TN HTB Spherical Lenses

7/23/2019 TN HTB Spherical Lenses

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Lenses are an integral component in most optical systems,

where they are used to focus, collimate, expand, collect

and image light. Many optical tasks require several lenses

in order to achieve an acceptable level of performance.

This selection guide will review the singlet spherical

lens shapes offered by CVI Laser Optics and offer some

practical guidance on determining the best material type

and lens quality for your application. For a complete

discussion of lens theory, use, and aberrations please refer

to the Fundamental Optics and Gaussian Beam Optics

sections of the Technical Guide.

Lens Shape

CVI Laser Optics offers four lens types for converging or

focusing light. A bi-convex lens is the classic symmetric

lens, possessing two convex surfaces of equal radii.

Bi-convex lenses have positive focal lengths and form

both real and virtual images. It is the best singlet lens

for imaging at unit magnification; spherical aberration

is minimized, and coma, distortion, and transverse

chromatic aberration exactly cancel each other out for a

perfectly made lens (longitudinal chromatic aberrationis not corrected). This is true regardless of the material

used or wavelength, although use of a remote stop can

reduce the degree of cancellation. Aberrations increase as

conjugate ratios (object distance/image distance) depart

from unity. Bi-convex lenses can also be used for focusing

applications, in particular when a lower f-number (ƒ /CA) is

required, even if they do not have the best shape for this

conjugation. They are recommended for virtual imaging

of real objects and for positive conjugate image ratios

from approximately 0.2 to 5 (note that these values are

wavelength sensitive).

Away from unity, the singlet lens shape that best minimizes

spherical aberration at a given conjugate ratio is called

a bestform lens, in which the two convex sides are of

different radii. The marginal rays are equally refracted

at each of the lens/air interfaces for this shape, and

surface-reflection loss is minimized. Another benefit

is that absolute coma is nearly minimized for bestform

shape, at both infinite and unit conjugate ratios. It does

not, however, perform well in wide-field applications,

except in very specific configurations (with a meniscus for

instance). At infinite conjugate ratio, the best form lens

is not the optimum singlet shape, since it results in high

field curvature. Positive bestform lenses are of exceptionaperformance and provide the smallest spot size available

in a singlet lens.

When working at infinite or near-infinite conjugate ratio,

plano-convex lenses with the convex side toward the

infinite conjugate perform nearly as well as the best-form

lens. Convex on one side and flat on the other, these

positive focal length lenses cost much less to manufacture

than a bestform lens. This lens shape also exhibits near-

minimum transverse spherical aberration and near-zero

coma when used off-axis. Longitudinal aberration is low,

but is only minimized when using a best form lens.

If bulk light collection is required at minimal cost, an

aspheric glass condenser lens may be the solution.

Aspheric lenses provide better performance by reducing

aberrations when used in low f-number, high-throughput

applications. One surface is aspheric; the second surface

is flat, or spherical-convex. A flat second surface minimizes

aberration, while a spherical-convex second surface

How to Select a Spherical Lens

Fig 1: Bestform lens



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provides the lowest f-number and highest transmission.

Only single-layer MgF2 antireflection coatings are

recommended for these lenses due to the steep curvature

of the surface. The molding and felt-polishing processesused to manufacture these lenses economically, together

with the use of optical crown glass, makes these lenses

best suited to less demanding light collection applications.

They are not recommended for imaging, precise focusing,

or high power applications, but are ideal for gathering

low-power light with minimal aberration.

CVI Laser Optics offers two lens types for diverging or

expanding light. A bi-concave lens is a symmetric lens,

possessing two concave surfaces of equal radii. Bi-

concave lenses have negative focal lengths and form only

virtual images which can be seen through a lens. They areused in laser beam expanders, optical character readers,

viewers, and projection systems to diverge collimated

incident light.

Like their convex counterparts, plano-concave lenses can

reduce aberrations as compared to bi-concave lenses,

depending on the configuration. They have a negative

focal length and are often used to expand light or to

increase focal lengths in optical systems, as they diverge

collimated incident light. When working at infinite or near-

infinite conjugate ratio, plano-concave lenses with concave

side toward the infinite conjugate reduce sphericalaberration, and coma. Additionally, the negative spherical

and chromatic aberration that plano-concave lenses

exhibit can be used to balance the aberrations resulting

from other lenses within a system.

Lens Materials

Aside from lens shape, the material a lens is made from

has the greatest impact on its performance. Not only does

it determine the transmission properties, refractive index,

laser damage threshold, thermal coefficient, durability and

weight, but it also impacts cost. It even imposes practical

limits on manufacturing tolerances, especially surface

cosmetic quality. CVI Laser Optics utilizes six different

materials to manufacture our catalog singlet lenses, but

other materials such as magnesium fluoride, sapphire,

germanium, and Suprasil are available on a custom basis.

We can also manufacture many of our lenses with custom

dimensions and focal lengths.

Fig 2: Plano-convex lens

Fig 3: Aspheric glass condenser

Fig 4: Bi-concave lens



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N-BK7 is a lead- and arsenic-free borosilicate crown glass

that is used widely in the optics industry. It has excellent

transmission from 350 nm – 2.0 μm, good thermal

expansion coefficient, moderate laser damage threshold,

and is relatively low in cost. It is a hard glass that stands

up well to handling, with good chemical resistance.

Our fused silica is Corning 7980 UV-grade, a synthetic

form of fused silica manufactured by flame hydrolysis to

extremely high standards. Its ultra-low impurity content is

evident in the wide transmission range of 180 nm – 2 μmand its high laser damage threshold. It does not fluoresce

in response to wavelengths longer than 290 nm, and in

general exhibits good resistance to radiation darkening

from ultraviolet, x-rays, gamma rays, and neutrons. It also

boasts excellent thermal properties, including a wide

operating temperature range, low thermal coefficient,

and resistance to thermal shock. Fused silica lenses from

CVI Laser Optics have increased hardness and resistance

to scratching, resulting in better surface quality than their

N-BK7 equivalents.

