Advances in Color Measurement
David R. Battle, Harry Oana and Colman Shannon*, Datacolor International
A high-performance, dual-beam spectral analyzer, the SP2000, has been developed based on active-pixel complementary-metal-oxide-semiconductor (CMOS) technology. Each of the two diode arrays contain 256 pixels. The small size of the analyzer allows it to be used in portable spectrophotometers. A wavelength range of from 360 to 780 nm has been achieved with a wavelength resolution of 1.8 nm and a signal-to-noise ratio of 85 dB. Wavelength calibration is done separately from the instrument using eight narrow-band spectral lines. Fiber optics are used to connect the spectral analyzer with the instrument so that areas as small as 3 mm can be accurately measured.
Until recently, the spectral analyzers (spectrometers) in most high-grade color measurement spectrophotometers used discrete diode arrays with separate amplifiers and processing electronics in order to get the required signal-to-noise performance. This led to large arrays, 5 mm wide by 40 mm long with 40 diodes, that in turn led to large spectral analyzers (75 x 75 x 150 mm) and large color-measuring instruments. In addition, complex mechanical alignment techniques were required to obtain satisfactory inter-instrument agreement.
Several years ago Datacolor introduced the MC-90, a high performance, miniature, dual-channel spectrometer based on active-pixel complementary-metal-oxide-semiconductor (CMOS) technology. 1 The MC-90 was significantly smaller (40 mm diameter by 60 mm long with 128 pixels) than previous spectrometers and could fit into a portable color-measuring instrument.
For the last two years, Datacolor has been developing the SP2000. Similar in design to the MC-90, the SP2000 replaces the MC-90’s 128 pixel array with a CMOS diode array containing 256 pixels. The SP2000’s 256 pixels result in improved signal-to-noise performance, an increased wavelength range and an increased wavelength resolution. Wavelength calibration techniques are capable of achieving an inter-instrument agreement of 0.1 CIELAB units. In addition, the measurement array and the reference array are both integrated into the CMOS chip eliminating the need for a mechanical alignment.
2. DESCRIPTION OF THE SPECTROMETER
The SP2000 (figure 1) consists of three main parts: the grating, the cylinder and the sensor plate. An aberration-corrected, concave grating combined with the 256 pixel diode array allows a wavelength range of from 360 to 780 nm covering the range recommended by the CIE2 for most color-measuring applications. The 380 to 780 nm wavelength range conforms to illuminant/observer weighting tables listed in ASTM E-3083. Extending the wavelength range over the MC-90 also leads to improved instrumental discrimination among bright red colors. The spherical surface of the grating sits on a stainless steel cylinder with a tightly controlled outer diameter. A sensor plate is mounted at the other end of the cylinder. The slits and CMOS arrays are mounted in the same plane on the sensor plate. The 256 pixel CMOS array can achieve a wavelength resolution of 1.8 nm. Individual measurement and reference detector arrays are used in the MC-90. These must be mechanically aligned during spectrometer assembly. A single, integrated chip in the SP2000 contains both arrays. Thus, alignment is part of the chip design. Each array is only 1 mm by 6.5 mm. The zero and minus first order reflections from the grating are blocked by blackened baffles. Infrared reflections are blocked with filters in the pickup optics. Second order diffraction from the grating is corrected with the instrumental firmware. The SP2000 spectrometer has excellent light collection capabilities (f/1.6) and a more consistent band pass than previous analyzers. A single-to-noise ratio of 85 dB has been obtained. Fiber optics are used to interface the SP2000 with the instrument and can measure an area as small as 3 mm.
Spectrometers based upon detector arrays are preferred for portable instruments since they do not require any moving parts and are quite small. Charge-coupled device (CCD) technology is commonly used for this and similar applications. The CMOS detectors chosen for the MC-90 and SP-2000 have three major advantages over the CCD detectors. Perhaps the major advantage is the ability to integrate the photo diodes with other capabilities such as analog-to-digital conversion and random pixel access. After the pixels are exposed to light in a CCD array, the CCD transfers each pixel’s charge packet sequentially to an output structure, which converts the charge to a voltage, buffers it and then sends it off-chip for further processing. A CMOS detector has the charge-to-voltage conversion built into each pixel. Amplification is also built into the chip.
