Light control film on a backlight: sharp silhouettes on shiny parts

A diffuse backlight is built to be even, and it pays for that evenness with angle. Light leaves the panel over a wide cone, and the oblique part of that cone wraps around the flanks of a glossy round part, so the silhouette edge turns into a grey gradient several pixels wide and the diameter a subpixel edge finder reports drifts with the part's finish. A light control film over the panel passes the near-vertical rays and blocks the oblique ones, the way a microscopic set of louvers would. It is the low-cost fix for penumbra on shiny cylinders, pins and shafts, and it charges a predictable toll: 30-40% of your light, 50% in the worst case. Here is when to fit it, how to mount it, and how to pay the bill back.

What the film is, and the decision

Light control film is a transparent sheet carrying a microstructure of parallel louvers; commercial types either embed the louvers in a black matrix or shape transparent ribs that steer and absorb off-axis light. Fitted flat over the face of a standard backlight, it transmits rays travelling close to the panel normal and removes rays that leave at steep angles. The effect on the image is similar to telecentric backlighting, without the full collimation.

The decision:

• Glossy, round part on a standard backlight, soft grey edges, and you can spare or recover 30-40% of the light: fit the film.
• The silhouette must be hard in every direction, or the gauging tolerance is a few micrometres: specify telecentric backlighting. The film filters stray light; it does not align the light.
• Matte, flat parts whose edges are already crisp: skip the film. You would pay light and gain nothing.

For the wider picture of transmitted-light techniques, the lighting topic page gathers the backlight and bright-field guides.

Why glossy round parts lose their edge

A backlight's diffuser is an extended source that radiates over a very wide range of angles, because that is what makes the panel look even from the camera's position. With a wide panel a modest distance behind the part, light arrives at the silhouette edge from far off the vertical; the arithmetic appears in the worked example below. On a matte part that merely softens the shadow. On a specular cylinder it is worse: the flank near the tangent reflects part of that wide cone straight into the lens, so pixels just inside the geometric silhouette arrive part-lit. The edge becomes a grey ramp instead of a step, and the wider the ramp, the more the interpolated edge position depends on the part's reflectivity rather than its size.

That is also why chasing contrast by pushing exposure towards saturation backfires: the ramp carries the measurement. The diagnostic method for low-contrast images is covered in machine vision stray light: diagnose low image contrast; the same grey-edge fight on turned steel is worked through in backlight for width measurement on shiny turned steel; and liquid level detection: backlight contrast at the meniscus chases the identical transition problem at a liquid edge.

How the louvers work, and why direction matters

Each louver behaves like a microscopic blind. Rays inside a narrow cone around the panel normal thread between the louvers and reach the camera; rays outside that cone strike a louver wall and are absorbed or reflected away. Light the panel, lay the film over it and walk around: near normal the sheet is clear, and from a steep viewing angle it goes almost black, because no straight path exists through the structure at that angle. That black-at-an-angle behaviour is also the fastest bench check that a sheet is a louver film rather than a plain diffuser.

The microstructure runs in one direction only, and that sets the mounting rule: align the louvers parallel to the longitudinal edge of the part. Blocking is strong for oblique light whose tilt is measured across the structure and much weaker along it, so rotating the film 90° keeps most of the brightness but gives back the penumbra. Pins and shafts are measured across their axis, which is why the louvers usually end up running along the axis.

The polarizers and bandpass filters that pair with transmitted light live in lens accessories; the film itself is optical sheet, ordered cut to size and fitted to the panel you already run.

The brightness bill: plan for 30 to 40 percent

The rays the film removes were contributing to your exposure. Count on the image dropping by about 30-40%, with 50% the number to design the exposure and controller budget around.

At a typical T = 0.65: t_film = 500 µs / 0.65, about 770 µs, and the loss is log2(1/0.65), about 0.62 stops. Three ways to pay it back:

1. Exposure. Multiply by about 1.5 (500 µs to 770 µs). Free for static parts; on a moving line the extra integration time may cost blur.
2. Aperture. Open by about two thirds of a stop. A full stop from f/5.6 to f/4 over-compensates, since 0.65 x 2 = 1.3x the original brightness, so shorten exposure to about 385 µs to land back on the original grey levels.
3. Brighter light. When exposure and aperture have no room, step up the panel. The LS-BKL-0865 is a 400 x 300 mm high power backlight rated at 92.2 W.

One safety note: a strobed high-power panel is a strobe source, so avoid staring directly into it while aligning the station.

Where the film goes: flat on the panel, centimetres from the part

The sheet is not optically dead material: the louver structure, and micro-cracks in the film, live in its plane. If that plane sits near the plane the lens is focused on, the structure prints into the bright field as faint lines and speckle. The effect is worst for small imaging areas, where the temptation is to tuck a small panel close behind a small part at high magnification. The rule from practice: keep several centimetres between the film and the part, and let defocus average the structure into the uniform glow a backlight is supposed to be. The worked example puts numbers on how far out of focus the film ends up.

