Why does one piece of optical glass need single-side grinding, double-side grinding or double-side polishing?
A professional selection guide covering optical path, surface roughness, TTV, parallelism and industry applications
For: purchasing / optical engineering / R&D prototyping / precision processing / supplier communication
Figure 1 | Three typical surface finishes and their effect on light transmission (technical illustration)
| The conclusion first: Choose a ground finish when you need scattering, light diffusion or glare reduction; choose double-sided polishing when you need low scattering, high transmission, coatability, precise measurability and control of TTV, parallelism or surface form. Surface finish is not a cosmetic option — it is part of the functional design. |
01 | Why does surface finish decide which industry the same piece of glass ends up in?
In precision optics, "what the material is" answers only half the question; the other half is "what state the surface is processed into". Take the same borosilicate, aluminosilicate, fused silica or optical crown glass and finish it as single-side ground, double-side ground or double-side polished, and its scattering, transmission, glare, coatability, cleanability, thickness uniformity and compatibility with downstream assembly processes all change noticeably.
So engineering selection cannot stop at "ground or not ground". Ask first: what function does this glass perform in the system — to make light more uniform, or to disturb light as little as possible? Is it a viewing or diffusing surface, or a window, filter substrate, wafer, carrier, cover plate or laser aperture?
| A note on terminology: In industry usage, "ground" may refer to lapping, sandblasting, acid etching or other processes. This article focuses on a scattering surface produced by controlled roughening. If the drawing specifies Ra, scattering angle, haze, grit number, particle contamination or chemical state, the exact process must be pinned down — "ground" alone is not enough. |
02 | Single-side ground: one side handles scattering, the other stays smooth and controllable
2.1 Structure and optical effect
Single-side ground is often written as G/P (Ground/Polished), or "ground face + smooth face". The roughened face scrambles the direction of incident light through micro-scale random relief, producing diffusion; the other face keeps a polished, fire-polished or original high-quality surface to control transmission and to make cleaning, bonding, inspection or later coating easier.
2.2 Which industries and positions is it normally used in?
| Industry / System | Typical Position | Why Single-Side Ground | Engineering Notes |
| Machine Vision / Industrial Inspection | Lighting diffusers, backlight module diffusion faces, calibration and viewing targets | Reduces hot spots and local highlights for more even illumination, while keeping one smooth face for easy assembly and cleaning | If it sits in the imaging optical path, evaluate the contrast loss caused by scattering |
| Microscopy / Scientific Instruments | Diffusers in illumination systems, beam-spot viewing screens | Turns a concentrated beam into a more uniform light field and reduces directionality | Scattering strength must be defined by grit number, BSDF/scattering angle or haze |
| LED / Light Source Modules | LED front-end light diffusion, indicator lighting, area light source diffusion | Suppresses bright spots and the granular look of the light source | Coarser grinding usually diffuses more strongly, but effective transmission drops |
| Laser Alignment / Optical Path Set-Up | Low-power beam-spot viewing, target screens | A rough surface scatters light over a wider angle, making the beam spot position easier to see | High-power lasers must not be used with ordinary ground parts; absorption, damage threshold and contamination must be calculated |
| Equipment Viewing / Glare Reduction | A specified side of a non-imaging viewing window | Reduces specular reflection and glare while the other side can keep an easy-to-clean surface | Glare reduction is best confirmed with haze, gloss or scattering metrics — do not just write "ground" |
Engineering judgement:If you need diffusion but still want to keep one side at higher optical quality, single-side grinding often controls system loss more easily than double-side grinding.
