Selecting a suitable coupler to connect a camera to a microscope is an important step in the instrument’s configuration. Another term for a coupler is a C-mount adapter, and the terms are often used interchangeably. When selecting a coupler, several factors must be considered in order to maximize the camera’s field of view, without compromising image quality.
This blog post will provide a general guideline and tips for coupler selection, touching on the basics required to produce an evenly illuminated image. The field of view of the camera is dictated by three factors: the field number of the microscope (FN), the sensor format of the camera, and the magnification of the coupler. The FN is dictated by the microscope and typically varies from 20 to 26 mm. The sensor format of the camera is another way of referring to the size of the image sensor. Common sensor formats for microscopy cameras are 1/3", 1/2", 2/3", and 1".
A good rule of thumb to achieve even illumination across the entire field of view of the camera is to use the following reference, which assumes a relatively low FN to ensure compatibility with a wide range of microscopes:
|Sensor Format||Coupler Magnification|
If the selected coupler magnification is too small, a phenomenon called vignetting will occur. Vignetting occurs when the outer regions of the image, starting with the corners, start to have a drop in illumination. This is due to the fact that the optics in the microscope produce a circular image and the camera’s image sensor is rectangular in shape. When the magnification of the coupler projects the circular field of view onto a sensor that is smaller than the entire area of the image sensor, the corners are darkened, creating the vignetting effect.
Variation of Coupler Magnification with 40x Objective and INFINITY3-3URC Camera with 2/3" Sensor
By following the above recommendation, vignetting should be avoided for most microscopes. If you need help selecting the right coupler, another resource you can check out is Lumenera’s coupler selection app note.