Worked example: a 20 mm shiny pin at 200 mm

The case: a ground steel pin, 20 mm diameter, glossy, tolerance ±0.05 mm on diameter, presented at a fixed height. The camera is a 2/3-inch monochrome sensor with 2448 x 2048 pixels of 3.45 µm, a common gauging format, giving a sensor width of 2448 x 0.00345 mm = 8.45 mm.

Field of view. Give the 20 mm pin 2 mm of margin each side: a 24 mm field. Magnification m = 8.45 / 24 = 0.352.

Focal length. At 200 mm working distance, f = 200 x 0.352 / 1.352 = 52 mm. The nearest standard lens is 50 mm, which delivers the same 24 mm field at WD = 50 x (1 + 1/0.352), about 192 mm.

The numbers above are the ones the lens focal length calculator works through when you trade field of view against working distance.

Resolution check. Object-space pixel size = 3.45 µm / 0.352 = 9.8 µm. The ±0.05 mm tolerance spans ±5.1 px, and a 0.1 px subpixel edge is worth about 1 µm, so the sampling has margin.

Light and penumbra. The panel is the LS-BKL-1468, a 150 x 150 mm red flat backlight.

It sits 100 mm behind the pin. Rays from the near edge of the panel reach the pin's edge at atan((75 - 10) / 100) = atan(0.65), about 33° off vertical; the specular flank reflects part of that cone towards the lens and smears the edge over several pixels. With the film fitted and the louvers running along the pin axis, only near-normal rays leave the panel and the transition collapses towards a one-to-two-pixel step. If you are weighing a purpose-built panel against sheet goods, DIY versus catalogue backlights for a red panel runs that trade-off.

Paying the bill. A realistic starting point before the film is 500 µs at f/5.6. After the film, at T = 0.65: 500 / 0.65, about 770 µs, or open to f/4 and expose about 385 µs. The line presents one part per second, so 770 µs is not a constraint; keep f/5.6 for its depth of field and pay in exposure.

Film distance check. At f/8 with a one-pixel circle of confusion (3.45 µm), the lens's total depth of field is 2 x 8 x 0.00345 x (1 + 0.352) / 0.352², about 0.6 mm. The film 100 mm behind the pin sits more than 150 times that range outside focus, so nothing of its structure can print. This is the arithmetic behind the several-centimetre rule.

Decision. Fit the film, expose about 770 µs, and expect the diameter reading to stop following the part's finish. One guard remains: a 50 mm entocentric lens makes magnification track part height. One millimetre of height change moves magnification by 1 / 192, about 0.52%, which is 0.10 mm on a 20 mm diameter, double the tolerance. Hold the part height within ±0.25 mm (about 26 µm of induced error, half the tolerance) or move to a telecentric lens; when you need a telecentric lens for gauging draws that line.

Film, collimated panel or telecentric backlighting

The film is the only one of the four you can cut to size and re-align in minutes, which is exactly its role: an upgrade to a panel you already own. A collimated panel builds the angular control in at the factory; the LS-BKL-0830 is a 120 x 120 mm collimated backlight, and collimated or diffuse backlight for edge gauging plus collimated backlight set-up: working distance and light size cover choosing and sizing one. Telecentric backlighting goes further and aligns the light rather than filtering it, so the silhouette is hard from every direction, the standard for round shiny parts shown in telecentric illumination for measuring round shiny parts, and it is the technique that images transparent parts as solid black: collimated backlighting for transparent parts.

Browse backlights filtered to the Collimated type for the factory version of what the film approximates.

When the film is the wrong answer

• Edges in every direction. Louver blocking is one-directional. A square profile with four gauged edges, or a ball with no long axis at all, gets full treatment only from collimated or telecentric light.
• Translucent parts. Removing oblique light darkens translucent material towards a solid silhouette. When the internal features are the measurement, that is the wrong direction, and wavelength does more than angle: backlight wavelength selection for translucent parts.
• No exposure, aperture or brightness headroom. If the line cannot afford x 1.5 exposure, the lens is already wide open and the controller has no strobe headroom, buy the angular control instead of paying the toll on every frame.
• The soft edge is not angular. Profile the edge first. If the transition is already one to two pixels, the problem is focus, diffraction or pixel size, and film buys nothing.
• Varying part height. The film sharpens edges; it does not touch the perspective term from the worked example. Fix the height or fit a lens from telecentric lenses.

Checklist before you fit film

• Profile the edge with a line plot. Several grey pixels between bright and black means angular stray light, and the film is the right tool; one to two pixels means looking at focus and pixel size instead.
• Budget the toll: 30-40% typical, 50% for the worst case, in exposure, aperture or controller headroom.
• Order the sheet to cover the lit area with clamping margin, and mark the louver direction before cutting.
• Fit it flat on the panel face, louvers parallel to the part's long edge, several centimetres from the part.
• Re-run exposure (x 1.5, or open about two thirds of a stop) and confirm the bright field stays below clipping.
• Image the part with the film rotated 90° once, and keep the orientation with the steeper gradient.
• If the reading still drifts with batch or finish, check part height against the perspective arithmetic above before blaming the light.

When you would rather buy the angular control than add it, the backlight finder filters by Collimated and High power types, and the lighting configurator folds the choice into a full station.