2.3 How is the data usually specified?
For diffusion-type ground glass, the common approach is to describe coarseness with a grit number or abrasive grain size. Publicly available optical products list grades such as 120, 220, 600 and 1500 grit: coarser grit usually gives a wider diffusion angle but lower transmission efficiency, while finer grit usually gives a narrower diffusion distribution and higher transmission efficiency. Note that different grit systems (for example CAMI and FEPA) and different processing methods cannot be equated one to one.
| Do not convert grit number directly into Ra: Grit number describes the processing medium or grain grade; Ra is the statistical roughness of the finished surface. Both are affected by material hardness, pressure, time, slurry, machine path and the filter conditions of the measurement. If a customer specifies Ra, the value measured by a profilometer or white-light interferometer must govern. |
03 | Double-side ground: both faces scatter, prioritising uniformity and the removal of specular reflection
3.1 When do you really need both faces ground?
Double-side ground can be written as G/G. Its core value is not "coarser" but that neither side keeps an obvious specular reflecting surface. For parts that are visible from both sides, receive light from both sides, need stronger diffusion or need lower directional sensitivity, roughening both faces has advantages; but it brings higher scattering loss, more cleaning difficulty and is less suited to high-precision imaging or laser transmission faces.
3.2 Common applications
- Backlighting and lighting: diffusers, light-mixing plates and internal instrument lighting parts where specular reflection must be reduced on both sides.
- Display, indicator and viewing: diffusion screens, indicator windows and beam-spot viewing parts that do not carry high-resolution imaging.
- Industrial vision lighting: where structural space is limited and light may enter from different directions, two-sided scattering reduces directionality.
- Special fixtures and non-functional faces: a few equipment glasses specify two-sided roughening for anti-glare or visual identification, but it must be confirmed that they carry no bonding, coating or precision optical path function.
| Not recommended for direct use in: Semiconductor temporary bonding faces, lithography faces, TGV functional faces, precision filter transmission faces, high-quality FA/FAU cover plates, high-power laser windows and imaging system windows. These positions generally place more value on low scattering, flatness, TTV, parallelism and surface defect control. |
04 | Double-sided polishing: the core surface state for genuinely high-precision optics, optical communications and semiconductors
4.1 Double-sided polishing is not just "polishing both faces until they shine"
Double-sided polishing (P/P) typically uses double-side lapping and double-side polishing equipment so that the upper and lower surfaces are removed simultaneously. Its value lies in putting micro-roughness, thickness uniformity, parallelism and surface form control into a single processing chain. For thin glass, wafers, windows and precision substrates, this usually stabilises TTV and parallelism better than repeated single-side flipping.
Figure 2 | How common purchasing specifications for a double-sided polished part differ
4.2 Which industries and positions is it normally used in?
| Industry | Typical Component / Position | Why Double-Sided Polishing | Metrics Usually Watched |
| Optical Communications | FA/FAU cover plates, V-groove mating cover plates, OSA/TOSA/ROSA windows, filter substrates | Reduces scattering and wavefront disturbance and ensures assembly datums and transmission consistency | Ra, TTV, parallelism, S/D, dimensional tolerance, cleanliness |
| Semiconductor / MEMS | Glass wafers, TGV substrates, carriers, temporary bonding carriers, package lids | Supports lithography, bonding, coating, deposition, inspection and wafer-level processes | TTV, Bow/Warp, Ra, flatness, particles, edge chamfer |
| Laser / Precision Optics | Protective windows, laser windows, filter and beamsplitter substrates | Low scattering, low absorption, coatable and with controlled wavefront error | S/D, surface form, parallelism, Ra, material absorption, coating requirements |
| Machine Vision / Sensors | Camera protective windows, vision windows, filter substrates | Maintains image contrast and MTF and avoids scattering haze | Surface form, S/D, transmission, coating, appearance defects |
| Medical / Microfluidics | Chip cover slips, reaction chamber lids, detection windows (functional faces) | Eases bonding, imaging, fluorescence detection and cleanliness control | Ra, TTV, flatness, chemical stability, cleanliness |
05 | Typical data: how can it be defined at the purchasing stage?
