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Designing And Validating The Performance Of
Free-Form Street Lighting Optics By
Andrew Dennington BSc Optical Design Manager
Polymer Optics Ltd.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
Introduction LED light output and efficiency has been increasing significantly every year for the last 20 years.
Up until recently the optics used to focus light from LEDs were limited to producing circles and ellipses of light. However very few areas that require lighting are circular.
Free-Form optics allow far more control over where light can be directed. Optical designers can efficiently produce arbitrary shaped beams so wide areas can be illuminated more efficiently.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
Free-Form Optics
What are they?
There is no universally accepted definition of what constitutes a Free-Form optic but for the purposes of this presentation it is:
“An optic that has one or more reflecting or refracting surfaces that cannot be formed by sweeping a 2D curve.”
Designing And Validating The Performance Of Free-Form Street Lighting Optics
Why are Free-Form optics appearing now?
• Increased Computing power allows complex surfaces to be calculated • Spheric surfaces defined by 1 variable • Aspheric surfaces defined by up to 10 variables • Free-Form surfaces defined by 100+ variables
• Very high accuracy high speed 5-axis machining has developed • Optical surfaces have to be very smooth and very accurate
• Significant increases in LED output power • More powerful LEDs illuminate much larger areas with fewer LEDs. • Fewer sources makes even illumination more difficult.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
S Class Street Lighting Example
• Illuminates an area 6m wide by 30m long when mounted at 6m. • Needs just 3200 Lumens to meet S2 class illuminance levels. • Up to 85% efficiency – LED output to light on roadway. • Very low stray light levels. • Can be used without a secondary window.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
ME Class Street Lighting Example
• ME3a conformance with 12250 Lumens compared with 17000 lumens needed from a conventional ceramic light source.
• 8m mounting height, 7.5m road width, 30m between columns
• Peak luminance level significantly reduced.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
Free-Form Optic Design And Validation
The design of the Free-Form optic surfaces is very complex and can’t be covered in a 20 minute talk. However a number of issues that effect the design of SSL street lighting optics are important to understand before starting the design process.
• LED Selection
• Optic Size
• Optic Mounting Angle
• Validating the design
• Disability Glare
Designing And Validating The Performance Of Free-Form Street Lighting Optics
LED Selection For Free-Form Optics
• It is not just the total number of lumens emitted by the LED that is important or even the lumens per watt.
• The following LED optical characteristics are vitally important in selecting the best LED for a particular application .
• Emitting area size • Emitting area uniformity • Radiation intensity variation with angle • Radiation colour variation with angle
Designing And Validating The Performance Of Free-Form Street Lighting Optics
LED emitting area size The emitting source size is one of the most important LED parameters because it sets the limit on how sharp the edges of the illumination pattern can be.
The concept of ’Etendue’ or ‘The Lagrange Invariant’ is often cited in determining how narrow a beam can be produced from a given size source from a particular sized aperture. It is not the beam width but the rate of transition from dark to light that is constrained by an optical systems Etendue.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
LED Emitting Area Uniformity • Ideally LEDs should have uniform Luminous Intensity across the whole emitting area.
• LEDs with many chips placed under a thick layer of phosphor make poor sources for Free-Form optics.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
LED Radiation Intensity Variation With Angle Packages with beam shaping optics on the packages make poor sources for use with secondary optics because:
• The LED lens is not as accurate as the secondary optic.
• The LED lens has a very short focal length. This means that chip position errors significantly change the angular distribution of light from the LED and this in turn alters the output from the secondary optic.
Unfortunately some LEDs that would otherwise make good sources like the Osram Oslon can’t be recommended because they are not currently available in a package with a Lambertian angular distribution.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
LED Radiation Colour Variation With Angle Many high power LEDs have a colour temperature that varies with output angle
The effect of the LEDs colour variation in the Free-Form optic output is usually noticed around the edge of the illumination area. The outside edge of which can be seen to have a distinctly orange cast compared to the centre.
Free-Form optics work on the edge ray principle. Light that leaves the LED at the largest angles to the axis is sent to the edge of the area being illuminated.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
Free-Form Optic Mounting Angle - 1
If the light distribution required is not symmetrical about the LED axis, the Free-Form optic will need to be tilted to obtain the highest possible efficiency.
The reason for this is that a simple optic with two refracting surfaces can only bend light by 45°, (sometimes less), with reasonable efficiency.
The total region of lost light appears to be the same in both cases but is not!
Designing And Validating The Performance Of Free-Form Street Lighting Optics
Free-Form Optic Mounting Angle - 2
Tilting the optic increases the amount of light the optic can collect from the LED. This is because the LED output is Lambertian and light lost at large angles has less intensity.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
Free-Form Optic Mounting Angle - 3
If an offset distribution is required and horizontal PCB mounting is essential the optic must be wedged in cross-section:
• The optic has lower efficiency.
• The optic is thicker, takes longer to solidify and therefore costs more.
• The thicker optic shrinks more in the mould and consequently the illumination quality is lower.
The consequences of needing to bend the light more are:
Designing And Validating The Performance Of Free-Form Street Lighting Optics
Optic Size
It is a general principal in optics that the narrower the beam of light that you want to make from a light source, the bigger the optic you will need.
Because the further out from the centre we illuminate the further the light must travel.
BUT ALSO The wider the area you need to evenly illuminate, the larger the optic required.
Why?
A wide angle 1100 optic is really a narrow angle optic with an 110 full angle beam width.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
Disability Glare 1 Threshold Increment
The roadway luminance is a function of how much the light reflected from the road has to change direction to reach the drivers eye.
Although possible to design a luminaire that has uniform roadway luminance Threshold Increment limits
how even the road luminance can be.
ME3c class requires less light than ME3b than ME3a because light can be directed to angles that reflect more light towards the driver. This increases the average luminance but lowers uniformity.
Maximum Illuminance on roadway required to meet Threshold Increment Requirement.
Illuminance on roadway required give uniform luminance.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
Disability Glare 2 Glare Class
The luminous intensity glare classes G1 to G6 are defined in terms of the maximum luminous intensity per kilolumen of light output from the luminaire.
Unfortunately it is NOT a good definition for the purposes of defining the performance of the luminaire from the drivers perspective.
Why? Because SSL using Free-Form optics are so much more efficient at getting light to the roadway than conventional luminaires they emit less than half the lumens.
Consequently a G6 rated conventional luminaire causes more glare to the driver than a G4 rated SSL luminaire. However purchasing authorities often insist on G6 rated luminaires to the point of refusing to consider the most efficient product.
We can design optics that give exceptional even roadway illuminance with very wide column spacing but the light leaving the optic is very close to 70° which makes meeting the G6 glare class very difficult.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
Validating Performance POL uses the goniophotometer at NPL where we can obtain measurements of everything from complete luminaires to single optics. Free-Form optics generate optical beams with exceptional rapid transitions from light to dark. This requires measurements to be made at much smaller angular intervals.
Results from NPL are exported as IES or Illumdat files and imported in to Reality Roadway software for analysis.
Customers need to have confidence that the measurements made are accurate and will be reflected in their products. Having results generated by NPL gives us that credibility.
Designing And Validating The Performance Of Free-Form Street Lighting Optics
Conclusion
Free-Form optics allow luminaire manufacturers to significantly increase the efficiency and quality with which their products can illuminate wide areas compared with products based on TIR optics or reflectors.
They are not without their own limitations and understanding how to get the best from them is vital if all the possible gains that this technology allows are to be realised.
Free-Form optics are currently in the early stages of adoption but the advantages of using them in wide area illumination are significant.
A rapid increase in their use in future years can be expected.