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Equation 5.10 from Real Time Rendering book

We will use $E_L$ for irradiance perpendicular to $l$

Where $l$ is the light vector.

What does this mean? How do I calculate $E_L$?

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The line you quoted says it: $E_L$ is the incoming 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.

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  • $\begingroup$ 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 ? $\endgroup$
    – v.oddou
    Apr 5, 2017 at 9:14
  • $\begingroup$ @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). $\endgroup$ Apr 5, 2017 at 13:37

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