A request for an optical glass ball may look simple at first.

The customer may only need a small sphere with a specified diameter. However, once the ball becomes part of an optical sensor, safety light curtain, infrared detection system, or other optical assembly, diameter alone is no longer enough to define the product.

A typical RFQ may include specifications such as:

• Material refractive index: 1.475 ±0.001

• Ball diameter: 7.14 ±0.02 mm

• Sphericity: less than 18 μm

• Surface quality: 80/50

• Coating: uncoated

• Working wavelength: 850–860 nm

For buyers who normally purchase mechanical precision balls, terms such as 80/50 surface quality or working wavelength may be unfamiliar.

They also represent very different characteristics.

Diameter tolerance controls size.
Sphericity controls geometric form.
Surface quality controls visible defects.
Refractive index determines how light bends through the glass.
Working wavelength tells us where the optical system actually operates.

This guide explains each specification and, more importantly, how they work together when selecting a precision optical glass ball.

1. What Is an Optical Glass Ball?

An optical glass ball is a precision spherical glass component used to transmit, redirect, focus, couple, or otherwise control light.

Depending on the optical design, a glass sphere may function as a simple optical element or as a ball lens.

Typical applications include:

• optical sensors;

• safety light curtains;

• infrared transmitters and receivers;

• fiber-optic coupling;

• LED optical systems;

• inspection equipment;

• measuring instruments;

• optical detection assemblies.

Unlike decorative or general industrial glass balls, an optical glass ball may require control of not only diameter and roundness, but also:

refractive index, optical transmission, surface defects and wavelength-dependent properties.

That distinction is important when sending an RFQ to a precision ball manufacturer.

2. What Does Refractive Index Mean for a Glass Ball?

Refractive index, normally represented by n, describes how strongly a material bends light as light passes from one medium into another.

For an optical glass ball, refractive index directly affects the path of light through the sphere.

In simplified terms:

A higher refractive index generally causes light to bend more strongly at the glass-air interface.

This can change the focal behavior, beam path and optical performance of the sphere.

For this reason, a specification such as:

n = 1.475 ±0.001

should not be treated as just another material property.

It can be a fundamental part of the optical design.

Why is the ±0.001 tolerance important?

A requirement of:

1.475 ±0.001

means the acceptable nominal range is approximately:

1.474 to 1.476

If the optical design was calculated around this value, replacing the material with another borosilicate glass having a refractive index of 1.48 or 1.49 may change the optical result even though both materials are commonly described as "borosilicate glass."

This leads to an important purchasing rule:

Glass family alone does not determine optical equivalence.

Two borosilicate glasses can have different refractive indices, dispersion curves and transmission characteristics.

3. Why Is Refractive Index Specified at a Particular Wavelength?

This is one of the most frequently misunderstood parts of an optical glass specification.

You may see a drawing stating:

Refractive index: 1.475 at 587.6 nm

Why include the wavelength?

Because the refractive index of glass changes with wavelength.

This phenomenon is known as optical dispersion.

Therefore, simply saying:

Refractive index = 1.475

is incomplete for a precision optical application.

The reference wavelength should also be known.

What is 587.6 nm?

587.6 nm is close to the helium d-line and is widely used as a reference wavelength when reporting the refractive index of optical glass.

You may therefore see the term:

n?

in optical material specifications.

For example, published technical data for SUPRAX? 8488 give a refractive index of approximately:

n? = 1.482 at 587.6 nm.

Corning's technical data for PYREX? Code 7740 list a refractive index of approximately:

1.474 at the sodium D line.

These numbers demonstrate why the exact material and reference wavelength should be verified before approving a substitute.

4. PYREX 7740 vs. SUPRAX 8488: Can One Replace the Other?

Both materials belong to the broad borosilicate glass family, but that does not automatically make them optically interchangeable.

Consider their published refractive-index values:

Published Corning information identifies Code 7740 as low-expansion Type I borosilicate glass and reports a refractive index around 1.474 at the sodium D line. SUPRAX 8488 data report n? around 1.482.

Therefore, if a customer's drawing strictly requires 1.475 ±0.001, it would not be correct to assume SUPRAX 8488 is an exact equivalent simply because both products are borosilicate glasses.

Before production, a supplier should confirm:

• Is 1.475 ±0.001 a strict optical requirement?

• At what wavelength is this value specified?

• Can an alternative refractive index be accepted?

• Is the customer specifying a glass brand or an optical property?

• Does the design require refractive-index data at the actual operating wavelength?

This kind of clarification can prevent a material that is mechanically suitable but optically incorrect from entering production.

5. What Does an 850–860 nm Working Wavelength Mean?

The working wavelength is the wavelength, or wavelength range, in which the optical system is designed to operate.

