Two different strategies for taking a wide LED output and turning it into the beam your design needs. A lens redirects most of the flux; barn doors block the edges. Knowing when to use each saves energy, and buys precision only where the project justifies it.
Four ideas to carry into the next specification conversation you have.
TIR (Total Internal Reflection) and Fresnel lenses redirect most of the LED's output. Typical optical loss is about 5-10%. The beam is clean, with a defined edge.
Mechanical flags block unwanted spread. Loss can reach roughly half the output on a tight cut, depending on beam geometry and flag position, but the beam edge can be hand-set on site.
Streetlights, downlights, panels, beam is locked at manufacture. Lens does the work.
Where the cut needs to be tuned on site to miss a curtain, a chair, or a face.
Metric: optical efficiency, the share of luminaire flux leaving the optic. Values are indicative typical figures, not product data; real values depend on lens material and barn-door geometry. Verify against the photometric file for the exact fixture.
| Method | Efficiency | Best use |
|---|---|---|
| TIR lens (PMMA/PC) | 85-94% [5][7][8] | Streetlight · spot |
| Fresnel lens | 80-90% [5] | Stage · concert · spotlight |
| Refractor (textured) | 78-88% [5][6] | Office panel · diffused |
| Reflector (specular) | 80-90% [9] | Highbay · downlight |
| Reflector (diffuse white) | 70-85% n/a | Office troffer · glare control |
| Barn doors (mech.) | set by the cut n/a | Theatre · hospitality accent |
| Honeycomb louvre | set by the cut [12] | Museum · gallery glare control |
Five framing decisions that pick between refractive optics and mechanical control.
Total Internal Reflection lenses use a refractive shape with a hollow centre. The LED sits in the cavity; photons either pass straight through or hit the side at an angle exceeding the critical angle and reflect internally. Efficiency above 90% with clean cut-off.
A grid of small hexagonal cells in front of the optic. Cuts off-axis light at the cell wall angle, which is what tames the high-angle rays that veil computer screens. The cost is easy to under-estimate and hard to generalise: on one soft light measured open and then louvred under LM-79 [12], a 90° louvre took 9,046 lm down to 1,700 lm while peak intensity fell only 3,300 cd to 2,942 cd. A louvre removes luminous flux, not centre-beam intensity, and how much it removes depends on how much wider the source is than the cell cut-off. Take the figure from the accessory's own photometric file, never from a rule of thumb. A common fix where spots or downlights push glare past the office UGR ≤ 19 limit [10].
Theatre, film, hotel hospitality, the wall you light tonight is not the wall you light tomorrow. A pre-set lens has no answer. Four hinged flags around the aperture do. Spec barn doors only where reconfigurability is non-negotiable.
A premium retail spot may run a TIR lens for the primary beam (efficient) plus a removable snoot or flag for spill control on the wall (precise). Cost more, look like the work of someone who cared.
An RFQ that says "barn doors" forecloses TIR alternatives. An RFQ that says "beam 15° ± 1°, hard cut-off at 35° from axis, < 50 cd outside" lets the supplier pick the optics, and usually returns a better answer.
Our MAGIC-T track spotlight pairs a lensed core (17°-55°) with snap-on barn-door, honeycomb, cross-louver, elliptic and wall-washer accessories, and the wider track-spot range adds louvre and snoot options for fine glare control.
Efficiency figures in the table above are indicative planning bands, not product data. Optical efficiency here means the share of source flux leaving the optic [7]. At an air/PMMA boundary about 4% reflects at each surface, so a flat clear acrylic window transmits about 92% [5]; a finished optic is measured, not derived from that, and published pairings run roughly 85% to 94% [8]. Barn-door and honeycomb-louvre losses are shown as bands because no primary measurement was found for them: the real figure depends on beam geometry and how far the flags or cells close, and must be taken from the photometric file for the exact fixture and accessory.
A 13-slide explainer, ready to post straight to LinkedIn as an image carousel. Same content, faster reading.
13 slides · LinkedIn-ready. Download all 13 as high-res PNGs and post them straight to LinkedIn as a carousel. PDF version still available.
Practical lighting know-how from NLC Academy. No spam, unsubscribe anytime.
By subscribing you agree to receive NLC emails.