The figures below are common engineering purchasing reference ranges in precision glass processing; they are not guaranteed values that every material or dimension can achieve unconditionally. Actual capability depends on glass type, outline dimensions, thickness, aspect ratio, equipment capacity, batch size and inspection method.
| Item | Single-Side Ground G/P | Double-Side Ground G/G | Double-Sided Polished P/P |
| Surface function | One side diffuses, one side keeps a smooth, controllable surface | Both sides diffuse, reducing specular reflection | Both sides low-scattering, high-precision transmission |
| Common ways to describe it | Grit number, Ra, haze, diffusion angle + smooth-face quality | Grit number/Ra/haze or diffusion angle on both sides | Ra + S/D + TTV + parallelism + surface form/Bow-Warp |
| Example grit numbers | 120 / 220 / 600 / 1500 grit (common in published diffuser specifications) | Can be customised with the same or different grades on each side | Not applicable |
| Typical reading of Ra | Usually sub-micron to a few microns; must be defined by measurement | Usually sub-micron to a few microns; must be defined by measurement | Common targets in precision processing ≤1–5 nm; higher grades can be tightened further |
| S/D surface quality | Generally only meaningful for the smooth face that is retained; as agreed on the drawing | S/D logic from a polished face should not be carried over to a ground face | Commonly 60/40, 40/20, 20/10, depending on the optical grade |
| TTV | For diffuser parts this is usually not a key metric; it must be specified separately for precision substrates | Same as left | Commonly ≤5–20 μm; precision wafers and thin substrates may require ≤5 μm |
| Parallelism | Defined by assembly requirements | Defined by assembly requirements | Precision windows commonly ≤1–3 arc min; tighter requirements can go below 1 arc min |
| Surface form / flatness | Usually not a primary metric | Usually not a primary metric | Can be defined as λ/4, λ/2, 1λ, 2λ or μm-level form; wafers more often use Bow/Warp |
| Suitability for coating | Coating is normally applied to the smooth face first; coating a rough face needs validation | Coating uniformity and scattering on a rough face must be evaluated | Well suited to AR, filter and reflective precision thin-film processes |
| Cleaning difficulty | A rough face retains particles more easily than a polished one | Both faces are harder to clean | Precision cleaning and clean packaging are more straightforward |
| One key distinction: S/D (Scratch-Dig) describes visible surface defects such as scratches and digs; Ra describes micro-scale height variation; TTV describes thickness variation across the whole sheet; Bow/Warp describes overall warpage. The four are not interchangeable. | |||
Figure 3 | Working backwards from the industry position to the surface finish
06 | Different materials call for different surface finishing strategies
| Material System | Typical Advantages | More Common High-Value Surface States | Typical Directions |
| Borosilicate Glass | Low thermal expansion, heat resistance, chemical resistance, good machinability | Double-sided polishing; single- or double-side grinding for diffusion uses | Optical communications, laboratories, microfluidics, instrument windows, optical substrates |
| Aluminosilicate Glass | Mechanical strength and chemical strengthening potential, suitability for thin sheets | Double-sided polishing / precision thin sheets | Displays, sensors, semiconductor carriers, precision cover plates |
| Fused Silica | Low thermal expansion, high UV/visible/NIR transmission, temperature resistance | High-grade double-sided polishing; diffuser parts can also be roughened | Lasers, UV optics, semiconductors, inspection windows |
| Optical Crown Glass / Low-Iron Glass | Visible-light transmission, a balance of cost and processing | Ground for diffuser parts; double-sided polished for imaging and windows | Machine vision, lighting, general optical windows, diffusers |
Take micro-float borosilicate sheet as an example: the raw sheet itself can have a mirror-grade surface and good flatness, but once it becomes a high-precision wafer, filter substrate or a part with strict TTV requirements, further lapping and double-sided polishing may still be needed to turn "raw sheet quality" into "part-level geometric precision".
07 | How to choose? Use this decision logic directly in engineering
- If the core purpose is to scatter light, eliminate hot spots or view a beam spot, start with a ground finish; then choose single-side or double-side depending on whether one face must stay smooth.
- If the core purpose is to transmit light as close to its original direction as possible, preserve imaging quality, apply precision coatings or run wafer processes, start with double-sided polishing.
- For precision functional faces in semiconductors, MEMS, TGV, lithography, bonding, FA/FAU or laser windows, do not write only "polished" — also specify Ra, TTV, parallelism, S/D, Bow/Warp or surface form.
