# R/subt.spectra.r In photobiology: Photobiological Calculations

#### Documented in subt_spectra

#' Subtract two spectra
#'
#' The wavelength vectors of the two spectra are merged, and the missing
#' spectral values are calculated by interpolation. After this, the two spectral
#' values at each wavelength are added. This is 'parallel' operation between two
#' spectra.
#'
#' @param w.length1 numeric vector of wavelength (nm).
#' @param w.length2 numeric vector of wavelength (nm).
#' @param s.irrad1 a numeric vector of spectral values.
#' @param s.irrad2 a numeric vector of spectral values.
#' @param trim a character string with value "union" or "intersection".
#' @param na.rm a logical value, if TRUE, not the default, NAs in the input are
#'   replaced with zeros.
#'
#' @return a data frame with two numeric variables \item{w.length}{A numeric
#'   vector with the wavelengths (nm) obtained by "fusing" w.length1 and
#'   w.length2. w.length contains all the unique vales, sorted in ascending
#'   order.} \item{s.irrad}{A numeric vector with the sum of the two spectral
#'   values at each wavelength.}
#' @details If trim=="union" spectral values are calculated for the whole range
#'   of wavelengths covered by at least one of the input spectra, and missing
#'   values are set in each input spectrum to zero before addition. If
#'   trim=="intersection" then the range of wavelengths covered by both input
#'   spectra is returned, and the non-overlapping regions discarded. If
#'   w.length2==NULL, it is assumed that both spectra are measured at the same
#'   wavelengths, and a simple addition is used, ensuring fast calculation.
#' @export
#'
#' @family low-level functions operating on numeric vectors.
#'
#' @examples
#'
#' tail(zero.data)
#'
subt_spectra <-
function(w.length1, w.length2 = NULL,
bin.oper = -))