Various laser lenses have been introduced following Goldmann 3- mirror and Goldmann fundus contact lens for retinal photocoagulation.

Below described some of the time-tested lenses in widespread use. Precise knowledge of these lenses is necessary for safe retinal photocoagulation.

Activation of the laser can be done by :

• Lens-mirror contact lens system used with a slit lamp

• Fiber optic system (endophotocoagulation)

• Indirect ophthalmoscope with a handheld +20 or +28 diopter(D) lens

For optimal visualization and treatment of retinal structures, the accessory lenses are used.

They are used to image these structures at a point where they can be reimaged by the slit lamp.

In a Haag- Streit Model 900, the stereoscopic slit lamp is in focus at about 9 mm in front of the objective lens of the microscope or about 280 mm from the examiner's eye.

The slit lamp cannot focus directly on the patient's retina due to the intervening optical system of the patient's eye.

So, accessory optical aids are required in the form of lenses in front of the patient's eye to nullify the optical effects of the various intervening optical systems of the patient's eye.

Laser should be delivered through a Slit-Lamp system for optimal viewing.

Coupling solution like Hydroxypropyl methylcellulose (2- 2.5%) is commonly used to form a bond between the cornea and the contact lens.

Several contact lens solutions have been described that provide excellent post contact lens examination clarity for subsequent clinical examinatio or photography.

1% carboxymethylcellulose sodium has a viscosity and adherence greater than saline and at the same time provides excellent optical clarity after contact lens removal.

Types of Lens Design

Non- contact: Hruby lens, +90 diopter (D) lenses and +60 (D) diopter lenses.

+90 and +60 lenses has been used for retinal photocoagulation in retinal tears, post-vitrectomy laser treatment of a giant retinal tear and for treating proliferative diabetic retinopathy.

Treatment of macular lesions is not encouraged as the surgeon has less control over patient eye movement.

The Hruby lens (Figure 1 & 2) is a high minus (originally - 58.6 D) non-contact lens mounted on the slit lamp for stability.

It provides a high resolution, upright image of structures in the posterior pole.

But, the small field of view makes it difficult to be certain of the relationship of nearby structures.

Visualization of more peripheral structures is limited by the entrance pupil, which is minified by the negative optics.

This lens should be used only for observation purposes and not laser surgery.

The +90 D lens (Figure 3 & 4) is also more commonly being used as a diagnostic lens as it is a noncontact lens thereby avoiding contact lens solutions and potential compromise in corneal clarity.

It shows a relatively wide field of view with a good resolution.

However, subtle amounts of clinically significant fluid associated with a choroidal neovascular membrane or central serous retinopathy may be missed.

Contact: Pan retinal laser lenses, Focal laser lenses.

Two types of laser lenses are available to assist in slit-lamp delivery of photocoagulation:

Plano-concave lenses: provide an upright image with high resolution of small retinal area.

The plano-concave lenses have mirrors angulated at 590 , 690 and 730.

High plus power lenses: provide an inverted image with mild loss of fine resolution, but provide a wide field of view, making these lenses very suitable for pan-retinal photocoagulation. These are also called indirect lens.

Some common characteristics of the laser lenses are:

Concave posterior surface conforming to the corneal curvaturev and a flat or convex anterior surface

Planar mirrors allowing observation of the anterior chamber angle or peripheral retina.

A prism to allow visualization of the mid-periphery of the retina.

A flange to stabilize the lens and prevent blinking

Knurled edge to facilitate lens manipulation.

Laser lenses generally consist of a conical polymethylmethacrylate or aluminium shell

Glass anterior surface, lenticular elements and mirrors.

Antireflection coatings

They are usually applied to each optical surface in a laser lens that reduces reflected white light (from the slit lamp source) that could decrease contrast or the slit lamp image, and laser light (from the treatment beam) that could pose a potential hazard to an observer standing behind the laser operator.

