# What does $E_L$ mean in the context of shading equations from the book “Real time rendering”? We will use $E_L$ for irradiance perpendicular to $l$

Where $l$ is the light vector.

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

• For reference: Previously asked 2 hours earlier on an external site – trichoplax Jul 29 '16 at 13:20
• 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. – trichoplax Jul 29 '16 at 13:33

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.
• 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 Apr 5 '17 at 9:14
• @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 Apr 5 '17 at 13:37