L&T: RxPertise

Feb
2008

The Lighter Side

Understanding The Science of Light Tints


Eyecare practitioners commonly prescribe light tints for both function and fashion. Recently, light tints have had a resurgence in popularity thanks in part to celebrities who combine them with flash mirrors to wear indoors while clubbing or posing for paparazzi.
The use of light tints raises some interesting questions. Is there a scientific basis for using a light tint in a lens? Do patients truly benefit from light tints or are they simply giving a placebo effect? Most importantly, what rationale should be applied when a light tint (i.e. one that reduces lens transmission by about 5 to 10 percent) is added to the patient’s eyewear?

Historically, many pioneering eyecare practitioners came to the belief that patients benefit from having even small amounts of refractive error corrected. These practitioners also discovered that including an overall light tint, especially in low-powered lenses, seemed to give patients more comfort and perhaps even more clarity than if non-tinted lenses were used. Understanding why these light tints seemed to have such an effect is central to understanding how and when light tints can be used effectively.

Certainly a placebo effect could be a factor with some patients. A low-powered lens has little magnification and thickness, so perhaps the addition of a tint does reassure the patient that the lens has some additional benefit. However, a careful look at the lens reflections that can affect visual performance reveals there is also a physical basis beyond simple attenuation of light used for vision. In Fig. 1A and 1B light from behind the patient is reflected into the eyes. In Fig. 1A, light simply reflects from the back surface of the eye and would be unaffected by using a tint, but in Fig. 1B, the light passes through the back surface of the lens and its path transverses the lens twice, allowing the tint to do passes through the lens once, so the brightness of the light from behind is relatively more darkened than the darkening of the major ray bundle used for vision.

In Fig. 2 light entering the lens from the front transverses the lens twice before entering the eye. This has important implications for two common causes of reduced lens performance. During night driving it is this reflection that causes annoying “ghost images.” When patients are presented with extended bright light sources such as light-colored walls in an office or a store with relatively bright lighting, this reflection causes a veiling glare. These effects are both reduced when a light tint is used because the light pathway passes through the lens multiple (in this case three) times. In fact, although the reflectance of a 1.58 index lens is only about 30 percent higher than the reflectance of a standard plastic lens (1.49 index), the brightness of internally reflected images during night driving is 65 percent higher with the greater index. Using a 5 percent tint for the 1.58 index lens reduces the brightness of the “ghost images” by about 12 percent and using a 10 percent tint reduces the brightness of the “ghost images” by about 29 percent.

The advantage of using a light tint in both Figs. 1 and 2 mimics the use of an anti-reflective lens, although a tint is not nearly as efficient in eliminating the unwanted reflected light. If a light tint is included when anti-reflective lenses are prescribed, the reduction in these unwanted reflection is even greater than if AR alone is used. Dispensers and prescribers should be aware that labs may have difficulty with combining AR and tints. This is because the lens tint must be darker initially since the AR process tends to lighten tints in organic (plastic) lenses. Nevertheless using AR in combination with a light tint is more effective in reducing unwanted reflection than AR alone and should be considered for patients most likely to have reflection related problems.

Gradient Tints
Gradient tints offer some very unique applications. If a full gradient is used, it is usually darkest at the top and then fades to a lighter shade at the bottom. The darkest top part of the lens shields the eyes from useless light from above and can be very helpful in work environments with bright, overhead lighting. Light from overhead fixtures can cause light scatter from opacities or haziness of the lens of the eye or the cornea. Older patients are particularly prone to this problem, which they see as a veiling glare or reduced contrast. An additional advantage of a gradient tint lays in the fact that patient can “self-adjust” to a certain extent by tipping his head up or down as needed to momentarily cause his line-of-sight to pass through a lighter or darker portion of the lens as conditions dictate. Because glare recovery time lengthens with age, this can allow older patients to effectively reduce excessive light during short-term exposure so that glare-recovery is less of an issue. If eyewear is to be worn during night driving, the tinted portion of the lens for straight ahead vision should never be darker than a total of about 82 percent transmission or about the equivalent of a #2 tint. Experts agree that tints darker than this are unsafe for night driving.

