diff --git a/hyperSpec/R/fileio.optional.R b/hyperSpec/R/fileio.optional.R
index b83c1bdd7..ee0265a36 100644
--- a/hyperSpec/R/fileio.optional.R
+++ b/hyperSpec/R/fileio.optional.R
@@ -32,7 +32,7 @@
 ##' @include unittest.R
 .test (.fileio.optional) <- function (){
   context (".fileio.optional")
-  skip ("unittests not yet implemented.")
+  #skip ("unittests not yet implemented.")
   options.state <- .options
 
   flu [[3]] <- 0
diff --git a/hyperSpec/R/spc.NA.approx.R b/hyperSpec/R/spc.NA.approx.R
index 1e72a04e5..26631befb 100644
--- a/hyperSpec/R/spc.NA.approx.R
+++ b/hyperSpec/R/spc.NA.approx.R
@@ -1,53 +1,133 @@
 ##' Impute missing data points
 ##'
-##' Replace \code{NA}s in the spectra matrix by linear interpolation.
-##' @rdname spc.NA.linapprox
+##' Replace \code{NA}s in the spectra matrix by interpolation. With
+##' less than 4 points available linear interpolation of the 2 neighbour points is used. For larger numbers of
+##' neighbour points, smoothing interpolation is performed by
+##' \code{\link[stats]{smooth.spline}}.
+##' @note  The function has been renamed from \code{spc.NA.linapprox} to  \code{spc.NA.approx}
 ##' @param spc hyperSpec object with spectra matrix containing \code{NA}s
-##' @param neighbours how many neighbour data points should be used to fit the line
-##' @param ... ignored
+##' @param neighbours how many neighbour data points should be used to fit the
+##'   line
+##' @param w,df,spar see \code{\link[stats]{smooth.spline}}
 ##' @return hyperSpec object
 ##' @export
 ##' @author Claudia Beleites
-spc.NA.linapprox <- function (spc, neighbours = 1, ...){
+##' @examples
+##' spc.NA.approx (fluNA [,, min ~ 410], debuglevel = 1)
+##' spc.NA.approx (fluNA [1,, min ~ 410], debuglevel = 2)
+##' spc.NA.approx (fluNA [4,, min ~ 410], neighbours = 3, df = 4, debuglevel = 2)
+spc.NA.approx <- function (spc, neighbours = 1,  w = rep (1, 2 * neighbours), df = 1 + .Machine$double.eps, spar = NULL,
+                           debuglevel = hy.getOption("debuglevel")){
   chk.hy (spc)
   validObject (spc)
-  
+
   all.na <- which (apply (is.na (spc@data$spc), 1, all))
   if (length (all.na) > 0){
     warning ("Spectra containing only NAs found. They will not be approximated.")
   }
-  
+  stopifnot (neighbours >= 1L)
+
   ispc <- which (is.na (spc@data$spc), arr.ind = TRUE)
 
   ispc <- setdiff (unique (ispc[,"row"]), all.na)
 
+  if (debuglevel == 1L)
+    plot (spc [ispc], col = "gray")
+
   for (i in ispc){
+    if (debuglevel == 2L)
+      plot (spc [i], col = "gray")
+
     nas <- which (is.na (spc@data$spc[i,]))
-    
+
     start <- c (0, which (diff (nas) > 1)) + 1
-    end   <- c (start [-1] - 1, length (nas)) 
+    end   <- c (start [-1] - 1, length (nas))
 
     for (j in seq (along = start)) {
       pts <- nas [start [j]] : nas [end [j]]
 
       xneighbours <- c ( -(1 : neighbours) + nas [start [j]],
-                          (1 : neighbours) + nas [end   [j]]) 
-      xneighbours <- xneighbours [xneighbours > 0]
-      xneighbours <- xneighbours [xneighbours < nwl (spc)]
+                          (1 : neighbours) + nas [end   [j]])
+      mask <- xneighbours > 0 & xneighbours <= nwl (spc)
+      xneighbours <- xneighbours [mask]
+
+      if (sum (mask) == 0) {# should not happen as all NA-only spectra were excluded
+        stop ("No data to interpolate from.")
 
