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Analyze your color measurement

In this article the author shows you the advantages and disadvantages of the technologies present in the market.

By: Octaviano Fernández

For companies that operate within the field of color, the measurement of this over time, has become an imperative necessity, since due to the culture of quality, having the ability to guarantee the consistency of color in their products is an indispensable requirement to achieve the correct displacement of these. Initially, this requirement was covered by the 'measurement' of color by the expert and well-trained human eye, with the consequent variations inherent in this 'method'. With technological advancement and the standardization of requirements and standards, the use of instrumentation to measure this parameter has become practically indispensable.

Over the years, the tools for measuring color have undergone important transformations, all aimed at being able to measure this parameter more accurately. From the beginning with the use of simple colorimeters that measured color through filters,  to spectrophotometers, the evolution of technology development has modified them. This article is intended to introduce the reader  to the advantages and disadvantages of the color measurement tools that are currently used and available on the market.

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The beginnings

The first equipment that was used for color measurement were colorimeters,  instruments that meant a breakthrough in color quality control that companies had to apply to their products. This allowed us to move from  subjective evaluations of color,  to an objective measurement of it, and  although this was already a breakthrough, it was not for long. The needs  of the market demanded a stricter control of the color, and the colorimeters did not have the necessary instrumental characteristics for this, so they were exceeded for the required or necessary applications. This allowed  the development of more complex equipment reaching spectrophotometers .

Spectrophotometers are instruments designed to measure colors capable of determining the reflectance at each wavelength, making it possible to determine the spectral curve of a color, something comparable to the possibility of measuring a fingerprint, which makes them very accurate instruments.

Since the appearance of the first spectrophotometers to the present day, they have undergone an important evolution. Among the most notable advances or changes could be mentioned the resolution of their measurements and the lighting sources they use. Before starting to explain the main technological advantages and disadvantages of color measurement tools (spectrophotometers), it is important to establish the differences between the different optical systems used in these instruments, as this topic still causes confusion to some users.

Optical systems

One-way lighting system (45/0 or 0/45)
These systems provide illumination from one direction. With a geometry of 45/0, the surface to be measured is illuminated at an angle of 45 with a detector at an angle of 0 degrees; with a geometry of 0/45 it is the same situation, only the other way around (Figure 1 and2).
                                                                                 
The spectrophotometers 45/0 and 0/45 are used when you want the differences in the values obtained from the measurements to correlate with what we observe visually.

With this type of optical systems, the color is measured taking into account the brightness and texture of the sample, and when there is a difference in these characteristics, the values thrown by the instrument will be different.
In Figure 3, we show an example to better understand what was said in the previous paragraph. This plate is made of the same material, but has differences in texture between the two areas shown.
If this plate is measured with a 45/0 or 0/45 spectrophotometer, differences in instrumental readings will be obtained because the system will take into account the texture and perceive it darker, as it happens with the vision of a human.

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These systems are mainly used when assembling components of a product is required.

Diffuse Illumination Integrative Sphere System
These  systems employ an integrative sphere to illuminate samples  uniformly from all directions (an integrating sphere is a spherical device coated with a white material, almost always barium sulfate so that light diffuses evenly). (Figure 4)

Instruments with spherical systems can be used under different measurement conditions:

1)SCE (Specular component excluded)
2)SCI (Specular component included)

The SCE is an imitation of the 45/0 or 0/45 measurement

In the SCI the color is measured independently of the brightness and texture, so although there are differences in the samples to be measured  in the structure or brightness of these,  if the color is the same the values will be the same

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In this panel (figure 3) although the side with structure appears darker if measured with a spectrophotometer d / 8 with the SCI the differences will be 0.These systems are mainly used to measure color changes due to aging, to formulate colors or measure production, as well as dyeing power among many other applications.

The resolution
It is important to emphasize that no geometry is better than the other, only that its use is for different purposes. The technology for measuring color today is very even, but there are some important points to consider.

