BK7 B270 Superwhite Borofloat Fused Silica Soda Lime Float Glass

Technical Background:
About Optical Quality & Specifications

When specifying an optical component, three things must be clearly understood and defined in order to make the most informed choice for a particular application:  material, configuration and surface quality

Choosing appropriately will provide the designer with the best cost vs quality while assuring the optic will perform as required.

Optical Material

Wavelength vs transmittance

 

The majority of optical components are used as transmitting optical components in photonics applications.  It follows that the proper initial choice of candidate materials will be ones that transmit photons efficiently in the wavelengths important to the performance of the device for which the optical component will be used.

Transmittance vs materials

 

At this writing, transmitting optics are used in wavelengths from .157 micron to about 30 microns.  Transmitting materials are generally divided into two general categories: amorphic and crystalline.

Crystalline materials

 

Crystalline materials are used for all wavelengths from .157 micron to 30 microns.  These materials can be single or polycrystalline in structure and are synthetically grown in special environments.  Single crystal materials are subdivided and sliced into specific orientation of crystalline structure in order to further optimize optical performance.  Crystalline optical materials are expensive to synthesize and manufacture into optical components but offer certain optical and physical qualities not achievable in amorphic materials.

Amorphic materials

 

In general amorphic materials are used in wavelengths from .185 microns to 2.6 microns.  These materials include sheet glasses, bulk optical glasses and filter glasses.  Glasses are generally the preferred optical materials in these wavelengths: they are less costly and a wide range of material sizes are readily available.  There are several glasses that transmit to 12 microns.  However these glasses are very expensive and difficult to process into components.  Generally, these glasses are only used in limited application by the military.

Operating environment

 

Optical materials are exposed to the most extreme environments imaginable.  Deep space thermal cycles of extreme hot and cold, high ultraviolet and gamma radiation, deep sea extreme pressures, and prolonged chemical exposure are just a few of the extremes that optical components are routinely expected to survive for extended periods of time. In choosing an optic, environmental stability of the material is a critical qualifying consideration.

Configuration (Shape)

There are four basic transmitting optical component configurations:  converging, diverging, redirecting, and barrier.

Converging and diverging optics

 

These types of optics are used to either condense or to spread the transmitted stream of photons. The most common component of this type of optic is the concave or convex lens. Applications of this type of optic are common among both consumer and professional microscopes and binoculars.

Redirecting optics

 

This optical configuration is most commonly the prism.  Utilizing refraction, internal reflection or both, redirecting optics are the most spatially efficient and inexpensive way to redirect photons in instruments and other optical systems where space and weight are important.

Barrier optics

Barrier optics, often referred to as window or dome optics, perform one or two very important utilitarian tasks.  First, they provide the medium that separates one environment from another.  Second, they provide the medium on which optical thin film coating can efficiently be applied and incorporated into an optical system.

Edge Bevel and Corner Chamfer

Their purpose is either to protect the optic from damage in handling (chips) or to be a functional attribute to be utilized for locating the placement of an optical component into a fixture (hard stop).  The design specification criteria for the features of these two attributes can add significant cost to the manufacture of an optic.

The edge bevel

is placed along the horizontal length and width or the circumference of a diameter.  When specifying this feature as a protection for the optic, it is important to keep the specifications wide enough to be easily manufacturable and still perform the required task.  The typical specification on an optic whose diameter or length and width is between 12.5mm and 200mm would be as follows:  bevel edge .25mm leg width +/- .125mm at 45 degrees +/-5 degrees.  The leg width will be nominally .707 X the hypotenuse.

The corner chamfer

on a rectilinear optic can be specified in one of two forms.  The least expensive is a 45-degree straight angle.  A radius may also be specified but is two to three times more expensive to successfully fabricate and more difficult to maintain a tight tolerance.  In both cases a tolerance of +/- .25mm to .3mm of leg length or radius is reasonable.

Surfaces

Optical surfaces have three important features that are important to the application:  beam distortion, smoothness, and surface defects.

Beam distortion

is the result of a lack of flatness of the optical surfaces. It is typically measured by interferometry. Surfaces are specified in total allowable interference waves (or fractions of waves) over the area.

The following are proper examples of flatness specifications:

One wave per inch over 85% clear aperature (CA) of the optical component measured at 632.8 nanometers.  This example specifies one full wave (approximately .000025 in.) per inch as measured over 85 in. of the net usable area of the optical surface measured with a helium neon laser based interferometer.

One-tenth wave over 90% CA measured at 632.8 nanometers.  This example specifies one-tenth wave (approximately .0000025 in.) over the entire net usable area of the optic as measured with a helium neon laser based interferometer.

One-tenth wave transmitted wave front (TWF) over 90% CA of the optical component measured at 632.8 nanometers.  This example takes into account the net surface distortion of both surfaces on the transmitted wave.

The following table is the standard of surface flatness by application:

Commercial: Fewer than two waves/inch
Semi-precision: Less than one wave/inch
Precision: Less than 1/4 wave/inch
High Precision: Less than 1/10 wave/inch
Super Precision: Less than 1/20 wave/inch

Smoothness: RA (average roughness) or RMS (root mean squared) 

A smooth surface is required for applications where low scatter of photons is important. The following table is a typical specification by application:

Commercial: Less than 40 angstroms RMS
Semi-precision: Less than 20 angstroms RMS
Precision: Less than 10 angstroms RMS
High Precision: Less than 5 angstroms RMS
Super Precision: Less than 3 angstroms RMS

Surface Defects

Like smoothness, the quantity and size of surface defects on an optical surface are important for applications where low scatter of photons is important.  There are three types of common surface defects:  scratches, digs, and stains

The traditional method of specifying maximum allowable scratches and digs is using the decades old military specification (MIL-O-13830). See below for a more detailed description of the scratch dig MIL specification.  

The original specification required that the inspection be done with the unaided eye under a 40-watt incandescent light source.  This was a highly subjective process and has proved unreliable for optics that require low scatter surfaces.  At this time, precision and better optics are routinely inspected using magnifications up to 50 power.

Milspec MIL-O-13830

According to the MIL-O-13830 numbering system, the first number represents the scratch tolerance and the second number represents the dig tolerance.

 

The scratch number represents the maximum width of scratches in microns.

#80 Max width equals 8 microns
#60 Max width equals 6 microns
#40 Max width equals 4 microns
#20 Max width equals 2 microns
#10 Max width equals 1 micron
#5 Max width equals 1/2 micron

The second number defines the average maximum diameter of a dig.

#50 Max diameter equals .50 millimeter
#40 Max diameter equals .40 millimeter
#30 Max diameter equals .30 millimeter
#20 Max diameter equals .20 millimeter
#10 Max diameter equals .10 millimeter
#5 Max diameter equals .05 millimeter

Below is a table that represents common scratch/dig specifications vs application.

Commercial: 80/50
Semi-precision: 60/40
Precision: 20/10
High Precision: 10/5
Super Precision: 0/0

Stains

Stains are the result of an adverse chemical reaction on the polished surface and are easily detected with the unaided eye.  The common specification for stains is that there will be none visible to the unaided eye.

How to Specify (Quick)

Download MIL-O-13830

More MIL-SPEC Info


Contact VPG

Address
Volume Precision Glass, Inc.
150 Todd Rd., Bldg 100,
Santa Rosa, CA 95407
Phone
(707) 206-0100
Fax
(707) 206-0105
info@vpglass.com