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Jai Ganesh

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Registered: 2005-06-28

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Dioptre / Diopter

Dioptre

Gist

It is a unit of refractive power, which is equal to the reciprocal of the focal length (in metres) of a given lens.

A diopter is the unit of measurement used for an eye prescription. The focusing strength of your glasses or contacts is measured in diopters. In other words, diopters measure how powerful your prescription is. A higher prescription will have larger numbers, whereas a milder prescription will have smaller numbers.

Summary

Diopter, in optics, is a unit of magnifying power of a lens or lens system. Because the power of a lens is proportional to unity (one) divided by the focal length (see lens), the power of a lens in diopters is numerically equal to 1 m divided by the focal length in metres. The algebraic sign of the magnifying power indicates whether the lens causes an incident pencil of parallel light rays to converge or to diverge. Thus, a diverging lens having a focal length of 1 m has a power of -1 diopter, and a converging lens of focal length 0.5 m has a power of two diopters. The power of a combination of two or more thin lenses in contact is equal to the sum of the powers of the individual lenses. For example, a lens of -10 diopters combined with a lens of 30 diopters gives a converging lens of 20 diopters (of 5 cm focal length).

Details

What is a diopter?

A diopter is a unit used to calculate the focusing strength of a pair of glasses or contact lenses. And it is the measurement unit for an optical prescription. If you know the power of a lens in diopters, taking the inverse will give you the focal length — the distance where light is focused.

What does a diopter mean?

You may have wondered what the numbers on your glasses or contact lens prescription mean. You may also have tried deciphering the numbers on the diopter chart while choosing the correct reading glasses at the drugstore. These numbers describe the focusing strength of the lenses.

The unit that is used to calculate focusing strength is a diopter. To grasp the meaning of a diopter, you first have to understand the metric system. In the metric system, distance is measured in meters. A meter is approximately the same length as a yard.

What is the relationship between diopters and meters?

If a lens has one diopter of focusing power, it will focus light from very far away — let’s say from the sun — at a one-meter distance behind the lens. The distance where the light is focused is called the lens’s focal length.

Now, if we double the lens’s focusing power to two diopters, sunlight will be focused twice as close, at half a meter, behind the lens. The focal length is now half a meter.

What is the relationship between diopters and focal length?

If you know the power of a lens in diopters, taking the inverse will give you the focal length.

The concept of diopter and focal length helps us understand why some people need to use glasses or contact lenses and others do not.

The eye contains two focusing lenses; the cornea and the crystalline lens. The cornea has about 45 diopters, and the crystalline lens has about 15 diopters for a combined 60 diopters of focusing power. This is a very strong set of lenses that will focus at roughly 1/60th of a meter.

For those whose eyeball is exactly 1/60th of a meter in length, the light is focused precisely on the retina in the very back of the eye. In this case, additional lenses are not needed to help focus the light.

Like the eye, a camera has lenses that may be measured in diopters. These lenses help set the camera’s focus at a particular distance.

Why do some people need glasses and some don’t?

Many of us have either too few or too many diopters of focusing strength in the eye.

Too much dioptric power

In the case of too many diopters (in relation to the length of the eye), the focal length will be less than 1/60th of a meter, and light is focused in front of the retina. This is the case in people that are nearsighted.

The more nearsightedness, the farther in front of the retina light is focused, and the more out of focus an image will be.

To get light to focus on the retina again, some of the extra focusing strength of the cornea and lens needs to be taken away. This is done with negative or minus lenses that neutralize the excess focusing power of the eye.

Too little dioptric power

In the case of farsightedness, the opposite is true. The lens and cornea have too little power. In this case, the focal length is longer than the eyeball, and light is focused behind the retina. This requires correction with lenses that have positive or plus diopters.

Why do we need a prescription with more diopters when looking up close?

The eye requires more focusing power — diopters — when looking up close than far away. All the previous examples explain what happens when the eye is trying to focus light from very far.

Things work slightly differently when looking up close. In this case, the eye requires even more diopters to focus light on the retina. This process is called accommodation and is done by the ciliary muscle that pulls on the crystalline lens to change its shape.

Why do we need a higher prescription in diopters as we age?

Unfortunately, between age 40 and 50, the crystalline lens hardens up and loses the ability to provide those extra diopters needed when looking up close. This age-related loss of focusing ability is called presbyopia.

This is when reading glasses or bifocals become needed to see up close. The older we get, the more the reading glasses need to supplement the weakening focusing power of our own eyes.

It’s also why, in some stores that sell reading glasses, the recommended power is based on age. Because the lens required to focus up close is stronger than that for distance, it is called an “add.” The more diopters that are needed to help focus up close, the stronger the “add.”

What is a diopter chart?

A diopter chart contains lines with letters of different sizes. Some stores have a diopter chart to help choose the correct strength of the reading glasses. These lines are read without reading glasses. Each line of letters has a reading strength in diopters next to it. A lower reading power is required when the smaller letters can be read. A stronger reading power is recommended if the larger letters are the only ones that can still be read.

