However, it was not until 1909 that the Þrst stereoscopic photographs were published by W. Thorner, who ßipped the camera between exposures to capture a stereo image. In 1926, Metzger described the technique of side-to-side shift, which is still commonly used today. Eventually, simultaneous stereophotography was developed in the 1950s [25]. Although slide Þlm is still considered the gold standard, it is gradually yielding to the many beneÞts of digital imaging including ease of storage and sharing, internet transmission and reduced cost. These beneÞts are timely in that many manufacturers have announced an end to the production and sale of slide Þlm [6, 16, 22, 29]. In addition, the recent introduction of high-resolution, full-frame digital imaging has eliminated many of the detriments once associated with digital imaging.

3.3.1Non-simultaneous Versus Simultaneous

Non-simultaneous or sequential photography is the technique that is most commonly employed by ophthalmic photographers. It is known as the Allen Stereo Technique as it was initially described by Lee Allen in 1964 [25]. In this technique, the camera is positioned as for a normal fundus photograph. The camera is then shifted to the left and right of central to take the photographs needed from separate positions. The desired stereo base will determine the distance the camera is shifted, and can vary due to media opacities (such as cataract) or the degree of pupillary dilation. For example, if the pupil is dilated to 8 mm, then a displacement of the camera by 3.5 mm will allow a convergence on the retina of 15¡, which is sufÞcient for a stereoscopic view [13]. One problem

The basic element of stereophotography is to obtain two separate images of an object from two different positions in order to simulate the viewing position of an individualÕs eyes. The distance between the two positions at which the pictures are taken is known as the stereo base. Changes in the stereo base determine the stereo effect. The ideal stereo base would be approximately 65 mm, as this is the average interpupillary distance. When each unique image of the same object is presented to the appropriate eye, a three-dimensional image is observed. The techniques used to capture the two photographs needed for a stereo image can be done by a horizontal shift of the camera, or by rotation of the camera towards a single point on the object. The latter technique can cause some distortion, which is reduced if the object is fairly ßat and close to the camera [25]. Stereo fundus photography uses the horizontal shift technique, by corneal-induced parallax [22]. This is successful because of the optics of the eye itself induce convergence of light to a single point on the fundus, which allows photography of one point on the fundus from two different locations. Today, paired retinal images are captured on conventional slide Þlm or using a digital camera and computer [9].

pher and patient movement may create an inconsistent stereo base. Thus, Lee Allen developed the Allen Stereo Separator, which is a plano glass lens placed in front of a fundus camera [25]. Changing the angle of this plane displaces the optical position of the camera horizontally. Other cameras have a locking mechanism which prevents lateral movement of the camera beyond a certain point.

Simultaneous photography has the advantage of a consistent stereo base. Thus, all stereo photos will have identical stereoscopic depth. This method is also easier on the patient since one ßash will take two photographs. Disadvantages of this technique are that it can be difÞcult to illuminate two images evenly and that both images must be aligned simultaneously. In most simultaneous cameras, the stereo base is Þxed; however, others will allow adjustment to avoid lens opacities if needed [25].

3.3.2Mydriatic Versus Non-mydriatic Imaging

The need for pupillary dilation when obtaining stereo images is controversial. Mydriasis allows retinal photographs to be captured more easily and with improved image quality [20, 27] while facilitating peripheral retinal image capture.

Others argue that these beneÞts are outweighed by the increased risk of angle-closure glaucoma, patient time and discomfort. There are stereoscopic retinal imaging systems described in the literature that incorporate both mydriatic [10, 24] and non-mydriatic photography [2, 11, 21].

3.3.3Imaging Fields

The retinal imaging standard for photography of the retina for diabetic retinopathy was deÞned by the Airlie House ClassiÞcation to be seven Þelds of 30¡ stereo images [1]; this standard is often referred to as Early Treatment Diabetic Retinopathy Study (ETDRS) standard photography. However, it is time-consuming to capture all seven Þelds in stereo for both photographers and patients. Thus, many systems have evaluated modiÞcations to the standard seven Þelds. Moss et al. [19] compared two, three and four Þelds of stereoscopic images to the standard seven and found that the sensitivity and speciÞcity of detecting the level of diabetic retinopathy increased as the number of Þelds captured increased. The Joslin Vision Network (JVN) system [6] demonstrated substantial agreement (k =0.74) when comparing three Þelds of nonsimultaneous 45¡ stereo images to ETDRS standard photography for the detection of clinically signiÞcant macular oedema (CSME) and fair to almost perfect agreement (k =0.31Ð0.85) for the detection of levels of diabetic retinopathy. Salti et al. [26] compared two Þelds of 45¡ stereoscopic photographs along with a single monochromatic image to a dilated fundus examination. They found that the addition of a red-free photograph increased the sensitivity and speciÞcity in detecting the presence of diabetic retinopathy. At the University of Alberta, seven 30¡ images are obtained but only the anterior segment, disc (Þeld 1) and macula (Þeld 2) are captured in stereo [24].

3.3.4Image Viewing Techniques

also be a split frame 35 mm slide, in which twohalf frame images are placed into one 2 × 2 mount. A View-Master disc is a format that was used in the 1950s with stereo pairs on a circular disc and a hand-held viewer [25].

Electronic images can be viewed using the same principles but with a computer monitor. They can be viewed using side-by-side images and a four-mirror stereoscope and can also come with a viewing hood. Another method utilizes a polarized liquid crystal display (LCD) panel over a computer monitor with the viewer wearing polarized glasses. With the proper Þlters, even red-cyan glasses can be used. The method that is most commonly utilized when viewing stereoscopic digital images of the retina involves LCD shutter glasses linked to a cathode ray tube (CRT) monitor. This involves a computer monitor that alternately displays each image of a stereo pair with a high refresh rate (for example, 120 Hz). The computer then sends a signal to LCD shutter glasses through an emitter to synchronize the opacity of the lenses with the display of the contralateral image [25]. Thus, the right eye views only the right image, and the left eye views only the left image. Most recently, LCD monitor technology has evolved to allow high refresh rates, thereby enabling the same technology to be utilized.

The validity of stereoscopic digital photography when compared to stereoscopic slide Þlm photography has been well established in clinical studies. Rudnisky et al. [22] demonstrated that high-resolution stereo digital imaging correlates with contact lens biomicroscopy and is both sensitive and speciÞc when identifying clinically signiÞcantmacularoedema(CSME).Stereoscopic digital imaging has also been compared to slide Þlm photography for Age-Related Eye Disease Study (AREDS) grading of macular degeneration, and correlated well [29]. High-resolution digital imaging has also been shown to be accurate for the evaluation of ETDRS levels of diabetic retinopathy [10, 24].

3.3.5Image Compression

The traditional way of viewing stereoscopic slide Þlm utilizes a 2 × 2 stereo viewer over slides placed side by side on a light table. There can

There have been concerns regarding the effect of digital image compression on picture quality and,