Optical Domes: How to Choose the Right Size and Material

By UQG Optics | Published: 2026-10-13

Category: Buying Guides

A practical buying guide to optical domes: match diameter and thickness to your housing, then choose sapphire or fused quartz based on transmission, durability, and budget.

Quick answer

Start with the mechanical envelope: measure the housing seat diameter and the required wall thickness, then confirm the dome's outer diameter and thickness match. For material, choose sapphire when scratch resistance and broad-band transmission matter most, and fused quartz when you need deep UV transmission or a lower-cost, larger dome. If you need a small precision cover for microscopy rather than a full dome, UQG Optics fused quartz coverslips in 0.17 mm, 0.20 mm, and 0.50 mm thicknesses show how thickness and diameter trade-offs work in practice.

Measure the housing before you compare materials

The most common ordering mistake is picking a material first and a size second. A dome only works if its outer diameter seats correctly in the housing and its thickness clears the internal optical path. Measure the seat diameter, the maximum allowable dome height, and the wall thickness your pressure or impact requirement demands. If the dome is bonded or clamped, note the bond line width, because a dome that is 1 mm too small will not seal and one that is 1 mm too large will not seat.

Thickness and diameter are linked. A larger diameter dome needs more thickness to keep the same stiffness and pressure tolerance, which increases weight and can shift the optical center. If your housing is already fixed, treat the seat diameter as a hard constraint and adjust thickness within the allowed range rather than the other way around.

Sapphire vs fused quartz: match the material to the job

Sapphire is the default choice when the dome faces abrasive particles, high-speed impact, or repeated cleaning. It is very hard and resists scratching, which keeps transmission stable over a long service life. The trade-off is cost and weight: sapphire domes are heavier and more expensive per unit than fused quartz, and they are not the best option when you need transmission deep into the UV.

Fused quartz transmits well from the UV through the visible and near-infrared, handles high temperatures, and has a low thermal expansion coefficient. It is lighter and less expensive than sapphire, but it scratches more easily, so it suits protected environments or applications where the dome is not exposed to abrasive contact. For UV sensing, spectroscopy, or high-temperature windows, fused quartz is often the more practical material.

Specifications that decide the purchase

Beyond diameter and thickness, confirm the surface quality and transmitted wavefront your system can tolerate. A dome used for imaging needs a tighter wavefront specification than one used only as a protective window. Check the operating temperature range, because bonding materials and coatings may fail before the dome itself does. If the dome will be coated, ask whether the coating is applied to the convex, concave, or both surfaces, since this affects both cost and handling.

When a specification is not listed, treat that as a decision limitation rather than assuming a value. Ask the supplier for the missing number, or choose a standard stock size that is documented. For small precision covers used in microscopy, UQG Optics lists fused quartz coverslips with explicit thickness and diameter values, which makes them a useful reference point for how those two dimensions interact.

Decision rules for optical dome size and material

Rule 1: Seat diameter is a hard constraint: Measure the housing seat and choose the dome outer diameter that matches it within the bond or clamp tolerance. Do not adjust diameter to compensate for thickness. If the seat is fixed, change thickness, not diameter.
Rule 2: Thickness follows pressure and stiffness: Increase wall thickness when the dome must resist pressure, impact, or vibration. Accept the added weight and any optical center shift that comes with it. Thicker is stronger but heavier and sometimes optically different.
Rule 3: Material follows environment: Choose sapphire for abrasive or high-impact exposure and fused quartz for UV transmission, high temperature, or lower weight and cost. Match the material to the harshest condition the dome will actually see.
Rule 4: Do not buy an undocumented specification: If surface quality, wavefront, or temperature range is missing from the listing, request it before ordering. A dome that meets the size but not the optical spec will still fail. Missing data is a reason to ask, not to assume.

Product facts to ground your size decision

Fused Quartz Coverslip #No1.5: Fused Quartz Coverslip #No1.5, part CFQ2417, thickness 0.17 mm, diameter 24 mm, catalog price USD 25.20. This is the thinnest and largest-diameter option listed, so it shows how a wide cover can still be very thin when stiffness is not the priority.
Fused Quartz Coverslip #No5: Fused Quartz Coverslip #No5, part CFQ1250, thickness 0.50 mm, diameter 12 mm, catalog price USD 6.27. A small diameter paired with a thicker wall, useful when the seat is narrow and you need more mechanical rigidity than a thin cover provides.
Fused Quartz Coverslip #No2: Fused Quartz Coverslip #No2, part CFQ1520, thickness 0.20 mm, diameter 15 mm, catalog price USD 11.36. A middle thickness at a small diameter, a reasonable starting point when you need more stiffness than 0.17 mm but cannot fit a 0.50 mm wall.
Fused Quartz Coverslip #No5: Fused Quartz Coverslip #No5, part CFQ2555, thickness 0.50 mm, diameter 25 mm, catalog price USD 9.10. The largest diameter with the thickest wall, showing that increasing both dimensions together is how you maintain rigidity at a larger size.

Is an optical dome the right choice for you?

Best for

Housings where a domed window must seal or clamp into a fixed circular seat.
Applications needing UV or high-temperature transmission where fused quartz is preferred.
Exposed installations where scratch resistance makes sapphire worth the higher cost.

Not for

Flat window applications where a dome adds unnecessary height and cost.
Microscopy coverslip work, where flat fused quartz coverslips in 0.17 mm, 0.20 mm, or 0.50 mm thickness are the correct form factor.
Systems with no defined seat diameter, where a custom housing should be designed first.