RedSDK Rect Physical Luminance

RedSDK Rectangular Physical Light Luminance

About Rectangular Physical Light

From the help files - A Physical Rectangle light uses the definition of light sources from physical parameters issued from lamp manufacturers. A Physical Rectangle light uses a rectangle that defines the light surface that will emit light in the scene. Physical lights are rendered on the CPU using sampling and are approximated on the GPU. There are two possible approximations for a physical light on the GPU:
  • If the physical light is linked to a rectangular mesh, then the physical light is automatically turned into an Area light.
  • If the physical light is linked to a mesh with any other geometric shape, then the physical light is automatically turned into a Point light.


The rectangular Physical light has more parameters than the standard physical light, it has position and direction which can be set.


  • Power – Sets the power of the light in Watts.
  • Luminous Efficiency – Sets the luminous efficiency of the light in percentage (%).
Power is simply like changing a lightbulb for higher or lower wattage, for Luminance efficiency See internet search for full description, in TC it appears like a diffuse value, lowering the efficiency of the light produces a reduction in brightness.

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  • Intensity decay – Set the rate at which the lights power diminishes.
Not much choice here as its fixed at Natural Quadratic.

  • Color – Sets the color of the light.
Changes the light colour for specific affect, normally one would leave at a greyscale (black to white) colour, ts is possible to change it to produce other effects like serpia, below its changed to red to show the dramatic change.


  • Color Temperature – Alternate color value which supports definition of the color by temperature in Kelvin.
Instead of changing the colour , one can set the temperature of the light, however different websites use various ideas of what the temperature represents, so the image below was produced as an approximation with TC, obviously this will vary on different computer monitors. Using luminance color values - unlike using 'color', can produce a variable of colour rather than a single colour.

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  • Color Affect – Sets the relative intensity (power) of the color light.
Lowering the colour affect diffuses the colour, increasing the colur affect increases the brightness of the colour.


  • Samples Count – Sets the maximum number of light samples.
Increasing the sample may (or may not) produce a more accurate render but can increase rendering time.

  • Double Sided - sets whether the light will emit from one face or opposite faces, it is simply personal preference as to which works out best.

  • Pos – Sets the X,Y,Z position of the light relative to the object to which it is attached.
As the name suggests it moves the light away from the centre of the source object, sometimes it is necessary to move the light outside the source for the light to operate correctly, but it can also be used to position the light when the source object cannot be moved.

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  • Dir – Sets the X,Y,Z direction of the light relative to the position.
DIR uses the source object extents axes to aim the light, however because on occasions the light is automatically converted to a point type light, it doesn't direct the light in the same way as a beam or spot.

If the luminance is attached to a fixed rendered object, many times one cannot rotate the source because this would alter the way the material looks, therefore DIR allows the light direction to be altered without affecting the source.

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  • Top – Defines a second axis which is perpendicular to the direction, used for the rotation, if the lights display is not circular.
Being a second axes, Top should not be the same as DIR else RedSDK will likely throw up an error.

  • Samples Count – Sets the number of samples used to populate the light surface. Must be greater than 0

  • Width – Sets the width of the defining rectangle.
  • Height – Sets the height of the defining rectangle.
These alter the shape of the light, For a normal 'realistic' light they should be set at the same value, they can be altered for special effect,

Image Image

  • Shadows – Sets whether the light will generate shadows, or not.
  • Shadow Color – Specifies the color of shadows created by the light.
Shadows and shadow colour are use together, the colour slider is normally left black to shite, and used to make the shadow lighter or darker, changing the shadow colour would normally only be done for special effect.


  • Shadow Map - Shadow maps are generated by testing whether or not each specific pixel is visible from the light source. This is accomplished by comparing each pixel to a depth image (z-buffer) of the light source's view, stored in the form of a texture file. Shadow maps can accelerate the rendering of shadows, but usually as the cost of some quality
Shadow maps in this luminance are automatic with no user resolution settings, they can produce good images but can be unrealistic when the shadow is created behind glass,

In the image below, the shadow is too dark behing the glasses.

  • Texture File Name – Rather than using a single constant color for the whole light, you can setup a texture from which light color will be read. Using a texture, you can simulate projectors or complex light filters.
Although it can be used as to project a textured light it should not be confused with the standard definition of a 'projector, in that the projected light is a flat image of the light, it will not fold itself onto varying surfaces i.e. it is projecting the image to be used as a rectangular light source, not a wrapped image.


  • IES File Name – Manufacturers measure the behavior of their bulbs and store their directional contribution into IES files. Those files are available for most of the bulb models directly from the manufacturers' web sites. Even if IES files store information about the shape of the emitter, they are not used here.

  • Caustics – Specifies whether the light will generate caustic effects.
Caustics require a bit more thought, in that they only work if there is a material applied to an object which has its own caustics checkboxes selected, it also requires a object to catch the caustic effect which normally does not have its caustics checkbox ticked, caustics works well with GI rendering but will also work with quality raytrace.


  • Visible Geometry - Specifies whether the physical geometry of the associate 3D objects will be seen.

  • Intensity Decay - allows alteration to the quadratic equation.
To access the adjustment, click the +' sign against the luminance name in the render manager, it allows adjustment to the quadratic equation, which is trial and error as to what suits a particular scene.


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Contributors to this page: AndyUK .
Page last modified on Tuesday 07 of March, 2017 05:05:52 PST by AndyUK.