Making Anti-Newton Ring Glass For Slide Scanning
Newton rings are coloured or dark bands that appear when a smooth film surface sits extremely close to another smooth surface, usually a glass carrier. They are caused by reflected light interfering in the tiny air gap between the two surfaces. With mounted 35mm slides and unmounted negatives, the effect can obscure fine grain, soften detail and create patterns that are difficult to remove in software.
A home-made anti-Newton ring insert can be useful for a Nikon Coolscan, especially when the original holder is missing, damaged or unsuitable for a particular batch of film. The reliable approach is to buy suitable micro-textured optical glass and cut or mount it to match the carrier. Ordinary window glass, picture-frame glass or a piece of sanded glass may prevent rings, but can introduce haze, scratches and reduced resolution.
| Glass or carrier option | Ring control | Image quality | Cost and availability in Australia | Suitability |
|---|---|---|---|---|
| Standard window or frame glass | Poor to fair | Usually clear, but rings remain | Low; easy to find | Temporary testing |
| Commercial ANR optical glass | Very good | High when correctly oriented | Moderate to high; often sourced through framers or online suppliers | Best general choice |
| Fine etched or micro-textured glass | Good to very good | Can be high, depending on texture | Moderate; specialist glazing may be needed | DIY insert material |
| Sanded or blasted glass | Good ring suppression | Often visibly hazy | Low, but inconsistent | Avoid for archival scans |
| Open film holder with no glass | No glass rings | Excellent if film stays flat | Low | Best for naturally flat film |
Why Newton Rings Appear During Film Scanning
A Newton ring is not a flaw in the emulsion itself. It forms when two highly smooth surfaces are almost touching. A slide’s glossy emulsion, a clear film base and the polished face of a glass insert can all create the required conditions. Slight pressure from a holder can make contact more likely, particularly around the centre of a bowed or curled frame.
The bands may appear as broad arcs, concentric circles or irregular patches. They are often most obvious in skies, studio backdrops and other areas with smooth, even tones. A preview scan may show the problem clearly, while a full-resolution scan reveals finer interference patterns across the frame.
Dust should be treated separately. Dust creates sharp spots and hairs, while Newton rings create broad interference patterns. Digital ICE can sometimes remove dust and scratches from colour film, but it will not reliably repair Newton rings. If the rings are present in the raw scan, rescanning with a better film-to-glass arrangement is usually more successful than attempting extensive retouching.
Nikon Coolscan holders were designed for specific film dimensions, so an insert that works in one model may interfere with another. The LS-30, LS-40, 4000 ED, 5000 ED and later scanners use different holders and clearances. Measure the actual holder rather than relying on a photograph or a dimension found in a forum post.
Selecting Glass For A Homemade Insert
The most important feature is a very fine textured surface on one side. Commercial anti-Newton ring glass, often called ANR glass, uses a controlled microscopic texture to prevent broad areas of direct contact. The surface is not meant to look visibly rough. If it looks frosted from normal viewing distance, it is likely too coarse for high-resolution film scanning.
A proper insert should be thin enough to fit the holder without bending it or narrowing the optical path. The exact thickness depends on the carrier and scanner. A glass supplier, picture framer or specialist glazing business may describe suitable stock by thickness, etched surface, anti-glare finish or optical grade. Ask specifically for fine anti-Newton ring glass for film or photographic use, rather than generic non-reflective glass.
Surface orientation matters. Usually, the textured side faces the film and the smooth side faces away from it, but this is not universal. Some products are designed to work with the texture toward the scanner lens, while others are supplied for a particular carrier design. Mark one corner before testing, then compare both orientations with the same slide and scan settings.
Avoid applying sandpaper, abrasive powder or a rotary tool to optical glass. These methods create uneven pits and scratches that scatter the scanner’s light. A diamond blade may cut glass, but it requires appropriate equipment and leaves edges that can chip or shed small fragments. In Australia, custom cutting through a local glazier or framing shop is often safer and more economical than replacing a damaged scanner holder.
Building The Insert At Home
The simplest design is a thin rectangular piece of ANR glass held in a rigid frame or fitted into an existing film carrier. It should cover the image area without obscuring registration holes, magnetic strips, hinges or the scanner’s calibration openings. A glass insert should never be allowed to slide into the transport mechanism.
Measure the opening at several points with digital calipers. Check the holder for moulded ribs, retaining clips and changes in thickness. Allow enough clearance for the glass to sit flat, but do not leave a large gap that allows the insert to move during scanning. A loose fit can place the glass at an angle, causing uneven focus across the frame.
Have the glass cut with slightly rounded corners or carefully softened edges. Sharp corners can damage a plastic holder and make handling hazardous. Do not use household tape across the optical area. If retention is necessary, use small edge tabs outside the image path, or make a thin frame from archival polyester or another stable material.
