Optical units are dimensionless units of length used in optical

microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micr ...

. They are used to express distances in terms of the

numerical aperture In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the proper ...

of the system and the

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...

of the light used for observation. Using these units allows comparison of the properties of different microscopes. For example, the diameter of the first minimum of the Airy disk is always 7.6 optical units in the

image plane In 3D computer graphics, the image plane is that plane in the world which is identified with the plane of the display monitor used to view the image that is being rendered. It is also referred to as screen space. If one makes the analogy of taking ...

of a

diffraction limited The resolution of an optical imaging system a microscope, telescope, or camera can be limited by factors such as imperfections in the lenses or misalignment. However, there is a principal limit to the resolution of any optical system, due to th ...

microscope.

Equations

There are two types of optical units. ''Radial optical units'' are measured in the image plane, and ''axial optical units'' are used to measure distances between the image plane and the observer. The number of optical units

v

in a given radial length

r

is given by:

v_\mathrm = \frac \frac r

where: *

\lambda

is the wavelength *

n \sin \alpha

is the

M_\mathrm

is the total

magnification Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a calculated number also called "magnification". When this number is less than one, it refers to a reduction in siz ...

Axial optical units are more complicated, as there is no simple definition of resolution in the axial direction. There are two forms of the optical unit for the axial direction. For the case of a system with high numerical aperture, the axial optical units in a distance ''z'' are given by:

u_z = \frac \frac z

where: *

\eta

is the index of refraction of the medium above the optical plane. For systems with low numerical aperture, the axial optical unit is:

u_z = \frac \sin^2(\frac) z

References

Microscopy Units of length {{optics-stub