Once the correct diopter power is chosen, test the glasses by holding the reading material at a comfortable distance, and make sure that the whole chart appears sharp. If the smaller letters do not come into focus, move the material a little further away and then a little closer. If the glasses do not seem to focus at a comfortable distance, then an increase of the diopters will allow for a closer focus, and a reduction of the diopters will allow for a focus slightly farther away.

A common mistake is to get reading glasses that have too many diopters and try to use these to focus on the computer. Since the computer is usually farther away, the diopters actually need to be decreased.

Reading glasses are also called “magnifiers,” which can be very confusing. Reading glasses do indeed cause images to appear slightly larger. However, allowing the eye to focus at the correct distance is the main way reading glasses work, not magnification of the letters.

Do I need different diopter strengths for different distances?

For many, lenses of different diopter strengths are needed to see sharp at far, close and intermediate distances. Bifocals, trifocals and progressive addition lenses (PALs) incorporate these different strengths into one pair of glasses.

Less well-known are adjustable diopter glasses that allow one to adjust the diopter power of the lenses in the glasses with a small dial on the side of the frame. While these glasses can be very useful, there are restrictions on the available range of diopter strengths.

Reading glasses and adjustable diopter glasses are available without a prescription and can help many people. Limitations to their use occur when the diopter power between the eyes is different or when a significant amount of astigmatism is present.

Typically, if someone has an eye condition such as a cataract or age-related macular degeneration, there is no diopter strength that will provide sharp vision. In these cases, the glasses or contact lenses focus light precisely on the retina, but the eye structures interfere with the correct processing of light. Therefore, it is highly recommended to see an optometrist or ophthalmologist when over-the-counter glasses do not provide sharp vision up close.

Additional Information

A dioptre (British spelling) or diopter (American spelling), symbol dpt, is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dpt = {1 m}^{-1}. It is normally used to express the optical power of a lens or curved mirror, which is a physical quantity equal to the reciprocal of the focal length, expressed in metres. For example, a 3-dioptre lens brings parallel rays of light to focus at 1/3 metre. A flat window has an optical power of zero dioptres, as it does not cause light to converge or diverge. Dioptres are also sometimes used for other reciprocals of distance, particularly radii of curvature and the vergence of optical beams.

The main benefit of using optical power rather than focal length is that the thin lens formula has the object distance, image distance, and focal length all as reciprocals. Additionally, when relatively thin lenses are placed close together their powers approximately add. Thus, a thin 2.0-dioptre lens placed close to a thin 0.5-dioptre lens yields almost the same focal length as a single 2.5-dioptre lens.

Though the dioptre is based on the SI-metric system, it has not been included in the standard, so that there is no international name or symbol for this unit of measurement—within the international system of units, this unit for optical power would need to be specified explicitly as the inverse metre ({m}^{-1}). However most languages have borrowed the original name and some national standardization bodies like DIN specify a unit name (dioptrie, dioptria, etc.). In vision care the symbol D is frequently used.

The idea of numbering lenses based on the reciprocal of their focal length in metres was first suggested by Albrecht Nagel in 1866. The term dioptre was proposed by French ophthalmologist Ferdinand Monoyer in 1872, based on earlier use of the term dioptrice by Johannes Kepler.

In vision correction

The fact that optical powers are approximately additive enables an eye care professional to prescribe corrective lenses as a simple correction to the eye's optical power, rather than doing a detailed analysis of the entire optical system (the eye and the lens). Optical power can also be used to adjust a basic prescription for reading. Thus an eye care professional, having determined that a myopic (nearsighted) person requires a basic correction of, say, −2 dioptres to restore normal distance vision, might then make a further prescription of 'add 1' for reading, to make up for lack of accommodation (ability to alter focus). This is the same as saying that −1 dioptre lenses are prescribed for reading.

In humans, the total optical power of the relaxed eye is approximately 60 dioptres. The cornea accounts for approximately two-thirds of this refractive power (about 40 dioptres) and the crystalline lens contributes the remaining one-third (about 20 dioptres). In focusing, the ciliary muscle contracts to reduce the tension or stress transferred to the lens by the suspensory ligaments. This results in increased convexity of the lens which in turn increases the optical power of the eye. The amplitude of accommodation is about 11 to 16 dioptres at age 15, decreasing to about 10 dioptres at age 25, and to around 1 dioptre above age 60.

Convex lenses have positive dioptric value and are generally used to correct hyperopia (farsightedness) or to allow people with presbyopia (the limited accommodation of advancing age) to read at close range. Over the counter reading glasses are rated at +1.00 to +4.00 dioptres. Concave lenses have negative dioptric value and generally correct myopia (nearsightedness). Typical glasses for mild myopia have a power of −0.50 to −3.00 dioptres. Optometrists usually measure refractive error using lenses graded in steps of 0.25 dioptres.

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