Transmittance accuracy refers to the difference between the measured value of a sample with a standard transmittance value and the nominal transmittance value of the standard sample by a colorimeter without considering random errors.
When the instrument is in a working state, the light source should be stable without flickering. When the wavelength is set at 580nm, a normal yellow light spot can be seen in the sample chamber and at the position of the sample to be tested. The sample chamber of the instrument should be well sealed. There is no light leakage phenomenon, the sample holder should be pushed and pulled freely, positioned correctly, and the light transmittance of each light-transmitting hole should be the same.
For instruments with sensitivity conversion gears, the sensitivity should be set to the lowest gear during normal use, as long as the transmittance value of this gear can be adjusted to full scale when air is used as the measurement reference. If the sensitivity block is set too high, the dark current of the instrument will increase, and the zero drift of the instrument will increase accordingly, resulting in poor transmittance accuracy.
The spectral neutral filter in the visible region used to verify the accuracy of transmittance is designed with the size of the sample chamber of a digital instrument such as ATO-CM-NH300. Therefore, when calibrating pointer-type instruments such as the model, the spectral neutral filter must be placed in the center of each light-transmitting hole in the sample chamber to avoid position deviation, and the incident monochromatic radiation flux is blocked, which makes photoelectric detection. The transmitter does not fully receive the monochromatic radiation flux that should be transmitted, resulting in poor transmittance accuracy.
The effect of stray light. There are two main types of stray light: one is the light that deviates from the normal optical path and reaches the photodetector other than the wavelength used for measurement; the other is the light with the same wavelength as the measurement wavelength, but does not pass through the sample and directly reaches the photodetector. This kind of stray light is the so-called non-absorbing stray light. Stray light is a major source of error that causes errors in spectrophotometric measurements. Suppose the proportion of stray light to the total incident radiant flux is S, and the amount of stray light is SIo: then the measured value As after bringing in the stray light is:

When the stray light is not absorbed by the sample, the absorbance measurement value is always lower than its true value, that is, there is a negative deviation from the Lambert-Beer law. The higher it is, the more obvious its impact will be.
When part or all of the stray light is absorbed by the sample, the influence on the measured value of absorbance is more complicated. It is not only related to the wavelength distribution of the stray light, but also to the spectral characteristics of the measured sample. But in the end, the measured value is higher than the true value, and there is a positive deviation from the Lambert-Beer law.
In the above two cases, no matter whether the stray light is absorbed by the sample or not, it will inevitably affect the measurement results using the absolute measurement method.