tinyusdz/doc/lte_spectral_api.md

LTE SpectralAPI Extension Proposal

Revision History

Version	Status	Date	Notes
0.9	Draft	2024	Initial proposal

Extension Name

LTESpectralAPI

Overview

This extension introduces spectral data support for USD, enabling physically-based rendering with wavelength-dependent material properties. The wavelength: namespace is reserved for all spectral attributes.

Stage/Layer Metadata

Metadata	Type	Default	Description
unitForWavelength	string	"nanometers"	Global unit for wavelength values in the USD Layer/Stage

Supported Units

• "nanometers" (nm) - Default, typical range [380, 780]
• "micrometers" (um) - Typical range [0.38, 0.78]

Attributes

The wavelength: namespace is introduced for spectral data representation.

wavelength:reflectance

Property	Value
Type	float2[]
Description	Spectral reflectance as (wavelength, reflectance) pairs

• Wavelength range: Typically [380, 780] nm (visible spectrum)
• Reflectance range: [0.0, 1.0]

Example:

float2[] wavelength:reflectance = [(450, 0.2), (550, 0.4), (650, 0.9)]

wavelength:ior

Property	Value
Type	float2[]
Description	Spectral index of refraction as (wavelength, IOR) pairs

• IOR range: Typically [1.0, 4.0]
• Fallback: When only a scalar ior value exists, it is interpreted as the IOR at wavelength 550 nm (green light)

Example:

float2[] wavelength:ior = [(450, 1.52), (550, 1.50), (650, 1.48)]

wavelength:emission

Property	Value
Type	float2[]
Description	Spectral power distribution (SPD) for light sources as (wavelength, irradiance) pairs

• Wavelength range: Typically [380, 780] nm (visible spectrum)
• Irradiance unit: W m^-2 nm^-1 (watts per square metre per nanometre) when unitForWavelength = "nanometers"
• Irradiance unit: W m^-2 um^-1 (watts per square metre per micrometre) when unitForWavelength = "micrometers"

This attribute is intended for use with UsdLux light primitives (DistantLight, RectLight, SphereLight, etc.) to define physically accurate spectral emission.

Example:

def RectLight "SpectralLight" {
    float2[] wavelength:emission = [
        (400, 0.1), (450, 0.8), (500, 1.2), (550, 1.5),
        (600, 1.3), (650, 0.9), (700, 0.4)
    ]
}

Example (D65 Illuminant approximation):

def DistantLight "Sunlight" {
    float2[] wavelength:emission = [
        (380, 49.98), (400, 82.75), (420, 93.43), (440, 104.86),
        (460, 117.01), (480, 117.41), (500, 109.35), (520, 104.79),
        (540, 104.41), (560, 100.00), (580, 95.79), (600, 90.01),
        (620, 87.70), (640, 83.29), (660, 80.03), (680, 80.21),
        (700, 82.28), (720, 78.28), (740, 69.72), (760, 71.61),
        (780, 74.35)
    ]
}

Attribute Metadata

For wavelength:reflectance

Metadata	Type	Description
unitForWavelength	string	Per-attribute wavelength unit (overrides global setting)

For wavelength:ior

Metadata	Type	Default	Description
iorInterpolation	string	"linear"	Interpolation method for IOR values

Interpolation Methods

Value	Description
"linear"	Piecewise linear interpolation (default)
"held"	USD Held interpolation (step function)
"cubic"	Piecewise cubic interpolation (smooth)
"sellmeier"	Sellmeier equation interpolation

Sellmeier Interpolation:

When iorInterpolation = "sellmeier", the IOR values are interpreted as Sellmeier coefficients:

wavelength:ior = [(B1, C1), (B2, C2), (B3, C3)]

Where C1, C2, C3 have units of [um^2]. The Sellmeier equation:

n^2(lambda) = 1 + (B1 * lambda^2) / (lambda^2 - C1)
                + (B2 * lambda^2) / (lambda^2 - C2)
                + (B3 * lambda^2) / (lambda^2 - C3)

For wavelength:emission

Metadata	Type	Default	Description
unitForWavelength	string	"nanometers"	Per-attribute wavelength unit (overrides global setting)
emissionInterpolation	string	"linear"	Interpolation method for emission values
illuminantPreset	string	none	Standard illuminant preset name (use with empty attribute value)

Standard Illuminant Presets

When illuminantPreset metadata is specified, the attribute value can be left empty. The renderer should use the built-in SPD data for the specified illuminant.

Preset	Description
"a"	CIE Standard Illuminant A (incandescent/tungsten, 2856K)
"d50"	CIE Standard Illuminant D50 (horizon daylight, 5003K)
"d65"	CIE Standard Illuminant D65 (noon daylight, 6504K)
"e"	CIE Standard Illuminant E (equal energy)
"f1"	CIE Fluorescent Illuminant F1 (daylight fluorescent)
"f2"	CIE Fluorescent Illuminant F2 (cool white fluorescent)
"f7"	CIE Fluorescent Illuminant F7 (D65 simulator)
"f11"	CIE Fluorescent Illuminant F11 (narrow-band cool white)

Example (using preset):

def DistantLight "Sunlight" (
    float2[] wavelength:emission (
        illuminantPreset = "d65"
    )
)
{
    float2[] wavelength:emission = []
}

Example (preset with intensity scale):

def RectLight "StudioLight" (
    float2[] wavelength:emission (
        illuminantPreset = "d50"
    )
)
{
    float2[] wavelength:emission = []
    float inputs:intensity = 500.0
}

When both illuminantPreset and explicit SPD values are provided, the explicit values take precedence.

Irradiance Units by Wavelength Unit

unitForWavelength	Irradiance Unit	Description
"nanometers"	W m^-2 nm^-1	Watts per square metre per nanometre
"micrometers"	W m^-2 um^-1	Watts per square metre per micrometre

Interpolation Methods

Value	Description
"linear"	Piecewise linear interpolation (default)
"held"	USD Held interpolation (step function)
"cubic"	Piecewise cubic interpolation (smooth)

For Spectral Textures (assetInfo)

Metadata	Type	Description
wavelengths	double[]	Wavelength assignment for each channel/layer in multichannel textures

Applicable to multichannel/multilayer texture formats (TIFF, EXR). This metadata is used when the image file does not contain embedded wavelength information.

Example:

asset inputs:spectralTexture = @spectral.exr@
asset inputs:spectralTexture.assetInfo = {
    double[] wavelengths = [450.0, 550.0, 650.0]
}

Future Work

• Support for spectral textures using multiple single-channel images (similar to UDIM texture patterns)
• Fluorescence support (wavelength shifting materials)
• Blackbody radiation preset with color temperature parameter

References

• CIE Standard Illuminants
• Sellmeier Equation (Wikipedia)