diff --git a/DESCRIPTION b/DESCRIPTION
index 681be9a6..a7119cac 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -210,6 +210,7 @@ Collate:
     'trellis.factor.key.R'
     'vandermonde.R'
     'wl.R'
+    'wl_fix_unit_name.R'
     'wl_convert_units.R'
     'wl_sort.R'
     'write_txt_long.R'
diff --git a/R/wl_convert_units.R b/R/wl_convert_units.R
index 799c6fd9..266c7112 100644
--- a/R/wl_convert_units.R
+++ b/R/wl_convert_units.R
@@ -22,6 +22,8 @@
 #' @return Object of the same class as input `x`.
 #'
 #' @concept wavelengths
+#' @include wl_fix_unit_name.R
+#' @include hyperspec-class.R
 #'
 #' @export
 #'
@@ -78,69 +80,28 @@ wl_convert_units.hyperSpec <- function(x, from, to, ref_wl = NULL) {
 
 # Helper functions -----------------------------------------------------------
 
-wl_ev2freq <- function(x, ...) wl_nm2freq(wl_ev2nm(x))
-wl_ev2invcm <- function(x, ...) q * x / (100 * h * c)
-wl_ev2nm <- function(x, ...) 1e9 * h * c / (q * x)
-wl_ev2raman <- function(x, ref_wl) 1e7 / ref_wl - x * q / (100 * h * c)
-wl_freq2ev <- function(x, ...) wl_nm2ev(wl_freq2nm(x))
-wl_freq2invcm <- function(x, ...) wl_nm2invcm(wl_freq2nm(x))
-wl_freq2nm <- function(x, ...) 1e-3 * c / x
-wl_freq2raman <- function(x, ref_wl) wl_nm2raman(wl_freq2nm(x), ref_wl)
-wl_invcm2ev <- function(x, ...) 100 * x * c * h / q
-wl_invcm2freq <- function(x, ...) wl_nm2freq(wl_invcm2nm(x))
-wl_invcm2nm <- function(x, ...) 1e7 / x
+wl_ev2freq     <- function(x, ...)    wl_nm2freq(wl_ev2nm(x))
+wl_ev2invcm    <- function(x, ...)    q * x / (100 * h * c)
+wl_ev2nm       <- function(x, ...)    1e9 * h * c / (q * x)
+wl_ev2raman    <- function(x, ref_wl) 1e7 / ref_wl - x * q / (100 * h * c)
+wl_freq2ev     <- function(x, ...)    wl_nm2ev(wl_freq2nm(x))
+wl_freq2invcm  <- function(x, ...)    wl_nm2invcm(wl_freq2nm(x))
+wl_freq2nm     <- function(x, ...)    1e-3 * c / x
+wl_freq2raman  <- function(x, ref_wl) wl_nm2raman(wl_freq2nm(x), ref_wl)
+wl_invcm2ev    <- function(x, ...)    100 * x * c * h / q
+wl_invcm2freq  <- function(x, ...)    wl_nm2freq(wl_invcm2nm(x))
+wl_invcm2nm    <- function(x, ...)    1e7 / x
 wl_invcm2raman <- function(x, ref_wl) 1e7 / ref_wl - x
-wl_nm2ev <- function(x, ...) 1e9 * h * c / (q * x)
-wl_nm2freq <- function(x, ...) 1e-3 * c / x
-wl_nm2invcm <- function(x, ...) 1e7 / x
-wl_nm2raman <- function(x, ref_wl) 1e7 * (1 / ref_wl - 1 / x)
-wl_raman2ev <- function(x, ref_wl) 100 * h * c * (1e7 / ref_wl - x) / q
-wl_raman2freq <- function(x, ref_wl) wl_nm2freq(wl_raman2nm(x, ref_wl))
+wl_nm2ev       <- function(x, ...)    1e9 * h * c / (q * x)
+wl_nm2freq     <- function(x, ...)    1e-3 * c / x
+wl_nm2invcm    <- function(x, ...)    1e7 / x
+wl_nm2raman    <- function(x, ref_wl) 1e7 * (1 / ref_wl - 1 / x)
+wl_raman2ev    <- function(x, ref_wl) 100 * h * c * (1e7 / ref_wl - x) / q
+wl_raman2freq  <- function(x, ref_wl) wl_nm2freq(wl_raman2nm(x, ref_wl))
 wl_raman2invcm <- function(x, ref_wl) 1e7 / ref_wl - x
-wl_raman2nm <- function(x, ref_wl) 1e7 / (1e7 / ref_wl - x)
+wl_raman2nm    <- function(x, ref_wl) 1e7 / (1e7 / ref_wl - x)
 
