LED-Equipped Spectrophotometers Find Their Way to the Factory Floor

Companies that manufacture products ultimately used by consumers — interior trim for cars, vinyl siding for homes, decorative stone for landscaping, interior wall paints, etc. — are often discovering that they need to use precise spectrophotometers to ensure first-time color quality and lotto-lot consistency.

But to be effective, these optical instruments must perform their color measurements accurately, quickly and under some truly demanding shop floor conditions. The instruments need to show acceptable repeatability and reproducibility even within an environment of uneven ambient lighting, high levels of humidity, varying temperatures and even significant shock or vibration. Finally, the instruments also need to be inexpensive enough to yield an acceptable return on investment if they are placed on individual production lines to measure color on a real-time 100% inspection basis.

By leveraging technology that was commercialized for other industries, color instrument manufacturers have invented robust, in-line spectrophotometers for the factory floor that measure colors accurately at a fraction of the cost of older technology. The result is that manufacturers now have the means to obtain reliable online data from a high-speed production line that can be used for lean manufacturing techniques such as statistical process control.

The advent of small, intense, and highly reliable illumination sources based on light-emitting diodes (LEDs) has put solid state illumination at the heart of this new generation of spectrophotometers. Spectro manufacturers typically strive to use D65 standard illumination sources to take their measurements. D65 is the specification of white light that spectrally simulates daylight with a 6500K color temperature. D65 is the standard illumination that is commonly used in the graphics arts, paints, coatings, and other industries in which color matching is critical.

Although the phenomenon of electro-luminescence from a semiconductor was observed in the early 20th century, a major breakthrough occurred in the 1990s when experimenters finally developed a high brightness LED made from indium gallium nitride that produces a high intensity light at the blue end of the electromagnetic spectrum. By coating this LED with YAG phosphors that emit light at longer wavelengths when excited by blue light, engineers were able to produce a “white” light that had output across most of the visible spectrum.

While it has not yet developed a single LED that emits a true D65 illumination, the semiconductor industry has developed sufficient white and chromatic LEDs that the D65 standard can be approximated when they are properly mixed together.

LEDs originally developed just a few years ago for the flashes that are built into high-end cell phone cameras have now found their way into spectrophotometers. An LED light source that is approximating D65 standard light may have a basic white light LED and up to six other chromatic LED die that supply different colored lights to supplement weak spectral ranges for the white light LED.

The compact, intense sources of light generated from LEDs have been a boon to manufacturers of non-contact color measuring instruments because they represent the holy grail of optics — a near “point source” of light that doesn’t suffer the same degree of aging or degradation over time, nor the catastrophic failure modes, that incandescent and xenon flash light sources experience.

Older spectrophotometer technology relies on the use of xenon flash lamps or incandescent bulbs that cannot match LEDs as a point source. Coupled with a few other innovations in optics and software, the uniform burst of light is one reason why the new generation of inexpensive spectrophotometers can read samples from a distance of several inches, which is critical for companies that want inline measurement of color during production.

The light from LEDs has a very flat degradation curve over time, meaning that the amount of each constituent color does not change much over millions of flashes. The same cannot be said for light from xenon flashes or incandescent bulbs. Over time the constituent colors of light from these older technologies markedly shift and the intensity of the light diminishes as metallic atoms are deposited on the glass walls of bulbs.

Some LEDs are designed to withstand the application of a high electrical current in a very short period of time to produce bright flashes. The LED efficiency of converting electrical energy to light is now typically four to 10 times greater than incandescent and other forms of lighting. And because they rely on solid state physics rather than heated filaments in a vacuum or an electrical discharge through a gas, LEDs are very resistant to shock and vibration.

However, instrument designers need to be mindful that the optical energy outputs and wavelengths of light emitted by LEDs are greatly affected by the junction temperatures of the devices. One way to control temperature variation and thus maintain a chromatically stable light source is to mount the LED dies on an alumina nitride substrate that is an extremely efficient thermal conductor. A small thermistor chip is placed next to the LED die to provide real-time temperature feedback to a microprocessor that actively drives the substrate to the desired temperature set point.

