solid angle Please tell me why does power P has derivaties two times d2P ? Homework...
  21. mnb96

    Example of calculation of radiance

    Hello, I found http://omlc.ogi.edu/classroom/ece532/class1/radiance_flashlight.html" an example of calculation of radiance. I found this example quite confusing, and now I am not sure whether it's me who didn't understand some concepts, or the author who silently made a bunch of simplifying...
  22. F

    Understanding the Difference between Radiance and Irradiance

    Radiance, Irradiance? Homework Statement Hi, just wondering if my understanding of the difference between these two is correct?? Homework Equations If radient energy is the energy transported by electromagnetic radiation then radiance is the radient energy crossing a surface per unit...
  23. B

    Blackbody Radiance: Calculate Absorption & Emission Rates & Equilibrium Temp

    Homework Statement Consider a slab of matter contained within two infinite planes a distance of s = 1m apart. absorption coefficent =1m2kg-1 density of slab 1kgm-3 heat capacity 103^3jk-1kg-1 assume the absorption coefficient to be independent of wave length 1.Suppose one face of...
  24. B

    Blackbody Radiance Homework: Calculating Warming & Cooling Rates

    Homework Statement Consider a slab of matter contained within two infinite planes a distance of s = 1m apart. absorption coefficent =1m2kg-1 density of slab 1kgm-3 heat capacity 103^3jk-1kg-1 assume the absorption coefficient to be independent of wave length 1.Suppose one face of a...
  25. B

    Question on blackbody radiance

    Hi, just looking for some pointers in how to solve the following question, such as formulas, etc, any help at all so as i can proceed in the right direction will be much appreciated, thanks in advance. not sure how to start it Consider a slab of matter contained within two infinite...
  26. F

    What is the Radiance of Solar Radiation at Earth Orbit?

    Homework Statement given that T(sun)=5771.4, S(Earth)=1370, D(Earth)=1.5x10^8km, R(sun)=7x10^5km calculate the radiance of solar radiation at Earth orbit Homework Equations Irradiance=Radiance/Solid Angle Solid Angle=Area/Distance^2 I=sigma*T^4 The Attempt at a Solution...
  27. C

    How Does Water Affect Solar Radiance Observed Underwater?

    Homework Statement An underwater observer is looking directly into the sun. His head is near, but below the surface, and he is not wearingsun glasses. Let Ls(x) be the solar radiance at the water surface, on the air side, when the sun is at a zenith angle x. What is the ratio of the solar...
  28. J

    What are Radiance and Irradiance and how do they differ?

    Hey, I'm having trouble working out what Radiance and Irradiance are, all the definitions I get are along the lines of something really technical like: "For a point of radiant energy, Radiant Intensity, in a specified direction, per unit projected area: Le=(dIe/dA)*Cos(theta) where A is the...
  29. S

    How Do Radiance Units and F-Number Relate in Optics?

    While reading a paper on optics, I came across "the radiance of the projector is 20 mW/cm2, expanding out onto an f/18 cone." What does the "expand out onto an f/18 cone" part mean? I thought that radiance is just in units of power/area/steradians. What does f-number have to do with anything?
  30. A

    Reflected Radiance: Calculating with 100% Albedo

    Imagine that the mean solar irradiance reaching the Earth falls on a high altitude, low-latitude fresh snow field that acts as a perfect lambertian reflector. It has albedo of 100% with radiance being reflected isotropically into the upward hemisphere. Assume that you can completely ignore...
  31. G

    Confusion over concept of 'radiance'.

    I have some confusion about what 'radiance' means as used in radiometry; specifically, I don't understand what it is a function of. For example: radiant energy is specified over some region, radiant energy density is specified at a point, radiant flux is specified across some surface...
  32. U

    Spectral Radiance and it's Units

    I'm told the spectral radiance of a source is in the following units W ---------------- cm^2 * sr * nm And I try to use this to figure the spectral output of monochromator (in W), but I can't seem to cancel out the units. I multiply the spectral radiance by four other terms...