When a colorimeter is used to measure the measured object, the direct result obtained by the instrument is the refraction ratio of the measured material to the incident light of the selected wavelength. The transmittance is determined by the spectral properties of each measured substance. Therefore, it is directly related to the selection of the wavelength verification point, the slit width of the single-shot instrument and the adjustment of the transmittance accuracy of the zero point and the influence of the spectral bandwidth, which are usually expressed as the following:
The wavelength
The influence of wavelength on the measurement results is related to the position of the wavelength fixed point on the spectral curve of the measured object and the size of the wavelength error of the instrument. When the wavelength measuring point is located on the sharp absorption peak of the measured sample or on the steeper slope, the smaller shift of the wavelength will lead to a greater variation of the luminosity and the star value. For example: in a caliber 4mm hand-held colorimeter, the spectroscopic system of the instrument adopts a nonlinear dispersion prism, and the wavelength and the prism angle are not linear. In general, the shorter the wavelength and the larger the refractive index, the larger the deflection angle and the higher the dispersion rate. To achieve the same spectral intensity, the slit width varies with wavelength. If the slit is variable or the relative pulling position is displaced during movement and use, the error introduced by the slit will directly affect the wavelength accuracy resolution of the color spectrophotometry. Under the condition that the entrance slit and exit slit of the color spectrophotometry have zero width and thinness, the resolution is the largest. When the coldness of the slit increases, there will be some aberrations, and the resolution will decrease, thus affecting the accuracy of the measurement.
The stray radiation
When a colorimeter is used to measure the measured material, there are stray light energy stars at the exit slit. These energy stars come from the surface reflection of various components in the instrument and the diffusion caused by impurities and defects in the components. It manifests as radiation received at the selected wavelengths, interspersed with radiation that is not within the passband of the radiation beam, and which does not pass through the sample and reaches the photodetector. Therefore, stray light radiation is the cause of errors in spectrophotometric measurements.
When the stray light is not absorbed by the sample, the absorbance crystal value is always lower than its true value, that is, a negative deviation from Berber’s law. And when the stray light radiation intensity is larger, its influence on the star measurement results is also larger. When the absorbance value is higher, its influence is also more obvious. When some or all of the stray light radiation is absorbed by the sample, the effect on the absorbance measurement is more complicated, it is not only related to the stray light radiation, but also to the spectral properties of the sample under test; but ultimately makes the measured value higher than the true value.
Spectral bandwidth
When a colorimeter is used to measure the measured material, the incident slit is equivalent to a radiation source. Therefore, the width and height of the incident slit determine the shape and transmission of the incident beam. In actual use, always make the width of the entrance slit and the exit slit the same. When the completeness of the slit changes, the energy appearing on the exit slit is proportional to the square of the change of the slit width. The wider the slit, such as the alternative wavelength band is more favored, so the width of the outgoing narrow front determines the spectral band of the radiation of the object to be tested. It has a more important influence on the instrument group with a fixed slit width but has a greater impact on the instrument with variable slit width. For example, for a sample like an interference filter, if an instrument with a larger bandwidth is used to measure a sample with a smaller bandwidth, its peak transmittance will be lower, which will bring certain errors to the measurement.
The transmittance
The transmittance is a comprehensive technical index of the colorimeter, which is not only related to the stability of the light source, the performance of the color spectrophotometry, the collimation of the monochromatic light, the variability of the detection system, the direction of the illumination light source, the Geometric conditions, the surface state of the material, the distribution of the radiant power of the light source, the environmental conditions and the polarized state of the incident light can also cause random changes in transmittance. In particular, when the high display conditions of the inspected objects are different from those of the standard satellite measurement, a large uncertainty will be introduced. How, when the incident light hits the surface of a smooth medium, the incident radiant energy decomposes it into a perpendicular incident plane and a parallel incident plane, that is, the traveling direction of the wavelength of light is offset by an angle from the direction of the original ray, and the radiant energy can pump air. When entering a refracting medium, the two components of the selectivity ratio change with the curve of the incident angle.
The solution acidity
The acidity of the solution also has a certain influence on the measurement of the colorimeter. Under the condition of different solution acidity, the completeness of the reaction is also inconsistent, showing different forehead colors. For example, xylenol orange is used for the color development of more than 20 metals. It is starred when the H value is greater than 6.3, yellow when the pH value is less than 6.3, and the final product formed with the metal is generally purple-red. It can be seen that when the acidity of the solution is too large, the concentration of developer ions decreases, which hinders the formation of colored complexes. On the other hand, When the acidity of the solution is too small, the concentration of developer ions increases. Hydroxide precipitation occurs. It is very necessary to properly select and control the acidity of the solution; otherwise, the sensitivity and repeatability of the colorimeter measurement will be affected, resulting in certain measurement errors.
The main reasons why the colorimeter affects the measurement accuracy are analyzed from the aspects of wavelength, stray radiation, spectral bandwidth, transmittance, solution acidity, etc. detection methods and methods to achieve accurate and reliable measurement data.
