Any kinds of optical instrument's use and service conditions will have a requirement for their optical systems, they can be summarized as the following aspects.
1. The basic characteristics of optical system
The basic characteristics of optical system are: numerical aperture or relative aperture, linear field and field angle, magnification and focal length of system. Besides, there are some characteristic parameters that have something to do with these basic characteristics, like the size and position of pupil, back working distance and conjugate distance.
2. The boundary dimension of system
The boundary dimension of system means its lateral dimension and longitudinal dimension. When designing the complicated optical system, the calculation of boundary dimension and the connection of pupil between light groups are all important.
3. Imaging quality
The requirement of imaging quality has something to do with the use of optical system. Different optical can have different requirement for imaging quality according to its use. For telescopic system and common microscope, they only require the imaging quality on central field. But for photographic lens, they need whole field with good imaging quality.
4. The using conditions of instrument
We must consider the possibility of achievement in technology and physics when having requirements for optical system. Such as the magnification of biological microscope has to meet 500NA≤Г≤1000NA, the visual magnification of telescope must take limiting resolution of telescopic system and eyes into consideration.
The design process of optical system
Optical system design means to decide various data that meet the needs of use according to use conditions, we can also say decide the performance parameter, boundary dimension and light group's structure of optical system. We can divide optical design process into steps: calculation of boundary dimension, calculation of initiating structure, correction and balance of aberration and image quality.
1. Calculation of boundary dimension
We should protocol the schematic diagram of optical system, make sure the characteristics of optics to meet the needs of technology, that means to make sure magnification, focal length, linear field and field angle, numerical aperture or relative N1N aperture, conjugate distance, back working distance, diaphragm place and boundary dimension. Therefore, we always call this step calculation of boundary dimension. It is calculated according to ideal optical system theory and computational formula. We must pay attention to mechanical structure and electrical system in case it will fail in mechanical structure when calculating. Making sure every performance should be reasonable, too high requirement will cause complicated design result and high cost while too low requirement will cause unsatisfactory design.
2. Two ways for calculating and choosing and ensuring of initiating structure:
(1) Solving initiating structure according to primary aberration theory.
This way of solving initiating structure means to meet the imaging quality of initiating structure according to the basic characteristics and primary aberration theory by calculating.
(2) It is a practical and successful way to choose initiating structure from existing data, so that it is widely used by many optics designers. But it requires designers with much knowledge about optics theory and rich design experience, so that they can choose simple and reasonable initiating structure from many complicated structures. Model selection plays a very important role in design. A successful design with good automated layout programs and experienced designers can't without a good initiating structure.
3. Correction and balance of aberration
After choosing initiating structure, we should calculate optical path through optical program by computer and get all aberration and aberration curve. We can find which aberration will have influence on the imaging quality of optical system by analyzing aberration data, then finding some ways to correct aberration. Correction and balance of aberration is a repeating process until meet the requirement of imaging quality.
4. Image evaluation
The imaging quality of optical system has something to do with the size of aberration, the purpose of optical design is correcting aberration of optical system. However, it's impossible and unnecessary for any optical system to correct aberration to zero, there must be residual aberration, different residual aberration has different imaging quality. So the optical designers must know the allowable value and aberration formula of various optical system, so that we can judge the imaging quality of optical system by the size of residual aberration.