In aphalo/photobiologyFilters: Spectral Transmittance and Spectral Reflectance Data

knitr::opts_chunk$set(fig.width=8, fig.height=4)

Introduction

This package, is a data only package, part of a suite, which has package 'photobiology' at its core. Please visit (https://www.r4photobiology.info/) for additional information. For details on plotting spectra, please consult the documentation for package 'ggspectra', and for information on the calculation of summaries and maths operations between spectra, please, consult the documentation for package 'photobiology'.

library(photobiology)
library(photobiologyWavebands)
library(photobiologyFilters)
library(ggspectra)
theme_set(theme_bw())

In this brief User Guide we describe how to access individual spectra or subsets of spectra.

The objects containing the data

The data are stored in objects of classes defined in package 'photobiology' grouped into four collections of spectra. Functions and operators in the packages that are part of the R for photobiology suite recognize these classes, which allows automation of some computations and validation of the data values returned by operations on the data.

Individual spectra are stored in clases are derived from tibbles, which in turn is derived from data frames with additional metadata stored as attributes of the objects. Being the objects derived from data.frame means that the data can be used with native R functions and operators and with other R packages. As package 'photobiologyInOut' provides functions that facilitate the exchange of spectral data with R packages 'colorSpec', 'hyperSpec' and 'pavo', the data in these package can also be easily used with them.

The individual spectra are stored in collections as objects of classes derived from R's list class.

filters.mspct contains spectral transmittance data for r length(filters.mspct) optical filters and other similar transparent and semitransparent sheets and films.

class(filters.mspct)

class(filters.mspct[[1]])

metals.mspct contains spectral reflectance data for r length(metals.mspct) metals.

class(metals.mspct)

class(metals.mspct[[1]])

materials.mspct contains spectral reflectance data for r length(metals.mspct) materials and surfaces, including different types vegetation.

class(materials.mspct)

class(materials.mspct[[1]])

refractive_index.mspct contains spectral refractive index data for r length(metals.mspct) materials. These values allow the computation of reflectance at different angles of incidence.

class(refractive_index.mspct)

class(refractive_index.mspct[[1]])

Spectral reflectance and transmittance values are stored as fractions of one, and wavelengths expressed in nanometres (nm).

colnames(filters.mspct[[1]])

colnames(metals.mspct[[1]])

colnames(materials.mspct[[1]])

Refractive index values are:

colnames(refractive_index.mspct[[1]])

The transmittance and reflectance data

Spectral data are included both for optical filters, sold as such, and for materials that either on purpose or by accident may be interposed in the "light" path in photobiological experiments, including glass panes, plastic sheets and films. It must be kept in mind that, 1) absorptance depends on the thickness of a filter in addition to the properties of the material it is made off, and that 2) reflectance depends on the angle of incidence of the light beam. Spectral transmittance can be expressed either as internal or total, depending on how the effect reflections at surfaces are accounted for. The conversion between total and internal transmittance requires reflectance to be known.

We provide reflectance estimates for several of the filters in this collection, but we do not have such data available for all of them. For filters made of ionic glass and coloured plastics, reflection is very weakly selective, and amounts to about 9 to 10\% of radiation incident at an angle close to perpendicular to the surface (when both interfaces with air are considered). Anti-reflection coating (ARC) reduces reflections, and multi-coating (MC) even further. Such coatings are not equally effective at all wavelengths and, consequently, their use can modify the spectral properties of a filter. In contrast to absorptive filters described above, dichroic or interference filters reflect the "rejected" radiation. It is also possible to produce filters that have an absorbing glass as substrate and a dichroic coating deposited onto one or both of its surfaces. In the metadata filters are tagged as belonging to one of three attenuation.mode types: absorption, reflection and mixed. Mixed includes those filters where wavelength-selective attenuation is brought about both by absorption and reflection, including scattering media and dichroic filters deposited on absorptive ionic glass.

The spectral transmittance or absorbance data have been acquired with an assortment of different instruments. Some data are measured by the authors with spectrophotometers, others have been provided by filter manufacturers. Whenever these metadata were available an approximate reflectance for normal incidence angle, material thickness, and the mode of attenuation have been stored as attributes. In the case of filters sold assembled in frames for use in photography and industrial automation the thickness is rarely disclosed by suppliers, but in some cases we have been able to measure it.

For absortive filters with no coatings reflectance was computed from the spectral refraction index if available, or copied from manufacturer's specifications or for some standard materials based on published sources. In the case of filters with anti-reflection coatings, in most cases the estimated reflectance is based on the maximum total transmittance. Most of the data are approximations given as a single value for the whole spectrum following Schott's practice. (We aim to add in the future actual measured and/or computed spectral reflectance for some filters or clear glass to serve as examples).

