Light-bulb Filaments: Brightness and Internal Reflections in Cycles

Question

I'm working on making an "Edison bulb" for part of a photo-realistic scene.

The filament is composed of extrusions of a very small cylinder (if that's a poor method, please correct me; I'm very new to modeling) and that seems to have an effect on how bright it can be as an emission surface - whether it's set to 40 or 40,000 it doesn't make a noticeable difference.

The problem is that I can't get the internal reflection on the bulb to show up. Increasing the brightness has no effect. I tried to fake it by duplicating the filament, scaling it up, and making it invisible to the camera but that didn't have the intended effect.

Here's a quick render of the bulb - notice the reflections from outside light sources but none from the internal filament. There's even a bit of reflection from the internal support structures. Similar renders without any other light source have the same problem.

Here's a photograph of a similar bulb with great reflections.

Thanks in advance!

Answer 1

A few things you might try:

1. The color of your background will make significant difference when trying to see the internal reflections. I recommend making the background color the same color as the photo. With a light background like you have now, any light reflecting inside the bulb will be greatly diminished in terms of visibility. Here's an example:

   

2. The shape of the bulb will play a very important role in the amount of reflection you get inside the bulb. For example:

   

   So, if you can model your lightbulb to have the exact same shape as the one in the photo, you should get similar reflections.

3. In addition, the IOR (Index Of Refraction) setting plays a large role in how much reflection is shown inside the bulb:

   With an IOR of 1.0:
   

   The same bulb with IOR of 3.0:
   

4. Yet another factor that can influence the amount of reflection is the thickness of the glass. If the bulb is a simple lightbulb-shaped mesh with a Glass material, then Blender will consider it a solid piece of glass, like a crystal ball, and this will significantly influence how light passes through it. In addition, if the bulb is a solid piece of glass, that could be one reason why there is no reflection inside the glass -- there is no inside face for the light to reflect off of. Ironically, in order to make the bulb hollow, the solution here is to add a Solidify modifier (as described below) to the bulb -- this will give the bulb mesh a very thin thickness and leave the rest hollow, thus allowing light to pass through the glass like a normal lightbulb rather than as a solid piece of glass:

   

5. The camera angle in relation to the faces of the bulb will also play a significant role in how much light reflects into the camera:

   

6. In the end, I found the following settings to give a significant improvement in the internal reflections:

   • Add a Solidify modifier to the bulb and set the Thickness to 0.01 or something similar:
     Select bulb → Object Properties panel → Modifiers tab → Solidify → Thickness → 0.01

   • Then setting the IOR on the glass bulb to 5.0 or so:
     Select bulb → Object Properties panel → Materials tab → IOR → 5.0

   • Setting the filament's emission Strength to 3000
     Select filament → Object Properties panel → Materials tab → Surface section → Strength → 3000

   • Changing the filament's color to something like the following RGB and HSV values:
     Select filament → Object Properties panel → Materials tab → Color

   

   This resulted in the following render:

   

7. The fact that the shape of the glass and also the thickness of the glass play significant roles in creating more realistic reflections made me think that in order to achieve photo-realistic results, the physical properties of the mesh models should be physically accurate representations of the objects being modeled. I'm no expert on photo-realism at all, but this just occurred to me and might help in your quest for realism.

   In this regard, you might explore setting the filament's color based on it's temperature in Kelvin with the Blackbody node in the Node Editor:

   

   I'm not sure this is necessary, though, but you might consider it.

Answer 2

After learning my way through Blender and Cycles I devised my own method for achieving my desired results (kudos to Thom for providing the first answer which guided me early on):

If the bulb is modeled properly, then there should be internal reflections but they will be very dim in a normal scene. As rendered on a gray background they are barely visible (img A) but when rendered with enough passes on a black background with no other light sources (img B), they shine brilliantly.

Use the compositor to combine a render of the bulb (img B) with a render of the rest of your scene (img C). Use a mix node set to screen and it will enhance the reflections of your final render (img D).