![]() standard of the He d line of the spectrum (587.56 nm) is +10 diopters the power of the lens for blue light will be about +10.16 and for red light will be about +9.84 diopters. That means although the power of the lens using the U.S. A lens made of a material with an Abbe of 30 and with a power of 10 diopters (either + or -) will have a dioptric spread of 10 ÷ 30 or 0.33 diopters. The width of this spread of colors is 6.10Δ - 5.90Δ, or 0.20 prism diopters (0.2Δ). If the contour that is being viewed is vertical, and the base-apex line of the prism is horizontal (i.e., BO or BI), a red fringe will show on one side of the contour, and a blue fringe will appear on the other. If the patient’s line-of-sight is 1 cm away from the optical center, the red component of your patient’s retinal image is displaced by 5.90Δ (1cm x 5.90 diopters), and the blue component is displaced by 6.10Δ (1 cm x 6.10 diopters). standard.įor a +6.00 lens with a dioptric spread of 0.20 diopters (i.e., 6 ÷ 30), the power for red light is +5.90, and the power for blue light is +6.10. The background of your lensometer is green to help you read the lens power in the yellowgreen range of the spectrum-around the helium d (He d) line of the spectrum, our U.S. “Dioptric spread” can be used when referring to this range. The range of powers for any lens is expressed in diopters. The color fringes are wider from red to blue for a low Abbe lens and narrower for a high Abbe lens. It is important to recognize that a lens has not one power, but differing powers depending on the wavelength that is being considered. The color fringes are further apart from red to blue for a low Abbe lens and closer together for a high Abbe lens. The width of this rainbow is related to the Abbe value and the prism powers for the various wavelengths as determined by Prentice’s Law. The result is a rainbow of blur or color fringes at the edge of contours within the patient’s field of view. When the patient looks away from the optical center, these stacked images are displaced by differing amounts, sort of like spreading a deck of cards across a table (Fig. When the patient’s line-of-sight passes through the optical center, no prism is encountered. Prentice’s Law is the basis for understanding how Abbe value affects the performance of your patient’s lenses. Will this range of powers cause problems for your patient? Usually not-as long as your patient looks through the optical center of the lens! The reason is because the images formed by the various wavelengths of light passing through the lens are essentially stacked directly in front of and behind one another (Fig. That means the lens will have a power of about +6.10D for blue light (486.13 nm), +6.00D for yellow-green light (587.56 nm) and +5.90D for red light (656.27 nm). For example a +6.00 lens made of a lens material with an Abbe of 30 will have a dioptric spread of 6 ÷ 30 or 0.2 diopters. Simply divide the power of the lens by the Abbe value of the material. It is quite easy to determine the dioptric spread of any lens material. ![]() This difference in power can be called the “dioptric spread” between red and blue light for that particular combination of power and lens material. A lens that measures +5.00, for yellow-green light will measure slightly more than +5.00 for blue light and slightly less than +5.00 for red light. This standardization keeps us all in step when considering the power of lenses. yellowgreen (the exact wavelength is 587.56 nm) is used throughout most of the ophthalmic industry for measuring lens power. The amount of slowing (and bending) varies depending on the color (wavelength) of the light passing through the material. IOphthalmic lens materials bend light by slowing it down. The effects of chromatic aberration result from a combination of the Abbe value of the lens material, the prescription, the point at which the patient’s line-of-sight intersects the lens and the distance from the object being viewed. Much more common comments are, “I can’t see with these,” or “These lenses are just not right.” It’s frustrating to you as an eyecare provider and disappointing to your patient when aberrations of any kind interfere with the performance of new eyewear. ![]() It is rare that a patient will actually mention the colored fringes or “rainbows” caused by chromatic aberration. Pots of gold and bluebirds aside, rainbows are generally unwelcome visitors when it comes to your patients’ spectacle lenses and retinal images. ![]() At the end of the rainbow is a pot of gold, and over the rainbow is where bluebirds fly, or so they say. ![]()
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