As first would be the resolution at which spectrophotometers can make their readings. In the market there are equipment that can measure every 10 or 20 nanometers. Nanometers are the units of measurement used to measure the wavelength of light.

It is worth mentioning that instruments capable of measuring every 10 nanometers are more accurate in their measurements because they obtain more information to analyze from the object. Instruments that measure with differences in wavelength of 20 nanometers are already very scarce, but there are still some and  are usually used in simple applications, such as in the measurement of natural materials (sand, dust, food, etc.).

Light source
As a second point to consider when talking about advantages and disadvantages of color measurement systems,  and in our opinion the most important, would be the source of iluminación.La lighting source is the heart of the spectrophotometer. Mainly the reproducibility of the instrumental readings depends on the quality of this.

To measure the color of a coating, the spectrophotometer emits a beam of light that collides with the object to be measured and the light that is returned to the instrument is analyzed by it and transformed into numerical values, therefore, if the light emitted by the spectrophotometer has variations, therefore the values obtained from the measurements will also have variations.

The main causes for which a lighting source can vary are: the decay of the lamp itself and the influence of the temperature of the environment.

Over time different sources of illumination have been used in spectrophotometers, currently only three types are used.

1)Tungsten Halogen
2)Pulsed Xenon
3)LEDS

Tungsten Halogen
Halogen tungsten lamps were the first to appear on the market, although those that were used at first were different from those used today. At first these lighting sources required a preheating to be able to function properly, today this is no longer necessary.

The halogen tungsten lighting source performs well in relation to the temperature of the environment; the changes of this do not significantly modify the results,  which allows users to work with this equipment not only under strict laboratory conditions. However, the biggest problem is the decay that the lamp suffers through its life time, which is usually short.

The measurements obtained with a spectrophotometer with a halogen tungsten lamp change over time, since these lighting sources are losing intensity, so the measurements obtained at the beginning and end of their life, will show great variability. This situation is accentuated at the end of the useful life of one of these lamps and the values obtained tend to appear darker. This situation becomes a little more complicated when it is necessary to work with multiple instruments, especially if the control of these does not have a single person, since the measurements between one instrument and another can vary considerably depending on the life in which your lamp is located.

In practice, an alternative solution  to this problem is solved by taking a strict control in the time elapsed between each change of lamps of the spectrophotometer,  because if they are changed before this decay begins (approximately every year, under normal conditions of use) the differences in the readings will be less significant. Some will argue the  inconvenience of the cost that this generates and the time that the device is no longer available.

Pulsed xenon
The pulsed Xenon lighting sources considerably improved the reproducibility in the data or readings thrown by the spectrophotometers, since the life time of this type of lamps is much longer than those of halogen tungsten and although they also suffer a decay with the passage of time, its life span is approximately three times that of halogen Tungsten. The drawback of Xenon lamps is that they are much more sensitive to temperature changes, for this reason it is more common to find this technology in places with conditions similar to those of a laboratory.

LEDS
This type of lighting is the latest in this type of technology used in color measurement. This type of lighting has gained many followers over time, as it does not present the most common drawbacks of the other sources of lighting.

LEDs are lighting sources that do not have decay, which is why the measurements obtained in a spectrophotometer with this technology are always constant. LeDs when they lose strength melt.

The life span of this light source is very long, usually exceeding 10 years. On the other hand, this lighting source is practically not affected by changes in the temperature of the environment, so it can not only be used in laboratory conditions but also outside it.

From what we have considered in this article, we can realize the instrumental advantages offered by a spectrophotometer with an LEDS type lighting source. We can aspire to have lower coefficients of variation, that is, a greater reproducibility of our instrumental data and at the same time have the advantage that variations in the environment, such as temperature, would  not affect color measurements.

These advantages would impact on better color quality control of the company's products, as well as greater flexibility in the measurement conditions of the instrument.

*Manager of Exacolor. [email protected]

Author: Vanesa Restrepo

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