The lens collects light from a source and refracts the light to form a usable image of the source. The source may either produce light itself or may be an illuminated object.
The article describes the working of lenses. This is to help select the best lens for one’s particular application.
Types of Lenses - Simple, Compound, Complex
The term “lens” is applicable to a number of configurations. The most basic, the simple lens, is a single element. The compound lens consists of a single group of two or more simple lenses, and the complex lens is one made up of multiple groups of lens elements, such as UV Microscope Objective Lenses.
Figure 1 shows the three types of lenses
Figure 1. Three Lens Types
Basic Lens Characteristics
In order to predict lens performance, it is important to understand certain characteristics of lenses.
Effective Focal Length
A lens forms an image of the source on a target on which it is focused at an appropriate distance away. The distance along the optical axis of the lens from the focal point back to the effective bend point of rays near the axis (paraxial rays) is the effective focal length (EFL) of the lens.
Figure 2 shows this concept for a compound lens.
Figure 2. Basic Lens Characteristics
Lens speed is expressed in the form of F/#. It is defined as:
EFL = Effective Focal Length
D = Lens Diameter
Field of View
The concept of angular field of view (FOV) is important when the usable image size is limited. FOV is the angle subtended by the source, producing the maximum usable image size. The FOV is often determined by the size of a detector.
Light is collected by the lens from a point on the source, and focuses it to a corresponding point (a conjugate point) on the image. It is important to choose a lens system with a small enough blur circle to offer the image quality or resolution required.
Spherical aberration is the imaging error found when a lens is focusing an on-axis (FOV = 0) bundle of monochromatic light. It is usually the most significant monochromatic aberration. Spherical aberration is reduced in simple lenses by choosing the lens and orientation which “balances the work between the two surfaces,” e.g. choose a plano convex lens to focus a collimated beam, with the convex surface facing the beam. This results in approximately equal angular bending at each of the surfaces. Use a bi-convex lens for a 1:1 image, as both surfaces do equal work.
Figure 3. Spherical aberration in a Plano Convex Lens
Coma affects off-axis light bundles in a similar manner to the way in which spherical aberration affects on-axis bundles. The coma of two simple lenses combined symmetrically is canceled. There is no net effect on final image quality. In a lens system where coma has not been canceled, the residual coma combines with other off-axis aberrations, making it difficult to determine the individual contribution of coma to final image quality.
Figure 4. Off-axis abberation, coma. AC is the optical axis, AB is the direction of the incoming bundle of rays
It is desired that the final image is formed on a plane surface. However most optical surfaces form the image in a curved surface. The nominal curvature (1/radius) of that surface is referred to as the Petzval Curvature of the lens. For simple lenses, this curvature is roughly equal to 2/3 of the lens power (1/f).
Figure 5. Tangential and Sagittal Field Curvature
When astigmatism is present in a lens system, fans of rays of differing orientations at the lens aperture tend to focus on different curved surfaces. Astigmatism is, by definition, the difference between the tangential and sagittal field curves. If the tangential and sagittal surfaces are coincident then the lens is free of astigmatism, and the image is formed on the Petzval surface.
Distortion is an aberration that affects the shape of an image, rather than the sharpness. The image differs from a true scaled duplicate of the source.
The following chromatic aberrations can be important when a lens is used over an extended spectral bandwidth.
The index of refraction of all optical materials varies as a function of wavelength. The index is typically greater for shorter (blue) wavelengths. The rate at which it changes is also greater for the shorter wavelengths. In a simple lens, this means each wavelength is focused at a different point along the optical axis. This chromatic spreading of light is called dispersion.
The Point Spread Function
Point Spread Functions show the image quality formed by a lens system or a lens. The X and Y dimensions of the PSF represent the size and shape of the image. The height of the PSF is proportional to the power density focused at that point on the image plane.
About Oriel Instruments
Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.
Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.
This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.
For more information on this source, please visit Oriel Instruments.