An optical window has one job: let your wavelength through while keeping the environment out. The substrate you choose decides quietly whether that works. It decides in the transmission curve, in the survival rate, and on the quote.
Four filters do the choosing: wavelength range, operating environment, mechanical load, and cost. Run them in that order and each one eliminates materials fast. You end up comparing two or three survivors instead of reading ten datasheets.
This guide walks through each filter in turn, shows the numbers that matter, and ends with a checklist you can run before requesting a single quote.
Start with the wavelength range - it eliminates the most options
Start with wavelength because it disqualifies more materials than any other filter. A substrate outside your band fails outright, no matter how tough or cheap it is. One micron is a micrometer, one thousandth of a millimeter.
For visible and near-infrared work, N-BK7 crown glass covers roughly 350 nm to 2.0 µm [1]. Newport describes it as the common choice for visible and near-IR optics up to about 2.1 µm [2]. Below 350 nm, it is out.
Ultraviolet work moves you to fused silica, which transmits down to about 195 nm [2]. Grade matters here. UV-grade fused silica absorbs at 1.4, 2.2, and 2.7 µm because of hydroxyl impurities, while IR-grade fused silica carries less hydroxyl and transmits farther into the infrared [3]. One selection guide lists IR-grade fused silica as useful from 0.18 to 3.5 µm, and notes the low hydroxyl content pushes usable transmission to 3.6 µm [4].
Beyond about 3 µm you are choosing among crystals. The same guide’s comparison table gives the working ranges [4]:
| Material | Transmission range | Typical use |
|---|---|---|
| N-BK7 crown glass | 350 nm – 2.0 µm | Visible, near-IR |
| UV fused silica | 185 nm – 2.1 µm | UV through near-IR |
| IR-grade fused silica | 0.18 – 3.5 µm | Near-IR extension |
| Sapphire | 0.17 – 5.0 µm | UV through MWIR |
| Calcium fluoride (CaF2) | 0.15 – 9.0 µm | Deep UV to MWIR |
| Magnesium fluoride (MgF2) | 0.13 – 7.0 µm | UV to MWIR |
| Germanium | 2.0 – 17 µm | MWIR, LWIR thermal imaging |
| Silicon | 1.2 – 10 µm | MWIR |
| Zinc selenide (ZnSe) | 0.55 – 20 µm | Broad IR, CO2 lasers |
| Zinc sulfide (ZnS) | 0.37 – 14 µm | Visible plus IR |
| Sodium chloride (NaCl) | 0.23 – 12 µm | FTIR spectroscopy |
MWIR means mid-wave infrared, roughly 3 to 8 µm; LWIR is the long-wave band beyond it. Count every wavelength in your system, not just the design wavelength. An alignment laser at 635 nm in front of a 3 to 5 µm thermal camera disqualifies germanium, which does not transmit visible light at all.
Let the operating environment narrow the shortlist
Two windows can both pass your wavelength, and only one survives the job. Environment is the filter that tells them apart.
Scratch resistance and chemical exposure
Sapphire takes abrasion that destroys everything else. It sits at Mohs 9 — second only to diamond on that scratch scale — while fused silica manages 5.5 to 6.5 and calcium fluoride about 4 [5]. Fused silica therefore scratches during aggressive cleaning; sapphire shrugs off sand, wipes, and most impacts [6].
Chemically, fused silica and sapphire are the inert pair, resisting common solvents, dilute acids, and bases [5]. The crystals are not. Calcium fluoride dissolves in mineral acids and degrades under prolonged water exposure [5]. Barium fluoride is less water-resistant still, though it serves to 800°C in dry air [7]. Sodium chloride is outright water-soluble [4].
Heat, cold, and thermal shock
Rapid temperature change breaks windows by differential expansion. Fused silica is the benchmark here. Its thermal expansion coefficient of 0.55 x 10-6 per kelvin is extremely low, and it holds excellent thermal shock resistance up to 1100°C [5]. High-expansion calcium fluoride sits at the other end, prone to thermal-shock cracking [5].
Mounting compounds the problem. A rigid metal mount gripping a high-expansion crystal will stress it on every temperature cycle. If your window sees thermal cycling, either pick a low-expansion material or design a compliant mount.
Check pressure and mechanical load before you buy
A window is also a structural part. Pressure differential — vacuum on one side, atmosphere on the other, or subsea and aerospace service — turns a flat plate into a loaded plate, and that drives thickness.
How pressure drives thickness
Pressure window design starts with a question about mounting: is the window clamped at both edges, or resting unclamped like a manhole cover [8]? The answer changes the stress calculation and the thickness it demands.
Next comes a safety factor. Esco Optics describes a scale of 1 to 7, where 4 is a modest value suitable for many applications and 7 is the most conservative [8]. Required thickness then grows with aperture and pressure differential. Work from the pressure ratings vendors publish for standard viewports instead of guessing your own number.
When sapphire earns its price
Esco reports that customers often specify sapphire by default for pressure windows, drawn by its hardness and strength [8]. The preference has a basis: sapphire works in pressure-vessel applications where quartz would fracture [5].
The default is not always right, though. The same note points out that alternative materials can perform as well as sapphire for a fraction of the cost [8]. A gentle laboratory vacuum on a visible-wavelength instrument is standard glass viewport territory. Save sapphire for the pressure, abrasion, or chemical exposure that actually demands it.
What each material costs to buy and to finish
Material price and fabrication price are two different filters, and both move with the substrate. This is why two quotes for the same size window can differ by a factor of ten.
The raw material ladder
Crown glass sits at the bottom of the ladder. N-BK7 is a high-volume optical glass with straightforward manufacturability, which keeps it cheap for visible and near-IR work [2]. Fused silica costs more but stays modest: one selection guide prices typical 25 mm stock fused silica windows around $10 to $30, cheap relative to sapphire and calcium fluoride [5].