For example:

Working wavelength: 850–860 nm

means that the optical component is intended to operate primarily with light in this range.

850 nm lies in the near-infrared region.

This wavelength is commonly used in industrial photoelectric and safety detection systems. For example, KEYENCE specifies an 850 nm infrared LED as the light source in several of its safety light curtain systems.

So a request for an 850–860 nm optical glass ball in a safety light curtain is technically reasonable.

Working Wavelength Does Not Simply Mean "Transmission Range"

One common explanation is:

"Working wavelength means the wavelength range where the glass has good transmission."

That is only part of the story.

The correct interpretation is broader.

The working wavelength describes where the complete optical system is intended to function.

Material transmission is one consideration, but engineers may also need to consider:

• refractive index at that wavelength;

• dispersion;

• reflection loss;

• absorption;

• coating performance;

• detector sensitivity;

• LED emission wavelength;

• focal behavior.

This distinction becomes particularly important when the published refractive index is specified at 587.6 nm but the actual system works at approximately 850 nm.

6. 587.6 nm Refractive Index vs. 850 nm Working Wavelength: Is This a Problem?

Not necessarily.

These two values describe different things.

587.6 nm may be the standard reference wavelength used to characterize the glass material.

850–860 nm may be the actual wavelength at which the finished optical system operates.

There is nothing unusual about seeing both on the same technical drawing.

However, because glass exhibits dispersion:

the refractive index at 850 nm will not be exactly the same as the refractive index reported at 587.6 nm.

For a non-critical application, the published material grade may provide sufficient information.

For a sensitive optical design, however, the customer or optical engineer may need the actual refractive index or dispersion data near the operating wavelength.

Therefore, a good supplier should ask rather than assume.

7. What Does 80/50 Surface Quality Mean?

This is probably one of the most confusing specifications for buyers new to optical components.

An optical surface specification such as:

80/50

normally refers to Scratch-Dig surface quality.

The first number describes scratches.

The second describes digs, or pit-like surface defects.

So:

80/50 = Scratch 80 / Dig 50

The Scratch-Dig system is commonly associated with MIL-PRF-13830B.

What Does "Scratch 80" Mean?

A common mistake is to interpret Scratch 80 as:

"The maximum scratch width is 0.08 mm."

That is not technically correct under the traditional MIL specification.

The scratch number is primarily based on visual comparison of scratch brightness against calibrated reference standards under specified illumination conditions.

It is therefore a comparison grade rather than a direct dimensional measurement of scratch width.

In the traditional system, common scratch designations include:

10, 20, 40, 60 and 80.

A lower number represents a more stringent cosmetic requirement.

Therefore:

Scratch 20 is more demanding than Scratch 80.

What Does "Dig 50" Mean?

The dig number is more directly related to defect size.

The dig designation represents the diameter of the largest allowed dig in units of 1/100 mm.

Therefore:

Dig 50 corresponds to a maximum individual dig diameter of approximately 0.50 mm.

For example:

• Dig 10 → approximately 0.10 mm

• Dig 20 → approximately 0.20 mm

• Dig 40 → approximately 0.40 mm

• Dig 50 → approximately 0.50 mm

Edmund Optics describes this distinction clearly: scratch ratings are based on comparison to calibrated reference scratches, while the dig designation is a dimensional quantity.

8. Is 80/50 Good Surface Quality?

The answer depends on the application.

In general optical-industry terminology:

80/50 is therefore not "poor quality."

It is simply a less restrictive surface-defect requirement than 60/40 or 20/10.

Edmund Optics similarly classifies 80/50 as typical standard quality, 60/40 as precision quality and 20/10 as high-precision quality.

This matters for cost.

Specifying 20/10 when the optical system only requires 80/50 can increase:

• polishing difficulty;

• inspection requirements;

• rejection rate;

• manufacturing time;

• final component cost.

A tighter specification is therefore not automatically a better specification.

The correct goal is:

the surface quality required by the optical system — no more and no less.

9. Surface Quality vs. Surface Roughness: Are They the Same?

No.

This is another important distinction when purchasing optical glass balls.

Surface Quality

Surface quality evaluates defects such as:

• scratches;

• pits;

• digs;

• visible imperfections.

An 80/50 requirement falls into this category.

Surface Roughness

Surface roughness describes the much smaller-scale texture of the polished surface and is normally expressed using parameters such as:

• Ra;

• Rq;

• nanometers;

• micrometers.

Therefore:

80/50 Scratch-Dig does not tell you the Ra value of the glass ball.

A surface can satisfy a specified scratch-dig level while still having a separate surface roughness requirement.

For critical optical applications, both may need to be specified.

