Fog Lamp and Foreground Illumination - Better Automotive Lighting

Fog Lamp SS3 Pro Case Study

Most recently, a very powerful TIR collimator optic-based fog lamp system, SS3 Pro was introduced by Diode Dynamics.  Its peak intensity approaches almost 12,000cd which is near the legal max limit of 14,400cd. Boasting ±45 degree horizontal beam stretch and ±6 degree vertical tight spread.

I had the opportunity to observe and was able to carefully recreate the optical model for simulation-based analysis that revealed a more high-resolution understanding of its performance and observation I would like to share.

SS3 Pro Fog – TIR collimator optic-based high power fog lamp

Typical TIR collimator Optic[Image 1: Typical TIR collimator Optic ]

TIR collimator optics (= Total Internal Refraction) optic is made by high refractive index optical material such as PMMA or polycarbonate plastic.  Contrast to conventional parabolic reflector optics (traditional collimator optic) uses mirror surface, TIR uses boundary surface between refractive lens material and air as a mirror by controlling entry angle of ray, this allows exit surface of optics also to carry optical parameter to work with refraction, achieving higher light source usage. In short, it can capture more light emitted from an LED than a conventional reflector optics.

[Image 2: Reflector vs TIR optic]


In the image above, to the left is a conventional parabolic reflector collimator optic. Light source position at the focal point of the parabola, reflected ray turn into a parallel beam. However, as you can see in the image, a stronger forward-directed ray group which is indicated in red is not hitting the reflector surface, resulting in a non-collimated beam scatter.  To capture more light emitted from an LED in reflector optic, the reflector must have a much deeper parabola shape is required.  To the right is the ray behavior of the TIR collimator optic. Ray group that was indicated in red in reflector optic also is captured at center aspheric condenser lens to convert into a parallel beam.

In general, TIR collimator boasts great efficiency to convert LED light source into moderate to tight spot beam depend on available optic size. (Larger the optic, stronger collimation)

SS3 Pro uses TIR collimator optic as a primal optical component. Then, by adding exit surface secondary optical parameters, such as X-axis selective diffusing optic (typically seen as “rib shaped” surface texture), it can create a horizontally stretched beam pattern.

[Image 3]

SS3/SS3 Pro consist of 4 of TIR collimator + X-axis selective diffuser which is powered by powerful Cree XP-L High intensity emitters (SS3 Pro).

Using carefully measured geometrical information, applying refractive material parameters and light source information, here I recreated a detailed optical model of SS3 Pro.

[Image 4]

As Diode Dynamic's website describes, it does achieve over 11,000cd of peak beam intensity, 2,500cd + main hot spot strip stretches to ±40 degree horizontal span, generally visible range can be as much as ±45 degree horizontal span.

Simulated beam pattern, beam intensity appeared to be identical to actual beam pattern and measurement.  

Beam quality observation

I noticed SS3 Pro has a very tall vertical gradient, in short, at this high power configuration, it appears to have a large amount of glare above horizontal line as of default 0 degree installation.

What we tend to recognize in the wall shot picture is cut off “impression” that is defined by very subjective visual contrast recognition between highly illuminated area and less illuminated area.

Running optical simulation [Image 5] revealed above observation to be something we all should be aware of as glare comes from such a powerful lamp can be potentially hazardous if the lamp is not aimed properly.

[Image 5]

At default installation of 0 degree adjust, beam quality was evaluated according to SAE J583 Rev. Nov 2011

In this evaluation specification, L1~L5 covers what is typically described as “glare zone”.

Notable reading we can learn from the simulation result, which I highlighted in the orange box, is showing L2~L5 simulated value is exceeding their specification allowance max by  11.7 to 23.5 times of intensity.

To be able to contain glare level below allowance max ended up requiring a lamp to be aimed as much as -6 degree vertical adjustment. [Image 6]

Adjusting -6 vertical degree contains a consequence of excessively strong immediate foreground illumination, that fails to satisfy another aspect of the test point of the foreground zone illumination requirement.   This essentially means users will experience either causing a large amount of glare to others, or compromising mid-distance view with excessive foreground illumination. 

[Image 6]

[Image 7] shows driver simulated road illuminance distribution and impression of SS3 Pro fog lamp beam appears to driver eyes.

Lamp height is 0.4m     Driver view height is 1.2m

At 0-degree default aim, it was observed to have a very high level of glare above the horizontal line. This is due to the overall beam pattern is based on a typical circular-shaped spotlight type of beam distribution.  The gradient of below H line is actually very much desirable, if we consider this lamp as AUX high beam support or off-road driving lamp where strict cut off is not regulated.

However, it is not reasonable to consider this beam distribution as SAE regulated fog beam compatible beam pattern.