Materials And Safety Checks
- Fine ANR optical glass cut to the measured dimensions
- Digital calipers, lint-free gloves and a blower
- A clean, rigid frame or compatible film-holder insert
- A padded storage box for the finished glass
Checks Before The First Scan
- Confirm that the insert cannot touch the scanner’s moving parts
- Inspect both faces under a bright, angled light
- Test the textured side against a spare slide
- Record the orientation that gives the clearest result
Wear eye protection when handling cut glass and keep children and pets away from the work area. Glass fragments can be almost invisible on a dark bench. A damp disposable cloth is safer than blowing fragments around the room, and the work surface should be cleaned before film is brought near it.
In Australia, a local picture framer in Melbourne, Sydney, Brisbane, Adelaide or Perth may already have access to non-reflective glazing and precision cutting services. A framing shop may use “non-glare” to describe several different products, so show a sample holder and explain that excessive diffusion is unacceptable. Prices can vary substantially once cutting, postage and GST are included, particularly if the glass is ordered from overseas and sent through Australia Post.
Fitting The Glass To Film And Scanner
Clean the film first with a blower and an appropriate anti-static brush. Do not wipe a valuable slide repeatedly just to remove one stubborn speck. If liquid cleaning is required, use a product and method known to be safe for the film type. Some old slides have fragile mounts, loose labels or deteriorating emulsions that can be damaged by pressure.
Place the film in the carrier without forcing it flat. The purpose of the ANR surface is to control contact and reduce interference, not to clamp a curled original aggressively. If the slide bows, use the lightest retaining pressure that keeps the frame within the scanner’s focus range. Excessive pressure can leave marks on the mount or emulsion.
Begin with a low-resolution preview. Scan a frame containing both a smooth tone and fine detail, such as a pale sky with foliage or lettering. Compare the ANR insert with the original holder, and rotate or reverse the insert if the product permits it. Look for three things: fewer rings, no broad veil over the image and consistent sharpness from corner to corner.
Do not assume that the textured side should face the film merely because that is common with one product. The texture can scatter light differently depending on the scanner’s lens, the film’s curl and the distance from the sensor. A simple orientation test on a duplicate or expendable slide provides more useful evidence than a general rule.
Scanning Settings And Long-Term Care
Once the insert is working, make a fresh scanner calibration or prescan. A glass layer changes the optical path by a small amount and may alter autofocus behaviour. On a Nikon Coolscan, compare autofocus with a fixed focus position if the software allows it. The best setting is the one that resolves grain and edge detail without exaggerating scratches or film curl.
Keep Digital ICE, ROC and GEM decisions separate from the anti-Newton ring test. ICE can be helpful with colour negatives and slides, but infrared-based dust removal has limitations with traditional black-and-white films and some unusual emulsions. Save an unprocessed master scan in TIFF format before applying colour correction, sharpening or noise reduction.
Glass must be stored clean and protected from pressure. A soft sleeve or archival box is preferable to leaving it inside the scanner holder, where dust can accumulate and the insert may be knocked against the bench. In dry inland parts of Australia, static can attract dust quickly; in humid coastal areas such as Brisbane or Darwin, fingerprints and condensation need equal attention. Let cold equipment reach room temperature before sealing it in a storage container.
Strong Australian sunlight is another reason to preserve original slides carefully. A digital copy is useful, but it does not replace stable physical storage. Keep film and the glass insert away from heat, direct sun and sheds where summer temperatures can rise sharply. For family slides collected from a house clearance, label the scanned files with dates, locations and original frame numbers before the history disappears.
Troubleshooting Common Problems
If rings remain, first inspect the film for curl and check whether the wrong face of the insert is against the emulsion. A glass surface can also appear effective in one area and fail in another if it is not flat. Try a different frame, then compare the insert outside the scanner by holding it against the film under a single light source.
If the entire scan looks milky, the texture is probably too coarse, the glass is dirty or the insert is scattering light into the lens. Generic picture-frame anti-glare glass is a common cause. Replace it with finer ANR optical glass rather than trying to polish or clean away the texture.
If the scan is sharp in the centre but soft at the edges, check that the insert is not tilted. A retaining tab that is too thick, a chip under one corner or a warped plastic frame can change the film height across the image. The scanner’s depth of field is limited, so a tiny height difference can matter at high resolution.
If the holder binds, stop the scan immediately. Do not push a modified carrier through the transport path. Trim the insert only through a competent glass service, or redesign the frame so the glass is supported outside the moving mechanism. The cost of a replacement sheet is small compared with the cost and difficulty of repairing a damaged Coolscan.
For most hobbyists, the best home-built solution is a carefully measured ANR glass insert rather than glass made from scratch. Use fine optical material, keep the textured face correctly oriented, test it at preview resolution and store it like a photographic accessory. A practical rule is simple: if the insert suppresses rings without adding visible haze or changing edge sharpness, it is ready for full-resolution slide scanning.