10. What Does Sphericity <18 μm Mean?

Sphericity describes how closely the actual ball approaches a theoretically perfect sphere.

For a specification such as:

Sphericity <18 μm

the allowable deviation from ideal spherical form is limited to less than 18 micrometers according to the agreed measurement definition.

ISO terminology for precision balls distinguishes deviation from spherical form from diameter and other surface irregularities.

Sphericity is important because the optical path through a sphere depends on its geometry.

An irregular sphere can affect:

• beam direction;

• focal position;

• optical symmetry;

• repeatability;

• assembly alignment.

For an optical ball, dimensional accuracy alone therefore does not guarantee good geometry.

11. Sphericity and Diameter Tolerance Are Not the Same

Consider this example:

Diameter: 7.14 ±0.02 mm

Sphericity: <18 μm

These specifications control two different characteristics.

Diameter tolerance

7.14 ±0.02 mm means the specified size range is:

7.12 to 7.16 mm

It answers the question:

How large is the ball?

Sphericity

Sphericity answers a different question:

How closely does the surface follow a true spherical form?

A ball may be within the required overall diameter range while still having unacceptable form error.

Conversely, a very spherical ball can still have the wrong nominal diameter.

That is why precision glass-ball drawings may specify both values separately.

12. Why Is Sphericity Important for an Optical Ball?

Imagine two balls with almost the same measured diameter.

Ball A is very close to a perfect sphere.

Ball B contains more significant geometric variation around its surface.

Mechanically, both balls might fit into the same holder.

Optically, however, they may not behave identically.

For systems using the sphere to redirect, collect or focus light, tighter sphericity can improve consistency between components.

This is especially relevant when many identical optical channels are installed side by side, as can occur in sensor arrays or light-curtain assemblies.

13. Does an Optical Glass Ball Need AR Coating?

Not always.

AR stands for anti-reflective or anti-reflection coating.

Whenever light passes from air into glass, part of the light is transmitted and part is reflected at the interface.

An AR coating can be designed to reduce reflection over a specified wavelength or wavelength range.

However, it is incorrect to assume that every optical glass ball should automatically be coated.

Whether AR coating is necessary depends on factors such as:

• working wavelength;

• required optical throughput;

• acceptable reflection loss;

• optical architecture;

• surface geometry;

• cost;

• environmental requirements.

If the original optical design specifies:

Uncoated

then adding an AR coating without engineering approval can actually change the intended optical behavior.

14. Why Can Coating a Glass Ball Be More Complicated Than Coating a Flat Window?

A glass ball has a fully curved three-dimensional surface.

This creates manufacturing challenges that do not exist to the same extent on a flat optical window.

Depending on the process, coating uniformity around the sphere can be more difficult to control.

For this reason, when a customer requests an optical ball, the supplier should first determine whether:

• the entire sphere needs coating;

• only a functional zone needs coating;

• a specific AR band is required;

• the original design assumes an uncoated surface.

If coating provides no meaningful system benefit, avoiding unnecessary coating may reduce manufacturing complexity and cost.

15. Why Material Selection Cannot Be Based on Refractive Index Alone

Suppose two glass materials both appear close to the requested refractive index.

That still does not mean they are automatically interchangeable.

Other properties may matter, including:

Optical properties

• transmission at 850–860 nm;

• dispersion;

• absorption;

• refractive-index tolerance.

Manufacturing properties

• ability to form and polish a 7.14 mm sphere;

• achievable sphericity;

• surface quality;

• consistency between production batches.

Mechanical and environmental properties

• hardness;

• thermal expansion;

• thermal shock resistance;

• chemical resistance.

The best material is therefore not simply:

"the glass with the closest number on a refractive-index table."

It is the material that satisfies the complete optical, dimensional and manufacturing specification.

16. Example: How to Review a Safety Light Curtain Glass Ball RFQ

Consider the following anonymized specification:

At first glance, this appears to describe one product.

From a manufacturing perspective, however, it actually contains six separate technical questions.

Question 1: Can the required diameter be produced consistently?

The nominal size and ±0.02 mm tolerance need to be confirmed.

Question 2: Can the required spherical form be achieved?

Diameter and sphericity must be evaluated separately.

Question 3: Which glass actually satisfies the refractive-index requirement?

Published material data must be compared at the specified reference wavelength.

Do not choose only by the words "borosilicate glass."

Question 4: Can the polished surface achieve 80/50?

The inspection method should also be agreed with the customer where necessary.

Question 5: Is the material suitable around 850–860 nm?

Transmission and wavelength-dependent optical properties should be reviewed.

Question 6: Is an uncoated surface intentional?

If yes, the supplier should not automatically introduce an AR coating simply because the component is optical.

