Use an instrument or instruments to measure an illuminant spectrum, including estimate its Ultra Violet content. A combination of direct illumination readings and readings from a piece of paper having some FWA content are used for this. (If the UV content is not needed, or a suitable instrument is not available, then spotread should be used instead.)

Usage Summary

illumread [-options] illuminant.sp
 -v                    Verbose mode

 -S                    Plot the readings in a graph window.
 -c comport            Set COM port, 1..4 (default 1)

 -N                    Disable initial calibration of instrument if possible
 -H                    Use high resolution spectrum mode (if available)
 -Y r                  Set refresh measurement mode
 -W n|h|x              Override serial port flow control: n = none, h = HW, x = Xon/Xoff
 -T                    Test mode - restore & save measurements to
                        *_i.sp, *_r.sp, *_p.sp, *_mpir.sp, *_cpir.sp files
 -D [level]            Print debug diagnostics to stderr
  illuminant.sp         File to save measurement to

Usage Details and Discussion

illumread uses a suitable instrument to read an illuminant spectrum, and uses an indirect method to estimate the Ultra Violet content of the illuminant, so as to provide better accuracy with FWA compensation. An instrument or combination of instruments capable of spectral measurement of both emissive measurement and reflective measurement without a U.V. filter is required for this.

The -v flag causes extra information to be printed out during chartread operation.

The -S flag enables the plotting of the spectral reflectance/transmittance values. You must select the plot window and strike a key in it to continue with another measurement.

The instrument is assumed to communicate through a USB or serial communication port, and the initial port can be selected with the -c option, if the instrument is not connected to the first port. If you invoke illumread so as to display the usage information (i.e. "illumread -?" or "illumread --"), then the discovered USB and serial ports will be listed. On UNIX/Linux, a list of all possible serial ports are shown, but not all of them may actually be present on your system.

-N Any instrument that requires regular calibration will ask for calibration on initial start-up. Sometimes this can be awkward if the instrument is being mounted in some sort of measuring jig, or annoying if several sets of readings are being taken in quick succession. The -N suppresses this initial calibration if a valid and not timed out previous calibration is recorded in the instrument or on the host computer. It is advisable to only use this option on the second and subsequent measurements in a single session.

The -H option turns on high resolution spectral mode, if the instrument supports it. See Operation of particular instruments for more details.

The -Y r option turns on refresh mode measurement, if the instrument supports it. This may improve the repeatability of measurements of illuminants that have a repetitive flicker.

The -W n|h|x parameter overrides the default serial communications flow control setting. The value n turns all flow control off, h sets hardware handshaking, and x sets Xon/Xoff handshaking. This commend may be useful in workaround serial communications issues with some systems and cables.

The -T flag invokes the test mode. In test mode the three measurements are saved to files illuminant_i.sp (Illuminant spectrum), illuminant_r.sp (Illuminant off paper spectrum), and illuminant_p.sp (Instrument measured paper reflectance spectrum), and these will be loaded if discovered, allowing a replay of the calculation without requiring any measurement. In addition, two diagnostic files illuminant_mpir.sp (Measured paper under illuminant spectrum) and illuminant_cpir.sp (Computed paper under illuminant spectrum) will be saved.

The -D flag causes communications and other instrument diagnostics to be printed to stdout. A level can be set between 1 .. 9, that may give progressively more verbose information, depending on the instrument. This can be useful in tracking down why an instrument can't connect.

The illuminant.sp is the name of the file to save the resulting illuminant spectrum to. The format used is .sp.

Unlike the other measurement utilities, illumread doesn't connect to the instrument until it is about to make a measurement. This allows for the possibility of using a different instrument for each measurement.

It will display a menu:

Press 1 .. 6
1) Measure direct illuminant
2) Measure illuminant reflected from paper
3) Measure paper
4) Select another instrument, Currently 1 'usb:/bus4/dev2/ (GretagMacbeth i1 Pro)'
5) Compute illuminant spectrum, average result with 0 previous readings & save it
6) Compute illuminant spectrum from this reading & save result
7) Exit

There are three measurements to be made, after which the illuminant can be computed and saved. Before each measurement, the instrument may need calibrating.

The first measurement needs a spectral instrument capable of reading in an ambient or emissive mode. For instance, a Spectrolino, Eye-One Pro or ColorMunki would be suitable instruments.

The second measurement needs a spectral instrument capable of reading in an projector or emissive mode. For instance, a Spectrolino, Eye-One Pro or ColorMunki would be suitable instruments.

The third measurement needs a spectral instrument capable of reading in reflective mode with UV included. For instance, a Spectrolino, Eye-One Pro, DTP20, DTP22 or  DTP41 would be suitable instruments, as long as they are not fitted with UV filters.

To be able to estimate the level of Ultra Violet (UV) light in the illuminant, a reasonable sized piece of white paper needs to be used. The paper should have some noticeable level of FWA (Fluorescent Whitener Additive, or Optical Brightening Agents) in it, so that it responds to UV light. A piece of cheap copier paper is ideal, since cheap paper is typically whitened with large amounts of FWA. If the paper is thin (less than 160 gsm) then two or three sheets should be used to prevent any background showing through. If the intention is to use the illuminant spectrum for proofing to a particular paper, then it's best to use the intended paper for this purpose.

The first measurement 1), is to use either the ambient or emissive measurement mode to measure the illumination directly.

If the instrument supports an ambient measurement capability, then it will be used. If the insrument does not have an ambient mode, then an emissive measurement mode can be used, although typically many illuminants are too bright to directly point the instrument at. A work-around is to reflect the illuminant from a spectrally flat white surface. A good candidate for this is a piece of white, fine textured polystyrene foam, or piece of white Tyvec. [The suitability of a reflector can be checked using spotread -S to check that the reflection characteristic is close to flat.]

Measuring Ambient Measuring Ambient  Measuring Ambient

The second measurement 2), is to measure the illuminant after it has reflected from the paper.

This is done by placing the paper such that it is uniformly illuminated with reasonable brightness, and then placing the instrument so that it receives the reflected light from the paper. This is typically achieved by placing the instrument close to the paper at about 45, so that it's aperture has a clear view of the illuminated paper, but avoiding shadowing the region that is in view, and also avoiding measuring specular reflection if the paper is glossy.

Measuring via
        PaperMeasuring via Paper

The third measurement 3), is to measure the paper directly using the instrument reflective mode measurement.
Measuring Paper
If a different instrument is needed, use 4) to select from the available instruments attached to your computer.

Once these three measurements have been made, then the illuminant readings spectrum can be computed and save using 6), or a series of readings can be made with each reading being averages with the previous readings before saving it by using 5). Note that the averaged readings will be weighted by their absolute intensities, and that while the direct and indirect illumination needs measuring for each reading, the same paper measurement can be used each time.

If plotting is enabled, a plot of the measured (black) and with estimated UV (red) is plotted. This is followed by a plot showing measured paper reflectance (black) and the FWA calculated paper reflectance (red).

Illumread can then be terminated using 7).


If For a full description of the approach that illumread uses, see:

"Estimating illuminant U.V. without a U.V. capable instrument",
Graeme W. Gill, IS&T 19th Color and Imaging Conference Final Program and Proceedings, pp. 58-60(3)