Fig. 3: The lens on the left is a full gradient with about 50 percent transmission at the top that fades to about 85 percent transmission straight ahead. The lens on the right has about 18 percent transmission at the top and fades to about 85 percent transmission straight ahead. This is sometimes called a “fast-fade” gradient, and is particularly appreciated by older patients with a lot of lenticular and/or corneal light scatter and reduced contrast sensitivity.

Fig. 4. Double gradients are sometimes used in situations in which useless light is entering the eye from above and below, such as on a boat, but when little or no restriction of light from objects straight ahead is needed.

Fig. 5 A subtle, lateral gradient can improve the appearance of a unusually narrow PD.

Unusual Gradients
A lateral gradient is darker temporally and fades toward the lens center. This is strictly a cosmetic application and has the effect of making unusually narrow PDs less notice ble. The effect is subtle and the amount of tint should be limited so that it is not blatantly noticeable. For the best cosmetic result, lateral tints should be coupled with frames that have very plain, unadorned bridges and a relatively “busier” appearance laterally near the temple attachment. At its darkest point (i.e. laterally) a lateral gradient should absorb about 5 percent or about the same as a #1 tint. The color of the tint should be similar to the patient’s complexion. Patients who are given this type of tint should be cautioned that under some conditions distance judgment may be affected so caution should be used until they are fully adapted. This distance judgment effect is due to the fact that when gaze is shifted laterally, the line-of-sight of one eye passes through a darker portion of the lens while the other passes through a lighter portion of the lens. Most patients experience little problem with this, however it could lead to incidents such as the unwanted removal of one side of the family auto by the wall of the garage, so patients should be particularly cautious about judging distances as they adapt to this lens.

Another unusual application for gradients is to apply a sun tint everywhere in the lens except the lower 15 percent or so. This type of gradient offers special advantages in some avocations or vocations. For example, mechanics who are exposed to bright sun, but who must then look into a dark engine compartment for the dipstick opening or to diagnose a problem can be especially appreciative of this kind of tint.

Color
Pink tints, sometimes referred to as “soft-light” tints, are usually used for patients who complain of discomfort working under fluorescent light. The pink tints selectively absorb more blue light, which tends to be more scattered by ocular opacities and haze. This may account for their apparent success in making office workers more comfortable. A #2 pink tint can have subtle masking effect for patients whose conjunctivas are chronically injected. Most gradient tints (other than lateral gradients) are neutral or gray in color, reducing transmission more or less equally across the spectrum.

Computer Vision
A few years ago light tints were touted as being especially helpful for computer users. Perhaps these had some validity when computer screens were dark and the letters on the screen were orange or green. Light tints probably offer limited benefits other than the placebo effect when patients are using today’s computers. More beneficial ways of helping computer users include careful placement of the MRPs, careful determination of the add requirements and possibly repositioning the computer screen. Because the light output of most computer screens is quite low, it can be helpful to reduce the office’s ambient lighting. This creates less need for the visual system to adjust from the lower level light from the screen to the brighter illumination on copy and in the surrounding office environment.

Caution
A basic tenet of health care is “Do No Harm.” Using a light tint in lenses for constant wear is a decision that should be made by the prescribing doctor. Light tints offer little risk with the exception of night driving or using dangerous equipment or entering a dangerous environment under reduced illumination. Most experts feel lens transmission should be at least 82 to 88 percent when operating a vehicle at night.

It is good practice to always check your patient’s old lenses for a tint when prescribing new eyewear because the patient who has “always had a tint” often feels they must continue with the tint. When the lenses are found to be without the tint at the dispensing, disappointment may arise. Tints in plastic lenses do fade with time and it is especially easy to miss a light tint if it has faded. This fading is a particular problem if you must order just one replacement lens. The best way to get a match is to send the eyewear to the lab or do the tinting in-office if you have the equipment.

Remember, light tints can be beneficial for some patients, as long as the eyecare practitioner keeps these basic concepts in mind when prescribing or dispensing them.


 Palmer R. Cook, OD, is director of professional education for Diversified Ophthalmics in Cincinnati.

 

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