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spectra2rgb

Convert hyperspectral reflectance signatures to sRGB hex codes and CIE Lab* colour coordinates, using the CIE 1931 2-degree standard observer and the D65 standard illuminant.

Developed for leaf-level plant phenotyping data from the PSI PlantScreen platform, but applicable to any reflectance spectra in the 400-700 nm range.

Installation

# install.packages("devtools")
devtools::install_github("yourname/spectra2rgb")

Quick start

library(spectra2rgb)

# A single spectrum
wl  <- seq(400, 700, by = 10)
ref <- 0.1 + 0.6 * exp(-((wl - 550) / 60)^2)   # synthetic green peak

hs_to_xyz(wl, ref)         # CIE XYZ tristimulus values
xyz_to_rgb(hs_to_xyz(wl, ref))  # sRGB (0-255 + hex)

Data-frame workflow

library(spectra2rgb)

# df: rows = samples, columns = wavelengths (nm), named numerically
set.seed(1)
wl  <- seq(400, 700, by = 10)
mat <- matrix(runif(5 * length(wl), 0.1, 0.9), nrow = 5)
colnames(mat) <- as.character(wl)
df  <- as.data.frame(mat)

# Convert to sRGB + CIELAB in one call
spectra_to_rgb_lab(df, id_col = paste0("leaf_", 1:5))

Missing values

Three policies via na_action:

"zero" (default) — replace non-finite reflectance with 0
"drop" — remove all-NA rows with a warning
"keep" — preserve NA in the output for downstream filtering

df2 <- df; df2[2, ] <- NA

spectra_to_rgb(df2, na_action = "zero")   # silent
spectra_to_rgb(df2, na_action = "drop")   # warns, removes row
spectra_to_rgb(df2, na_action = "keep")   # keeps NA

Functions

Function	Purpose
`hs_to_xyz(wavelengths, reflectance)`	Reflectance -> CIE XYZ
`xyz_to_rgb(XYZ)`	CIE XYZ -> sRGB (0-255 + hex)
`rgb_to_lab(R, G, B)`	sRGB -> CIELAB
`xyz_to_lab(XYZ)`	CIE XYZ -> CIELAB
`spectra_to_rgb(df, id_col)`	Data frame of spectra -> sRGB
`spectra_to_lab(df, id_col)`	Data frame of spectra -> CIELAB
`spectra_to_rgb_lab(df, id_col)`	Data frame -> both

Method

Reflectance spectra are integrated against the CIE 1931 2-degree standard observer and the D65 standard illuminant to obtain CIE XYZ tristimulus values. XYZ is transformed to linear sRGB via the IEC 61966-2-1 matrix, gamma-encoded, and quantised to 8-bit RGB. CIELAB coordinates are computed from sRGB using the D65 white point (Xn = 95.047, Yn = 100.000, Zn = 108.883).

Wavelengths outside the CIE CMF range are extrapolated flat; for visible-colour applications the standard 400-700 nm range is recommended.

Citation

If you use spectra2rgb in published work, please cite:

Elguera, J. (2026). spectra2rgb: Convert spectral signatures to sRGB and CIELAB colour coordinates. R package version 0.1.0.

License

MIT (see LICENSE).

These binaries (installable software) and packages are in development.
They may not be fully stable and should be used with caution. We make no claims about them.