What is the Effect of Stray Light on a Colorimeter?

The colorimeter is a common analytical instrument in the laboratory; it is widely used in industry, agriculture, environmental testing, medical and health, food inspection, and other fields. The principle of the instrument is that the substance in the solution has an effect on light absorption under the excitation of light, and the substance absorbs light selectively. Different substances have their own absorption spectra, so when a monochromatic light passes through the solution, its energy will be absorbed and weakened, and the degree of light energy weakening has a certain proportional relationship with the concentration of the substance.

In colorimeter verification, stray light is an important indicator that must be checked, and its size determines whether the instrument is qualified or not, and the difference in stray light has a great impact on the measurement results of the spectrofluorometer Stray light is the interference of light other than the light required by the application. When the light emitted by the light source passes through the color spectrophotometry, it may be reflected from the surface of the color spectrophotometry cabin and other optical elements. The surface of the optical element and the dust in the atmosphere can also be scattered. The existence of stray light will affect Lambert-Beer. The law is biased. This article mainly discusses the effect of stray light on the spectrofluorometer

Test methods of the stray light

At present, there are four kinds of measurement methods for stray light: cut-off filter method, series transmission method, optical method, and convolution method, among which the cut-off filter method is the most simple. According to the measurement method specified in the UlG 17-2007 metrological verification regulation, the ultraviolet region of A section was measured with Rui iodide standard solution or loaded with stop filter at 220 nm, and the sodium nitrite standard solution or the cut-off filter at 360 nm. The 10-mm standard adaptive absorption tank was used, and the distilled water was used as the reference. The spectral bandwidth was 2 nm, and the transmission reference value was measured. The visible region of the B section was measured with a cut-off filter at 420 nm. The transmission ratio was measured with air as the reference. The near-infrared region of section C was measured with H2O at the wavelength of 1 – 420 nm and air as the reference. According to JJG 178-2007, the stray light of the instrument should meet the requirements of stage I not more than 0.1 %, B stage I not more than 0.1 %, C stage I not more than 1.2 %.

The stray light has a great influence on the measured data. The spectral instruments used for chemical analysis comply with Lambert-Beer law, requiring a better monochromatic and accurate transmission ratio. The existence of stray light makes the transmission ratio increase and the absorption rate decrease, resulting in a deviation of the measurement results. The upper limit and measurement linearity of the concentration of the measured sample is directly limited.

Influence of stray light on measurement results

As long as the stray light of the instrument is known, the measurement error caused by the stray light of the instrument can be obtained. To analyze a sample by colorimeter, the absorbance is about 1.95, and the accuracy of the analysis and test is required to be 1.0 %. However, when a caliber 8mm portable colorimeter with 0.2 % stray light is selected, the instrument cannot meet the requirements. Because when the stray light is 0.2 %, the absorbance is 1.95, and the relative error of measurement is 3.6 %. The analysis and test must select a colorimeter with stray light better than 0.05 % to meet the requirements. Therefore, in the analysis and test of samples, the influence of stray light on the absorbance measured by the colorimeter cannot be ignored. Especially for high concentration samples, the higher the absorbance is, the faster the error increases, and the greater the deviation of Lambert-Beer extension is. In order to measure the accuracy and reliability of the results of stray light must be limited, as far as possible to reduce stray light into the measurement error. In addition, stray light also has a great influence on the measurement linearity of the colorimeter.

Causes of stray light

Poor design of the light source system of the instrument, rough machining of mechanical parts, misalignment of optical components, and falling off of the anti-glare black paint on the inner wall of the instrument can all cause stray light.

  • Gratings, prisms, lenses, and collimators are moldy in a humid environment;
  • The optical original is damaged or the optical components are defective, such as prism scratches, air bubbles, etc.;
  • Reflected light from the edge of the clapboard inside the collimation system or on the optical path;
  • The optical path system and detector are not well shielded, and the sample chamber leaks light;
  • Secondary electron emission caused by thermal radiation or fluorescence;
  • Defects and inaccurate positioning of slits;
  • The blackening of the inner wall of the color spectrophotometry is improperly handled.

Among the above reasons, the stray light generated by prisms and gratings accounts for the largest proportion of the total stray light.

Measures to reduce stray light

Temperature and humidity are important factors that affect the performance of the instrument. They can cause corrosion of mechanical parts, reduce the smoothness of the metal dysprosium surface, increase the error of the mechanical part of the instrument or reduce the performance. The aluminum film is isolated, resulting in insufficient light energy, stray light, noise, etc., and even the instrument stops working, which affects the life of the instrument. It should be corrected regularly during maintenance. Constant temperature equipment should be configured, especially in laboratories located in the south. Dust and corrosive gases in the environment can also affect the flexibility of the mechanical system and reduce the reliability of various limit switches, buttons, and photoelectric couplers. Therefore, regular cleaning is necessary to ensure the environment and indoor sanitary conditions of the instrument.

In addition, the optical components are easily loosened and displaced after being vibrated, and the beam cannot be positioned accurately. It is believed that the light source and reflector must be carefully adjusted to return to the normal working position. The stray light is increased due to optical zero-secondary surface pollution and dust accumulation. You can use a clean soft chop and zero blowing ball to gently blow the cotton wool to remove the whole process, or use a degreased soft cloth to mix Z ether and alcohol. , carefully wipe the surface of the optical parts other than the reflector. If it is due to the high humidity of the instrument, the surface of the optical parts is dehydrated, or the reflective mirror surface is closed, the film is damaged and the stray light increases, so be very careful when wiping, and do not touch it with your hands, because most of the reflection is used. It is made by the method of vacuuming aluminum plates, and the surface coating layer is easily damaged. In daily work, you should always pay attention to protecting the instrument from vibration), especially dust-proof, moisture-proof and pollution-proof, so do not easily open the cover plate of the color spectrophotometry and colorimeter.

The colorimeter has the difference between stray light and no stray light

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