Fifty million years ago, an asteroid composed almost entirely of the precious metal Iridium (Ir-77) struck the earth creating the Gulf of Mexico. From this catastrophe came a metal with properties ideal for spark plug construction. Using this element, Denso has evolved spark plug design to a new level.
Below is a cross-section of the highlights of the Iridium plug. 
1. Iridium Plug Insulator. The insulators are made of a highly purified alumina powder, which gives exceptional dielectric strength and thermal conductivity at high temperatures, as well as greater physical strength.
2. Five-Rib Design. The five-rib design prevents flashover and decreases voltage loss.
3. Copper-Glass Seal. A special mixture of copper and glass powder bonds the center electrode and insulator together for a gas tight seal. Proper heat is maintained and hot combustion gases are excluded.
4. Copper-Cored Center Electrode. A special center electrode incorporates copper into the core of the nickel chrome binary alloy. This gives superior thermal conductivity and increases the operating range.
5. U-Groove Ground Electrode. The originally patented U-Groove allows the spark to fill the gap formed by the U. This ball of fire develops a larger and hotter flash front so combustion is complete.
6. Patented Iridium Center Electrode. Utilizes Iridium to create the world's smallest center electrode tip for maximum firing performance and reduced voltage requirements.
7. Tapered Cut. Improves firing performance by reducing quenching.
8. 360° Laser Welding. Denso perfected the Laser Welding Process, which provides a 360° seal so reliable the electrode resists wear even in the harshest conditions.
9. 100% Pre-Fired. All Denso spark plugs are pre-fired at the factory to assure highest possible quality.
In a never-ending effort to improve upon success, Denso developed an Iridium alloy that is superior to platinum based on hardness, strength and melting point. The new Iridium alloy is composed primarily of Iridium with rhodium mixed in for oxidation wear. Iridium Alloy Pure Iridium has a melting point of almost 700C degrees higher than platinum, and has the greatest potential for resistances against wear. Iridium by itself is susceptible to oxidation wear at temperatures over 900C degrees, however, ordinary driving around town an engine would never reach these temperatures. Further testing proved that Iridium alone would not meet Denso's demanding standards. After testing other pure metals combined with Iridium, rhodium proved to be the best choice to meet the needs of Denso's spark plug engineers. 
By harnessing the power of Iridium, Denso is suddenly able to shrink the surface area of the center electrode - the critical component of a spark plug. Unlike a typical platinum plug that has a 1.1mm diameter center electrode, Denso Iridium Power plugs have the world's smallest center electrode measuring 0.4mm in diameter. This means that the voltage requirements are decreased while firing performance is greatly improved. 
The 0.4mm Iridium tip that generates the high performance of Iridium Power is an alloy with a very high melting point. Therefore, ordinary resistance welding cannot be used because the Iridium alloy does not melt enough and adequate weld strength cannot be guaranteed.
The Denso patented 360° Laser Welding process employs a high-energy laser so reliable that the electrode resists wear in the worst conditions. 
Patented U-Groove design changed the industry standard in the mid 1970s by providing a better design that virtually eliminates erratic starts caused by poor fuel mixture. The U-Groove results in less flame dissipation making the spread of the flame nucleus easier and therefore yielding greater firing energy. It gives the flame a groove to grow in, thereby creating a larger flame front for a complete combustion.
On a conventional plug, the flat electrode design crushes the spark, and the overall volume of the flash front is smaller.
The conventional method to improve firing was to increase the spark gap. This has a negative effect on the spark. With a U-Groove, firing can be greatly improved without increasing the spark gap.
U-Groove electrodes are featured in Iridium Power™ Series spark plugs. 
To help improve ignitability Denso minimized the portion of the ground electrode that comes in contact with the flame nucleus. Because the area of the ground electrode where the tapered cut comes into contact with the flame nucleus is small, the heat lost to the electrode is lessened and firing performance improves dramatically. The surface area and weight of the ground electrode tip is reduced along with the load in terms of heat and vibrations. This means the spark plug can cope with heavier driving conditions. 
The figure on the above contains photographs of the discharge part of an ordinary spark plug, a platinum spark plug and the two types of iridium spark plugs developed here. 
Use of 0.4 mm diameter iridium center electrode and a tapered cut and a U-groove in the ground electrode has enabled Denso to achieve previously unparalleled firing performance in the IRIDIUM POWER.
Ordinarily, the leaner the air mix the more difficult the firing.
The greater the ignitability limits, that is the leaner the air mix in which a plug can spark, the better firing performance a plug is said to have. The following graph shows some examples of ignitability limits (*) for various spark plugs.
*: Ignitability limits (Air/Fuel);
1)Lean limits to be able to fire
2)As this figure is getting higher, it will become easier to fire.
Compared to normal spark plugs, with a spark gap of 0.8mm, the Ignitability limits is better by 2.5 with Iridium Power.
Only the BEST
Denso Iridium Spark Plugs use the highest quality materials, and are produced in our state of the art manufacturing facilities.Below is a cross-section of the highlights of the Iridium plug.
Iridium Alloy
Ultra Fine Electrode
Laser Welding
The Denso patented 360° Laser Welding process employs a high-energy laser so reliable that the electrode resists wear in the worst conditions.
U-Groove Technology
U-Groove electrodes are featured in Iridium Power™ Series spark plugs.
Tapered Cut
SAE Comparison of electrode sizes and voltage requirements
Firing Performance
Ordinarily, the leaner the air mix the more difficult the firing.
The greater the ignitability limits, that is the leaner the air mix in which a plug can spark, the better firing performance a plug is said to have. The following graph shows some examples of ignitability limits (*) for various spark plugs.
*: Ignitability limits (Air/Fuel);
1)Lean limits to be able to fire
2)As this figure is getting higher, it will become easier to fire.
Compared to normal spark plugs, with a spark gap of 0.8mm, the Ignitability limits is better by 2.5 with Iridium Power.