- For diffuser parts, do not write only "ground" — specify at least: single- or double-side ground, process method, grit number or Ra, haze/scattering angle (if the optical function matters), dimensions and thickness, and allowable appearance variation.
- If the customer only says it should look "hazier" or "brighter", first confirm its position in the system and its optical path function, then decide the process route.
08 | A professional enquiry or drawing should specify at least these fields
| Recommended format: Material + outline dimensions (L×W×T) + quantity + surface state (G/P, G/G, P/P) + grit number or Ra/haze/scattering requirement for the ground face + Ra for the polished face + S/D + TTV + parallelism + surface form/Bow-Warp + edge break/chamfer + cleaning cleanliness + coating + packaging. |
| What the customer says | A more professional engineering statement |
| "I want single-side ground, as fine as possible" | Confirm the grinding process, grit number or target Ra; if it is for light diffusion, also confirm the diffusion angle/haze and the operating wavelength band. |
| "I want double-sided polished, with a good surface" | Specify Ra, S/D, TTV, parallelism, surface form/Bow-Warp, and whether clean cleaning and coating are required. |
| "As long as the glass transmits light, it is fine" | Confirm whether it sits on an imaging, laser, filter, bonding or lithography functional face; requirements for scattering and geometric precision differ completely by position. |
09 | The five most common mistakes
- Confusing "ground" with "lapped, awaiting polishing". The former may be the final functional surface; the latter is only an intermediate operation.
- Treating the grit number as Ra. Grit number is only a process or abrasive grade; the finished surface must be confirmed by measurement.
- Watching only Ra and ignoring TTV, parallelism, surface form and S/D. For wafers, windows and cover plates this often leads to a surface that looks smooth but still fails once installed in the system.
- Assuming double-side grinding is automatically "better" than single-side. In reality it is simply a stronger two-way scattering option, and whether it suits depends on the system function.
- Assuming double-sided polishing must reach 20/10 or Ra<1 nm. Tighter specifications increase cost and yield pressure; match them to the real optical function rather than piling on parameters.
10 | Conclusion: surface state essentially decides how this piece of glass relates to light, equipment and process
Single-side ground, double-side ground and double-sided polished: none is inherently superior. Single-side grinding emphasises "one side scattering, one side keeping a smooth function"; double-side grinding emphasises "two-way diffusion and removal of specular reflection"; double-sided polishing serves low scattering, high transmission, precision geometry and subsequent coating or bonding. Genuinely professional selection starts with understanding where the glass sits in the system, then expresses the surface requirements as engineering language that can be measured, produced and accepted.
References and Technical Basis
1. SCHOTT, BOROFLOAT® Technical Details / Microfloat process: describes how micro-float forming produces a mirror-grade surface, good flatness and optical quality, and provides the material's optical, mechanical and thermal data. https://www.schott.com/zh-cn/products/borofloat-p1000314/technical-details
2. SCHOTT, Glass Melting and Hot Forming – Microfloat: the surface formation mechanism and process characteristics of float and micro-float. https://www.schott.com/zh-cn/expertise/glass-melting-and-hot-forming
3. Edmund Optics, Diffuser Selection Guide / Ground Glass Diffusers: published 120/220/600 grit ground glass diffusers and the selection relationship in which coarser grit gives stronger diffusion and finer grit gives higher transmission. https://www.edmundoptics.com/knowledge-center/application-notes/optics/diffuser-selection-guide
4. Thorlabs, Ground Glass Diffusers: published 120/220/600/1500 grit optical ground glass diffusers and their scattering characteristics. https://www.thorlabs.com/NewGroupPage9.cfm?ObjectGroup_ID=1132
5. Edmund Optics, Understanding Optical Specifications: definitions and applications of surface quality metrics such as Scratch-Dig. https://www.edmundoptics.com/knowledge-center/application-notes/optics/understanding-optical-specifications/
Note: the "typical data" in this article is intended as a reference for engineering communication and purchasing selection and does not constitute a guaranteed value for all materials, dimensions and batch sizes. Formal production should be governed by drawings, inspection specifications, sample validation and a quality agreement confirmed by both parties.
