Color Measurement Types and Instruments (3)

Third, spectrophotometer and three color filter colorimeter

Most of the commercially available spectrophotometers are not true spectrophotometers but are simplified spectrophotometers. With this spectrophotometer, only a predetermined wavelength can be measured, and not all wavelengths of the visible spectrum can be continuously measured.

Figure 2-7 is a schematic diagram of the optical path of the spectrophotometer. The spectrophotometer is different from the eye, and the eye evaluates the received light energy at all wavelengths of the sensation at the same time, and the measurement of the reflection curve must be performed wavelength-by-wavelength. This requires that the light of the light source be decomposed at various wavelengths, which can be decomposed into monochromatic light before the sample is irradiated, or can be decomposed after being reflected from the sample. Almost all new instruments work in the latter way. Only in this way can the correct measurements be made on samples with fluorescent properties.

1. light source. The illumination used for the measurement must contain all the wavelengths of the visible spectrum. When measuring a fluorescing sample, the reflectivity curve and the tristimulus value calculated from the reflectivity curve must correctly reproduce the visual color. The radiation distribution of the used light source is measured to meet the radiation distribution required by the color matching.

2. Dispersion of light. In order to measure the reflectivity of a sample wavelength-by-wavelength, the light reflected by the sample must be dispersed. The traditional components in astigmatism are prisms, but in modern instruments curved gratings are commonly used. The spacing of the different wavelengths obtained by the latter can be equal, which can make the aperture structure not necessarily variable. The third optional element for dispersive light is a color interference filter.

3. The geometric condition of the measurement. When people judge a sample visually, in other words, when people match the color of the sample visually, the sample being judged should be placed on a desk facing the north window and be illuminated by diffused natural light. At this time, only light reflected in a certain direction and reaching the eyes is observed. Instead of daylighting, the light is measured using a light source that simulates sunlight. The relationship between the two is similar.

Most spectrophotometers have a component called an integrating sphere. The inner wall of the ball is painted white, and a small hole is made in the wall to allow the light necessary for lighting the specimen. The light source is placed inside the ball or at least beside the ball so as to illuminate the ball wall with diffused light. Therefore, there are one or two small holes in the ball so that the test sample or standard whiteboard can be placed. The hole for the sample has different dimensions due to the different measuring instruments. In most instruments, the hole diameter of the hole can be continuously changed to adapt to the size of the measured sample. The most commonly used aperture is 2 to 3 cm, and the aperture is less than 0.5 cm only as a special accessory. For technical reasons, apertures larger than 5 cm are not commonly used. The size of the sample to be measured is often not the same, and the aperture should be made as large as the sample, but when the aperture is larger than 5 cm, the measurement result is likely to be inhomogeneous (not stable like a small hole).

In a spectrophotometer with an integrating sphere, the sample is usually measured in a slow-illuminated state and reflects light reflected from a certain direction of the sample. The general measure of light reflected in the 8° direction (Figure 2-8) has the advantage that a so-called glossy absorber cake can be mounted next to the other hole of the integrating sphere, thereby shielding the sample from illumination from 8°. The gloss of the sample was excluded during the measurement. However, the measurement method with a glossy absorption trap is only used in the case of high-gloss specimens. It is always always measured at the same time and then subtracted by calculation. The amount of deduction depends on the optical properties of the interface between the air and the sample, and the average amount of gloss correction is 4%. This is of special significance in the case of calculating network coverage.

In the case of measuring with an integrating sphere, the structure of the upper surface of the sample is only a minor factor. This means that when the sample is measured in different directions under the measuring hole, the change in the measured value is smaller. Even so, the sample should still be placed in the same direction.

Although the inside surface of the integrating sphere is whitened, it absorbs a small part of the light, causing changes in the radiation distribution of the light source, especially when measuring fluorescent samples. Therefore, in some new spectrophotometers, the 45° circular light source is used instead of the integrating sphere, and the measurement is performed in the 0° direction, so that the gloss is always excluded. The ring light source has all the advantages of an integrating sphere, overcoming its disadvantages.

There are spectrophotometers for measuring glossy specimens. With this instrument, the specimen is directionally illuminated, unlike in the case of circular illumination. Illumination is only incident from the 45° direction, measured in the 0° direction (see Figure 2-9), ie with a 45°/0° geometric condition. This figure clearly shows the reason why the gloss is excluded under this measurement method. As long as the gloss of the sample is small, the influence on the measurement result is negligible. For glossy samples, the effect of gloss is generally ruled out by calculation. The 45°/0° geometric condition is less applied because the measurement result is significantly affected by the irregularity of the upper surface of the sample, and the repeatability of the result is not as good as that measured with an integrating sphere.

4. sensor. In spectrophotometers, sensors are assembled using photocells, photodiodes, or photomultiplier tubes. Previously only one sensor was usually used. Monochromatic light was irradiated on the sensor in chronological order and quantified. In order to improve the measurement speed, some new instruments equipped with 16 sensors can measure 16 wavelengths at the same time (from 400 to 700 nm with a spacing of 20 nm).

