<p>The line you quoted says it: $E_L$ is the incoming <a href="https://en.wikipedia.org/wiki/Irradiance" rel="nofollow">irradiance</a> at the surface due to the light source under consideration. Less technically, it's a vector representing the intensity and color of the incoming light at that point.</p>

<p>The equation you mention is for a punctual light source (directional, point, or spot light), so in this case $E_L$ would simply be the user-defined RGB color of the light source, multiplied by any relevant attenuation factors—such as shadows, distance attenuation for point/spot lights, and cone angle attenuation for spot lights.</p>

CC BY-SA 3.0
2017-04-05T09:14:00.997Z
wasn't the original `El` name derived from "emissive light" ? which was not a convolved term, but an additive term in front of the integral. Irradiance should be named `Ir` no ?
v.oddou
CC BY-SA 3.0
2017-04-05T13:37:02.617Z
@v.oddou E is the usual variable for irradiance that I've seen used in texts and papers. Emissive light is usually denoted $L_e$ (L is radiance).
Nathan Reed
CC BY-SA 3.0
2016-07-29T16:20:33.077Z
2016-07-29T16:20:33.077Z
Nathan Reed
<p><a href="https://i.sstatic.net/j9ani.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/j9ani.png" alt="Equation 5.10 from Real Time Rendering book"></a></p>

<blockquote>
  <p>We will use $E_L$ for irradiance perpendicular to $l$</p>
</blockquote>

<p>Where $l$ is the light vector. </p>

<p>What does this mean? How do I calculate $E_L$?</p>

CC BY-SA 3.0
2016-07-29T13:20:42.510Z
For reference: Previously asked 2 hours earlier [on an external site](http://www.gamedev.net/topic/680892-does-anyone-know-what-el-means-in-the-context-of-shading-equations-from-the-book-real-time-rendering/)
trichoplax is on Codidact now
CC BY-SA 3.0
2016-07-29T13:33:51.560Z
I've edited the title to include MathJax to show the subscript. If anyone finds this causes a problem please contribute to the [discussion on meta](http://meta.computergraphics.stackexchange.com/questions/238/should-we-use-mathjax-latex-in-question-titles).
trichoplax is on Codidact now
<rendering><lighting><shading>
What does $E_L$ mean in the context of shading equations from the book "Real time rendering"?
Maik Klein
# What does $E_L$ mean in the context of shading equations from the book "Real time rendering"?

(Asked by: username_0 on 2016-07-29)

[![Equation 5.10 from Real Time Rendering book](https://i.sstatic.net/j9ani.png)](https://i.sstatic.net/j9ani.png)

> We will use $E\_L$ for irradiance perpendicular to $l$

Where $l$ is the light vector.

What does this mean? How do I calculate $E\_L$?

**Tags:** rendering, lighting, shading

> **Comments:**
> **username_1**: For reference: Previously asked 2 hours earlier [on an external site](http://www.gamedev.net/topic/680892-does-anyone-know-what-el-means-in-the-context-of-shading-equations-from-the-book-real-time-rendering/)  
> **username_1**: I've edited the title to include MathJax to show the subscript. If anyone finds this causes a problem please contribute to the [discussion on meta](http://meta.computergraphics.stackexchange.com/questions/238/should-we-use-mathjax-latex-in-question-titles).  

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## 1 answers

### Answer by username_2 on 2016-07-29. Score: 4 | Accepted answer

The line you quoted says it: $E\_L$ is the incoming [irradiance](https://en.wikipedia.org/wiki/Irradiance) at the surface due to the light source under consideration. Less technically, it's a vector representing the intensity and color of the incoming light at that point.

The equation you mention is for a punctual light source (directional, point, or spot light), so in this case $E\_L$ would simply be the user-defined RGB color of the light source, multiplied by any relevant attenuation factors—such as shadows, distance attenuation for point/spot lights, and cone angle attenuation for spot lights.

> **Comments:**
> **username_3**: wasn't the original `El` name derived from "emissive light" ? which was not a convolved term, but an additive term in front of the integral. Irradiance should be named `Ir` no ?  
> **username_2**: @username_3 E is the usual variable for irradiance that I've seen used in texts and papers. Emissive light is usually denoted $L_e$ (L is radiance).  

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