All correcting contact lenses must have a valid prescription from an ophthalmologist or optometrist. Contact lens wearers are usually surprised and happy with the level of comfort that lenses provide. There are 75 million contact lens wearers worldwide and 31 million in the U.S. alone.

Now it's easy to order the perfect contact lenses online at a great discount. If you've always wanted to change your eye color, color contact lenses can provide baby blues, gorgeous greens, heavenly hazels -- even various patterns and designs.

Extended wear lenses are usually soft contact lenses; made of flexible plastics that allow oxygen to pass through to the cornea. The length of continuous lens wear depends on the lens type and your doctor's evaluation of your tolerance for overnight wear. Some doctors prescribe disposables as planned-replacement lenses, which are removed, disinfected, and reused before being discarded.

Newer soft lens materials include silicone-hydrogels to provide more oxygen to your eyes. People who have good distance vision but need help for reading can get a monovision reading lens for one eye. You can mark rigid lenses to show which lens is for which eye; they don't rip or tear, so they're easy to handle.

Rigid gas permeable lenses tend to be less expensive over the life of the lens since they last longer than soft lenses. While the ability to hold water increases the oxygen permeability of soft lenses, it increases their fragility quotient as well. There are several types of lenses including: soft contact lenses, rigid gas permeable lenses, extended wear lenses and disposables.

The expiration date for your prescription is currently set by your state requiring a one-year or two-year renewal; if your state hasn't set a minimum expiration date, government regulation sets a one-year date unless your eye doctor determines there's a medical reason for less than one year. There are also a few rigid gas permeable lenses that are designed and approved for overnight wear.

Make sure you have a current, valid prescription when you order contact lenses. Compare prices - get quotes from two or three online and offline suppliers. Get a feel for how the retailer handles customer service calls, in case you have a problem after your order arrives.

Wherever you buy, shop for quality and value and don't forget you want to do what's best for your eye health. Buy your contact lenses from a supplier you're familiar with and know is reliable or has name familiarity. Always ask what rebates are available.

Always buy from a reputable company; you can buy contact lenses without a prescription, but the company is selling you a prescription device as if it were an over-the-counter device violating FTC regulations by selling you contact lenses without having your prescription. Focus on value, not just the price; most people looking for the best price are really looking for the best value. When you place your order make sure your lenses are available and not out of stock, because you'll need them now.

When the eyes are open, tears carry adequate oxygen to the cornea to keep it healthy, but during sleep, the eye produces fewer tears, causing the cornea to swell. Soft extended-wear lenses bind down on the closed eye, but they are porous and allow some tears through during sleep; because they have so little form, their binding has very little effect on the shape of the eye. Don't wear lenses any longer than they're prescribed for, nor when sleeping unless you are otherwise directed.

The risk of corneal ulcers for people who keep extended-wear lenses in overnight is 10 to 15 times greater than for those who use daily-wear lenses only while they are awake. If your eyes become red or irritated, remove the lenses immediately and consult with your doctor. Heat disinfection is the only method effective against the microorganism Acanthamoeba keratitis, a common culprit for contact lens wearers, and it also kills organisms in and on the lens case.

Laser surgery and its risks can be frightening and too expensive for some people; contacts can provide a safe, comfortable and time-tested alternative. It's becoming easier and easier all the time to shop online and often the shipping is free. Remember to use only contact lenses that are FDA-approved and only if prescribed by a licensed eyecare professional.