The difference in resolution and slit function among instruments can give, for the same filter, measured spectra with "apparent" peaks and valleys of slightly different width, and slopes of slightly different steepness. This is an inevitable artifact of spectral measurements, which has little effect on the spectra from absorptive filters (with gradual slopes and bell-shaped peaks) but can be more significant for dichroic (= interference) filters with very steep cut-on and cut-off slopes.

Another important consideration is that some materials scatter transmitted and reflected light, and consequently such materials can be accurately measured only with an integrating sphere. Data included here have been in many cases measured without an integrating sphere; i.e., only by assessing the direct beam. For non-scattering materials this causes only minor errors. For scattering materials like polythene films, the data included have being measured with an integrating sphere.

Glass-filter properties vary to some extent among melt batches. Variation can also be expected among batches of plastic filters. Furthermore, filters age upon exposure to light and UV radiation, and in some cases even upon exposure to air. Aging is not limited to plastic filters and can also affect optical glass. Manufacturers occasionally may update the specifications for filters while maintaining the product code.

All the spectral transmittance data in this package are stored in a single R object, a collection of spectra of class filter_mspct with members of class filter_spct. Individual or subsets of spectra can be retrieved by name. The package includes also several character vectors of names, each one containing names for filters of a given color, a given type or from a given manufacturer. The names of all these vectors are available in vector all_filter_selectors. The names used are in most cases the codes used by the manufacturers for the given type with a code for the manufacturer of supplier prepended. Any dashes in these codes have been replaced by underscores to convert them into syntactically valid R names.

band_pass_filters

schott_filters

Help pages give additional information.

help(schott_filters)

In addition to these data for filters, the package contains spectral reflectance data for both metal surfaces and various man-made and natural surfaces. All spectral reflectance data in the package are for normally incident radiation and total values, that is including both especular reflection and scattering.

These collections are small, so we list the names of all members.

names(metals.mspct)

names(materials.mspct)

names(refractive_index.mspct)

The collection of filter spectra is large, so we list only the first 20 names.

head(names(filters.mspct), 20)

To obtain metadata in a tabular form, we can use function spct_metadata(). This function is available in package 'photobiology' version 0.10.5 or later. (This example is run only if a supported version is loaded.) The limit the number of columns we select two of them by name.

if (utils::packageVersion('photobiology') > "0.10.4.9001") {
  print(colnames(spct_metadata(filters.mspct[1:5])))
  spct_metadata(filters.mspct[1:5])[ , c("spct.idx", "what.measured")]
}

Accessing individual spectra

The filter_spct member objects in filters.mspct and the reflector_spct objects contained in metals.mspct and materials.mspct can be accessed through their names or through a numeric index. As the numeric indexes are likely to change with updates to the package, their use is discouraged. Names should be used instead. The names are listed in the documentation and be listed with method names(). All examples below use filters.mspct but operations on the other collections work in the same way.

We can use a character string with the name as an index to extract an individual spectrum.

filters.mspct$Schott_UG11

filters.mspct[["Schott_UG11"]]

Be aware that according to R's rules, using single square brackets will return a filter_mspct object possibly of length one. This statement is not equivalent to the one in the chunk immediately above.

filters.mspct["Schott_UG11"]

Of course, with single square brackets it is possible to use a vector of member names.

Accessing subsets of spectra

We can subset the filter_mspct object by indexing with vectors of character strings. The package provides several predefined ones, and users can easily define their own, either as constants or through computation. Here we use a vector defined by the package.

filters.mspct[petri_dishes]

The vector all_filter_selectors contains the names of the different vectors of names of members of filters.mspct.

all_filter_selectors

In addition to the predefined vectors it is possible to compute numeric indexing vectors using pattern matching with grep(). In this example we extract the member spectra with names containing the string "UG".

filters.mspct[grep("UG", names(filters.mspct))]

To generate the subset of names matching a pattern, we can also use grep().

grep("UG", names(filters.mspct), value = TRUE)

Querying metadata

The spectra are saved in objects of class "filter_spct", defined in package 'photobiology'. Specializations of several methods including print() and summary() include a summary of the metadata in the header of the printout. Two different definitions of transmittance exist, differing in how reflection is treated: for "internal" transmittance, the divisor is the radiation entering the material, and for "total" transmittance the incident radiation. For some materials reflectance (Rfr) does not vary much with wavelength, and some suppliers provide a constant value for it.

filters.mspct$Schott_UG11

Metadata can also be queried with other methods. Please, see the documentation for package 'photobiology' for the details.

filter_properties(filters.mspct$Schott_UG11)

what_measured(filters.mspct$Schott_UG11)

how_measured(filters.mspct$Schott_UG11)

is_normalized(filters.mspct$Schoot_UG11)

cat(comment(filters.mspct$Schott_UG11), "\n")

Of the different metadata items, the type of data is of great importance. Transmittance (and absorptance) can be expressed in two different ways: 1) taking incident radiation as reference, or 2) taking radiation entering the material as reference (i.e. discounting reflection). 1) is referred as total transmittance while 2) is referred as internal transmittance. Attribute `Tfr.type" is used to store this information.