Excimer grade fused silica is UV-grade fused silica thathas been manufactured with near-zero defects for use with

high power KrF lasers at 248 nm. This material has very

low levels of inclusions, bubbles, striation and striae, and

minimal variations in index of refraction. Its exhibits the

lowest level of laser induced fluorescence of all the Schott

glasses, strong resistance to radiation-induced defect

generation, and higher UV laser damage threshold than

CaF2. It is one of the more expensive UV material options,

particularly at larger diameters. ArF grade fused silica for

use at 193 nm is also available upon request.

N-SF11 is a type of lead- and arsenic-free Schott glasswith much higher refractive index than N-BK7, resulting in

greater focusing power and reduced spherical aberration.

Its transmission is best in the visible and near-infrared,

from 500 nm – 2.5 μm. Though its thermal characteristics

are similar to N-BK7, it is a slightly softer glass, resulting

in lenses with lower surface quality than their N-BK7

equivalents.

CaF2 is a cubic single-crystal material with transmission

spanning the deep UV through infrared, 150 nm – 8 μm. It

can be mined or manufactured synthetically, but is higher

in cost than other lens materials, particularly in the high-purity forms used for deep-UV applications. That being

said, its excellent transmission and high laser damage

threshold in the deep UV makes it the material of choice

for many excimer laser applications. Lenses manufactured

with CaF2 tend to have slightly lower surface quality than

their N-BK7 or fused silica equivalents.

Optical crown glass is a low-index, commercial-grade

glass. Its index of refraction, transmittance, and

homogeneity are not controlled as carefully as in optical-

grade glasses like N-BK7. It transmits from 200 nm – 2

μm, with best performance from 400 nm – 1.5 μm. Thoughits thermal expansion coefficient is reasonable and its

hardness makes it robust to handling, its laser damage

threshold is low.

Lens Quality

Once the lens shape and material has been selected, the

next step is to determine the lens quality required. This

will depend on the application and performance needed,

but factors to consider include laser damage threshold,

degree of scatter, surface figure, and focal lengthtolerance.

Standard lenses possess a surface irregularity of λ/4 to

λ/2 before coating, and are manufactured to a minimum

surface quality of 60-40 to 40-20 scratch and dig,

depending on material. They are an economical solution

for many applications, but lower surface accuracy impacts

resolution, and laser damage threshold is not as high.

Fig 5: Plano-concave lens



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CVI Laser Optics offers standard singlet lenses in both

N-BK7 and fused silica. These lenses are available

uncoated, or with a selection of antireflection coating

options from the UV to near-infrared.

Our laser quality lenses differ from standard lenses in

two key aspects. First, high cosmetic surface quality

(10-5 scratch and dig for fused silica, 20-10 for CaF2 and

N-SF11) reduces scatter, making them more suitable

for high energy laser applications. Second, careful

polishing to λ/10 surface figure and tighter paraxial focal

length tolerance yields the low wavefront distortion andaccuracy required for use in ultra-violet and performance-

critical applications. The term “laser quality” therefore

refers to both high laser damage threshold and to high

performance specifications. CVI Laser Optics manufactures

CaF2, N-SF11, and excimer grade UV fused silica spherical

lenses only in laser quality grade. All of our laser quality

lenses are available with a wide range of broadband,

dualband 1064/532, and narrowband V-coat antireflection

coatings at wavelengths from the UV through near-

infrared. These AR coatings offer high durability and

damage threshold, low loss, and peak performance

when applied to laser quality optics. For best laserdamage threshold performance, we recommend V-coat

antireflection coatings on laser quality 10-5 fused silica

substrates.

Our aspheric glass condenser lenses, in contrast, are

fabricated to light collection quality standards. This

involves a low-cost process in which the aspheric side is

precision molded and the second surface is felt polished

to achieve 80-50 scratch and dig surface quality. Light

collection quality is more than adequate for many low

f-number, high throughput applications in the visible

wavelength region, and provides an economical alternative

to standard N-BK7 lenses. These lenses are available with

an optional single-layer MgF2 coating for 400 – 700 nm.

Making the final decision

Choosing the right singlet lens or combination of

lenses for your application involves much more than

identifying the required lens material and quality. Detailed

performance analysis of an optical system is accomplishedby using computerized ray-tracing software, and CVI Laser

Optics applications engineers are available to provide a

ray-tracing analysis of simple catalog-component systems.

If you need assistance in determining the performance of

your optical system, or in selecting optimum components

for your particular application, please contact us.

Numerous other factors, such as lens manufacturing

tolerances and component alignment, impact the

performance of an optical system. It should be kept in

mind that if calculations indicate that a lens system only

just meets the desired performance criteria, in practiceit may fall short of this performance as a result of other

factors. In critical applications, it is generally better to

select a lens whose calculated performance is significantly

better than needed.

In addition to the array of singlet lenses described,

CVI Laser Optics also offers many cylindrical lenses,

as well as multi-element lenses that can simplify your



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optical design. No singlet lens can correct for chromatic

aberration, an important consideration when working

with multiple wavelengths. Achromatic lenses may be

needed to improve performance in this case. Additionally,a multielement lens system is often required at low focal

ratios when the aberrations of a singlet lens are too large

to satisfy performance requirements. Regardless of your

needs, we can work with you to find the right solution

by combining products from our catalog with lenses

manufactured to your exact specifications using custom

coatings.

Selection Guide:

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TN HTB Spherical Lenses