Monthly Archives: November 2016

Glass Lens vs. Silicone Lens in Street Light

What is the difference between a lens made from optical glass and the lens made from silicon, when used in street light application?

In this blog post, I will explain the pros and cons of both lenses. I will also use a case example to showcase the differences.

The Basics

First let me explain few basic terms related to optics in street light:

Light Pollution

Light that doesn’t go to desired direction and causes harm of anykind. It is wasted light, that isn’t used to its primary purpose. Light pollution can be divided to three different categories:

  • Glare

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Discomfort glare results in an instinctive desire to look away from a bright light source or difficulty in seeing a task. So as told by its name, discomfort glare causes discomfort.

  • Uplight
Light Pollution (Uplight/Skyglow)

Light Pollution (Uplight/Skyglow)

Uplight can be seen especially in cities: it makes sky glow and stars disappear.

  • Light tresspass
Light Pollution (Light trespass)

Light Pollution (Light trespass)

Light trespass is found in the vicinity of streets: it can prevent you from sleeping or disturb your garden lighting.

Optical glass

Pros:

  • Cheap to manufacture
  • Very high temperature range, sensitive also to stress

Cons:

    • Complex optical shapes can’t be done accurately or if the complex shapes are needed, it is expensive
    • Non-optimal light distribution in street light
    • Heavier than silicone (freight costs are more expensive)
    • Lower light transmission than in silicone lenses

 

Silicone

Pros:

  • Enables high precision manufacturing of complex optical shapes
  • High integration level in luminaire
  • Material weighs less than in case of glass lens

Cons:

  • Cost is higher than for glass lens

  • Lower temperature range
  • Lower fire rating

Glass Lens

Glass Lens Light Distribution in Street Light Application

Glass Lens Light Distribution in Street Light Application

In the image you can see the light distribution image taken from above. This application uses Glass lens.

      • Boom angle 15 deg
      • 10880 LED lm, eff 88%
      • Eav 9.0 lx (>9.0 lx)
      • Eav/Emin 2.2 (<4.0)
      • Lv max/Lav 0.3 (<0.4)

Silicone Lens

Silicone Lens (Stella DWC2) Light Distribution in Street Light Application

Silicone Lens (Stella DWC2) Light Distribution in Street Light Application

In the image you can see the light distribution image taken from above. This application uses Stella DWC2 Silicone lens.

      • Boom angle 10 deg
      • 8400 LED lm, eff 92%
      • Eav 9.0 lx (>9.0 lx)
      • Eav/Emin 2.3 (<4.0)
      • Lv max/Lav 0.3 (<0.4)

Results

Glass lens needs more lumens for the same application. In this case, around 20% more. This means that you generally speaking need more power to get the same amount of light out from the luminaire.

The reason behind the lumen need is the fact that glass lens generates more light pollution. You can see that the trespass light area is much larger in glass lens image (the red box). And on top of this, glass lens distributes light 10 meters away from road. In comparison, silicone lens only distributes 7 meters.

So I think I can end this blog post by stating that the silicone lens gives a lot of advantages over glass lens in street light application.

Vivid LEDs: Special Color Rendering With Spectrum Tuning

When we talk about color rendering, traditionally that conversation has been filled with CRIs and Ra-indexes. These traditional ways of telling how well certain light source represents sunlight have been criticised because they may not tell the whole truth.

In recent years, LED manufacturers have been trying to answer this criticism by creating different products. Terms like “premium white”, “crispy white”, “pearl white” and “vivid white” have come to LED markets.

Despite the different terms, they are all meant for the same purpose: To represent certain colors and make the lighting look better. I will be using term “Vivid” as it is the term Citizen Electronics uses. And to be honest, it describes the purpose of these LEDs quite well.

Color rendering

Color rendering means simply how well a white light source can show, or render, the true colors of different physical objects compared to sunlight. You know the effect when you buy a jacket in the clothes shop and it looks completely different in sunlight.

Colors are divided into 15 indexes (R1-R15). A general color rendering index (CRI or Ra-index) is defined as an average of the sum of first eight indexes (R1..R8). However, these first 8 indexes are rather less saturated colors, while indexes R9-R12 represent highly saturated colors (red, yellow, green, blue).

CRI 100 = Sunlight

For example, in grocery stores, a shopkeeper may want to highlight red color of meat or colors of vegetables. This means that the general color rendering index doesn’t really tell anything about the rendering capabilites of the light. In this case, high rendering index of some of R9-R12 indexes is necessary. It doesn’t matter how high the CRI is, R9-R12 can be anything.

CRI 83 (look at R9)

CRI 83 (look at R9)

The above image shows the index values of R1-R15. The CRI is 83, but look at the R9 value. Not very good.

So basically, the LED itself can have the CRI of 97 and you still have no idea how does it render red or green for example.

Vivid via Spectrum tuning

Vivid LEDs are made using spectrum tuning. In short, this means that the phosphorus of the LED has been modified. How it is modified, depends on the LED and the intended application. Note that spectrum tuning can be made also by using RGB-LEDs.

For example, Vivid White LED’s spectrum has been tuned so that it represents white and bright colors as well as possible. The colors are more saturated than under typical normal LED light.

Test report on Vivid White aLED Module

Test report on Vivid White aLED Module. Click the image to open it in new tab for better view.

These LEDs work very well for example, in clothing stores, where you have a lot of different colors that need to look good.

Here are two good principals when choosing LED:

  1. Think about your application, what colors do you want to highlight?
  2. Don’t stare at the CRI (unless you get a full report), it might not tell everything

Conclusion

Traditional way of thinking color rendering solely through the CRI should be updated. More importantly, you should know what you want to highlight and ask for a LED suited for your application.

Download More Information About Vivid