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When someone is thinking about installing fog lights, I start with questions: First of all, do you know what fog lights are? Secondly, what do you drive, where do you drive, what kinds of roads do you drive on? What kind of weather do you drive in? What do you want the fog lights for — do you live in a real foggy area? Are they for pea soup fog or light fog, for snow, rain? Or just to look cool when you’re cruising?
Fog lights aren’t necessarily amber lights, nor are amber lights always fog lights. There are amber driving lights, amber spotlights, and, in France, there are amber headlights. What makes a fog light is the light pattern, regardless of the type of lamp or color of the light. A real fog light — one that will do you good when you’re trying to see in the fog — has a wide beam pattern (70 degrees to 120 degrees) with a sharp, flat cutoff on top.
So beyond that, not all “fog lights” are created equal. There are lots of lights being marketed as fog lamps — some very low priced, some hundreds of dollars a pair — that, if I were driving in extreme weather, I would not turn on. They would make it harder to see.
Fog lights, of course, are just one kind of auxiliary light. There are also driving lights, midbeams (passing lights) and spotlights for other purposes. And fog lights have many other functions besides fog, such as other types of bad weather and increasing side light for cornering. While fog lights are beneficial in fog or any other glare-producing conditions, they can also help you see better when you drive on a clear night. Mainly when you think of fog lights, you think of fog.
Seeing through the fog?
You can’t see “through” fog. You see in foggy weather by lighting up the road under the fog, illuminating as little of the fog as possible to avoid producing glare.
Fog is defined as a thick cloud of water droplets, 0.00039 to 0.00156 inches in diameter, suspended in the atmosphere at or near the earth’s surface and reducing visibility to below half a mile.
Your high beam headlights produce a wall of glare — a whiteout — from the light bouncing off these droplets of water. You’re being blinded by the glare of your own lights reflected off the water vapor in front of you. Similar experiences occur in rain, snow, dust, etc.
One solution would be to buy heated blowers like giant hair driers to dry the air in front of your car — works great, costs a lot and uses a ton of energy. We may never deliver any of these, but you can start sending deposits in now (big ones).
The amount of glare from the airborne moisture is affected by the relationship of angles between your line of sight and the angle of your car’s lights. That’s why you want a wide, flat beam from your fog lights. What you want your fog lamp to do is light up the road in front of you without lighting up the fog that’s higher up, right in front of your eyes.
Lenses and reflectors
It helps to know something about the construction of the light — specifically its lens and its reflector. The cuts in the lens, called fluting, shape the pattern by how they deflect the light. Ever knocked the lens out of your fog light? And noticed how it turned into a spot light? That’s what it would be if it had a totally clear lens, with no fluting.
Since the light passes through the lens, obviously the material of the lens would have an effect on how much light passes through. Lead crystal, being the clearest, allows the most light to pass through. Both ordinary glass and hardened glass are full of dirt and impurities that affect the passage of light. Plastic lenses block even more of the light.
Most of the better lights are made with lead crystal lenses. Hardened glass doesn’t break as easily, but is still as impure as ordinary glass. Plastic is basically useless for lenses because of impurities and because it gets scratched up.
Breakage is, of course, an occasion for replacing lenses. And if fog lights are mounted in an air dam and you drive across the desert a lot, they get essentially sand blasted and all the little pits affect the flow of light and the light pattern. Even if you don’t drive across the desert, your lenses will still be scuffed up by grit and you likely will want to replace them sooner or later, especially if you’re using good quality lights.
You also will care about the quality of the reflector, the shiny part that gathers the light from the bulb and reflects it out at precise angles through the lens. Good reflectors aren’t chrome — chrome appears to be shiny and reflective to the eye, but for reflecting light, it’s not efficient!
Some reflectors are silver plated. This is from the olden days. Prior to the invention of sealed beam headlights, American cars had separate bulbs, reflectors and lenses, and the reflectors were silver. Silver is not optimally reflective even when first applied. And through tarnishing, it loses 40 – 50% of its reflectivity after about 48 hours. These reflectors were supposed to be polished monthly.
The preferred and most reflective material is an aluminized vapor coating. This is the neat stuff they use in microscopes and in mirrors for aiming lasers. Most of the higher quality lamps use this.
Some lights also use a bulb shield, like a cup in front of the bulb. It blocks direct light from the bulb to the lens so that only light from the designed shape of the reflector passes through the lens. This reduces stray light above the cut-off.