Flat, convex, and lens glass – how to recognize the right profile?

When selecting glass for a watch, the diameter alone is not enough. The profile of the glass, meaning the shape of its surface, is equally important.

The most commonly encountered glasses are:

• flat,
• convex – spherical,
• lens-shaped.

Although two glasses may have exactly the same diameter, their shape and height can be completely different. Therefore, they should not be treated as automatic substitutes.

Flat glass – what does it look like?

Flat glass essentially has a flat surface. It does not create a distinct arc or dome above the dial.

When selecting it, the most important factors are:

• diameter,
• thickness of the glass,
• material,
• method of mounting in the case.

For example, the glass may have a diameter of 30.4 mm and a thickness of 1.0 mm. Both parameters must match the watch's construction.

See example flat mineral glasses 1.0 mm

Convex glass – spherical

Spherical glass, often also referred to as convex glass, has a surface that is gently curved in an arc.

Unlike flat glass, the central part is higher than the edge of the glass.

A spherical profile may be needed, among other reasons, when:

• the original glass had a convex shape,
• additional space above the hands is needed,
• the design or style of the watch requires convex glass.

Kurant offers convex mineral glasses in many diameters and thicknesses; for example, the 1.0 mm category includes many sizes. :contentReference[oaicite:0]{index=0}

See convex mineral glasses 1.0 mm

Does convex and spherical glass mean the same thing?

In commercial practice, the terms convex and spherical often refer to the same type of glass with a gently curved surface.

However, it is crucial to distinguish such glass from lens-shaped glass, which has a distinctly different profile.

Lens-shaped glass – how does it differ from spherical?

Lens-shaped glass has a profile resembling a lens. Its central part is noticeably thicker than the edge.

Therefore, with lens-shaped glass, two thickness dimensions are very important:

• thickness at the edge,
• thickness in the center of the glass.

For example:

3.5 × 1.5 mm

means that the glass is about 3.5 mm thick at the highest point, i.e., in the center, while about 1.5 mm at the edge.

Your store has such categories, e.g., lens-shaped glasses 3.5 × 1.5 mm. :contentReference[oaicite:1]{index=1}

See lens-shaped glasses 3.5 × 1.5 mm

Why are two dimensions of lens-shaped glass important?

The thickness at the edge determines, among other things, how the glass interacts with the case, gasket, or mounting area.

The thickness in the center determines the height of the glass profile above the dial.

Two glasses can have:

the same diameter and the same edge thickness,

but a different height in the center. In that case, visually and structurally, they will be two different glasses.

Example: lens-shaped glass 4.0 × 2.5 mm

If the glass is described as:

lens-shaped 4.0 × 2.5 mm

then 4.0 mm indicates the height/thickness in the central part of the glass, and 2.5 mm the thickness at the edge.

Kurant also offers such profiles in various diameters. :contentReference[oaicite:2]{index=2}

See lens-shaped glasses 4.0 × 2.5 mm

Spherical glass vs. lens-shaped glass – the main difference

This can be explained simply:

Spherical glass has a gently curved surface.

Lens-shaped glass has a distinctly thickened center and a characteristic lens profile.

Therefore, with lens-shaped glass, we record the edge thickness and the center height separately.

Can spherical glass be replaced with lens-shaped glass?

This should not be assumed.

Even if both glasses have the same diameter, they may differ in:

• height,
• thickness at the edge,
• space above the hands,
• mounting method,
• the appearance of the watch after assembly.

It is best to replicate the type of glass originally used.

Can convex glass be replaced with flat glass?

In some constructions, physically it may be possible to mount glass with a different profile, but that does not mean it is the correct substitute.

Changing convex glass to flat glass may result in, among other things, a change in the appearance of the watch or too little space above the hands.

How to measure spherical or lens-shaped glass?

The glass must first be removed from the watch.

Then measure:

• the outer diameter,
• the thickness at the edge,
• in the case of lens-shaped glass, also the total height/thickness in the center.

The measurement should be done with calipers, as differences of hundredths of a millimeter can be significant when selecting glass.

How to identify the type of glass when the old glass is broken?

If the old glass is only partially intact, it is worth checking the edge fragment and profile.

In the case of complete absence of glass, measuring the case alone may not be sufficient to determine whether flat, spherical, or lens-shaped glass was originally used. Documentation of the watch, preserved fragments of glass, or comparison with an identical model may be helpful.

The same size, different profile – why is this important?

For example, three glasses may have a diameter of 30.4 mm:

• 30.4 mm flat 1.0 mm,
• 30.4 mm spherical 1.0 mm,
• 30.4 mm lens-shaped 3.5 × 1.5 mm.

All have the same diameter, but they are three different products.

Where to find the right type of glass?

In the Kurant watchmaker's warehouse, watch glasses are available in a very large number of diameters, thicknesses, and profiles, including flat, convex, and lens-shaped glasses.

When choosing, it is best to first determine the type of profile, then the diameter and the appropriate thickness values.

See also: How to choose glass for a watch – complete guide