Collimated backlight or diffuse backlight for edge gauging

Backlighting turns a part into a black silhouette on a white field, the cleanest image there is for measuring diameters, lengths, hole positions and profiles. But a diffuse backlight and a collimated backlight give very different silhouettes once the part has thickness, curvature or shine, and they suit different lenses. This comparison explains what changes, puts numbers on the light efficiency, and ends with a recommendation for each common case.

What each light sends out

A diffuse backlight is a flat panel of LEDs behind a diffuser. Every point on its surface emits in all directions, roughly following Lambert's cosine law. It looks evenly bright from any angle, which is exactly what a normal lens needs: each point of the field sends rays towards the lens's entrance pupil, and the diffuser supplies them at whatever angle that requires.

A collimated backlight puts a lens in front of a small LED source so that the light leaves as a beam of nearly parallel rays. It looks very bright when you look straight into it and dark from any other angle. It is built to pair with a telecentric lens, which accepts only rays nearly parallel to its axis.

Why edges look different

Think of a polished steel pin lying across a backlight. Near the silhouette edge, the pin's surface is almost parallel to the viewing direction.

With a diffuse backlight, light arrives at the pin from every angle. Some of it strikes the curved surface near the edge at a grazing angle and reflects towards the lens. The camera then sees a bright fringe inside the dark silhouette, so the pin measures smaller than it is, and the error changes with surface finish and colour.

With a collimated backlight and a telecentric lens, light arrives only along the axis, and the lens accepts only light along the axis. Light reflected off the curved surface is mostly bent out of the narrow acceptance cone and never reaches the sensor. The fringe shrinks to a thin band and the edge becomes a sharp step from dark to bright. The same holds for the walls of a hole and the sides of a thick part: a diffuse light can make the inside of a bore glow, while collimated light shows only the true through-opening.

The efficiency gap in numbers

A telecentric lens accepts light only within a small cone on the object side. Its half-angle follows from the image-side F-number and magnification:

The last line is the share of a Lambertian emitter's output that falls within a cone of that half-angle (sin² of the angle).

A worked example

Take a 0.158x telecentric lens with an image-side F-number of 4.9:

1. Image-side NA: 1 / (2 × 4.9) = 0.102.
2. Object-side NA: 0.102 × 0.158 = 0.016, a half-angle of about 0.9 degrees.
3. Share of a diffuse backlight's light inside that cone: 0.016² = 0.00026, about 0.03 percent.

So a diffuse panel behind this lens wastes more than 99.9 percent of its output. You can still get an image by raising the exposure or the panel brightness, but the exposure grows long enough to blur moving parts, and the stray light it adds lowers edge contrast. A collimated light concentrates its output into the cone the lens accepts. Here is a matched pair: the lens from the example and a collimated light with a beam of the same size as its field.

The light's data sheet names LS-TCL-1894 as its compatible telecentric lens, and its beam diameter of 72 mm equals the lens's object field. When you choose them separately, match the beam to the field the same way.

The telecentric lens guide for gauging explains when the lens itself is needed; this article assumes you already use one.

When a diffuse backlight is the right choice

A collimated light is not always better. A diffuse backlight wins when:

• You use a normal lens. A standard lens needs rays from every point of the field heading to its entrance pupil at different angles. A collimated beam lights only the centre of its field, so the image shows a bright spot and dark corners.
• The field is large. Collimated lights stop at a few hundred millimetres of beam. Panels are made far larger.
• The part is thin and flat. For stamped sheet, gaskets, film or printed circuit boards, there is no side wall to reflect light, and a diffuse panel gives the same edge as a collimated beam.
• The task is presence, count or rough position. Edges a pixel or two softer change nothing.

A flat diffuse backlight with a published irradiance makes exposure planning easy:

For larger fields or brighter needs, high-luminance panels in the backlights range go further:

Setting up a collimated backlight

1. Match the beam to the lens field. The beam diameter must be at least the lens's object field, with a few millimetres for alignment.
2. Respect the working distance. Collimated lights list a working distance range; place the part inside it.
3. Align the axes. Mount light and lens on the same rail or plate. Tilt or shift the light until the image is evenly bright from edge to edge. With an acceptance half-angle under one degree, a tilt of a fraction of a degree is visible.
4. Choose the colour. Blue or green gives slightly sharper edges than red, because diffraction blur scales with wavelength. Many collimated lights come in several colours:

5. Check brightness headroom. Use the shortest exposure that freezes the part, then set the light's brightness, rather than the other way round.
6. Calibrate with a reference. Measure a gauge pin or a calibrated glass scale and correct the system scale before measuring parts.

Recommendation by case

A compact round collimated light suits small telecentric set-ups on a test bench or in a machine:

Next step

Compare beam sizes in the collimated lights range, pair them with telecentric lenses of the same field, or build the complete gauging station in the configurator. More lighting articles are on the lighting topic page.