Lighting Theory: Radiometry and Photometry

Recent games have been heading towards Physically Based Rendering and this requires a solid understanding on lighting theory more than ever before. This time, I'm posting my note on Radiometry and Photometry.

Radiometry

Radiometry is basically ideas + mathematical tools to describe light propagation + reflection. Radiative Transfer is a study of transfer of radiant energy (which operates on geometric optics level - macroscopic properties of light suffice to describe how light interacts with objects much larger than light's wavelength).

Four Radiometric Quantities:

 1. Flux (Radiant Flux/Power) - total amount of energy passing through a surface or region of space per unit time (J/s or Watt). Total emission from light sources is generally described in terms of flux.
2. Irradiance (E) - area density of flux arriving at a surface (Watt/m2)
Radiant Exitance (M) - area density of flux leaving a surface (Watt/m2)
3. Radiant Intensity (I) - flux density per solid angle (Watt/sr)
4. Radiance (L) - flux density per unit area, per unit solid angle (Watt/m2-sr)

Photometry

Photometry is a study of visible electromagnetic radiation in terms of its (human) perception. Each spectral radiometric quantity can be converted to its corresponding photometric quantity.
Luminance (Y) measures how bright a spectral power distribution appears to a human observer. The unit is cd/m2 or nit. Cd is the photometric equivalent of radiant intensity (Watt/sr).

Connecting Radiometry with Photometry
Photometric quantity can be obtained by integrating spectral radiometric quantity against the spectral response curve V(lambda) (the relative sensitivity of the human to various wavelengths).

QuantityRadiometric NameRadiometric UnitPhometric NamePhotometric Unit
powerRadiant FluxWLuminous Fluxlumen (lm)
power per unit areaIrradianceW/m2Illuminancelm/m2 = lux (lx)
power per unit solid angleRadiant IntensityW/srLuminous Intensitylm/sr = candela (cd)
power per unit area per unit solid angleRadianceW/m2-srLuminancelm/m2-sr = cd/m2 = nit

References

1. Radiometry and Photometry FAQ - http://fp.optics.arizona.edu/Palmer/rpfaq/rpfaq.pdf
2. Philips Lighting Theory - http://www.lighting.philips.com/main/connect/lighting_university/internet-courses/lighting-theory/lighting-theory/
3. Moving Frostbite to PBR - http://www.frostbite.com/wp-content/uploads/2014/11/course_notes_moving_frostbite_to_pbr.pdf
4. IES File Format - http://www.cn-hopu.com/upload/file/IES.pdf
5. Radiometry Summary - http://cs.brown.edu/~ls/teaching/RadiometrySummary.pdf


Comments

Post a Comment

Popular posts from this blog

World, View and Projection Matrix Internals

Image
This convention below is applicable to Direct3D and XNA matrices World Matrix Given a  position  and basis vectors  right ,  up  and  look  of an  object , a world matrix can be formed by the following arrangement: View Matrix Given a  position  and basis vectors  right ,  up  and  look  of a viewer, a view matrix can be formed by the following arrangement: Projection Matrix Given field of view  FOV ,  aspect ratio , near clip plane  Zn  and far clip plane  Zf , a perspective projection matrix can be formed by the following arrangment:
Read more

GDC 2015 Links

Here are [growing] links to GDC 2015 slides/presentations: Update: GDC Vault for GDC 2015 is now up!  Programming Track Khronos Group ( Link ) Khronos OpenCL GDC Khronos Vulkan GDC Valve Vulkan Session GDC Live Stream of Vulkan Session (Youtube) The Future of High-Performance Graphics Virtual Reality VR Direct: How NVIDIA Technology is Improving the VR Experience ( pptx ,  pdf ) - Nathan Reed, Dean Beeler Advanced VR Rendering  - Alex Vlachos Far Cry 4 Fast Iteration for Far Cry 4 - Optimizing Key Parts of Dunia Pipeline  - Remi Quenin Rendering the World of Far Cry 4  - Stephen McAuley Adaptive Virtual Texture Rendering in Far Cry 4  - Ka Chen Mesh Cutting in Farming Simulator 15 - Gino van den Bergen Advanced Visual Effects in 2D Games - Viktor Lidholt Great Management of Technical Leads - Mike Acton Code Clinic: How to Write Code the Compiler Can Actually Optimize - Mike Acton Parallelizing the Naughty Dog Engine Using Fibers - Christian Gyrling
Read more

BASH: Reading Text File

BASH for C programmers: Read a text file word-by-word in BASH: #!/bin/bash for WORD in `cat filename` do echo $WORD done Read a text file line-by-line in BASH: #!/bin/bash while read LINE do echo $LINE done < filename Looping in BASH: i=0 while [ $i -lt 10 ] do echo $i # next i=$(($i+1)) done Update : another way to loop suggested in the comment.
Read more