Color Control in the Palm of Your Hand: New Advances in Portable Spectrophotometers for Plastics
New Advances in Portable Spectrophotometers for Plastics
A white paper by Datacolor
In manufacturing operations across a wide variety of plastics applications, color serves as a fundamental indicator of quality. Delivering material that is off-color can risk future business, and failing to get the color right "the first time" can drive up labor and raw materials costs significantly, reflecting the quality of the manufacturing process itself. With increasing competition and the move to bring products to market in record time, it is more important than ever to deliver an on-spec color faster and more efficiently. Integrating a color-measuring instrument will significantly improve the efficiency of the coloring process and the color consistency of the finished product. For the past 30 years, color-measuring instruments have been successfully introduced into color formulation, production correction, and quality control processes in manufacturing applications throughout the world. The discussion below covers the current uses of handheld spectrophotometers for quality control, and the technical improvements that are being made on these instruments.
The color process in production: a review of handheld instruments
Portable spectrophotometers appeared in the last decade as an answer to the logistical problems posed by traditional laboratory instruments. In many cases, portability has made it possible to replace a subjective visual inspection of the color with an objective instrumental evaluation as part of the plastics quality control process. Lighter, smaller, mobile instruments allow instrumental color evaluations to be performed on both components, and large finished products that cannot be moved to the lab for evaluation. In the past, the logistics of this situation often restricted the quality control procedure to a visual evaluation of the color. In other situations, portable instruments can now be brought to the production line. Through an on-the-spot instrumental color evaluation, plastics production personnel can know the instant that the current batch has satisfied the color approval tolerances, and can immediately begin preparing the equipment for the next production batch. The use of a portable instrument in this example offers the benefit of decreasing the turn-around time of a batch, reducing labor and raw materials costs.
One of the continuing challenges to users of handheld spectrophotometers however, is mastering the user interface. All portable models have in common that the interface is very cumbersome to use. They often employ switches that must be toggled in a precise order to customize sample names and screen selections. In order to collect the evaluation data quickly and easily, users commonly retreat to the default sequences provided with the units. In the case of sample identification, users find themselves depending on the auto-numbering sequences built into the software. Customizing sample names usually involves the extra step of uploading the data to a desktop PC, and then renaming each sample. When running an evaluation procedure, users often limit themselves to cycling though a default series of evaluation screens simply because it is it easier. Portable instruments currently on the market today offer a wide variety of software tools, many of which are never used simply because it is difficult for the user to navigate through the program to access them.
The touch screen interface advantage
The newest offering in the portable spectrophotometer market offered by Datacolor, the Mercury introduces a radical departure regarding the user interface. A PDA (Personal Data Assistant), containing software customized for color management applications has been integrated into this instrument. This innovation allows a customized program to retain all of the navigation features that are standard on a PDA. Not only does the new interface eliminate cumbersome toggle switches, but it also eliminates default selections. Using a stylus, the user simply taps the screen to input custom sample names or to change evaluation screens. This speeds the color evaluation process while it reduces errors in sample identification and evaluation selection.
Adapting the PDA to a color management application also takes advantage of the memory/storage capacity available with a PDA. In the past, the software offerings that have accompanied portable instruments were confined to basic quality control functions—simple color difference, pass/fail, and color indices--because of memory limitations. Data upload/download to and from a PC has been a mandatory feature of all handheld units. However, QC and color formulation systems based on PC platforms generate enormous databases of both samples and formulas. Memory limitations however, have prevented the full utilization of these databases in a handheld application. The integration of the PDA into the new instrument is an answer to that limitation. This memory capacity of this device makes it possible to accommodate the storage of large databases on the portable unit (a maximum of 30,000 samples), and to develop more complex programs that can search, retrieve, and manipulate the information that they contain. The navigational features of the PDA, literally make this information available at the touch of the screen.
The importance of a color-measuring engine in measurement accuracy
All color measuring instruments must separate the light being reflected from the sample into bands of wavelengths across the visible spectrum. Typically an optical sensor detects the amount of energy reflected by the sample within each of the wavelength bands using either a series of filters, or a diffraction grating. The greater the number of bands, the finer the resolution of the instrument. As the wavelength intervals get smaller, the precision of the instrument increases, because a variation in the energy detected for a single band affects a smaller portion of the spectrum. Typically, portables divide the spectrum into wavelength intervals of 10 nanometers.
