   Calculating Irradiance
  I've just starting getting into improved BRDFs (over Phong) and am a bit confused by irradiance.  From what I understand, irradiance is the amount of light that passes through a unit plane that is tangent to the surface.  I understand the concept, but I don't really understand how to apply it.In my scene, say I have a room with a 60W light bulb on the ceiling.  Ignoring falloff, I would like to pass this light into my BRDF.  In most BRDFs I've seen so far, the irradiance is modulated with some sort of specular and diffuse result.  What values would I use in my shader with this light bulb?  What if I wanted to move my scene outside and have the sun light it?   
  Typically you deal with normalized values (0 to 1), so in most apps you just tweak the light and/or materials to achieve the desired effect/lighting/shading.To actually use values like 60W, you have to do the computation with these 'physically correct' values and use hdr or a variation of it to map the values (may well be outside 0 to 1) back to 0 to 1.Another approach is to not use the actual physical values again, but to use a fake multiplier for your really intense lights and just tweak it along with the hdr;Quote:Original post by psquareTo actually use values like 60W, you have to do the computation with these 'physically correct' values and use hdr or a variation of it to map the values (may well be outside 0 to 1) back to 0 to 1.Actually, yes, I am planning on using only an HDR render path onto a 64-bit floating point surface.  I'm hoping these "realistic" BRDFs will help me learn more about HDR rendering since the values they output should be more realistic, and not just in the [0,1] range.  I have no concept of what the range of my input values should be though.  Does the sun have an irradiance of 100.0?  50000.0?  And then is a light bulb 15.0?So my diffuse calc:color_diffuse = irradiance * (N dot L)Am I anywhere near on the right track?;  You might be interested in reading a good ray tracing book, since this is the heart of the matter (I would recommend 'Physically Based Rendering' by Pharr).  When you mentioned 60W, that is a power value.  Irradiance is not a power per se, but rather a measure of the power per unit area.  So if you make a sphere surface around your light bulb, you could measure the irradiance (power per unit area) of the sphere.  If you made another sphere with a greater radius around the light bulb, the irradiance would be a smaller value since you have the same power output over a larger surface area.You can check out the wiki page on irradiance for more details here.   