17. Questions Buyers Should Include in an Optical Glass Ball RFQ

To reduce repeated emails and avoid misunderstandings, we recommend providing as much of the following information as possible:

1. Ball diameter and tolerance

2. Required glass material or permitted material alternatives

3. Required refractive index

4. Reference wavelength for the refractive index

5. Working wavelength

6. Sphericity or spherical-form requirement

7. Scratch-Dig surface quality

8. Surface roughness, if separately required

9. Coating requirement

10. Quantity

11. Drawing and inspection standard

12. End-use application

For optical projects, a technical drawing is always preferable to a simple message saying:

"We need a high-precision glass ball."

The word high precision means different things in different applications.

Actual numerical specifications make the requirement measurable.

18. Common Mistakes When Specifying Optical Glass Balls

Mistake 1: Treating all borosilicate glass as the same

Different compositions can have different refractive indices and optical properties.

Mistake 2: Reading Scratch 80 as a literal 0.08 mm scratch

Traditional Scratch-Dig scratch grades are comparison-based specifications, not a direct scratch-width measurement.

Mistake 3: Confusing surface quality with surface roughness

80/50 does not specify Ra.

Mistake 4: Confusing diameter tolerance with sphericity

One controls overall size; the other controls spherical form.

Mistake 5: Selecting material using refractive index at the wrong wavelength

Refractive index varies with wavelength.

Mistake 6: Assuming lower Scratch-Dig numbers are always necessary

A 20/10 surface may cost considerably more to manufacture than an 80/50 surface, while providing no useful benefit in some systems.

Mistake 7: Adding AR coating without checking the original optical design

Coating should be specified according to the optical requirement, not simply because the part is made of optical glass.

19. How Precision Ball Manufacturers Evaluate Optical Glass Ball Projects

For a standard mechanical ball, purchasing may begin mainly with:

material + diameter + grade + quantity.

For an optical glass ball, the discussion often needs to go further.

A manufacturer must translate the optical specification into measurable production requirements:

Material

↓

Refractive index

↓

Diameter

↓

Spherical form

↓

Polishing

↓

Surface quality

↓

Optical wavelength

↓

Inspection

This is why detailed RFQs are particularly valuable for customized optical sphere projects.

The earlier these parameters are confirmed, the lower the risk of discovering incompatibilities after samples have already been produced.

20. Final Thoughts

A precision optical glass ball is a small component, but its specification can contain a surprising amount of engineering information.

When reviewing an RFQ, remember these six points:

• Refractive index determines optical behavior and must be associated with a wavelength.

• Working wavelength describes where the optical system operates.

• 80/50 is a Scratch-Dig surface quality specification, not a surface roughness value.

• Scratch 80 should not simply be interpreted as a 0.08 mm scratch width.

• Sphericity and diameter tolerance describe different geometric requirements.

• A borosilicate glass with similar chemical or thermal properties may still have a different refractive index.

For safety light curtains, optical sensors and near-infrared systems, evaluating these parameters together is much more reliable than selecting a glass ball based only on material name or diameter.

If you are developing a custom optical glass ball or precision glass sphere, send us your drawing, required diameter, refractive index, spherical-form requirement, surface quality, working wavelength and quantity.

We can evaluate the specification before sampling and help determine whether the requested material and manufacturing requirements are compatible.

FAQ

What does 80/50 mean on an optical drawing?

80/50 normally refers to Scratch-Dig surface quality. 80 is the allowable scratch designation and 50 is the dig designation. Under the traditional MIL system, the scratch value is evaluated by comparison with reference standards, while Dig 50 corresponds to a maximum individual dig diameter of approximately 0.50 mm.

Is 80/50 considered optical quality?

Yes. 80/50 is commonly considered standard or commercial optical surface quality. More restrictive specifications include 60/40, 40/20 and 20/10.

Is 80/50 the same as surface roughness?

No. Scratch-Dig describes visible surface imperfections. Surface roughness describes microscopic surface texture and is specified separately.

What does 850 nm working wavelength mean?

It means the optical system is intended to operate around 850 nm, which lies in the near-infrared region. 850 nm infrared LEDs are used in commercial safety light curtain systems.

Why is refractive index sometimes specified at 587.6 nm?

Refractive index changes with wavelength. 587.6 nm is commonly used as a reference wavelength for reporting optical glass refractive index.

Is SUPRAX 8488 the same as PYREX 7740?

No. Both are borosilicate glasses, but their published optical properties are different. PYREX 7740 is reported at approximately 1.474 at the sodium D line, while SUPRAX 8488 is approximately 1.482 at 587.6 nm.

What is sphericity in a precision glass ball?

Sphericity describes deviation from an ideal spherical form. It is different from nominal ball diameter and diameter tolerance.