The color measurement technology in the Mercury moves the resolution of portable instruments to a new level. It is based on an active pixel, CMOS integrated circuit technology that allowed the placement of two, 256 diode arrays on a single chip. This technology reduces the number of electronic components as compared to non-integrated designs producing a very low noise detector with a wavelength resolution of 1.8nm that offers the following benefits:
- Better inter-instrument agreement
- More accurate wavelength alignment
- Performance comparable to laboratory instrument.
Color measurement spectrophotometers typically use 45/0 or D/8 sphere geometry. 45/0 is sometimes used in plastics quality control applications, because it excludes gloss and more closely simulates how a colorist compares samples in a light booth. However, it exaggerates the influence of texture differences and surface defects on the color evaluation, and does not account for gloss differences.
In contrast, sphere geometry illuminates the sphere uniformly, minimizes any differences in the samples due to surface defects and texture differences. It also offers the ability to include gloss or exclude gloss in the evaluation of color. This capability becomes important when the standard and the batch are significantly different in terms of surface characteristics such as gloss or texture—a situation that arises in many production applications. When the surface of the samples is similar, including the gloss produces the most accurate evaluation. However, when the surfaces of the samples are different, excluding the gloss may be required to produce an accurate evaluation. The latest handheld spectrophotometer allows the user to make this selection automatically.
Maintaining the range of aperture sizes available on a laboratory instrument is also an important feature for a portable unit. Laboratory spectrophotometers typically come equipped with a range of aperture sizes to allow measurement of both large and small samples. Handheld advancements maintain this flexibility, offering four (4) aperture sizes ranging from 3 mm – 15mm. The units are available in either a single or dual aperture configuration.
The best handheld design also ensures that the instrument is remarkably easy to handle. The Mercury from Datacolor, for example, features a tower configuration that allows more flexibility when measuring samples of unusual shape or size. The unit is lightweight (2kg), and had been ergonomically designed for a natural fit in either the left or right hand. A stapler foot is also available for each aperture size offered, to help align the samples correctly over the instrument port.
Managing UV energy in the light source
All spectrophotometers have traditionally made use of one of two light sources - either a tungsten filament bulb similar to a common projector bulb, or a pulsed xenon flash lamp. Pulsed xenon is considered superior and has become the standard source for use in laboratory instruments because its high intensity allows for good measurement repeatability on dark and high chroma samples. The short pulse length minimizes the possibility of sample heating, which may change the color of the sample. Xenon sources are also preferred because they are easily filtered to simulate daylight, making it possible to measure UV-excited fluorescent samples.
Handheld measuring instruments are now capable of dealing with the UV light in the source. Mercury offers three approaches. The standard unit has an unfiltered xenon bulb that contains a UV component. Mercury can also be fitted with a cutoff filter, which keeps all of the energy in the UV region (wavelengths of 400nm or lower) from reaching the sample. Finally, the instrument can be equipped with an adjustable filter, which allows the user to adjust the UV content of the light source, which will vary as the instrument ages. This provides for accurate measurements of products that contain UV-excited fluorescing agents such as optical brighteners. Until now, this adjustable UV filter option was only available on laboratory instruments.
The need for precision in color control is widely acknowledged in plastics manufacturing operations throughout the global market. For several decades, the plastics industry has been integrating laboratory-based instrumental color management systems into processes, making the evaluation process increasingly more objective and repeatable. Combining a performance standard and features comparable to laboratory counterparts, with a user interface that is a cinch to operate, Innovations in handheld instrument design will expand the use of instrumental color management to every aspect of the coloring process. From the receipt of incoming raw materials, to the approval of the finished product, the control of color quality is now in the palm of your hand.
The most revolutionary feature of the Mercury is its user interface - a PDA (Personal Data Assistant) operating in a Palm Pilot OS™ (operating system). Its high resolution (160X160 pixels) and high contrast screen is easy to read and its touch screen and hand writing recognition capabilities make the Mercury even easier to operate. Naming standards and batches has never been so fast or simple.
Datacolor, 5 Princess Road, Lawrenceville, NJ 08648. Tel: 609-924-2189 ; Fax: 609-895-7472 .