Understanding Physical Lights in Redshift

Understanding Physical Lights in Redshift

Lights are a critical part of any 3D scene. They illuminate the objects and materials, create shadows, reflections, and refractions, and set the overall mood and tone. In Redshift, there are many different types of lights available, giving artists a high degree of control and flexibility. One of the most versatile light types is the Redshift Physical Light.

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Understanding Physical Lights in Redshift
Snip from the Maxon Training Team’s introduction to Lighting in Redshift 

Introduction to Physical Lights

The Redshift Physical Light is designed to simulate real-world lighting as accurately as possible. It supports a range of light types including point, spot, directional, area, dome, IES, and sunlight. The parameters and behaviors are based on physical properties like intensity, color temperature, decay, and unit types. This makes Physical Lights ideal for photorealistic rendering, architectural visualization, product design, and any application where realism is critical.

Physical Lights have many advantages over simpler light types:

  • Accurate light intensity and falloff based on inverse square law
  • Area lights with diffusion for soft shadows
  • Temperature-based color controls
  • Photometric IES light profiles
  • Sunlight simulation with angle-based color shifts
  • Bidirectional lighting from area sources
  • Light is visible in both reflections and refractions.

In this in-depth guide, we'll explore how to set up and use the various features of Physical Lights to achieve realistic and physically accurate lighting in Redshift.

General parameters

The general parameters control the overall behavior of the Physical Light.


The On/Off switch simply turns the light on or off. This is useful for quickly toggling a light for comparison.

Light type

This dropdown specifies the type of light. The options are:

  • Area - Emits light evenly across a surface. Used for soft lighting.
  • Point - Omnidirectional point light source.
  • Spot - Directional cone-shaped light beam.
  • Infinite - Directional sunlight style light.
  • Dome - 360-degree environmental light.
  • Photometric IES - Real-world light profile.
  • Physical Sun - Sunlight simulation.
  • Portal - Optimizes global illumination.

Each light type has its own unique controls and behaviors. The type can be changed at any time to turn a point light into a spot for example.

Preview options

The preview options control how the light appears in the viewport for interactive feedback:

  • Wireframe - Displays light shape and falloff.
  • Illumination - Shows illuminated areas.
  • Illumination Adjustment - Controls brightness of preview.


The intensity parameters determine the overall brightness and photon emission of the light:

  • Intensity Multiplier - Scales light brightness.
  • Exposure - Sets intensity in terms of f-stops.
  • Units - Physical unit types like lumens or watts.
  • Luminous Efficacy - For converting between radiant and luminous units.
  • Decay - Falloff mode (inverse square, linear, none).
  • Falloff Start/Stop - Range for linear decay.

Light color

The color of the light can be set directly to an RGB value, or by temperature in degrees Kelvin for physically accurate colors. Lower temperature values appear more yellow/red, while higher values are blue.

Area light shapes

Area lights have the following shape options:

  • Rectangle
  • Disc
  • Sphere
  • Cylinder
  • Mesh

The shape parameters control the size, aspect ratio, and appearance of the area light. Area lights naturally create soft shadows based on their size.

Mesh lights

Any mesh can be used as an area light source. The mesh defines the shape and appearance of the area light. Mesh lights work the same as other area light types.

Spot light controls

Spot lights have directional cone-shaped beams. The main spot controls are:

  • Cone Angle - Angle of the outer edge of the beam.
  • Falloff Angle - Rate of falloff at the edge of the cone.
  • Falloff Curve - Concentrates intensity in the center of the beam.

These parameters allow detailed control over the shape and intensity profile of the spot light cone.

Shadow parameters

Shadows are a defining aspect of lighting. The shadow controls for physical lights include:

  • Enabled - Turns shadow casting on/off.
  • Transparency - Sets the opacity of shadows.
  • Softness - Diffuse edge softening of shadows.
  • Affects Gobos - Matches softness to gobo projection.

Softer more diffuse shadows create a different lighting mood than harsh sharp shadows. The transparency setting is useful for creating faded shadows as if light is passing through a translucent surface.

Light linking and groups

Physical Lights in Redshift can be assigned to light groups for compositing and rendering control via AOVs. Lights can also be linked to specific objects for selective lighting.

Light linking allows precise control over which objects receive illumination and shadows from specific lights. This is useful for example to ensure a key light only lights characters and not the background.

Controlling light contribution

The contribution parameters give you full control over how much a light influences various lighting components:

  • Diffuse
  • Reflection
  • Refraction
  • Global Illumination
  • Volumetric Scattering

This offers enormous flexibility. For example, you could create a light that provides strong diffuse illumination but does not cast any specular highlights. Or a light that only affects volumetrics.

Caustics and photons

Physical Lights can emit photons to generate caustic effects like concentrated water reflections and glass refractions.

The main caustic parameters are:

  • Enable Caustics - Turns caustic photons on/off.
  • Intensity Multiplier - Scales caustic photon intensity.
  • Number of Photons - Quantity of caustic photons.

For best results, use high-quality photon mapping with a large number of caustic photons.


There are a few legacy options that can be enabled to match the lighting in old scenes:

  • Non-Area Intensity - Old intensity model.
  • Soft Shadow Technique - Old soft shadow method.

Generally, these should be left disabled for most scenes to ensure the highest quality.


In summary, Physical Lights provide a full-featured lighting toolkit for photorealistic rendering and maximum control. The accurate simulation of real-world lighting behavior, advanced parameters, and built-in physically-based presets allow fast setup of incredibly realistic lighting. Whether matching a photograph, designing a product, or creating an imaginary world, physical lights offer the power and flexibility to bring any lighting concept to life.

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