A typical bare LED die emits close to a Lambertian hemisphere, typically near 120° wide and completely uncontrolled. The lens is the part that turns that into a 10° spot, a 90° flood, a UGR-19 office wash, or a soft glow with no hotspot. Six lens families do almost all the work in modern luminaires. Here is what each one is for.
Four ideas that decide which lens belongs in your spec, and which one is the wrong answer dressed up.
Total-internal-reflection lenses sit directly over the LED and redirect light into a controlled cone using the lens wall itself, with no reflective coating. Efficiency is published per optic and LED pairing, not per lens: Carclo lists 83-93% across LED pairings for one 10 mm narrow-spot optic.
Frosted, opal, and micro-prismatic optics scatter light to soften hotspots and cut glare. You pay for that in transmitted lumens, by an amount set by the specific diffuser, and the payoff is lower luminance at the angles that reach an eye. UGR itself belongs to the room, not the lens.
Linear strips and wall-washers need beams shaped for a rectangle, not a circle. Asymmetric, batwing, and oval optics push lumens off-axis to light the wall or aisle the fixture is not directly above.
PMMA gives the cleanest beam, scratches easily. Polycarbonate handles impact, yellows in UV without stabiliser. Glass is permanent and inert, heavier and more expensive. The lens material decides the 10-year photo of your fixture.
Indicative planning bands, not product data. Optical efficiency is measured for one optic over one specific LED, so the published figure moves with the pairing; the only sourced band below is the TIR row. Always take the real numbers from the photometric file.
| Lens type | Beam angle | Efficiency (indicative) | Best use |
|---|---|---|---|
| TIR (PMMA / PC) | 10°-60° | 83-93% | Spots · downlights · street |
| Fresnel | 10°-50° | 82-90% | Stage · sports · long-throw |
| Micro-prismatic | 60°-90° | 82-88% | Office troffer · UGR ≤ 19 |
| Opal / frosted diffuser | 110°-120° | 75-85% | Panel · ceiling · ambient |
| Asymmetric / batwing | 30° × 120° | 85-90% | Wall-wash · linear · aisle |
| Honeycomb louver | set by the cut-off angle | set by the cut-off angle | Accent · gallery · anti-glare |
Six families, six physics. Read this once and you can tell at a glance which optic is in front of an LED.
A TIR lens is a moulded PMMA or polycarbonate piece with a hollow centre over the LED. Light entering the cavity either passes straight through the dome (refraction) or hits the angled outer wall above the critical angle and reflects internally down the lens to the aperture. The result is a tight, clean cone with a hard cut-off at the rated beam angle. It is the usual optic on directional fixtures, and the NLC spot and track range publishes fixed beam-angle options per SKU across 10° to 60°, selected at order rather than swapped on site.
A Fresnel collapses a thick plano-convex lens into a series of concentric grooves on a flat disc, same focal effect, fraction of the mass. The trade is a slight ring artefact in the beam edge, but you save weight and thermal mass. Theatre, stage, sports floods, and high-bay reflectors with secondary Fresnels still own this space, especially where the throw is > 6 m and the fixture must stay slim.
A flat acrylic sheet with thousands of tiny pyramidal prisms moulded into one face. The prisms refract grazing-angle light back toward the vertical, so the luminaire's luminance drops at the shallow angles that reach an eye or a screen while it stays bright straight down at the desk. That is the zone EN 12464-1:2021 polices: its Table 7 caps average luminaire luminance at elevation angles of 65° and above from the downward vertical at display-screen workstations. NLC's HARMONY offers the same body with either an opal or a micro-prismatic diffuser, and publishes UGR < 19 on the micro-prismatic option; SPECTRA panels list opal or prismatic. A UGR figure is always a calculation for a stated room, layout and observer.
An opal diffuser is a thicker, more diffusing version of frosted glass or acrylic, the LED disappears into a uniform luminous panel. Beam control is loose: scatter a beam enough and the distribution returns to Lambertian, which is 120° full width by definition, and NLC's diffuse and linear products publish 110° to 120°. You pay for it in transmitted lumens, but glare drops sharply and the ceiling reads as a soft surface, not a row of bright dots. Right for hospitality, residential, healthcare wards, and any ceiling where the fixture should recede.
Two situations break circular optics. Wall-washers need a beam that throws lumens at a vertical surface evenly from top to bottom, the optic is asymmetric, pushing light forward and up. Aisle and corridor luminaires need a beam that lights the long axis of the room without overlapping into the next bay, the optic is batwing or oval, with two off-axis peaks instead of one centred peak. Both are visible on the photometric polar plot before they are visible on the floor.
A honeycomb is a grid of small hexagonal cells fitted in front of the primary lens. It does not re-aim light: it intercepts whatever leaves the fixture above the cell-wall angle, killing visible glare on adjacent sightlines. Whether the rated beam angle moves depends on where that cut-off falls inside the distribution. The cost is not a fixed percentage. A louvre intercepts by angle, so what it removes depends on how much of the source sits outside its cut-off angle. On the one primary LM-79 measurement we could find, a 90° louvre fitted to a Kino Flo Celeb 250 took the fixture from 9,046 lm to 1,700 lm, 81% of the flux, while peak intensity moved only from 3,300 cd to 2,942 cd, about 11%. A louvre removes flux; it does not re-aim it. Standard kit on museum spots, art-gallery tracks, retail accents, and any application where the visitor must read the artwork, not the fixture. Specify a black-anodised honeycomb where the surround is dark, white where it is light.
Most packaged LEDs already carry a primary optic, a silicone or epoxy dome moulded directly on the die, which sets the raw distribution; chip-scale packages may omit it and emit directly from the die. What you spec is the secondary optic, the lens or lens-array bolted in front of the LED inside the fixture. The two work together: a tight-binned primary with a 10° secondary makes a museum spot. A wide-binned primary with a frosted secondary makes a panel. Always read the LM-79 photometric file with the secondary fitted, not the bare chip.
PMMA (acrylic) transmits about 92%, and that figure is not a marketing number: Röhm's own PLEXIGLAS data lists 4% reflection loss per surface and under 0.05% absorption in the visible range, and the Fresnel equation at its refractive index of 1.491 gives 3.9% per surface, so two surfaces put the ceiling at 92.4%. It has the cleanest optical surface but scratches and crazes under abrasion. Polycarbonate is the impact-tough choice, though an IK rating belongs to the assembled enclosure and is published per fixture, never per material; it yellows in UV unless stabilised, and Covestro's Makrolon 2408 measures 88% at 3 mm against a computed 90.0% ceiling, so the missing points are absorbed rather than reflected. Borosilicate glass is permanent and inert, used in IP68 marine and hazardous-area fixtures; at 2.23 g/cm³ against acrylic's 1.19 the same lens weighs about 1.9× as much, which is a lens-mass question rather than a fixture-weight one, and it breaks rather than deforms on impact. For Saudi outdoor and high-ambient use, NLC defaults to UV-stabilised PC on impact-prone fixtures and tempered glass on coastal and high-IP work, the spec sheet should always name the material, not just say "lens."
NLC product pages name the optic and its material on the spec sheet, TIR, micro-prismatic, opal, honeycomb, glass, or PC, and photometric files for the combination you specify are available on request. Our DIALux service models lux and UGR at the work plane for the optic you actually pick.
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Every figure on this page traces to one of these, to an NLC product page, or is derived from a value in one of them. Where a claim could not be sourced, it was removed rather than softened.
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