What is the anti-telephoto objective and what is its main purpose?

In the measurement system, the object distance often changes, which makes the image height change, so the size of the measured object also changes, that is, the measurement error occurs; On the other hand, even if the object distance is fixed, the measurement error will occur because the sensitive surface of CCD is not easy to adjust accurately on the image plane. To solve the above problems, a telecentric objective lens can be used. Among them, the image telecentric objective lens can eliminate the measurement error caused by the change of object distance, and the object telecentric objective lens can eliminate the measurement error caused by the inaccurate CCD position.

1) objective telecentric objective

The telecentric objective lens places the aperture stop on the image focal plane of the optical system. Figure 1. 1-23 shows that when the aperture stop is placed on the image focal plane, even if the object distance changes and the image distance changes, the image height remains unchanged, that is, the size of the measured object will not change; Fig. 1. 1-24 clearly shows the principle of object telecentric optical path, in which the aperture stop is located on the image focal plane and the main ray of the object is parallel to the optical axis. If the object B 1B2 is correctly located at the position A 1 where it joins the CCD surface M * *, its image on the CCD surface is M 1M2. If the object B 1B2 is not in position A 1 but in position A2 due to the change of object distance, then its image b? 1B？ 2 deviation from CCD plane, b? 1 and b? The projection of 2 points on the CCD plane is a diffuse spot, and its center is still M 1 and M2 point. According to this projection, the reading length is still M2M 1. That is to say, the above object distance change does not affect the measurement accuracy.

Figure 1-23

2) image telecentric optical path

The telecentric optical path of the image side is to place the aperture stop on the object focal plane of the optical system, and the main light of the image side is parallel to the optical axis. As shown in figure 1. 1-25. What about image b of object B 1B2? 1B？ 2 does not coincide with CCD plane m, so what is obtained on CCD plane m is b? 1B？ 2. The distance between speckle centers is m 1m2 = b? 1B？ 2。 Therefore, regardless of CCD planes m and b? 1B？ 2, and the length corresponding to the scale is always B 1B2, so there is no measurement error.

Figure 1-24

Figure 1. 1-25 image telecentric optical path

1. 1.6 long-distance objective lens

A long-distance objective lens is an objective lens with a long focal length and a short lens barrel. The distance from the front surface of the objective lens to the image plane is smaller than the focal length, which is beneficial to shorten the axial size of the objective lens for a long focal length objective lens.

The remote structure adopts the form of separation of positive and negative power, with the positive power lens group in front and the negative power lens group in the back, as shown in figure 1. 1-26. The main surface of the whole system moves out of the objective lens, which makes the barrel length of the objective lens-the distance from the front surface of the objective lens to the focal plane smaller than the focal length, thus reducing the structural size of the objective lens. Here, the ratio of tube length to focal length L/f? It's called distance ratio. The distance ratio is an important index of the distance objective. Usually the distance ratio is less than 1, and the smaller the better.

According to the above principle, the long-distance objective lens has many structures, especially the front group, which is generally more complicated than the rear group because of its large power.

Figure 1. 1-26 Gaussian optics of long-distance objective lens

The front group has a three-lens structure, as shown in figure 1. 1-27, and its relative aperture is 1: 4.

Figure 1. 1-27 telephoto objective

1. 1.7 reverse distance objective lens

The anti-telescope is an objective lens with short focal length and long intercept. In this way, a beam splitter can be added between the objective lens and CCD to realize the function of framing. The structure of this objective lens mostly has a negative front lens and a positive rear lens, as shown in figure 1. 1-28.

The aperture stop and the exit pupil of the anti-telephoto objective almost overlap and are located on the main plane of the second lens group (figure 1. 1-28a), or the aperture stop can be located near the front focus of the second lens group (figure 1. 1-28b). In this case, the design principle that the main beam of the image side is telecentric is the most ideal, but it is difficult to realize in an objective lens with a relatively large aperture.

Figure 1. 1-28 optical path diagram of anti-telephoto objective.

The anti-telephoto objective lens designed according to the above structure has various structural styles in the front group and the back group. The front group with negative power ranges from Dan Toujing to very complicated structures, while the back group with positive power often adopts Petzval type, tri-plate type, double Gaussian type and their complicated structures. As shown in figure 1. 1-29, the first group is a single lens, and the second group is a three-piece double Gaussian anti-far objective, which is the simplest structure of the anti-far objective. Their working distance is equivalent to the focal length, their viewing angle is about 60, and their relative aperture is 1: 3.5 ~ 1: 2.5.

Figure 1. 1-29 The front group is a single negative lens, and the rear group is three anti-distance objective lenses.

1. 1.8 distorted objective lens

The deformed objective lens will introduce a predetermined deformation into its image in advance. When the negative distortion of the objective lens is large, it can actually shoot the object space with the angular field of view exceeding180. This objective lens is used in aerospace research and gas image measurement. The size of the image is not according to formula y? =-f？ Tanω is definite, but, for example, by the following formula: y? =-f？ Sinω. In the latter case, when -ω = 90o, y? = f？ That is, the diagonal of the image frame is twice the focal length.

The schematic diagram of the distorted objective lens is shown in figure 1. 1-30a. 1930, Guiliou successfully realized a distorted objective lens with an angular field of view of 180o and a relative aperture of 1: 22 for the first time (Figure 1. 1-30b). According to the optical path diagram of the anti-distance objective lens, a distorted objective lens can be made. The first group consists of one or two lenses, which will produce great distortion (Figures C and D). The second lens group is used to correct aberration to obtain a clear image.

To develop ultra-wide angle objective lens, the fourth power influence of image angle cosine is the biggest obstacle. However, due to negative distortion, the beam shrinks deeply at the edge of the image field, so the actual optical density at the edge of the image field is not lower than the center of the field of view.

Fig. 1. 1-30 optical system diagram of distorted objective lens