The optical system of the patient's eye - the cornea and lens, together provide so much convergence that ordinarily we cannot see the retina using a slit-lamp. At most, the slit lamp can view upto the anterior vitreous. So, auxiliary lenses for slit-lamp examination of the retina are required to nullify these intervening optical effects caused by the patient’s eye.1 During indirect ophthalmoscopy the divergent rays from the patients eye are focused between the hand held condensing lens placed just above the patient’s eye.
The various lenses used in clinical ophthalmology can be divided in to Diagnostic(figure 1) and therapeutic (figure 2) lenses
1. Non-Contact Lenses
a. Convex Lenses - Examination of the retina using the slit-lamp with these lenses placed in front of the patient’s eye, is called indirect slit-lamp double aspheric lenses bio microscopy. Typically, the +60D, +78D, and +90D(Figure 4) fundus lenses are used for comprehensive fundus evaluation.
These are double aspheric lenses (so it does not matter which side is held towards the patient), forming a real, inverted and laterally reversed aerial image of the fundus(Figure 3).
POWER 8 MAGNIFICATION8 1/FIELD OF VIEW
High powered lenses provide a large field of view but lesser magnification, e.g. the +90D lens provides a bigger field of view but lesser magnification than +78D lens.
Magnification is calculated under the assumption that the patient’s eye is a +60D optical unit.
Brief Technique: With the patient seated comfortably at the slit-lamp, the illumination and observation system kept co-axial, and magnification kept at 10X or 16X, the light beam (4mm wide with brightest light intensity) is focused on the patient’s pupil. The condensing lens is then aligned at around 5-10 mm from the patient’s cornea and the slit-lamp is pulled backward gradually towards the examiner until the fundus is visualized. To view the peripheral retina, the patient is asked to look into appropriate positions of gaze as with standard indirect ophthalmoscopy.
+60 D lens – Introduced by Volk Optical in the early 1980s.4
- High magnification views of the posterior pole for detailed optic disc and macula imaging. (Figure 4A)
- Ideal for optic nerve head examination as lens offers higher magnification near 1x.
- Diameter -31mm, facilitates easy handling.
- Working distance from the cornea - 11mm.
+78 D lens – Single lens providing an ideal balance between magnification and field of view (Figure 4B)
- Working distance from the cornea - 7mm.
+90 D lens – Relatively wide field of view with a good resolution.(Figure 4C)
- Posterior pole examination for general diagnosis and small pupil examination,.
- Diameter ring - 26mm, ideal for dynamic fundoscopy. ie dynamic observation of the fundus in all directions possible by moving the condensing lens, scleral indentation etc.
- Working distance from the cornea - 6.5mm.
Advantages
- Stereoscopic, view of the retina
- Better image achieved when viewing through media opacities like cataract.
- Ease of viewing independent of pupillary size/dilation status.
- Allows for magnification of image with the help of slit-lamp filters and magnification.
- Image size less affected by the patient’s refractive error.
- “Up-close” look at the condition e.g. magnified view of the macular details and neuro-retinal rim tissue
- Non-Contact: avoids potential infection transmission.
Disadvantages
- Small field of view as compared to indirect ophthalmoscopy. Cannot visualise beyond the equator, hence not ideal for screening of peripheral lesions.
- Precise patient fixation required. Not ideal when patients cannot sit at the slit-lamp, in children and challenging situations like nystagmus.
b. Indirect Ophthalmoscopic Lenses: introduced by Nagel in 1864. The condensing lens used is aspheric with one surface less curved than the other which is kept facing the patient’s eye (indicated by silver ring).(Figure 5)
Optical Principle
To make the eye highly myopic by placing a strong convex lens in front of the patient’s eye so that the emergent rays from an area of the fundus are brought to focus as a real inverted aerial image between the lens and the observer’s eye.
+30D lens – High Dioptric power lens with the least magnification of retina but the largest field of view.
