Guide length is often treated as a convenience, ordered long “just in case” and coiled behind the lamp. That habit costs delivered UV on every cure and adds a bend that the guide has to survive for its whole service life.

Q: Does a Longer Light Guide Reduce UV Intensity at the Part?

A: Yes — Specify Length From Required Reach, Not From Spare Slack

Light intensity drops with distance along a lightguide, so every extra length of guide between the lamp port and the tip removes some of the energy the lamp produced. The practical rule is to measure the real routing path, add only what the fixture’s motion and service access require, and order that length. Then confirm delivered output at the tip, because the guide, not the lamp, is the last component to affect dose before the bond line.

Length and Working Distance Are Two Different Losses

Engineers sometimes merge two separate effects. The first is loss along the guide itself, which grows with guide length. The second is falloff in air between the tip and the part, which depends on working distance. Incure’s data for the L9000™ UV LED spot lamp illustrates how steep the second effect is: 7,500 mW/cm² at a 9 mm focal point falls to 2,300 mW/cm² at 17 mm and to 223 mW/cm² at 30 mm. A longer guide that lets the tip sit closer to a recessed bond can therefore recover more dose than it loses along its length. Length is worth adding when it shortens working distance, and worth avoiding when it only adds slack.

How Incure Defines Length on the L9000™

The L9000™ makes length an explicit selection rather than a free variable:

• Cool Guide (L9000-CG): 83 mm total length with a 24 mm cube head and integrated cooling fan, built for tight, confined fixtures where limiting heat transfer to a thermally sensitive assembly matters.
• Short Guide (L9000-SG): 66 mm fiber optic guide for standard bench-top curing and semi-automated dispensing lines.
• Long Guide (L9000-LG): 118 mm, for deep-reach positions inside complex fixtures that shorter guides cannot access.

The Long Guide is 52 mm longer than the Short Guide (118 minus 66). That extra reach is justified only when the bond line actually sits that deep; on an open bench fixture, the Short Guide reaches the same part with less guide in the path.

Email Us with a fixture drawing showing the lamp location and the bond line, and Incure can recommend a guide length and tip position for it.

Bend Radius Sets a Floor on Practical Length

Too short is also a problem. Every guide has a minimum bend radius, and exceeding it breaks glass fibers or kinks a liquid guide, both permanently. A guide cut so short that it must turn sharply to reach a fixture, or that pulls tight when a guard opens, will fail early. The right length includes a gentle service loop and enough slack for the full movement of any robot or slide it rides on, and nothing more.

A Hypothetical Sizing Exercise

Consider a hypothetical bench station where the lamp port sits beside the fixture and the bond line is near the top of an open part. Measuring the path shows the Short Guide reaches it with a relaxed curve, so the 66 mm option is chosen. Now consider a second hypothetical station where the bond line sits at the base of a cavity inside the fixture. Here the Short Guide can only reach the cavity opening, leaving a large air gap to the bond, while the Long Guide’s tip can sit close to the bond line. Given the working-distance falloff above, the longer guide is the better trade at that station even though it adds guide length.

Where Length Interacts With Tip Hardware

Angled bonds add another option. Incure’s LGT-Series™ terminators redirect output at 90° or 60° in Ø5mm and Ø8mm apertures, which can put the exit point at a side wall without routing the guide itself around a tight corner. A short guide plus a terminator can sometimes reach a face that would otherwise demand a longer guide and a sharper bend.

Verifying the Choice

After installation, measure delivered output at the tip with a radiometer, or on a Standard D, Ø5mm arc-lamp port with Incure’s LS217™ Lightguide Simulator, and record it as the station’s baseline. If two stations with different guide lengths must run the same cure recipe, the measured tip output, not the lamp setting, shows whether their doses actually match. For the diameter side of the same decision, see Incure’s article on choosing light guide diameter for a UV spot application, and for how guides work in general, the liquid light guide explainer.

Contact Our Team to size light guide length for your UV curing fixture.

Visit www.incurelab.com for more information.