-      if (length (xneighbours) == 0) # should not happen as all NA-only spectra were excluded
-        stop ("No data to interpolate from.") 
-      else if (length (xneighbours) == 1)
+      } else if (sum (mask) == 1) {
         spc@data$spc [i, pts] <- spc@data$spc [i, xneighbours]
-      else
+
+        if (debuglevel == 2L)
+          points (x = spc@wavelength  [xneighbours], y = spc@data$spc [i, xneighbours])
+
+      } else if (sum (mask) < 4) { # old behaviour using linear interpolation
         spc@data$spc [i, pts] <- approx (x = spc@wavelength  [xneighbours],
                                          y = spc@data$spc [i, xneighbours],
                                          xout = spc@wavelength  [pts],
                                          method = "linear",
                                          rule = 2)$y
-      
+        if (debuglevel == 2L)
+          lines (x = spc@wavelength  [xneighbours], y = spc@data$spc [i, xneighbours])
+      } else {# more neighbours: interpolation spline
+        spline <- smooth.spline(x = spc@wavelength  [xneighbours],
+                                y = spc@data$spc [i, xneighbours],
+                                w = w [mask], df = df, spar = spar,
+                                cv = FALSE, all.knots = TRUE, keep.data = FALSE)
+        spc@data$spc [i, pts] <- predict (spline, x = spc@wavelength [pts])$y
+
+        if (debuglevel == 2L) {
+          wlr <- seq (from = min (spc@wavelength  [xneighbours]),
+                      to   = max (spc@wavelength  [xneighbours]),
+                      length.out = 100)
+          lines (predict (spline, wlr))
+        }
+
+      }
+
+      if (debuglevel == 2L)
+        plot (spc [i,,xneighbours, wl.index = TRUE], add = TRUE, lines.args = list (type = "p", pch = 20), col = 1)
+      if (debuglevel >= 1L)
+        plot (spc [i,,pts, wl.index = TRUE], add = TRUE, lines.args = list (type = "p", pch = 20), col = 2)
     }
   }
 
   spc
 }
+
+##' @rdname spc.NA.approx
+## TODO: remove after 2017-06-01
+spc.NA.linapprox <- function (...){
+  stop ("spc.NA.linapprox has been renamed to spc.NA.approx")
+}
+
+.test (spc.NA.approx) <- function (){
+  context ("spc.NA.approx")
+
+  test_that ("linear interpolation", {
+    tmp <- spc.NA.approx (fluNA [-2,, min ~ 410])
+    expect_equivalent(tmp [[,, 406]], rowMeans (fluNA [[-2,, 405.5 ~ 406.5]], na.rm = TRUE))
+  })
+
+  test_that ("spline interpolation", {
+    tmp <- spc.NA.approx (fluNA [-2,, min ~ 410], neighbours = 2)
+    expect_equivalent (tmp [[,, 406]], rowMeans (fluNA [[-2,, 405 ~ 407]], na.rm = TRUE),
+                       tolerance = 1e-5)
+  })
+
+  test_that ("edge treatment and debuglevel", {
+    ranges <- list (405 ~ 407, 405.5 ~ 406.5, 405.6 ~ 406)
+    for (d in 0 : 2) {
+      for (r in ranges) {
+        tmp <- spc.NA.approx (fluNA [-2,, r], neighbours = 3, debuglevel = d)
+        expect_equivalent (tmp [[,, 406]], rowMeans (fluNA [[-2,, r]], na.rm = TRUE),
+                           tolerance = 1e-5, info = paste0 ("debuglevel = ", d, "range = ", r))
+      }
+    }
+  })
+
+
+  test_that ("spc.NA.linapprox deprecated",{
+    expect_error(spc.NA.linapprox (fluNA))
+  })
+
+}
diff --git a/hyperSpec/R/unittest.R b/hyperSpec/R/unittest.R
index 7a117e62d..b890a9490 100644
--- a/hyperSpec/R/unittest.R
+++ b/hyperSpec/R/unittest.R
@@ -1,5 +1,5 @@
 ##' hyperSpec unit tests
-##' 
+##'
 ##' If \code{\link[testthat]{testthat}} is available, run the unit tests and
 ##' display the results.
 ##'
@@ -12,24 +12,24 @@
 ##' @import testthat
 ##' @export
 ##' @examples
-##' 
+##'
 ##' hy.unittest ()
-##' 
+##'
 hy.unittest <- function (){
   if (!requireNamespace("testthat", quietly=TRUE)) {
     warning("testthat required to run the unit tests.")
     return(NA)
-  } 
-  if (! "testthat" %in% search ())
+  }
+  if (! "package:testthat" %in% search ())
     attachNamespace("testthat")
-  
+
   tests <- eapply(env = getNamespace ("hyperSpec"), FUN = get.test, all.names=TRUE)
   tests <- tests [! sapply (tests, is.null)]
-  
+
   reporter <- SummaryReporter$new()
   lister <- ListReporter$new()
   reporter <- MultiReporter$new(reporters = list(reporter, lister))
-  
+
   with_reporter(reporter = reporter, start_end_reporter = TRUE, {
     for (t in seq_along(tests)){
       lister$start_file(names (tests [t]))
@@ -42,12 +42,12 @@ hy.unittest <- function (){
 }
 
 ##' @noRd
-{   
-  `.test<-` <- function (f, value) {     
-    attr (f, "test") <- value     
-    f   
-  } 
-} 
+{
+  `.test<-` <- function (f, value) {
+    attr (f, "test") <- value
+    f
+  }
+}
 
 ##' get test that is attached to object as "test" attribute
 ##' @noRd