- Least magnification – 60/30 = 2X
- Largest field of view – 30*2 = 60°
- Stereopsis is half that of normal, 2/4 = ½
- Used to obtain a panoramic view when detail and stereopsis are not as important. Can be used with a small pupil. (Figure 6a)
+20D lens (FIg 6)–
- Retinal magnification – 60/20 = 3X
- Field of view – 20*2 = 40°
- Stereopsis is 3/4th that of normal.
- Most widely used, since it provides an adequate field of view, stereopsis, and magnification.(Figure 6c)
+14/15D lens –
- Retinal magnification = 60/15 = 4X
- Field of view – 15*2 = 30°
- Stereopsis is full i.e. 4/4.
- Most useful for a detailed view of the macula or optic disc or for determining the elevation of the retina in shallow retinal detachment. (Figure 6b)
Advantages
- Large field with 3D stereoscopic view of the retina and considerable depth of focus
- Lesser distortion of the image of retina.
- Ease of examination especially if the patient’s eye movements are present or if there are high spherical or astigmatic refractive errors.
- Easy visualization of retina anterior to equator, helpful in visualising retinal peripheral degenerations and breaks
- Useful in hazy media because of its bright light and optical property.
Disadvantages
- Relatively low magnification of 5x
- Visualization is difficult with very small pupils.
- Patient usually more uncomfortable with intense light of IO and scleral indentation.
- Steeper learning curve, requires extensive practice both in techniques and interpretation.
c. Concave Lens – Hruby Lens Biomicroscopy5
- Plano-concave high minus lens mounted on the slit-lamp for stability, with a dioptric power of -58.6D, which neutralizes the optical power of the eye (+ 60 D). (Fig 7)
- Placed 10-12 mm in front of the patient’s cornea.
- Provides a high resolution, virtual, erect image of the fundus. Image is formed 18mm in front of the patient’s retina.
- Small field of view with low magnification; cannot visualise the fundus beyond equator.
2. Contact Lenses = Combines stereopsis, high illumination and magnification with the advantages of slit beam5
a. Modified Koeppe’s Lens – Posterior fundus contact lens used to examine the posterior segment. Image formed is virtual and erect .
b. Goldmann’s 3-mirror contact lens – Consists of a central contact lens with 3 mirrors placed in the cone, each with different angles of inclination (Figure 5).5 Helps to visualise central as well as peripheral parts of the fundus, and even of the angle of the anterior chamber.
- Provides a virtual and erect image, located near posterior surface of crystalline lens.
- Can visualise the entire fundus by rotating the lens 360°. The 3 mirrors inclined at 59°, 67° and 73° gives view of the anterior peripheral retina (including ora serrata and pars plana), equatorial fundus and the area around the posterior pole, respectively. The central contact lens is used for posterior pole and vitreous. Figure 10a,b)
- Primarily used nowadays for therapeutic purposes like retinal photocoagulation
Disadvantages
Inconvenience to the patient (involves anaesthetising the cornea and direct contact). Limited field of view requiring rotation of the lens to visualize more than a small patch of the fundus.
c. Wide-field Panfundoscopic Indirect Contact lens – Field of view – upto 130°. Real and inverted image is produced. It is used for fundus examination and performing laser photocoagulation.
Therapeutic Purposes
Lenses used for Posterior Segment Lasers6,7,8
- Common Characteristics:
1. Concave posterior surface conforming to the corneal curvature and a flat or convex anterior surface
2. Planar mirrors allowing observation of the anterior chamber angle or peripheral retina.
3. A prism to allow visualization of the mid-periphery of the retina.
4. A flange to stabilize the lens and prevent blinking
5. Knurled edge to facilitate lens manipulation
6. Laser lenses generally consist of a conical PMMA or aluminium shell
7. Glass anterior surface, lenticular elements and mirrors.
8. Antireflection coatings applied to each optical surface reducing reflected white light (from the slit lamp source) that decreases contrast of image and laser light (from the treatment beam) that could pose a potential hazard to an observer standing behind the operator.
