Convert Spectral Signatures to sRGB and CIELAB Colour Coordinates

Converts hyperspectral reflectance signatures to device- independent sRGB hex codes and CIE L*a*b* coordinates using the CIE 1931 2-degree standard observer and the D65 standard illuminant. Includes bundled reference data so the package is self-contained. Handles missing and non-finite reflectance values via a configurable policy.

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

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).