Visible light consists of the wavelengths between 380nm and 760nm. If you’re recommending clear lenses for a patient, these are the wavelengths you want her to receive because these are the wavelengths that stimulate the retina and cause the sensation of sight. You’ll also want to know transmission in the UV region.
What kind of light transmission can you obtain through a clear lens? That depends on the lens material for the most part. Why? Because the index of the lens’ material is directly related to the how much the lens will reflect light. For example, a CR-39® lens will reflect 7.8% of the light that falls upon it while a polycarbonate lens will reflect 10.3% and 1.74 material reflects 14.1%.
Sun lenses, often called absorptive lenses, are a little more complicated because they not only reduce the light level, they often absorb different wavelengths of light at different percentages. This varying absorption is called selective absorption. If you’re recommending absorptive lenses, you’ll want to know what percentage of each wavelength the lens transmits and absorbs because this has a direct bearing on lens performance as the patient wears it.
READING SPECTRAL CHARTS
The way to know exactly what light wavelengths a lens is transmitting is to consult a spectral transmission chart. This chart is a graphical table that illustrates how each wavelength of light transmits (and is absorbed) by a particular lens.
On the horizontal axis of the chart you’ll see the wavelengths of light. On the vertical axis the light percentages are listed. For any wavelength, find its nanometer value on the lower axis and see where it intersects the transmission curve. When you find this point, run your finger to the left to determine what percentage that wavelength is transmitted.
CLEAR VS. ABSORPTIVE
Fig. 1 illustrates two clear lenses and a light pink glass sample. Notice the clear lenses transmit all of the wavelengths at the same percentage, about 92% in this case. That makes perfect sense because you want the full range of visible wavelengths to reach the prescription lens wearer’s eyes, just like in normal daylight. Notice also that the two clear lenses do not transmit at 100%. It’s because of surface reflections. If you want it near 100%, an anti-reflective treatment is needed.
The intent of sun lenses should be to simply reduce the intensity of bright light. Gray lenses have the tendency to do that. The lower portion of Fig. 2 depicts a darkened PhotoGray glass lens. Other colored lenses absorb wavelengths differently, sometimes dramatically, like yellow that absorbs the blue side of the spectrum and transmits the red side. This creates a high contrast filter.
When offering a lens for the first time, especially an absorptive lens, it’s good practice to review its spectral so you can learn more about its potential performance.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.