Optical design of spectrophotometer

The optical system is the core part of the spectrophotometer. Based on the basic principles of the spectrometer and optical design theory, with the specific design goal of portability, low cost, and meeting the design requirements of the spectral range and resolution, four feasible design schemes, namely the Littrow system, the Albert Fastian system, the Cherney Turner system, and the crossed Cherney Turner system, were compared and analyzed, Propose an asymmetric crossed Czerny Turner structure with a planar diffraction grating as the dispersion element as the system structure for this design.

The system is simulated and optimized by optical software. The design results show that the designed system has a spectral range of 360nm~740nm, a Spectral resolution of 10nm, a F number of 5.25, a spectral expansion of 44.1mm, and a system volume of about 80mm × 69mm × 62mm, meeting the design requirements of high accuracy, small size, and low cost.

Optical Design Theory:
Generally, the aberration of optical instrument can be divided into monochromatic aberration and chromatic aberration. For monochromatic aberration, there are spherical aberration, cometary aberration, astigmatism, field curvature and distortion. The main correction methods for aberration include spherical aberration, coma, and chromatic aberration. As the components used in the designed spectrophotometer are reflective elements, the system does not have chromatic aberration. Therefore, only spherical aberration and coma need to be corrected.

Spherical aberration is caused by the inability of parallel beams of different apertures to converge at a single point. On the contrary, due to spherical aberration, the collimator cannot convert all light from any point on the slit into a parallel beam. Spherical aberration can cause broadening of spectral line contours, blurring of spectral lines, and reduced resolution. The spherical aberration of the collimator and imaging mirror cannot be eliminated by adjustment, so it must be corrected within the aberration tolerance during design.

Due to coma, light emitted from a point at the height of a very close axis slit cannot become a parallel beam when passing through the collimating objective, and the beam structure is asymmetric. On the contrary, the imaging mirror cannot converge the parallel beams emitted from the dispersion system to a single point.

Comet aberration also has a serious impact on the spectral line contour, not only causing unilateral diffusion of the spectral line contour, reducing the resolution of the instrument, but also causing displacement of the maximum value of the spectral line contour, and even generating false companion lines. Therefore, coma must also be limited within the aberration tolerance.

The objective lenses of both collimation and imaging systems need to correct for spherical and coma errors. Based on experience, the Rayleigh criterion is generally used as the aberration tolerance. So the Rayleigh criterion is that the maximum wave aberration generated by residual spherical aberration and residual coma should be less than. According to the relationship between axial and wavefront aberrations, the tolerances for spherical and coma aberrations can be obtained as follows:

Axial spherical difference:

Deviation from sine condition:

In the formula: D is the effective aperture of the aperture, which in the spectral instrument is the effective width of the dispersion element; F ‘is the focal length of the objective lens;

Benchtop Spectrophotometer (Reflectance and Transmittance) DSCD-920 adopts 7 inches touch screen, full wavelength range, Android operate system. Illumination : reflectance D/8° and transmittance D/0°(UV included / UV excluded), high accuracy for color measurement, large storage memory, PC software, because of above advantages, it is used in laboratory for color analysis and communication.

Benchtop Spectrophotometer (Transmittance) DSCD-910 is good performance and specially designed for testing the transparent material’s transmittance, absorbance, chromaticity value and other parameters.

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Tags：DSCD-910 , DSCD-920