What is Candela, Beam Angle, and Optics in LED Street Lights?

Q: How do I know if a street light's beam angle is correct for my road width?

Ask the manufacturer for the IES file and import it into DIALux (free software). Set up your road cross-section and pole height. Run the calculation and check the lux distribution - if there are dark lanes on the far side of the road, the beam is too narrow. If there is severe glare in the calculation, the beam is too wide or the nadir angle is wrong. Alternatively, request a photometric report from Xera Tech showing calculated lux for your specific road dimensions.

Q: Is a higher candela always better for road lighting?

Not in isolation. Very high candela in the wrong direction (toward the horizon, directly at drivers' eyes) causes disability glare - worse than lower candela well-distributed along the road. The correct metric is not peak candela alone, but the combination of total lumens, beam distribution (IES type), and the resulting road lux map including uniformity ratio and TI (Threshold Increment for glare).

Q: What is the difference between IES files and LDT files?

Both are photometric data formats. IES (.ies) is the North American standard, widely used by manufacturers worldwide. LDT (.ldt) is the European format used primarily in German-speaking countries. Both can be imported into DIALux for calculations. Most Indian manufacturers provide IES files. If a manufacturer cannot provide either format, they likely do not have independently tested photometric data - a significant quality concern.

What is Candela, Beam Angle, and Optics in LED Street Lights?

Lumens tell you how much light a fixture produces. Candela and beam angle tell you where that light goes. For road lighting, the distribution of light is as important as the total quantity - and understanding photometric data helps you compare luminaires intelligently.

By Xera Tech Team · Updated 12 Jun 2026 · 4 sections

Candela - Intensity in a Direction

Candela (cd) is the unit of luminous intensity - the amount of light emitted in a specific direction per unit solid angle. A lamp that emits light in all directions has the same total lumens as a spotlight, but the spotlight has far higher candela in its beam direction. For road lighting: you want high candela along the road surface and low candela toward the sky or directly at drivers' eyes. The peak candela direction is called the nadir angle (downward, toward the road) for street lights. Candela values in different directions are listed in the IES photometric file - the data file that lighting design software uses to simulate real-world performance.

Beam Angle - Where the Light Spreads

Beam angle is the angle within which the luminaire emits at least 50% of its peak intensity (half-angle definition). A 60° beam angle is narrow and concentrated. A 120° beam angle is wide and spread. For street lights: the beam angle description is more nuanced - street light optics create asymmetric beams described by IES Type (I through V). Type I: very narrow, used for pathways. Type II: medium width, for roads up to 8m. Type III: wide, for roads 8–12m. Type IV: very wide with strong forward throw. Type V: symmetric 360° - not used for roads. For flood lights: single beam angle (e.g., 90° or 120°) describes a symmetric cone.

How to Read a Polar Diagram

A polar diagram (or candela distribution curve) is a circular graph showing light intensity in all vertical planes. The centre is the nadir (straight down). The outer edge is the horizon. The curve's distance from the centre at any angle shows the intensity in that direction. For a street light: the ideal polar diagram has a wide, flat distribution along the road axis (good road coverage) with a steep cutoff at 80°+ (no glare to approaching drivers). For a flood light: the curve is symmetric around the nadir. If you see a downloadable IES file from the manufacturer: this is the machine-readable version of the polar diagram. Import it into DIALux for photometric calculations.

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