Mirror Lenses
a. Goldmann’s 3-mirror lenses (Figure 10)
b. Yannuzzi fundus lens (Krieger lens) - Has better optics than a simple Goldmann fundus lens. Image produced is erect, virtual and located in the anterior vitreous. It is used for macular photocoagulation. (Figure 12)
Lenses without mirror
a. Mainster: Introduced in 1986, this lens has more field of view (58% greater than Goldmann) and a greater magnification. It has a biconvex aspheric anterior lens element which produces a real and inverted image.
• Mainster Standard Lens: For focal and grid laser, from posterior pole to mid-periphery.
Field of view = 90°/121°. Image magnification = 0.96X. Laser spot magnification = 1.05X.
• Mainster Wide Field Lens: Used for panretinal photocoagulation in proliferative diabetic retinopathy.
Field of View = 118°/127°. Image magnification = 0.68X. Laser spot magnification = 1.50X.
• Mainster Ultra Field PRP Lens:(Figure 13)
Widest field of view = 165°/180°. Image magnification = 0.51X. Laser spot magnification = 1.96X.
• Mainster Focal Grid laser Lens:
Used for Focal macular or grid lasers. (Figure 14,15)
b. Rodenstock Panfudoscopic Lens: Introduced in 1969 by Schlegen Schlegel. Provides panoramic view and produces a real and inverted image. Used for PRP from the posterior pole to beyond the equator without the use of mirrors.
c. Volk Lenses: Following are used as tabulated below (Table 1).
In conclusion, the myriad condensing lenses remain vital to everyday ophthalmological practice. Selecting an appropriate lens for a particular diagnostic or therapeutic posterior segment modality depends on the ophthalmologist’s experience with lens, its optics and the knowledge about new technological developments of lens designs and therapeutic strategies.
References
- Guyton DL, et al. Ophthalmic Optics and Clinical Refraction. Baltimore: Prism Press;1999. Illustration modified by Krishna Irsch, PhD.
- Brodie SE. Optical Instruments. In: Brodie SE, Mauger TF, Gupta PC. eds. Basic and Clinical Science Course – Clinical Optics. San Francisco: American Academy of Ophthalmology; 2020-21. p. 295-300
- Shah VA, Tripathy K, Do DV, Bhagat N, Lim JI, Karth PA. Binocular Indirect Ophthalmoscopy. AAO EyeWiki. 2021. Available from: https://eyewiki.org/w/index.php?title=Binocular_Indirect_Ophthalmoscopy&oldid=75743
- Kumar NKS. Lensopedia: Lenses in Ophthalmology. eOphtha. 2021. Available from: https://www.eophtha.com/posts/lensopedia-lenses-in-ophthalmology
- Walling PE, Pole J, Karpecki P, Colatrella N, Varanelli J. Condensing Lenses: Sharpen Your Skills in Choosing and Using. Review of Optometry. 2017. Available from: https://www.reviewofoptometry.com/article/condensing-lenses-sharpen-your-skills-in-choosing-and-using
- Khurana AK, Khurana AK, Khurana B. Optical Instruments and Techniques. In: Khurana AK, 4th ed. India: Elsevier; 2018. p. 460-8
- Mainster MA, Crossman JL, Erickson PJ, Heacock GL. Retinal laser lenses: magnification, spot size and field of view. Br J Ophthalmol 1990; 74:177-179
- Weingeist TA, Sneed SR. Contact and non- contact lenses in photocoagulation therapy. Laser Surgery in Ophthalmology: Practical Applications. 1992; 2: 7-14.
- Das T. Retinal laser optical aids. Indian Journal of Ophthalmology.1991; 39:3: 115-117.
- Sharma, Gitumoni & Dnb, Purkayastha & Deka, Hemlata & Bhattacharjee, Harsha. (2008). Commonly Used Diagnostic and Laser Lenses for Retinal Diseases-An Overview.