Spherical emitter does not illuminate very well?

Question

I remember when I was recreating my bathroom and I modeled the lamp and gave it a glass material, but when I came to the lighting it got confusing. I modeled a sphere in the lamp because in real life the lighting comes from the lamp obviously. And then when I render the scene, no matter how strong the sphere light is I always get a pretty low light.

Is Cycles a realistic or close to realistic render engine? Or what lamps should I use? Do I have to fake the light? How can I replicate the lighting as in real life?

Edit: I'm not sure if the spherical emitter will light up the room better if it is not inside a glass material

Answer 1

The problem

The issue here is that cycles, which uses backwards path tracing (tracing light rays from the camera to the light sources), is not very good at rendering caustics in these kinds of situations.

This is because it's not very likely for a ray to bounce off a surface, transmit through the glass bulb, and hit the element within. Multiple Importance Sampling (MIS) usually helps with this by guiding more rays to hit light source(s), however the glass bulb surrounding the light source effectively prevents this from working.

From left to right: Element with MIS, Element without MIS, Element with MIS and glass bulb.

To properly render such a situation, solutions like Bi-direction path tracing and Metropolis Light Transport (MLT) exist. Unfortunately, cycles does not yet have this functionality, and it is considered "not a priority" according to the cycles roadmap.

Even so, there is a working patch for MLT (and adaptive sampling/stopping) which is being actively developed as of the time of this posting. You will have to compile blender yourself (it's really not hard) to use this patch. There is also an older patch for Bi-dir here.

Here is an example render of the element and bulb with the MLT patch:

It's better, but not at all the desired result (I don't really know any of the MLT related settings yet, so it's possible this could be improved some more).

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See also:

• How is Cycles different from Blender Internal?

• reflecting direct light on glossy surface in Cycles (this is a similar problem to yours, but with reflection instead of refraction)

• When should MIS be used and when should it be disabled?

Workarounds

• Use the fake glass setup (see How to illuminate the darkness inside glass objects in Cycles?):

  

  Basically, if the incoming ray is a reflected ray and/or a shadow ray (A shadow ray is a ray that hits a light source), it is sent straight through as if the glass wasn't there (via the transparent node). This allows MIS to work as it normally does, so you get a nicer result like this (there are some fireflies, but you can probably clamp them out, or combine multiple renders with different seeds, etc.):

• Use Luxrender, an open source, unbiased, spectral renderer which has Bi-directional path tracing and MLT support. It also has support for GPU rendering with OpenCL.

• Use a point lamp for doing the actual illumination, and make the bulb and filament only visible to the camera by mixing the glass or emission shader with a transparent shader based on the is camera ray output of the light path node.

Notes:

There are a few things which can prevent indirect bounces from transmitting through objects entirely:

• No Caustics enabled in Render Settings > Light paths.

• MIS disabled on lamps (does not apply for mesh lamps, see What is multiple importance sample option for more details).

Make sure No caustics is disabled and any lamps which you want to shine through or reflect off transmissive or glossy materials have MIS enabled.