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:

CEN (European standard) uses a complementary M-class system that combines carriageway width, observer eye position, and uniformity requirements per EN 13201. In practice both systems appear in procurement: IES type in product datasheets and M-class requirements in technical specifications.

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)

An optically clear PMMA or PC lens transmits the light’s spectrum virtually unchanged. Optical transmittance is typically high — 90% or more per manufacturer data. Spectrally, the output is what the LED source delivers: a 4,000 K module gives 4,000 K output. Clear optics is the reference point for efficiency comparisons.

2. Absorption filter (amber lens)

A yellow or orange-tinted lens absorbs short-wavelength blue light within the PMMA material. Light in the blue/violet spectrum is converted to heat in the material rather than passing through. The result is a lower CCT and substantially reduced short-wavelength content — an absorption filter from LEDiL is stated by the manufacturer to block up to approximately 99% of light below 500 nm, with a CCT shift from, for example, 3,000 K to approximately 2,200–2,300 K.

Consequence: The absorption filter means a direct loss of luminous flux. Per LEDiL’s product datasheets, an amber lens typically loses 25–27% of luminous flux compared with a clear reference lens of the same geometric type. The installation therefore requires either higher input power or shorter pole spacing to meet the same luminance requirements.

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

A more recent approach places phosphor material within the lens PMMA, converting blue photons to longer wavelengths rather than absorbing them. The photon energy is utilised — short-wavelength radiation is redirected to yellow/green wavelengths where the eye’s sensitivity is higher.

Consequence: Manufacturer data suggests the total photopic luminous flux can be maintained at close to 100% — in some measurements marginally exceeding the clear-lens value by a few percent, because the conversion targets wavelengths where the V(λ) sensitivity curve peaks. Note: these figures originate from manufacturer datasheets and should be verified against independent accredited measurement before procurement decisions.

Spectral redistribution lenses are an option where blue light reduction is required without the luminous flux penalty, for example on higher-traffic roads where luminance requirements are more demanding.

Lens technologies compared

Transmittance and blue light figures above are manufacturer data from product datasheets. Independent accredited measurement should be requested in procurement.

What lens choice means for EN 13201 calculations

A lower optical transmittance (absorption filter) is equivalent to a power reduction: 1,000 lm with an amber lens corresponds to approximately 730–750 lm in the calculation when compared with clear optics. Installations dimensioned for clear optics do not therefore necessarily meet an M-class if the lens is changed to an absorption filter without adjusting other parameters.

Calculation implications of lens changes:

• Clear → amber: luminance contribution decreases by ~25–27%. Pole spacing may need to be reduced or input power increased.
• Clear → spectral redistribution: per manufacturer data, minimal flux loss. Verify that the IES/LDT file for the actual lens is used in the calculation — the photometric file must refer to the selected lens, not the clear reference.
• 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.

In DIALux and Relux: always import the product’s own IES or LDT file with the actual lens fitted, not a generic module curve. Clear optics will in most cases give the best calculated performance — choosing a spectrally filtered lens for other reasons always requires verifying that lighting requirements are still met with the correct file. See also Model vs reality — what DIALux does not calculate.

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:

• High S/P → better peripheral detection and mesopic visual performance, but increased blue light towards the sky, insects, and human circadian rhythm.
• Low S/P (amber/warm) → favours ecology, dark sky, and circadian health, but may reduce peripheral visual performance at low luminance levels.

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 IES type or M-class is required for the road geometry? Verify that the selected lens matches that type — not merely something similar.
2. Is spectral filtering a procurement requirement? If so: absorption filter (lower flux) or spectral redistribution (maintained flux per manufacturer data)?
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. IES. (2020). ANSI/IES RP-8-20: Roadway Lighting. Illuminating Engineering Society. (IES distribution types I–V.)
2. CEN. (2015). EN 13201-2:2015: Road Lighting — Part 2: Performance Requirements. (M-classes and luminance requirements.)
3. LEDiL. (2024). AMBER-2X2 Product Datasheet. LEDiL Oy, Salo, Finland. (Transmittance and blue light reduction per product datasheet.)
4. LEDiL. (2024). SPECTRE-Y-2X2: Eliminate Blue Light — Technical Whitepaper. LEDiL Oy. (Spectral redistribution and lumen data.)
5. CIE. (2010). CIE 191:2010: Recommended System for Mesopic Photometry Based on Visual Performance. (S/P ratio and mesopic adjustment.)
6. van Bommel, W. (2015). Road Lighting: Fundamentals, Technology and Application. Springer. (Role of optic types in road lighting design.)