Depth Of Field Of Endoscope Camera System

Introduction of medical endoscope optical system

At present, medical endoscopes mainly include three types: fiber electronic endoscopes, rigid endoscopes and capsule endoscopes. Hard endoscope plays an important role in many medical examinations, diagnosis and treatment because of its advantages of clear imaging, low price and high imaging resolution.

Hard endoscope minimally invasive surgery is to insert a hard endoscope into the human body (such as the abdominal cavity), use a cold light source to light through the optical fiber, and transmit the image to the digital camera through the optical system, which is displayed on the high-definition display in real time. Then the doctor analyzes, judges and operates on the patient's condition according to the image of the tissue organ on the display.

The optical system of medical rigid endoscope mainly includes four parts: rigid endoscope, coupling lens, splitting prism and CMOS image sensor. Hard endoscope can be divided into objective lens, relay system and eyepiece.

The objective lens of medical rigid endoscope has the characteristics of short focal length, large field of view and long optical length. Therefore, the optical system is generally designed with an inverse telephoto objective lens structure. The negative power lens group is used as the front group, and the positive power lens group is used as the rear group. When the parallel beam is incident, it is diffused by the front group and then formed into an image on the focal plane. The main surface of this optical system is removed from the objective lens to the rear, thus obtaining a working distance longer than the focal length. In addition, after the off-axis beam with a large field angle diverges through the negative power lens group, the divergence angle entering the positive power lens group becomes smaller, so as to achieve the goal of wide angle.

In order to penetrate the human body to observe the lesions, the medical rigid endoscope needs to be designed with a certain length, such as the laparoscope, which is generally 30 cm long. This requires that in addition to the imaging objective lens, the rigid endoscope also needs a rod mirror to act as a relay image transmission system, with a magnification of - 1. The use of odd groups can make the system become positive.
The function of the endoscope eyepiece is to pass the image formed by the objective lens and relay system through the eyepiece and enter the human eye in the form of parallel light.

Now, the adaptive lens is basically added on the basis of the visual endoscope, so that the outgoing light of the eyepiece can be imaged onto the CMOS target through the adapter lens. The exit pupil of the eyepiece is the entrance pupil of the adapter
To sum up, as far as the overall design is concerned, the essence of the endoscope in the entire optical system can be compared to a photographic objective. Referring to the depth of field evaluation in the camera lens, the depth of field of the endoscope imaging system focuses on the endoscope itself, not the lens.