In vertain applications, Single lenses may not be sufficient in cases where image quality is a concern. Multiple element lenses can offer a higher quality image, because dividing the lens power between several elements usually results in reduced aberrations. While building a prototype to sense incident laser energy, the instrument has three components as seen in Figure 1, which include an objective lens, a field stop, and a detector, with the following specifications:
|Objective lens EFL
|Field of view
||0.5 x 0.5 mrad
|Field stop size
||0.1 x 0.1 mm
Figure 1. Prototype of system to sense incident laser energy
Figure 2 shows that a good part of the focused energy falls beyond the limits of the field stop. Only about 40% of the energy collected is seen by the detector. An alternative to using the 40780 is to use two simple lenses with a combined EFL of 200 mm. An equal division of lens power is not critical in this application.
Figure 2. PSF shows that, when using a plano convex lens in the system in Fig. 20, the majority of the focused energy falls beyond the limits of the field stop.
The resulting lens power (1/EFL) is found by adding the individual lens powers:
θ total = θ 1 + θ 2*
θ total = 1/350 + 1/500 = 0.0049 *
EFLcombination = 1/0.0049 = 205.9mm
*Assumes the lenses are close to each other, otherwise:
θ total = θ 1 + θ 2 + d θ 1 θ2
Where d = Separation of the lenses
The image quality for this combination is greatly improved over that of the single 40780 Lens. For the two lens combination, the geometric blur circle diameter is 0.07mm, and the PSF shows that nearly 100% of the incident energy passes through the field stop onto the detector.
A three lens combination (two 40810, 500mm, and a 40820, 1000mm, lenses) produces a blur circle of 0.05mm.
Four lenses are required to get the low spherical aberration. Axial transmittance for these lenses is only 0.7, since no anti-reflection coatings are used.
Aspherics do not have spherical surfaces, unlike most lenses. Their surfaces are shaped to eliminate spherical aberration, even at low F/#s. High accuracy aspherics are quite expensive, but for non-critical, very low F/# applications.
An achromat is a lens constructed of two different element materials, usually flint and crown glass, cemented together. While generally selected for its ability to cover a broad spectral region, the achromat also offers superior correction of spherical aberration in monochromatic applications.
Figure 3 shows the advantages of the 42650 Achromat with monochromatic light. This 200 mm F/4 lens produces a blur circle with a diameter of about 1/4 that of the blur circle produced by using two simple lenses (Fig. 22).
Figure 3. PSF shows that using two simple lenses for the system in Fig. 20 results in nearly 100% of the incident energy passing through the field stop.
The MTF curves in Figure 24 show the advantages of the achromat with monochromatic light. If 0.1 is the minimum acceptable contrast for resolution, then you can see that the single lens is limited to 7 cycles per mm, the two lenses resolve to 18 cycles/mm, three lenses to 24, and the achromat has the dramatic increase to 150 cycles/mm.
If the object distance is more than 25 times the lens focal length, then a lens designed for infinite conjugates will function well. If the object distance is less than 25 times the lens focal length, a lens designed for infinite conjugates may not provide the required image quality.
Figure 4 shows a lens with a focal length of 200 mm, used to image objects at distances ranging from 5 to 500m. Since all object distances are greater than 25 times the lens focal length, a lens designed to image objects at infinity should provide adequate image quality, providing the focus is set correctly.
Figure 4. A 200mm focal length lens used to image objects at distances from 5 to 500m.
True Finite Conjugates
When the object distance is substantially less than 25 times the lens focal length, it is likely that the lens designed to function at infinite conjugates will not provide acceptable image quality, due to residual aberrations. One approach to solve this problem is to use two or more lenses each designed for, and working at, infinite conjugates.
Prototype System Design with Stock Lenses
There are many hardware/software combinations available for optical design and analysis, ranging from simple programs to very powerful and comprehensive optical system design packages.
There are several low cost options. It is possible to write a simple program for the computer that will handle basic thin lens calculations. With the lens specifications listed in the product description pages, it is possible to simulate and analyze a lens system.
Nomenclature for System Design
Aperture and Field Stop
An aperture stop limits the size, and thus the total power, of an incident beam on a collecting lens. You can vary the total flux using an aperture or an iris diaphragm. A field stop controls the size and shape of the image. It is important that the field stop have a sharp, well-defined edge, since it is that edge that determines the limits of the image. Figure 5 shows a lens system with an aperture stop and field stop.
Figure 5. Typical lens configuration showing aperture and field stop.
There is no interdependency between the aperture stop and field stop. As the aperture stop is enlarged or reduced, the image size remains constant. If the field stop is enlarged or reduced, the image size changes, but the power density in the image remains the same.
Entrance Pupil and Exit Pupil
The entrance and exit pupils of an optical system are important, because their size and location frequently enter into system calculations. The F/#, for instance, is found by dividing the system focal length by the diameter of the entrance pupil. Exit pupil characteristics are used to compute numerical aperture and other image brightness factors.
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.