There are two basic types of auxiliary lights: fog lights and driving lights.
Fog Lights
Switching on the high beams does not improve vision in heavy rain or fog, it makes it worse. It illuminates the fine water particles, making the fog shine back in your face.
For dense fog and heavy rain the beam angle must shoot sharply downward toward the ground.
Real fog, if you've seen it at Mahabaleshwar or Khandala sticks to the ground 1.5 feet above the surface.
Therefore fog lights must be mounted low in the front bumper apron or bolt above it.
The fog lamps must spill a 90-degrees wide fan-shaped beam of light down onto the road, lower than and into the fog sticking to the road. Only then can you see better and drive safer. The beam fanout must not extend above the bumper.
Aligning the fogs
By SAE standards, the fog lights should be mounted 1 foot plus or minus 2 inches above the ground.
When viewed from 8 metres away, the top of the beams should drop 0.75 degrees.
Now sin(0.75 degrees) is 0.01309. Multiplying that by 8 metres, you get 105 mm (10.5 cm).
Find a white wall on a flat ground.
Drive up to the wall until your car nearly touches the wall.
Mark points on the wall, where the centres of your fog lamps are.
Now reverse the car 8 metres away from the vertical white wall.
Switch on your fog lamps. Align the beams so that the tops of the beams are 10.5 cm below the dots you marked.
Your fogs are now aligned to SAE spec.
Driving Lights/Spot Lights
Unlike foglights which must spill their beam 90-degrees wide, driving spotlights shoot a 15-degrees wide beacon for fast driving.
They augment the brightness and distance-range of your high beams, illuminating your path with bright light.
That makes it most effective for night-time highway cruising at speeds above 80 km/hr.
As driving spotlights are so bright and focused, they can
blind oncoming drivers as well as the car you're following behind.
Correctly aiming them is therefore very crucial.
Aligning the spots
By SAE standards, the fog lights should be mounted 2 feet plus or minus 6 inches above the ground.
The beams should shoot out parallel to the road (never above it). 8 metres away from the vertical white wall, spot lamps should throw a hotspot 2.5 cm below the dots marked.
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The fallacy about yellow lights
In the old days fog lights were yellow. In the old days French law required all cars in France to have yellow headlamps.
Remember the 1953 Fiat 1100/103 TV (Turismo Veloce) colloquially known in Bombay as डुक्कर (Dukkar) Fiat, came with a yellow foglight in the center of the grille.
Rayleigh's scattering law does NOT apply to fog droplets! Why? Fog droplets are still huge compared with the wavelengths of visible light. Thus scattering of such light by fog is essentially wavelength(color) independent.
In order to get yellow light you need a yellow filter. If you place a filter over a white bulb, you get less transmitted light, and less penetration. Designers of headlights have known for a long time now, that there is no magic color that gives great penetration.
An article from the Journal of Scientific Instruments, says, "It seems to be becoming recognized that there is no filter, which, when placed in front of a lamp, will improve the penetration power of that lamp".
A Russian scientific article, stated, "Investigations and practices of automobile traffic do not confirm any substantial advantages of yellow light over white light. The advantages ascribed to it may take place only in very thin fog or may be subjectively received by some drivers owing to their individual peculiarities of vision. Therefore, it does not make any sense to switch over headlights to yellow light, although the use of yellow light in special fog lights does not raise any objections." End of quote. The phrasing is quaint, but the meaning is clear.
So, unless you just happen to like yellow, save your money and forget about so-called "fog-lights." They don't exist.