Numerical Aperture of Optical Microscope Performance
Numerical aperture abbreviation NA, numerical aperture is the main technical parameters of the objective lens and spotting scope, is to judge the two (especially for the objective lens) the performance of the important signs of high and low. The size of the value is engraved on the housing of the objective lens and the condenser lens, respectively.
Numerical aperture (NA) is the object before the objective lens and the object to be examined between the refractive index of the medium (n) and the aperture angle (u) half of the sine of the product. Expressed in the following formula: NA = nsinu / 2
Aperture angle, also known as the "mirror mouth angle", is the objective lens on the optical axis of the object point and the effective diameter of the lens before the objective lens formed by the angle. The larger the aperture angle, the greater the light flux into the objective lens, it is proportional to the effective diameter of the objective lens, and the focal point of the distance is inversely proportional.
Microscope observation, if you want to increase the NA value, the aperture angle can not be increased, the only way is to increase the refractive index of the medium n value. Based on this principle, it produces a water immersion objective and oil immersion objective, because the refractive index of the medium n value is greater than 1, NA value can be greater than 1.
The maximum value of numerical aperture is 1.4, a value that has reached its theoretical and technical limits. Currently, there is a high refractive index of brominated naphthalene as a medium, the refractive index of brominated naphthalene is 1.66, so the NA value can be greater than 1.4.
It must be pointed out here that, in order to give full play to the role of the numerical aperture of the objective lens, in the observation, the NA value of the focusing lens should be equal to or slightly larger than the NA value of the objective lens.
Numerical aperture has a close relationship with other technical parameters, it almost determines and affects all other technical parameters. It is directly proportional to the resolution, directly proportional to the magnification, and inversely proportional to the depth of focus, NA value increases, the field of view width and working distance will be correspondingly smaller.,,,
