Hi there.
I´m in my first Project and it´s involving a lamp. I´ve made a hollow housing, with a curved glass cover. In this I´ve placed the light source that´s already visible when starting a new project. What settings for the light source do I need for it to shine through the glass and onto it´s surroundings? Do I have to do anything to the glass, beside making it transparent? It´s a street lamp and I basically want it to shine on the ground.
Thanks in advance.
BR Knogen.
that is extremely inefficient and will at least 10x render times just make it look like that emitting light then make an invisible one actually lighting up the place
I see. I want the lamp to shine in a cone downwards to a spot on the ground at night time. Is it better to let the “glass cover” emitt light or to to actually create a lamp?
Generally speaking path tracers do not do well with light sources being enclosed by other geometry, it causes a lot of noise. If the lamp is frosted white then you could put a low level emissive material on the bulb, but use a spotlight right under the bulb to cast light on the ground. You can control the spread of the light easily and the render will clean up much more quickly.
I’m not quite sure if I’m on the right track here, but perhaps the glass node setup used in this tutorial could be helpful?
In essence, the setup treats the glass as being completely transparent to the light source, but it still appears to be frosted from the camera view.
That method is a bit of a classic when it comes to getting around Cycles’ poor performance with caustic light paths. While there’s still a shader being executed when the light path hits the glass, none of the light will be reflected or absorbed, so it helps keep noise down.
Well, you don’t actually say which renderer you are using here, so let me just speak in general terms and in reference to Blender Internal (BI).
You do not want to approach this problem by trying to “emulate reality.” Instead, you want to find the computationally cheapest way possible to produce a plausible-enough result. In this case, if what you really need is a cone of light hitting the street, stick a spotlight in there, pointing straight down. Set the color and intensity of the lamp appropriately. If you also want the lamp-housing to appear to glow, consider simply changing the material so that it’s an appropriate bluish-white color and that it is set to “emit” so that it will produce its color whether it’s being hit by another lamp or not. If you want the glowing lamp to appear to light up the lamp-stand itself, how about adding another weak lamp in the housing, an omnidirectional one this time, and make it layer-specific so that it doesn’t “see” the glass lamp-housing. (Or, so that it only “sees” the thing that you want it to light-up, if you need more control.) Adjust the light levels way down, as well as the fall-off of the lamp.
Remember that, in BI at least, you can never actually see a CG “lamp.” Even if it’s pointing straight at you. You only see whatever it “illuminates.”
Did I actually have light passing through glass? Umm, no. But I’ve tried to bring-together all of the most-critical “behaviors of light” to tell your eye that this is a glowing street-lamp.
Now, you say that it’s a foggy night and that you want a “cone of light?” Well, first consider whether the shot is actually improved by that sort of thing, then look for another “trick.” Namely, compositing. Like another RenderLayer containing … a cone … and maybe you use the alpha-channel of that cone to modulate a color (which will be “the light”) with an animated noise texture (the “moving fog”) and which also attenuates the hue-and-saturation . . . but, I digress. 
The main idea that I want to plant here is that you should “look at the light.” Not how the image in front of you happens in reality, but rather, what the light is doing, there in front of your eyes. Then, go out there and look for the computationally cheapest way to “fake” that. Good enough to persuade the viewer’s already-experienced eye that this is a street lamp. And, to achieve a good, well-balanced exposure.