The hazard distance is 7 meters for an uncoated lens and 1.6 meters for a coated lens. Most laser lenses use broad-spectrum, multilayer, antireflection coatings that reduce reflected light between 400 nm and 700 nm from approximately 4 per cent to less than one per cent.

Mirror lenses

Mirror lenses provide high magnification and high resolution but only a small part of the fundus or chamber angle can be viewed at any one time.

Therefore, the mirrors at various degrees of inclinations are necessary.

e.g.Goldmann 3 mirror lenses. The image formed in the Goldmann 3 mirror lens is the mirror image of the area focused.

Lenses without MirrorsThe field of view may be increased to a variable extent by use of biconcave contact lenses based on the simple Goldman lens (without mirrors).

Another entirely different way of increasing the field of view involves using contact lenses based on the EL Bayadi lens.

Both Rodenstock Panfundoscope and Mainster lens are excellent examples of such lenses.

All wide-angle systems of this category are derived from the principle of indirect ophthalmoscopy and the common denominator of all these lenses is a large and inverted field of view.

Thus both the panfundoscope and the Mainster lenses produce inverted real images.

Magnification and field of view

Magnification and field of view are critical parameters for determining which lens is best for a particular clinical problem. The three main lenses - Goldmann three mirror lenses, Panfundoscope and Mainster lens - are compared in Table.

Focal Laser Lenses

Goldmann Lens (Figure 5 & 6)

The premier lenses to be used for retinal lasers are the Goldmann fundus lens and the Goldmann three -mirror lens.

The Goldmann fundus lens with either a single mirror inclined at 620, or the three- mirror style with the Gonioscopy mirror angled at 590 provides a large field of view but must be rotated 3600 to view the angled structures.

It has a flat anterior surface and produces an erect, virtual ophthalmoscopic image located near the posterior surface of the crystalline lens.

The chief disadvantage of the Goldmann lens is its limited field of view without rotation.

Yannuzzi fundus lens (Figure 7).

It is a modification of an earlier model developed by Krieger in 1966 designed to facilitate macular photocoagulation.

It has a concave corneal surface which is steeper and of greater diameter, so also has a better optics than a simple Goldman fundus lens.

The concave corneal surface allows posterior lens pressure to be transmitted to the sclera without distorting the cornea.

It produces an erect, virtual ophthalmoscopic image located in the anterior vitreous humor.

Volk Area centralis lens (Figure 8).

This is an indirect contact lens that provides a good field of view with an excellent magnification.

The field of view is 700/840. Image magnification is 1.06x. Laser spot magnification is 0.94x.

Volk PDT lens (Figure 9).

The field of view is 1150/1370.

Image magnification is 0.67x.

Laser spot magnification is 1.5 x which allows treatment of Choroidal neovascular membranes upto maximum spot size of 6400um with providing excellent visualization of the CNVM.

The Volk PDT lens comes standard with SupraCoat whic covers the 689nm laser wavelength indicated in this type of procedure.

Volk Transequator lens (Figure 10)

It is designed for focal laser therapy and mid-to-far peripheral fundus diagnosis.

Its unique optical design presents a realistic contour of the retinal concavity, offering an impressive wide-field of view of the entire posterior pole extending to the equator.

Its superior optics allow dynamic movement on the globe, therefore increasing its functional field of view.

The field of view is 1100/1320.

Image magnification is 0.70x.

Laser spot magnification is 1.44x.

Mainster:

Introduced in 1986, this lens has more field of view (58% greater than Goldman) and a greater magnification.

Although the field of view is 14% less than the Panfundoscope, but the lateral and axial magnification are better which makes it useful for detecting retinal thickening.

It has a biconvex, aspherical anterior lens element and a concave lens element to fit the corneal curvature.

It produces an inverted, real image located in front of its biconvex aspheric anterior lens element (Figure 11a).

Mainster Standard lens (Figure 11b)

This lens is designed for focal and grid laser treatment from the posterior pole to the midperiphery.