When the lamp is adjusted -6 degrees vertically to contain the glare level of above H line, then it adds a very large amount of foreground illumination that is about 7.5 times more than the specification max allows. Peak illuminance on the road was observed at -16.1 degree point. At lamp height of 0.4m from the ground, this is about to equal to 1.39m of distance from the lamp, which, in the vast majority of the case, is out of the view of the driver, or display as very strong immediate foreground illumination to compromise relative distance view strength.

[Image 7: driver view]

To understand fog lamp performance in a practical scenario, it is important to observe together with low beams, as fog lamp does not come ON by itself.

Here is a configuration based on a sample LED low beam projector that was mentioned in my last article. SS3 Pro fog lamp was added to show a familiar near-wall shot image is displayed below. [Image 8]

[Image 8]

By looking at this image, many probably do not see the glare issue of SS3 Pro that I mentioned earlier. Because at near distance wall shot view hides fog lamp upper glare issue under the low beam’s beam pattern.  2 reasons that may make user confused are 1, fog lamp is installed 0.4m off from the ground and low beam is installed 0.75m off from the ground in this configuration. 2. At near distance wall shot image, the user will not be able to see how the above horizontal angle region of fog lamp glare develops over the distance.

When a low beam center is set at 0.75m from the ground and fog lamp is set at 0.4m from the ground, there is 0.35m of height difference. We learned the SS3 Pro fog beam can have +3 degree high-intensity gradient, and up to +6 degree overall gradient above horizontal angle.

The target distance for glare to show on the wall above low beam cut-off line reference border can be calculated as below.

(low beam height 0.75m – fog height 0.4m)/tan6°=3.33m(overall gradient bleeding distance)

(low beam height 0.75m – fog height 0.4m)/tan3°=6.68m(high-intensity gradient bleeding distance)

In this calculation, 3.33m(11’) of distance to target wall show fog beam upper edge to reach above the low beam’s horizontal axis.

However, our eyes only can detect certain levels of illuminance “contrast”, especially considering higher intensity low beam cut off is the reference illumination border,  many still will not be aware of the edge of fog lamp beam is bleeding over the low beam cut-off line.  

Therefore, visible “bleed” of fog lamp beam ( glare) above the low beam cutoff line can be observed often as “when” high-intensity gradient part of beam edge bleeds above the low beam cut-off line border.


This means, unless the vehicle is parked at least 6.7m (22’)from the wall, we may not see “visible” bleed above the low beam cut-off line when the fog lamp is installed at 0-degree.

This highly subjective visual recognition nature of human eyes and lack of objective information amount about illumination basics often can give the user a false impressions of “ there seems to be no glare” or “it is aimed properly”

It is very important to understand, not seeing the fog lamp beam bleeds above the low beam cut-off line in the near distance wall shot view does not define it is free of glare, nor aimed properly.

Below is a conceptual image of how glare above the horizontal line can be observed over the distance. [Image 9]

[Image 9]

Simulated driver view with SS3 Pro fog lamp

[Image 10] are teaching us SS3 pro at 0-degree aim hugely benefit mid-distance illumination, it can be described as great AUX driving or high beam support lamp. However, as a fog lamp, its glare level to be extremely high that is not suitable for on-road use, I must question the line of “SAE J583 compatible fog beam”  which their website claims.

When SS3 pro is aimed -6 degree to prevent glare, immediate foreground illumination level becomes so high at a very close range. In many cases, fog beam illuminated area likely be outside of driver view, or experience excessively strong foreground illumination, that particularly cause mid-distance view evaporation.

[Image 10]

TIR Collimator optics are not the ultimate best choice for fog lamp optics

Going back to [Image 3], we learned SS3 Pro fog lamp beam pattern was created by below simplified 2 steps

  1. Collimate to create circular-shaped spot beam
  2. Stretch beam created by 1. In X-axis (horizontally) selectively

Therefore, in this combination design, the initial primal optics collimation character determines how the overall beam’s vertical gradient behaves.

Fundamental conflict lays under using typical TIR collimator optic only is it does not have above H line gradient control. It is simply because this kind of optics is specialized to create a circular-shaped focused beam. Below H line and above H line will have a symmetrical gradient.

And by adding exit surface optical parameter of x-axis selective diffuser only can alter the behavior of horizontal spread, but not the vertical control.

Secondary, in an ideal theoretical scenario where ideal point light source is available, TIR collimator optics will boast extremely high efficiency in both capturing the light source and collimation.  However, in real life application hugely varies based on available optic size and LED selection.

Here is 3 example of how size of LED’s light-emitting surface area size affects collimation level.

[Image 11]


SS3 Pro confidently carries a very ample amount of light output, which gives unbeatable strength as AUX spotlight or floodlight application in my opinion.

However, as fog lamp application, it lacks upper vertical gradient control which unfortunately contributes to an unsafe level of glare to oncoming traffic. Level of glare can be controlled by aiming lamp as much as -6 degree vertical adjustment, however, it then compromises proper performance as fog lamp application by creating excessively strong foreground illumination.

Next, I will talk about what kinds of other optics can be used for fog lamp applications. Stay tuned!!

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