5. Calibration of the instrument. The calibration of the instrument is now much simpler than it used to be, as almost all mechanical adjustments and mathematical corrections have been replaced. Despite this, don't forget about calibration, because the measurement of a spectrophotometer depends to a large extent on careful, regular calibration.

The calibration content is 100% line calibration (standard white calibration), 0% line calibration, and wavelength and optical elevator scale calibration if possible.

The 100% line is calibrated with standard white, which is to be calibrated to absolute white again. The commonly used standard white is pressed with barium sulfate powder and its absolute reflectance reaches 98% in total wavelength range. Although current spectrophotometers can remain stable for a long period of time, they should be calibrated at least once a day. It is best to calibrate multiple times a day.

The 0% line correction is performed in a manner similar to that described above. Generally, a blackbody is used for the 0% line calibration. Since the blackbody absorbs all the incident light energy, the reflectivity is 0. Calibration of the zero line is also performed every day, and some instruments do not do 0% calibration, but this is not recommended.

Wavelength inspection and optical elevator rule detection are generally no longer performed in newer instruments.

6. Spectrophotometer accuracy. The accuracy of the instrument can be measured by indicators such as short-term repeatability, long-term repeatability, and absolute accuracy.

The short-term repeatability of an instrument is important, especially when measuring chromatic aberration in quality control. A short-time instrument with poor repeatability is not in compliance with the technical requirements. The sample can be continuously measured under the head of the test. The reflectance value of the wood should be greater than 0.02-0.03%, and then The measured value is used to calculate the color difference ΔE (refer to ΔE*CIELAB value). The average value of all the measured values ​​or the first measurement value can be used as the reference value. The value of ΔE should not be greater than 0.05 to 0.01. Modern colorimetry systems can generally Meet this requirement. If the measured values ​​are continuously changing during the inspection, this is not an instrument problem but a sample. When the sample is placed under the detection head, it will become hot when irradiated with light. When the sample is heated, its color will change. Users should pay attention to purchase the instrument with low energy release from the probe, such as the use of the instrument with the atmosphere flash. Modern instruments measure very quickly, and the effect of heat from the sample is much smaller than in previous instruments.

Long-term repeatability is important for formulation calculations because colorants and samples are measured at different times (without even one year).

The long-term stability can be checked by a long-term stable sample, and the color difference should not be greater than 1. ΔE=0.5 for good instruments. The significance of long-term stability in inspections and supervision is often underemphasized. For long-time stable instruments, calibration steps can be omitted when doing 10,000 calculations, and only statistical quality management can be used. At this time, the measured values ​​are only affected by the production conditions and are not affected by the instrument.

The absolute accuracy of a colorimetric instrument is far less than its repeatability. Absolute accuracy can be checked with calibrated standard samples. If you use a different instrument to measure the same long-term stable sample, you will get an inconsistent result. Before standard setting, the Standards Committee has been comparing the color values ​​measured by different systems. Through the study, it was found that when measuring the same sample with the same type of instrument, it can show several units of color difference. The darker and brighter the measured sample is, the larger the deviation is, and the larger the deviation is when comparing different types of instruments. It is hopeless to reduce the absolute deviation at present.

From these facts, it is inferred that if only the reflectance value and the tristimulus value are transmitted as a sample, matching the color with a desired accuracy is impossible.

There must also be a warning that has nothing to do with the absolute accuracy of the instrument, because it involves other factors that are not easy to determine, so it is impossible to achieve a consistent color match for a color in the chromatogram.

7. Colorimeter (three color filter colorimeter) accuracy. Three color filter color measuring instruments have been widely used in the industry as a color measuring instrument in the past. In order to imitate the human visual process in order to provide standards-compliant measurements, the standard light source (switching of the radiation distribution of the light source is to use a color filter) to illuminate the sample to be evaluated, and the sensitivity of the sensor is also converted into a viewer with a color filter. The visual acuity is consistent, but most of them only install a set of filters and accomplish both tasks. The colorimeter usually has only one sensor, juxtaposed with three, preferably four color filters in front of the sensor (four filters are used to better adjust the x(λ) curve of the short-wavelength part). In this case, the read value is easily converted to a tristimulus value, and sometimes the tristimulus value or a converted color space value is directly read. The advantage of this kind of instrument is that it has very good short-term repeatability. The disadvantage is that because the relationship between the visual sensitivity and the filter-sensor is difficult to adjust correctly, the absolute accuracy of this instrument is not good. Although the absolute accuracy of the chromatic aberration measurement is considered as a high-order deviation, in some colorimeters, the deviation is so large that it can only be used to measure a sample with a small color difference and a very small metamerism. This is usually the case with product quality control. In this case, the older three filter colorimeters can also achieve good results.

Due to the development of modern electronics and modern devices such as optical fibers, it is entirely possible to manufacture small-portable three-color filter colorimeters, which have the disadvantages described above but are inexpensive.

Due to limitations in measurement accuracy and sample accuracy, the calculation results should not be output in too many digits. Reflectance values ​​(%), tristimulus values, a*, b*, and L* values, and color difference values ​​are usually accurate to two decimal places. The last one is already unreliable.