The field of view is 900/1210. Image magnification is 0.96 x. Laser spot magnification is 1.05x.

High resolution, high magnification of image allows appreciation of subtle intra-retinal details and retinal thickening.

So, it is excellent for diagnosis and treatment of macular oedema, branch retinal vein occlusion, choroidal neovascular membrane in age-related macular degeneration and presumed ocular histoplasmosis.

PRP Lenses

Mainster wide field lens (Figure 11c)

This allows a very wide range of slit lamp magnification to be used.

It has excellent ophthalmic resolution and image binocularity is maintained across the entire field of view.

It is used for panretinal photocoagulation in proliferative diabetic retinopathy.

The field of view is 1180/1270. Image magnification is 0.68x. Laser spot magnification is 1.50 x.

Mainster Ultrafield PRP lens:

This lens has the widest field of view available for pan retinal photocoagulation. It has a unique optical design to provide a clear, bright image across the entire field.

It is light- weight, has a secure fit flange for easy manipulation besides having a high efficiency laser light anti-reflective coating.

The field of view is 1650/1800. Image magnification is 0.51 x. Laser spot magnification is 1.96x.

Rodenstock Panfundoscopic lens (Figure 12 & 13)

Introduced in 1969 by Schlegel this lens is used for panretinal photocoagulation from the posterior pole to beyond the equator without the use of mirrors.

It gives a panoramic view, produces an inverted, real image located in its spherical biconvex anterior lens element. Thus, the biomicroscope must be located further from the patient's eye than using a Goldman lens.

This low biomicroscopic magnification produces adequate magnification with a large field and acceptable depth of focus.

The working field is 84% greater than a Goldman but lateral magnification is 24% lesser than a Goldman.

The spot size is 40% larger than the photocoagulator setting or twice large than the conventional contact lens.

Disadvantage of this lens is that it produces peripheral distortion.

It can produce marked laser beam astigmatism while treating the peripheral retina.

Reflexes compromise retinal image thereby causing oblong burns when treating through the periphery of the lens.

Common Features of Mainster and Panfundoscopic lens

Large areas of the fundus may be treated in Panretinal photocoagulation without lens rotation.

Visualization of the optic disc and macula during peripheral treatment prevents disorientation.

The experienced laser surgeon can achieve a more peripheral view by tilting the lens off-axis.

The field of view is increased in myopes and decrease in hyperopes and will lead to differences in how far peripherally laser photocoagulation can be applied.

Working distance on these lenses are greater.

Anterior segment irradiance becomes excessive with large spot sizes (1000μm) but should be acceptable at a spot size of 500μm.

Volk Quadraspheric lens (Figure 14)

The original 130 Quadraspheric lens has grown in popularity since its introduction in 1989 as the preferred wide field fundus laser lens for diagnosis and treatment of the retina.

The four aspheric surfaces also employ high-efficiency antireflection coatings thereby improving lens performance by reducing astigmatism across the entire field of view.

It also enhances visualization through a small pupil. It produces an inverted and reversed image.

Its sleek 28.6mm diameter housing provides a definite advantage over competitive wide field lenses for peripheral retinal viewing, reflection displacement and ease of use.

The laser spot magnification is 1.97x and the image magnification is 0.51x.

Volk Super Quad 160 lens (Figure 15)

This lens offers the widest field of view. Its ideal 0.5x image magnification provides simultaneous visualization of the posterior pole to the peripheral retina providing a greater margin of safety even during extreme wide angle panretinal photocoagulation.

The field of view is 1600/ 1650 , image magnification is 1.97x and laser spot magnification is 2.0x.

This has become the ideal lens for visualization and treatment of proliferative diabetic retinopathy, ischaemic retinal vein occlusion and peripheral retinal holes and tears.

Laser beam transmission and fundus image quality are sharp and undistorted to the full extent of the viewing field.