As with any seen you will create in every software, you'll need to light your scene to see it.
Houdini will render your scene regardless if it has lights or not, but your scene will look flat and uninteresting. Lighting can set the mood, tone, and time of day of a shot. It is important to understand that when using the light types available in Houdini.
As of Houdini 17, there are many different light types:
Point Light: It emits light like a light bulb.
Area Light: This light source spreads light over a certain area. This light type can be changed into many different shapes. Such as a tube,disk,sphere,or grid.
Spot Light: Is a cone light source that you can transform to face any direction.
Geometry Light: Creates light from the selected geometry. Light will emit from this object into the rest of the scene. It uses the selected object's shader to color the created light.
Volume Light: Somewhat similar to the Geometry Light, where it will connect your geometry to your light source. However, this light will copy any volume shaders your geo has, and make sure the light matches the shader. This light source also creates a point cloud, so your render will be faster. This light is best for creations such as nebulae.
Distant Light: Acts like sun beams. Good for simulating the sun.
Environment Light: This source will simulate a globe of light all around your scene. This is also where you will plug in your HDRIs/Environment maps. Houdini comes with four free HDRIs that are ready to use in the drop down folder menu.
Sky Light: This node creates a sky environment around your scene, and uses it as a light source.
GI Light: This is your Global Illumination Light. This node uses Photon maps to light a scene. It is best used for lighting scenes that are indoor room that require no environment light, and have no windows or doors.
Caustics Light: This light is used for lighting indoor scenes , such as inside a house or a room. This light once placed in a scene, will slow down your render time, as Houdini will need to calculate the caustics before rendering the scene.
Portal Light: This node simulates light coming in from a door or a window.
Ambient Light: This node radiates light all around your scene.
Now we can move onto understanding some tips and tricks we use while lighting our simulations in Houdini.
Point Light Tips
You can use this light source like a center piece light. In case you would like to draw the viewer's attention to a particular place.
As we've mentioned before, this light source mimics a light bulb. Therefore, you have the freedom to use it for almost anything that emits a source of light.
This is the most time and render efficient light in the Houdini system. Therefore, you can use a lot of them. However, always keep in mind the what the require lighting setup your scene needs to be.
Ambient Light Tips
A good lighting setup for a scene will have a key light, a bounce light, a fill light, and an environment light. You can use the Ambient light as a fill light for your scene.
Since it is always a good idea to have a source of indirect lighting in your scene, an Ambient light might work well for this.
GI Light Tricks
Since this node only generates photon maps, the GI light may not appear to work if there are no other light sources in your scene. The photon maps require other lights to be present in order to work.
You can use this node for basement scenes, caves, tunnels, etc.
This node is similar to the Environment Light as the two nodes are practically identical when created. However the GI light will turn off the add background light parameter, and turn on the photon maps instead.
Distant Light Tricks
In case you have a computer that has outdated specs, or is slow at rendering. Sometimes you can create a faster render time by using two distant lights to fake an environment light. By turning one slightly yellow, and the other slightly blue. However, using an environment light is still preferred.
Area Light Tips
Area lights are very system heavy in Houdini, so if you are using them. Make sure they have a good purpose in your scene.
Area Lights are great with PBR. (Physically Based Rendering)
When an area light is a sphere shape, an object inside that sphere will have softer shadows and a greater GI look.
If you have noisy shadows, you can place a small area light under your object, and that will help.