getTfrType(filters.mspct$Schott_UG11)

Conversions

When metatdata are available and the mode of attenuation is "absorption" it is possible to compute the expected transmittance for a different thickness of the material. In the example we compute transmittance for a thickness of 4 mm.

convertThickness(filters.mspct$Schott_UG11, thickness = 4e-3)

We can also convert "internal" transmittance into "total" transmittance.

convertTfrType(filters.mspct$Schott_UG11, Tfr.type = "total")

Conversion between transmittance, absorptance and absorbance is also possible. In package 'photobiology' fractions of one are used to express transmittance, reflectance and absorptance.

any2Afr(filters.mspct$Schott_UG11, action = "replace")

For expressing absorbance, base-10 logarithms are used. In some fields, natural logarithms are used instead. Expressing A on this base is not supported by this package and any input absorbances must be first converted to log10 based A.

any2A(filters.mspct$Schott_UG11, action = "replace")

Plotting

Spectra can be plotted in the same ways as other data stored in data frames, using base R graphics, package 'lattice' or 'ggplot2'. However, another package in our suite, 'ggspectra', built as an extension to 'ggplot2' makes plotting spectra extremely easy.

autoplot() methods use the metadata in the objects to set labels and decorations, as well as automatically setting the mapping of the x and y aesthetics.

autoplot(filters.mspct$MIDOPT_LP500)

autoplot(filters.mspct$MIDOPT_LP500,
         annotations = list(c("-", "peaks"), c("+", "wls")))

autoplot(filters.mspct$MIDOPT_TB550_660_850, 
         annotations = c("+", "title:none:none:what", "wls"), 
         w.band = VIS_bands(),
         range = c(500, 910),
         span = 11)

To graphically compare filters, we can pass a collection of spectral objects, such as subset of filters.mspct.

autoplot(filters.mspct[c("Schott_UG1", "Schott_UG11")], 
         range = c(200, 900),
         annotations = c("+", "boundaries"), 
         span = 11)

To graphichaly compare filter thicknesses we can pass a collection of spectral objects.

thin_and_thick.mspct <- 
  filter_mspct(list("1 mm" = filters.mspct$Schott_UG11,
                    "3 mm" = convertThickness(filters.mspct$Schott_UG11,
                                              thickness = 3e-3)))
autoplot(thin_and_thick.mspct,
         range = c(200, 900),
         annotations = c("+", "boundaries"),
         span = 101)

To graphically assess filter stacks with air gaps.

stack.spct <- filters.mspct$Haida_Clear_Night_NanoPro_1.6mm_52mm * filters.mspct$Firecrest_UVIR_Cut_0.96mm_52mm

autoplot(stack.spct,
         range = c(NA, 1400),
         w.band = c(UV_bands(), IR_bands("CIE")),
         annotations = list(c("+", "boundaries"), c("-", "peaks")),
         span = 21) +
  geom_line(data = filters.mspct$Haida_Clear_Night_NanoPro_1.6mm_52mm, colour = "purple") +
  geom_vline(xintercept = c(589, 589.6), linetype = "dotted") # Na emission lines

Package 'ggspectra' also defines specializations of method ggplot() for spectra that automatically maps the $x$ and $y$ aesthetics.

ggplot(filters.mspct$Firecrest_UVIR_Cut_0.96mm_52mm) +
  geom_line() +
  scale_x_wl_continuous() +
  scale_y_Tfr_total_continuous()

autoplot(metals.mspct$gold, range = c(NA, 800))

autoplot(materials.mspct$grass, range = c(NA, 800))

Calculating summaries

Here we use waveband defintions from package 'photobiologyWavebands' and summary functions from package 'photobiology'.

transmittance(filters.mspct$Firecrest_UVIR_Cut_0.96mm_52mm, UVA())

absorbance(filters.mspct$Firecrest_UVIR_Cut_0.96mm_52mm, list(UVA(), NIR()))

transmittance(filters.mspct[grep("UG", names(filters.mspct))], 
              list(UVB(), UVA()))

reflectance(metals.mspct, w.band = VIS_bands())

Using the data in other contexts

As filter_mspct is a class derived from list, and filter_spct is derived from tibble::tible which is a mostly compatible reimplementation of data.frame the data can be used very easily with any R function.

head(as.data.frame(filters.mspct$Schott_UG11))

Of course attach and with also work as expected.

attach(filters.mspct)
transmittance(Schott_UG11, UVA())
detach(filters.mspct)

attach(filters.mspct)
with(Schott_UG11, range(w.length))
detach(filters.mspct)

with(filters.mspct, transmittance(Schott_UG11, UVA()))

aphalo/photobiologyFilters documentation built on Feb. 28, 2024, 12:05 a.m.