This arrangement allows spectrophotometers to operate accurately with shop floor variations in temperature as wide as 32 degrees to 120 degrees F.

New inline spectrophotometers that use LEDs as the light source offer another advantage that is critical when measuring colors under varying lighting conditions on the shop floor. The instruments only detect the light from the LEDs and they ignore all other sources of ambient light. This fact is critical for accurate color measurement.

The source of light — incandescent, fluorescent, sodium vapor — greatly affects the way colors are perceived. Just think about how difficult it is to find a dark red car in a parking lot illuminated by sodium vapor lamps. Since sodium vapor lamps are nearly devoid of the red spectrum, red cars appear to be black under their illumination.

Inline spectrophotometers in the past have gotten around the problem of error caused by ambient light by either using the brute force of blinding flashes of light from a xenon flash that simply overwhelm any ambient light, or by costly shrouding to keep ambient light off the sample. But LED-illuminated spectrophotometers use a more refined technique. The instruments modulate the LED’s pulses at a high frequency and tune the photosensors so they recognize only the LED light source, similar to the way that a person tunes in one radio station from many available in a city.

The small, almost “point source” aspect of the LED source allows the spectrophotometers to employ “searchlight” illumination optics that assure constant illumination intensity even at varied distances from the instrument.

Even so, the light that is reflected back from a measured sample is scattered in a Lambertian, or omnidirectional, pattern and is subject to the inverse-square law of optical energy, so doubling the distance from sample to sensor effectively reduces the intensity of the light to onequarter of its original strength. However, the effect of such intensity variation on the light’s return path can be compensated for optically by use of a multi-element lens for the sensor that views a focused sample area that is proportional to the square of the measurement distance.

Engineers have thus extended the range of depth insensitivity for precise color measurement by using an imaging-type of fixed angle optic to collect light and view slightly larger or smaller areas as the depth varies, always sensing a corrected area of reflected light from the sample.

The result is a 1 cm depth tolerance window from a nominal measurement distance of 10 centimeters with errors of less than one-quarter ΔE, where one ΔE is defined as the distance in color space at which a human can typically begin to perceive a difference between a color and a slight variation of that color.

To assist with targeting the exact location of the area that the spectrophotometer illuminates on samples, instrument designers have come up with an ingenious method that simulates the way a single-lens reflex camera shows a photographer exactly what field of view will appear on film. By activating targeting software, the instrument will send a flashing ring of light through the collector optics to illuminate the precise area on a sample that will be measured, so technicians can see exactly what area will be measured during testing or setup for on-line measurement.

Instrument designers have greatly reduced the size of spectrophotometers on the factory floor by using a color analyzer engine consisting of a stepper motor and spinning wheel of transmission filters to separate light reflecting from the sample into its visible light components. Fiber optics transmit light from the sample to the rim of a small wheel that is fitted with transmission filters. In the case of a highly accurate 31-point spectrophotometer, the wheel has 31 different transmission filters, each manufactured to allow only a specific wavelength of light. A photosensor reads the relative intensity of the light as it is transmitted through each filter, transmitting the electrical pulses to a PLC to record and analyze the data.

While they are more expensive than other optical tools that break white light into its constituents, transmission filters are ideal for the task because they are chemically and physically stable enough to handle the demands of the factory floor.

Instrument manufactures have further hardened spectrophotometers for the factory under dusty or oily shop conditions by directing a constant flow of filtered compressed air past lenses and other optics.

By borrowing technologies from other industries, color instrument manufacturers have developed rugged spectrophotometers that are about one-fifth the cost of spectrophotometers based on older technologies. And companies that employ the non-contact, in-line spectrophotometers have found other valuable applications that can be used in implementing lean manufacturing initiatives, such as statistical process control, data gathering for continuous improvement and first-time quality, and mistake-proofing.

This article was written by Steven H. Peterson, Technical Development Manager-Industrial, X-Rite, Inc. (Grand Rapids, MI). For more information, contact Mr. Peterson at This email address is being protected from spambots. You need JavaScript enabled to view it., or visit http://info.hotims.com/10978-200 .