Contrast Sensitivity and Glare

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Contrast Sensitivity and Glare 

Fig.1 Herman Snellen

 

In 1862 Herman Snellen, a Dutch ophthalmologist, felt a need to document visual acuity, and published his 5X5 unit grid in which he determined that “standard vision” was defined as the ability to recognize one of his letters or optotypes when it subtended 5 minutes of arc on the fovea.  The following are the original charts developed by him.

 

 

                                                                                                

   Fig. 2. Snellen experimental charts, 1861. Snellen apparently experimented with various targets designed in a 5 × 5 grid before choosing letters as optotypes. This chart remains in the Museum of the University of Utrecht. (From Duane’s Clinical Ophthalmology on CD ROM 2004)

 

 

 

Fig. 3. Snellen's chart as published in 1862

(From Duane’s Clinical Ophthalmology on CD ROM)

 

Snellen’s “standard” has existed relatively unchanged into the late Twentieth Century, when it was felt that it was not an adequate predictor of functional visual acuity. Snellen high contrast letters were displayed on a bright background in a dimly lit room and with a bright projector light. Most clinicians know that patients with cataracts and excellent Snellen visual acuity have visual disability, especially at night against headlights, or in rain, mist, or in reduced luminance. This was supported by Hess and Woo (Vision through cataracts, Invest Ophthalmol Vis Sci 1078;17:428-445)  who demonstrated by patient testing and photographic simulation that Snellen visual acuity does not adequately reflect the debilitating consequences of cataract.

In 1994, Professor John Werner, Chair, Department of Psychology at the University of Colorado, Boulder and I wrote a paper describing the use of contrast sensitivity in the evaluation of patients with cataracts and a Snellen visual acuity of 20/50 or better. (20/50 means that the patient has to have letters as close as 20 feet that a “normal” person can see at 50 feet.)

This was preceded by work of Arthur Ginsburg, a vision scientist for the US Air Force, in evaluating “functional” vision at night in Air Force Pilots.  He had noted that the best predictor of “functional” vision was sine wave grating contrast sensitivity testing.

In our paper we found that patients could be significantly disabled from driving at night and other activities and yet have a vision of 20/50 or better. We demonstrated that these patients were highly significantly benefited, with a p value less than .01, by cataract surgery. Some of these patients even had 20/20 Snellen acuity, IE: a neurosurgeon, who verbalized that he was significantly disabled from the bright lights on a surgical field in an operating room. He was able to resume his career after cataract surgery. This paper is widely quoted in standard reference works of the world.

Contrast sensitivity is measured by the perception of sine wave gratings. The closer the gratings are together the higher the spatial frequency.

This is illustrated by the following chart.

 

 

 

Light Peaks Blending

 

 

With Dark Peaks

 
                          

                         

 

                                                      

                                (From Duane’s Clinical Ophthalmology on CD ROM 2004)

 

 

 

 

 

 

            Cataracts occur in several different types by general classification: cortical nuclear, and posterior sub capsular. These types are identified by slit lamp biomicroscopy.

 

 

                                    

 

The slit lamp creates a parallelepiped of light which can “cut” and optical section through the media of the eye. In this view the slit beam is coming from the right, and, images first the cornea, then through the clear aqueous humor, then the iris (above and below), and finally the front surface and then the substance of the lens. This is called oblique illumination.

One can also make the beam nearly parallel to the visual axis, bounce the light off the surface of the retina and get the view, common in photographs, of the “red eye” appearance or red reflex.. This can be very useful in silhouetting opacities in the media against this red reflex such as in the next photograph.

                                             

 

 

The following are illustrations of cataract types

                       

                                          Cortical Cataract

                                    (Photo from personal file, Pfoff)

 

 

                                                  Nuclear cataract

(Photos from Ciba Vision Publication and from Lerman: Radient Energy and the Eye, p 133, Macmillan, 1980

 

We see these commonly on mission trips to Mexico and are exacerbated by exposure to ultraviolet light.

On the left picture, the upper right corner photo is a slit lamp left oblique illumination showing the various elements of a nuclear cataract, with the slit beam hitting first the cornea, transiting through the clear aqueous humor, striking the anterior surface of the lens demonstrating the various delineations of the internal structure of the lens. The right photo is a compilation of nuclear cataracts after they have been removed by the intra-capsular technique. This shows the browning or brunescence of the lens with age. The upper left is from a patient 6 months old. The lower right is from a patient 91 years old.

 

 

 

                                

                  Posterior sub capsular cataract

                                 (Photo from personal file, Pfoff)

 

This type of cataract is common in diabetes and younger ages.

 

Cataracts cause glare or dazzle especially when facing oncoming headlights at night or in reduced contrast settings.

Glare Tests

Methods of testing glare can be done by several methods: The Brightness Acuity Test or BAT. This device, to the best of my knowledge, cannot be purchased anymore.

                                          

                                       (Photo from Google images 2004)

 

In this test, three levels of brightness illuminate the white bowl. The patient places the bowl toward the eye and looks through an aperture in the center of the bowl. This simulates different levels of light stress. In my experience and opinion this test has underestimated the degree of functional disability.

Another test, which has been used by me through the many years of my career, employs a muscle light shown into the eye nearly parallel to the visual axis. I feel this best simulates an oncoming headlight and seems clinically to correlate with the patients symptoms.

 

 

                                 

                                                Glare Testing

                                         (Personal photo, Pfoff)

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Pfoff Laser and Eye, 6881 South Yosemite Street | Centennial, Colorado 80112 | 303-588-7900|