Sapphire, germanium, and zinc selenide sit higher, and size limits push large apertures higher still. Zinc selenide is grown by chemical vapor deposition, and one vendor’s blanks stop at 200 mm diameter and 15 to 20 mm thickness [9]. A large ZnSe window is not just expensive; above a certain diameter it may not exist as a single piece.
Why hardness drives fabrication cost
Raw material explains only part of the sapphire premium. Sapphire’s hardness makes it slower to machine and polish, which increases manufacturing time and cost, especially for larger windows or custom geometries [6]. Fused silica polishes easily, which is one reason it costs less [6].
Soft crystals carry their own surcharge. Zinc sulfide is harder and more chemically resistant than zinc selenide [4]. Softer crystals still need careful handling through grinding, polishing, and coating, and those steps appear on the quote.
A five-minute decision checklist
Run these five steps before you request a quote. Each one ends with something written down, so the last step is a shortlist rather than a memory exercise.
List every wavelength the system uses. Include alignment lasers, diagnostic LEDs, and detector response bands, not just the design wavelength. You should end with a written band list, such as “635 nm and 3 to 5 µm”.
Write down the worst-case environment. Abrasives, solvents, humidity, and temperature swings on the harshest day, not the lab average. Cross out every material from step one that fails it — sodium chloride in humidity, calcium fluoride in acids, and so on.
Add the pressure and mounting facts. Pressure differential, clamped or unclamped mounting, and unsupported aperture. You should end with a required rating you can compare against published viewport specifications.
Set the size and quantity budget. Diameter, thickness, and how many units. This catches crystal-grown materials whose blank sizes cannot reach your aperture at any price.
Keep the two cheapest survivors and quote both. Ask each vendor whether a cheaper substrate meets the written requirements from steps one through three. If one says yes and one says no, you have learned something about one of them.
What choosing wrong looks like
Four failure patterns account for most mis-specified windows. Each one traces back to a filter that would have caught it.
Glass that fluoresces in UV work. Optical glass irradiated with ultraviolet light can fluoresce, and prolonged UV exposure causes solarization, a reddish-brown coloration inside the glass. Synthetic fused silica resists both [10]. The wavelength filter catches this: below 350 nm, crown glass was already disqualified [1].
Germanium that goes opaque as it heats. Germanium transmission degrades measurably from about 100°C and collapses between 200 and 300°C, which one vendor describes as catastrophic failure of the optic [11]. The environment filter catches it — a hot enclosure is not a benign one.
Water-soluble crystals that cloud. Sodium chloride dissolves; barium fluoride loses surface quality in humid air [4][7]. Humidity on the worst-case day should have crossed these off.
Thermal-shock cracks in high-expansion crystals. Calcium fluoride is prone to cracking under rapid temperature change, exactly where low-expansion fused silica survives [5].
Frequently asked questions
Is sapphire always the best optical window material?
No. Sapphire is extremely hard and abrasion-resistant [5], but it is birefringent, expensive as a raw material, and slow to polish [6]. For a benign visible or near-infrared application, N-BK7 or fused silica performs as well for far less money [2]. Sapphire earns its price under abrasion, pressure, or chemical attack.
Can N-BK7 windows transmit ultraviolet light?
No. N-BK7 transmits from roughly 350 nm, so UV-A wavelengths just below the visible get through weakly at best [1]. For ultraviolet work, use UV fused silica down to about 195 nm [2], or calcium fluoride and magnesium fluoride, which reach 150 nm and 130 nm respectively [4].
What is the best window material for a CO2 laser?
Zinc selenide. A CO2 laser operates at 10.6 µm, inside ZnSe’s 0.55 to 20 µm transmission range [4]. Zinc sulfide also transmits there and is harder and more chemically resistant, but ZnSe remains the standard for high-power CO2 beam paths.
Which optical window material is the cheapest?
N-BK7, for visible and near-infrared windows. It is a high-volume glass with straightforward manufacturability [2]. Fused silica is the next step up; typical 25 mm stock fused silica windows run about $10 to $30 [5].
Do IR crystal windows need protection from humidity?
Yes, most of them. Sodium chloride is water-soluble [4], barium fluoride is less water-resistant than calcium fluoride [7], and even calcium fluoride degrades under prolonged water exposure [5]. Store the crystals dry, and seal or purge them in humid service.
Choose in this order and the list writes itself
The order does the work: wavelength first, environment second, mechanical load third, cost last. Each filter removes materials for a reason you can point to, and the survivors are few by the time the price question arrives.
One habit prevents most mistakes: check every wavelength and the worst-case day, not the bench average. That means the sand storm, the solvent wipe, the heat soak. Write those down, quote the two cheapest survivors, and ask each vendor whether a cheaper substrate meets the written requirements. The list writes itself from there.
Sources
- N-BK7 Windows: The Optical Standard for Precision and Performance - CLZ Optics
- Optical Materials - Newport
- UV vs. IR Grade Fused Silica - Edmund Optics
- 9 Most Commonly Used Infrared Materials And Selection Guide - NONI Custom Optics
- Optical Windows: Quartz vs Sapphire vs CaF2 Selection Guide - MachinedQuartz.com
- Which Window Material Is Right for You? Sapphire vs Fused Silica - UQG Optics
- TYDEX Barium Fluoride (BaF2) - Tydex Optics
- Pressure window design - Esco Optics
- TYDEX Zinc Selenide (CVD - ZnSe) - Tydex Optics
- Glass materials - OptoSigma
- Germanium Windows: Optical Materials Focus - UQG Optics