The i1pro Hi Res. Mode
Author: Graeme Gill

## Does the i1pro High Resolution mode improve accuracy ?

A question that has been asked is : "You've extended the Eye-One Pro with a high resolution spectral mode, giving readings at 3.3nm spacing rather than the default 10nm. Does this mode improve accuracy ?"
This is a quite reasonable question. The following attempts to answer it.

#### Why would a higher resolution spectral mode improve accuracy ?

A spectrometer computes CIE tri-stimulus values by measuring spectral values and then weighing those values by the observer curves before summing the weigted values. The accuracy depends on the correct weighting being applied at each wavelength. If the color is composed of very narrow spectra peaks, as is sometimes the case for certain light sources and many display devices, then the exact positioning of one of the peaks on the observer curves may be influencial in the final color value, and too coarse a quanization of the spectral readings may lead to tri-stimulus errors. So in theory increasing the spectral reading resolution to 3.3 nm should lead to improved color accuracy with narrow spectra color sources.

#### Why may this not work in practice ?

The instrument spectral resolving power is set by a number of factors, and a critical one is the entrance slit width. By measuring a very narrow band source such a as a laser, using the default 10nm resolution indicates a FWHM (Full width at half maximum) of about 25nm. Doing a measurement at 3.3nm resolution reveals that the optical limit seems to be about 15nm, so there is some hope of improvement from that perspective.

Another factor is that the calibration data for the instrument is only given at 10nm intervals. So to produce calibrated readings at 3.3nm intervals, it is necessary to up-sample the calibration data with sufficient accuracy. If the calibration data is sufficiently smooth (indicating that the underlying device characteristics are also smooth), or any slight inaccuracy will get calibrated out (which is typically the case for reflective measurements) then this may not be a limitation either. In the case of the i1pro2, which seems to have a diffraction grating/light sensor with a less smooth spectral efficiency curve than the Rev A - D models, the task of up-sampling the emissive calibration data with sufficient accuracy is a more difficult.

#### The verification experiment

To give some indication of whether ArgyllCMS's high resolution spectral mode is capable of improving color measurement accuracy, or at least to indicate that it doesn't noticeably worsen it, the following fairly simple, real world experiment was performed:
A measurement target consisting of white + primary + secondary colors (White, Red, Green, Blue, Cyan, Magenta, Yellow) repeated 10 times was used. This target was displayed on a conventional LCD screen with a CCFL backlight (MacBook display), and measured using using ArgyllCMS V1.6.0 dispread:
1) Using a JETI specbos 1211 reference Tele-Spectro-Radiometer.
2) Using an i1pro2 in standard 10nm mode.
3) Using an i1pro2 in ArgyllCMS 3.3nm mode.
The resulting readings were then analyzed using verify.
The results were analyzed two ways, first in absolute value error terms, and secondly in brightness (Y) normalized terms, the latter corresponding to the typical way such readings are used for display calibration and profiling.
A second, similar experiment was run on a CRT type display.

#### Results:

LCD display:

Absolute errors of i1pro2 10nm mode to specbos 1211:

Total errors (CIEDE2000): peak = 3.070420, avg = 2.204137
Absolute errors of i1pro2 3.3nm mode to specbos 1211:
Total errors (CIEDE2000): peak = 2.108411, avg = 1.568577
White Y normalised errors of i1pro2 10nm mode to specbos 1211:
Total errors (CIEDE2000): peak = 2.419800, avg = 0.747926
White Y normalised errors of i1pro2 3.3nm mode to specbos 1211:
Total errors (CIEDE2000): peak = 1.595033, avg = 0.578270
So in this particular situation, hi-res mode improves accuracy by somewhere between 0.2 and 0.6 DeltaE 2K.
Example of white spectrum for the three measurements (red: 10nm i1pro2, green: 3.3nm i1pro2, black: specbos):

CRT display:

Absolute errors of i1pro2 10nm mode to specbos 1211:

Total errors (CIEDE2000): peak = 1.516886, avg = 0.965740
Absolute errors of i1pro2 3.3nm mode to specbos 1211:
Total errors (CIEDE2000): peak = 1.751776, avg = 0.887878
White Y normalised errors of i1pro2 10nm mode to specbos 1211:
Total errors (CIEDE2000): peak = 1.509129, avg = 0.654752
White Y normalised errors of i1pro2 3.3nm mode to specbos 1211:
Total errors (CIEDE2000): peak = 1.284044, avg = 0.622501

#### Conclusions:

The results for the conditions of this particular experiment indicate that ArgyllCMS High Resolution mode can very slightly improve colorimetric measurement accuracy of display devices. Accuracy may conceivably be improved a little more than indicated by this experiment for i1pro rev A-D instruments which have a smoother diffraction grating/light sensor characteristic, or it is also conceivable that an unfortunate combination of display spectra and the i1pro2 may result in reduced accuracy. The High Resolution mode is primarily useful for showing more spectral detail, and should probably not be used for colorimetric measurement when the highest possible robustness and reliability is desired. The potential for improved accuracy may be of benefit in other situations though.

#### Raw Data:

The raw measurement data is available in this .ti3 archive.