Optical Distribution Types and Spectral Filtering — The Lens Shapes the Light

What optical distribution type means in practice

A street luminaire does not send light equally in all directions. The distribution — the pattern in which light is projected onto the road surface and surroundings — is built into the lens geometry. All lighting calculations (DIALux, Relux) are computed against a specific distribution. Specifying the wrong distribution type gives the wrong result, regardless of how precise the calculation otherwise is.

IES (Illuminating Engineering Society) classifies the lateral distribution of street luminaires into five types: LEDiL — beam guide

EN 13201 uses M, C and P classes as performance requirements for complete lighting installations. They are not optical distribution types. In procurement, an IES type may describe the luminaire distribution, while an M, C or P class states the project requirement.

When pole height and pole spacing are fixed, it is the lens choice that determines whether the requirements for mean luminance (L_m) and uniformity (U₀, U_L) can be met. The wrong distribution type — for example Type IV on a narrow road — wastes light sideways and can fail uniformity requirements in the centre of the carriageway. See Model vs reality — what DIALux does not calculate.

Three lens technologies — what they do to the light

Beyond geometric distribution, the lens today increasingly affects the spectral composition of the light as well. There are three principally different approaches:

1. Clear optics (reference)

Clear optics directs light without intentional blue-light filtering. Luminous output and any small colour shift depend on material, geometry and LED. Compare the same LED and distribution when using clear optics as a reference.

2. Absorption filter (amber lens)

LEDiL states that AMBER reduces about 99% of LED light in the 380–500 nm range. Published CCT examples use AMBER-2X2 with Osram Duris S8, such as 3,000 K input and approximately 2,200 K output. These are component examples, not VALDUR measurements. LEDiL AMBER

AMBER reduces blue light and total photopic luminous flux. Component relative luminous flux is referenced to the same LED without a lens; it is not automatically the same percentage loss relative to a clear lens. The luminaire requires photometry for the selected LED, lens and drive-current combination.

Absorption filters are suited to environments where blue light reduction is high priority and lower efficiency is acceptable — for example residential areas, parks, and habitats where ecological light pollution is a primary concern. (Whether the actual ecological impact of blue light in field installations is quantified — see open questions, question 4.)

3. Spectral redistribution lens

SPECTRE reshapes the spectrum and, according to LEDiL, reduces the blue component by approximately 99%. The manufacturer describes the technology as proprietary. Public product information does not substantiate a particular phosphor coating or exact material construction. LEDiL SPECTRE

LEDiL’s published SPECTRE-Y-2X2-T2 examples use Luxeon 5050 Square and show photopic luminous flux close to the bare LED’s level. The manufacturer states that performance depends on the LED, spectrum and distribution. This is not 100% radiometric transmission or a measured VALDUR value.

Lens technologies compared

The table distinguishes component examples from complete-luminaire performance. Identify the LED, drive current and lens geometry when comparing values. LEDiL AMBER · LEDiL SPECTRE

What lens choice means for EN 13201 calculations

Calculation implications of lens changes:

• AMBER: use the filtered luminous flux and correct distribution in the calculation.
• SPECTRE: the component example does not replace the complete luminaire’s photometry.
• A lens change also changes the distribution profile if the lens geometry changes: a reclassification (e.g. from Type III to Type II) must be recalculated from the correct IES/LDT file.

Use the complete luminaire’s IES/LDT file for the correct lens, LED source and drive current. A component percentage cannot establish average luminance, uniformity or glare. Recalculate the installation after changing the lens; no lens family automatically guarantees class compliance.

Blue light reduction and the S/P ratio — a real trade-off

A high S/P ratio (scotopic/photopic) means the source contributes relatively more to rod-based vision and thereby peripheral detection under mesopic conditions. Blue light is an important component of what produces a high S/P. An absorption filter or spectral redistribution lens that removes blue light therefore lowers the S/P ratio.

That creates a trade-off:

• Higher S/P can benefit some peripheral visual tasks under mesopic adaptation. CCT alone cannot determine S/P, and the result is not a general road-safety metric. CIE TN 005:2016
• Low blue content can be one part of a site’s environmental requirements. Also assess light level, direction, operating hours and affected species. A warm colour or low S/P ratio does not guarantee a particular ecological or health effect. DarkSky / IES

Neither is objectively correct — the choice depends on the installation context. See Mesopic photometry and night vision for the background, and What don’t we know? question 1 for the current research position on CCT optimum.

Five questions to ask when selecting a lens

1. Which lighting class (M, C or P) applies to the traffic area, and which IES/LDT distribution is needed to meet it in the project geometry?
2. If spectral filtering is required, which spectrum, luminous flux and colour rendering are documented for the selected combination?
3. Does the calculation use the correct IES/LDT file — i.e. the file that actually refers to that lens, not the clear reference?
4. Has transmittance data been confirmed by independent accredited measurement, or is the comparison based solely on the manufacturer’s datasheet?
5. How does the lens choice affect the S/P ratio, and is that relevant to the installation’s mesopic performance requirements?

Sources

1. LEDiL — Product catalogue 2019, street and area beam guide
2. LEDiL — Create biologically sustainable lighting with LEDiL AMBER lenses
3. LEDiL — SPECTRE: Eliminate blue light, with efficiency you won’t believe
4. LEDiL — SPECTRE
5. CIE TN 005:2016 — Specifying Product Performance for Mesopic Applications
6. DarkSky / IES — Five Principles for Responsible Outdoor Lighting