CMOS detectors require less power to operate than CCD arrays, which can be a significant advantage in portable color-measuring instrumentation.
Lastly, CMOS detectors do not require the specialized manufacturing processes of CCD arrays. As a result they are less costly to manufacture.
4. WAVELENGTH CALIBRATION
Wavelength calibration is done after the spectrometer is assembled. Errors in wavelength as small as 0.1 nm can lead to color differences in the order of 0.1 CIELAB color difference units. Calibration is based on measuring a number of narrow spectral lines across the spectrum and is not dependent on the use of material standards. Two high power spectral lamps are used in the calibration – a mercury-cadmium lamp and a helium lamp. The lines that the lamps emit have a high signal-to-noise ratio and are clean, that is they do not have interfering lines present within the bandpass range. Eight lines from the lamps are spread uniformly across the spectral range of the analyzer. Table 1 lists the centroid wavelengths for the spectral lines. For each line, the data from each pixel in the array is recorded as a set of digitized diode photocurrents (counts). The centroid of the instrument bandpass function for each spectral line can be determined by fitting the line in pixel space. For example, the line at 508.582 nm spreads over seven pixels. Thus pixel numbers can be associated with each of the eight lines. Figure 2 illustrates the improved fit of the cadmium 643.847 spectral line by the SP2000.
|Line Number||Line Source||Centroid wavelength(nm)||Line number||Line Source||Centroid wavelength(nm)|
Table 1. Centroid Wavelengths of the spectral calibration lines
The next step of the calibration procedure is to fit the grating dispersion equation to the centroids in order to assign each diode a corresponding wavelength.
Lastly, the fitted dispersion equation is used to calculate the wavelengths that correspond to the center of each pixel to create a wavelength table for the array.
This highly accurate calibration technique used on a 256 pixel photo diode array achieves an inter-instrument agreement of 0.1 CIELAB units and a wavelength resolution of 1.8 nm.
The SP2000 is a high-performance, dual-beam spectral analyzer based on active-pixel complementary-metal-oxide-semiconductor (CMOS) technology. It represents a significant improvement over the charge-coupled device (CCD) technology used in many instruments and expands upon the CMOS technology first introduced by Datacolor in the MC-90 spectral analyzer. Both the measurement photo diode array and the reference photo diode array are integrated onto a single chip. By integrating both arrays onto one chip, the need for a mechanical alignment during assembly is eliminated. A holographic grating combined with the 256 pixel photo diode arrays allows a wavelength range of from 360 to 780 nm with a wavelength resolution of 1.8 nm and a signal-to-noise performance of 85 dB. The excellent light collection capabilities (f/1.6) of the spectral analyzer used with a fiber optic interface to the color-measuring instrument allows areas as small as 3 mm to be measured accurately. Wavelength calibration is done separately from the instrument using eight narrow-band spectral lines. This highly-accurate calibration technique used on a 256 pixel photo diode array achieves an inter-instrument agreement of 0.1 CIELAB units.
1. H. Oana, L. Jahreiss, D. Rich, and S. Trost, “Development and characterization of a miniature dual-channel spectrometer for spectrocolorimetry”, SPIE 1681 Optically Based Methods for Process Analysis, pp. 12-28, 1992.
2. Commission Internationale de l’Éclairage, Publication CIE No. 15.2, Colorimetry, 2nd ed., Central Bureau of the CIE, Vienna, 1986.
3. “ASTM E 308-99 Standard practice for computing the colors of objects by using the CIE system”, Annual Book of ASTM Standards, Vol. 6.01, American Society for Testing and Materials, West Conshohocken, PA. 2001.
* Correspondence: email@example.com; phone 609-895-7492; fax 609-895-7461; Datacolor International, 5 Princess Road, Lawrenceville, NJ 08648