 
-# Bring the argument to a conventional name
-
-# FIXME: This function should be documented.
-#        Even if it used for internal purposes.
-#
-#' @export
-.wl_fix_unit_name <- function(unit, null_ok = FALSE) {
-
-  # Allow NULL as the default value
-  if (isTRUE(null_ok)) {
-    if (is.null(unit)) {
-      return(unit)
-    }
-  }
-
-
-  unit <- gsub(" .*$", "", tolower(unit))
-  if (unit %in% c("raman", "stokes", "rel", "rel.", "relative", "rel.cm-1", "rel.cm", "rel.1/cm", "raman shift")) {
-    return("raman")
-  }
-  if (unit %in% c("invcm", "energy", "wavenumber", "cm-1", "cm^-1", "cm^{-1}", "inverted", "cm", "1/cm")) {
-    return("invcm")
-  }
-  if (unit %in% c("nm", "nanometer", "wavelength")) {
-    return("nm")
-  }
-  if (unit %in% c("ev", "electronvolt")) {
-    return("ev")
-  }
-  if (unit %in% c("freq", "frequency", "thz", "terahertz")) {
-    return("freq")
-  }
-  if (unit %in% c("pixel", "px", "sensor")) {
-    return("px")
-  }
-  if (unit == "file") {
-    return(unit)
-  }
-  stop(paste0("'", unit, "': Unknown unit type"))
-}
-
 
 # Some physical constants ----------------------------------------------------
 # @concept constants
@@ -187,23 +148,6 @@ hySpc.testthat::test(wl) <- function() {
   })
 }
 
-# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-hySpc.testthat::test(.wl_fix_unit_name) <- function() {
-  context(".wl_fix_unit_name")
-
-  test_that(".wl_fix_unit_name() works", {
-    expect_equal(.wl_fix_unit_name("raman"), "raman")
-    expect_equal(.wl_fix_unit_name("invcm"), "invcm")
-    expect_equal(.wl_fix_unit_name("nm"), "nm")
-    expect_equal(.wl_fix_unit_name("ev"), "ev")
-    expect_equal(.wl_fix_unit_name("freq"), "freq")
-    expect_equal(.wl_fix_unit_name("px"), "px")
-    expect_equal(.wl_fix_unit_name("file"), "file")
-    expect_error(.wl_fix_unit_name("ddd"), "Unknown unit type")
-  })
-
-  # TODO (tests): add more specific tests.
-}
 
 # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 hySpc.testthat::test(wl_convert_units) <- function() {
diff --git a/R/wl_fix_unit_name.R b/R/wl_fix_unit_name.R
new file mode 100644
index 00000000..ef95ab48
--- /dev/null
+++ b/R/wl_fix_unit_name.R
@@ -0,0 +1,126 @@
+#' Bring argument to a conventional name
+#'
+#' @description
+#' Fix a string with unit name (for wavelength axis) into a sting that
+#' can be used as a standardized argument value for other \pkg{hyperSpec}
+#' functions, e.g., [wl_convert_units()].
+#'
+#' **Note:** This function is usually used internally in other \pkg{hyperSpec}
+#'           functions.
+#'
+#' @param unit (sting):
+#'        A name, abbreviation or shot description of unit for wavelength axis.
+#'
+#' @param null_ok (logical):
+#'        If `unit = NULL` and `null_ok = TRUE`, the function will not fail and
+#'        will return `NULL`.
+#'
+#' @param on_failure (string):
+#'        The type of behavior in case unrecognized value of `unit` is passed
+#'        (i.e., in case of failure to convert to a standardized value):
+#' - `"fail"` -- the code is stopped with an error message.
+#' - `"warn"` -- a warning is issued and the value of `unit` is returned as
+#'               an output.
+#' - `"pass"` -- the value of `unit` is returned as an output and no error nor
+#'               warning is issued.
+#'
+#'
+#' @return
+#' The function returns one of these "standard" values:
+#' - `"raman"` for Raman shift in relative 1/cm;
+#' - `"invcm"` for inverted centimeters (1/cm);
+#' - `"nm"`    for nanometers (nm);
+#' - `"ev"`    for electron volts (eV);
+#' - `"freq"`  for frequency (THz);
+#' - `"px"`    for pixels.
+#'
+#'
+#' @seealso [wl_convert_units()]
+#' @author R. Kiselev, V. Gegzna
+#'
+#' @export
+#'
+#' @examples
+#' .wl_fix_unit_name("wavelength")
+.wl_fix_unit_name <- function(unit, null_ok = FALSE, on_failure = "fail") {
+
+  on_failure <- match.arg(tolower(on_failure), c("fail", "warn", "pass"))
+  unit0 <- unit
+  # Allow NULL as the default value
+  if (isTRUE(null_ok)) {
+    if (is.null(unit)) {
+      return(unit)
+    }
+  }
+
+
+  unit <- gsub(" .*$", "", tolower(unit)) # remove everything after space
+  unit <- gsub("[ ._]*", "", unit) # remove spaces, dots and underscores
+
+  if (unit %in% c("rel", "relative", "relcm-1",  "rel1/cm", "relcm",
+                  "raman", "ramanshift", "stokes", "stokesshift")) {
+    return("raman")
+  }
+  if (unit %in% c("1/cm", "cm-1", "cm^-1", "cm^{-1}",
+                  "invcm", "invertedcm", "inverted",
+                  "wavenumber", "wn")) {
+    return("invcm")
+  }
+  if (unit %in% c("nm", "nanometer", "wavelength")) {
+    return("nm")
+  }
+  if (unit %in% c("ev", "electronvolt")) {
+    return("ev")
+  }
+  if (unit %in% c("thz", "terahertz", "freq", "frequency")) {
+    return("freq") # FIXME: why `freq` and not `THz`?
+  }
+  if (unit %in% c("pixel", "px", "sensor")) {
+    return("px")
+  }
+  if (unit == "file") {
+    return(unit)
+  }
+
+  msg <- paste0("'", unit0, "': Unknown unit type")
+
+  switch(on_failure,
+    pass = return(unit0),
+    warn = {
+      warning(msg)
+      return(unit0)
+    },
+    fail = stop(msg)
+  )
+}
+
+
+#' # Unit tests -----------------------------------------------------------------
+
+hySpc.testthat::test(.wl_fix_unit_name) <- function() {
+  context(".wl_fix_unit_name")
+
+  test_that(".wl_fix_unit_name() works", {
+    expect_equal(.wl_fix_unit_name("raman"), "raman")
+    expect_equal(.wl_fix_unit_name("raman shift"), "raman")
+    expect_equal(.wl_fix_unit_name("raman_shift"), "raman")
+    expect_equal(.wl_fix_unit_name("raman.shift"), "raman")
+    expect_equal(.wl_fix_unit_name("invcm"), "invcm")
+    expect_equal(.wl_fix_unit_name("nm"), "nm")
+    expect_equal(.wl_fix_unit_name("ev"), "ev")
+    expect_equal(.wl_fix_unit_name("freq"), "freq")
+    expect_equal(.wl_fix_unit_name("px"), "px")
+    expect_equal(.wl_fix_unit_name("file"), "file")
+
+    expect_error(.wl_fix_unit_name("dDd"), "Unknown unit type")
+    expect_equal(.wl_fix_unit_name("dDd", on_failure = "pass"), "dDd")
+    expect_warning(.wl_fix_unit_name("dDd", on_failure = "warn"), "dDd")
+
+    expect_error(.wl_fix_unit_name(NULL), "argument is of length zero")
+    expect_silent(.wl_fix_unit_name(NULL, null_ok = TRUE))
+  })
+
+  # TODO (tests): add more specific tests.
+}
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+