Ugh! Exact same result!

This also clarifies why most people tend to set their photon intensity to 10 times their direct light intensity when using Maya lights with quadratic decay. Well, now they know that they actually should either divide the direct light intensity by 4*pi (=~12.566), or multiply the photon intensity by 4*pi. This way, physical_light can safely be 'emulated' in case of point, spot or directional lights.

Conclusion

Maya light intensity = physical_light intensity / 4*pi (=~12.566)

But what about (flat) area lights? Well, here's a simple point light, with physical_light attached (intensity 10000.000) and MentalRay area light set to true, type rectangle - it's sampling is set to 1/1 and the number of anti-aliasing samples is min/max 1/1 to avoid irregular noise for our test:


Let's try to apply what we learned above. Physical_light detached, intensity divided by 4*pi, decay rate quadratic:



Hmm, that didn't work out. Let's have a look at the physical_light's source code again:

[code] case miLIGHT_RECTANGLE: /* rectangular area light */
mi_query(miQ_LIGHT_AREA_R_EDGE_U, state, light, &u);
mi_query(miQ_LIGHT_AREA_R_EDGE_V, state, light, &v);
mi_vector_prod(&normal, &u, &v);
mi_vector_normalize(&normal);
mi_vector_to_light(state, &dir, &state->dir);
mi_vector_normalize(&dir);
/*
* Compute area-to-point form factor (except cos at
* the receiver). is cos at sender. Returning
* 2 means "no color and stop sampling".
*/
cosine = mi_vector_dot(&normal, &dir);
if (cosine <= 0)
return((miBoolean)2);
if (paras->cos_exp != 0 && paras->cos_exp != 1)
cosine = pow(cosine, paras->cos_exp);
/*
* cos term and distance attenuation. No area term
* since "color" of the light is energy, not radiance.
*/
f = cosine / (M_PI * r2);
break;[/code]

As you can see, in the line next to the last, pi only is used. So let's try that - intensity divided by pi only (10000/pi = 3183.099):