lightSpectra: compute standard light spectra

In colorSpec: Color Calculations with Emphasis on Spectral Data

compute standard light spectra

Description

Two families of standard illuminants that are parameterized by temperature are the Planckian spectra (black-body spectra), and daylight spectra. For the daylight spectra, a smoothed version is available. Illuminant E, a third and trivial spectrum, is also available.

Usage

planckSpectra( temperature, wavelength=300:830, normalize=TRUE, c2=1.4388e-2 )

daylightSpectra( temperature, wavelength=NULL, 
                    components=colorSpec::daylight1964, roundMs=FALSE )
                    
referenceSpectraTM30( temperature, wavelength=380:780, ... )                    

illuminantE( energy=1, wavelength=380:780 )

Arguments

temperature	a vector of temperatures, in Kelvin
wavelength	a vector of wavelengths. For planckSpectra() and illuminantE() this is required. For daylightSpectra() this is optional. The default wavelength=NULL means to use the wavelengths in components, and otherwise components is resampled at the given wavelength vector.
normalize	a logical value. If TRUE the Planck spectra are normalized to have value 1 at 560nm. If FALSE then the quantity returned is radiant exitance with unit W * m^{-2} * nm^{-1}.
c2	the value of hc/k in Planck's law. h is the Planck constant; c is the speed of light in m/sec; and k is the Boltzmann constant. The default value of 1.4388e-2 m*K was recommended by the CIE in 2005; in 1986 the CIE recommended c2=1.438e-2. If c2='calc' then c2 is calculated directly from the 3 physical constants, as recommended by CODATA 2014.
components	a colorSpec object with the daylight components S_0, S_1, and S_2. The default is daylight1964 and a smoothed version daylight2013 is also available.
roundMs	a logical value. The original CIE method for the daylight spectra requires rounding intermediate coefficients M1 and M2 to 3 decimal places. This rounding is necessary to reproduce the tabulated values in Table T.1 of the CIE publication in References.
...	other args - components and roundMs - passed to daylightSpectra
energy	a vector of energy levels

Details

For planckSpectra() the valid range of temperatures is 0 to Inf (\infty) K, but with exceptions at the endpoints. For a negative temperature the spectrum is set to all NAs.
If temperature=0 and normalize=TRUE, the spectrum is set to all NAs. If temperature=0 and normalize=FALSE, the spectrum is set to all 0s.
Conversely, if temperature=Inf and normalize=FALSE, the spectrum is set to all NAs. If temperature=Inf and normalize=TRUE, the spectrum is set to the pointwise limit (560/\lambda)^4 (which appears blue).

For daylightSpectra() the valid range of temperatures is 4000 to 25000 K. For a temperature outside this range the spectrum is set to all NAs.

For referenceSpectraTM30() the valid range of temperatures is 0 to 25000 K. The spectrum is computed from the recipe in standard IES TM-30, see ANSI/IES TM-30-20. For a temperature \le 4000K, the reference is the Planck spectrum. For a temperature \ge 5000K, the reference is the Daylight spectrum. And for a temperature between 4000K and 5000K, the reference is a weighted blend of the Planck and Daylight spectra at that temperature.

The equations for daylightSpectra() and planckSpectra() are complex and can be found in the References.

IlluminantE() is trivial - all constant energy.

Value

For planckSpectra(), daylightSpectra(), and referenceSpectraTM30() :
a colorSpec object with quantity equal to 'energy', and organization equal to 'matrix' or 'vector'. The specnames are 'PNNNN' or 'DNNNN' for planckSpectra() and daylightSpectra() respectively. And for referenceSpectraTM30() 'blended CCT=NNNN' is used.
The number of spectra in the object is the number of temperatures = length(temperature).

For illuminantE() :
A colorSpec object with quantity equal to 'energy'.
The number of spectra in the object is the number of energy levels = length(energy).

References

Günther Wyszecki and W.S. Stiles. Color Science : Concepts and Methods, Quantitative Data and Formulae. Second Edition. Wiley-Interscience. 1982. page 146.

CIE 15: Technical Report: Colorimetry, 3rd edition. CIE 15:2004. Table T.1, pp 30-32, and Note 5 on page 69.

Schanda, Janos. CIE Colorimetry, in Colorimetry: Understanding the CIE System. Wiley Interscience. 2007. p. 42.

ANSI/IES TM-30-20. Technical Memorandum: IES Method for Evaluating Light Source Color Rendition. https://store.ies.org/product/technical-memorandum-ies-method-for-evaluating-light-source-color-rendition/

See Also

daylight, resample, organization, quantity, materialSpectra

colorSpec documentation built on June 10, 2025, 5:11 p.m.