vessel, may cause spasm or closure of the vessel. However in clinical practice, although it is possible to close vessels, the main application of the laser is to the underlying pigment epithelium where it produces a visible burn. A burn if gently applied causes a blanching of the outer neural retina; a more intense laser burn will produce marked whitening of the entire retinal thickness, a pigment ring surrounding the laser spot develops later. The energy applied to the pigment C with a temperature gradient from the centre of the burn to the adjacent retina resulting in enlargement of the visible area of RPE cell loss, damage to the neural retina and progressive choriocapillaris atrophy. It is therefore important that the laser energy should be set to induce a minimal reaction at the time of application. Although the second peak from the Argon laser is the 488nm, this wavelength has now fallen into disuse. The disadvantage of this wavelength was that it is absorbed by the luteal pigment in the nerve fibre layer in the macular area risking damage to the perimacular nerve fibre layer. The 514 nm wavelength is minimally absorbed by the luteal pigment and therefore is safer for treatment of retina close to the macula(5-7) and should be used for macular laser treatment. 9.2.2 The Krypton Laser Krypton laser peaks at 647 nm and 568 nm and thus emits in the red and yellow wavebands. The red waveband was initially thought to be useful in treating parafoveal lesions because it is not absorbed by luteal pigment. However, this colour is no longer used because of the sensitivity of the pigment epithelium to changes in power. A small increase in power changed a white RPE reaction either to a haemorrhage or to disruption of the pigment epithelium. A slight tilting of the lens leading to a change in the spot size had the same effect. The yellow peak of the krypton laser is similar to the yellow of the dye laser which has become more readily available now. 9.2.3 The Dye Laser The dye laser (570nm – 630nm) was developed to provide a variable wavelength laser in the green, yellow and red wavebands. In the green waveband the dye laser has no advantage over the Argon laser and the red waveband is similar to the krypton laser and has the same complications. However the yellow wavelength (577 nm) has gained some popularity because it is absorbed by haemoglobin and therefore allows direct closure of microaneurysms and blood vessels. In addition, much less power is required compared to the argon laser to achieve a satisfactory burn and therefore in those patients with a low pain threshold or very thin retinas, this wavelength can be 74 very helpful(8,9). Some operators feel that they are most comfortable treating with this wavelength. 9.2.4 The Diode Laser The diode laser at 810 nm in the infrared or invisible spectrum is delivered via a portable machine. The lack of a bright flash provides increased patient comfort. Additionally, the laser producing minimal bleaching of the retina allows rapid recovery from the laser treatment. However with the diode laser the end point being a greyish lesion at the level of the pigment epithelium rather than more obvious white lesion produced by other wavelengths, it is more difficult to assess. If the laser surgeon is unaware of this difference, there may be a tendency to raise the power of the diode laser to produce a white lesion similar to that produced with the argon laser and that such more intense lesion may cause pain and excessive damage to the retina10,11. Around 9% of the energy from the diode laser is absorbed by the pigment epithelium, the remaining energy penetrates into the choroid to be absorbed by choroidal melanocytes compared to the 50% energy uptake by the RPE from the argon laser12,13 . The diode laser has been adapted to fire in a rapid sequence micropulse mode (Micropulse laser). In this mode there are short applications of laser of approximately 100 micro-seconds in duration with an interval in the region of 1900 micro-seconds. Thus 100 micro-bursts of the laser can be applied into an envelope of 0.2 seconds. The method of application of this laser is to increase the power of the laser to achieve a whitening of the retina and then to reduce the energy levels by around 50% to continue treatment. The effect of this laser is to raise the temperature within the retinal pigment epithelium only; thus minimising collateral damage to both neuroretina and choroid. In addition, unlike the conventional mode of diode laser in which pain may occur, usually there is no associated pain with the micropulse mode. Initial non-randomised clinical studies in particular for diabetic maculopathy are encouraging14,15 and there is currently a large multi-centre study in progress comparing this laser with the argon laser. 9.2.5 The Frequency-Doubled Yttrium Aluminum Garnet (YAG) Laser Recent application of the frequency doubled YAG laser has shown that it is as effective in treatment of diabetic macular oedema as other laser types and that it is gaining some popularity 16,17. In particular, the Pascal (PAttern SCAn Laser) frequency-doubled neodymium-doped yttrium aluminum garnet solid-state laser with a wavelength of 532 nm is increasingly used. Usual laser lenses appropriate for use are Mainster® 165 PRP, Mainster® Focal Grid, 1X Mainster® , Area Centralis® , Quadraspheric® & Super Quadraspheric® and similar lenses. A laser indirect ophthalmoscope can also be attached for single spot delivery only. Power settings for Pascal are in general twice that of argon for comparable treatments. However, pulse duration is one fifth that of conventiaonl argon laser treatment, [e.g. for Pascal laser PRP, 20ms (0.02sec) versus 100ms (0.10sec) for conventional argon laser]. 18 9.2.6 Lasers – Principles in Practice The goal of retinal photocoagulation is to target the RPE with minimal photoreceptor damage and RPE cell loss, and perhaps barely-visible scar formation within the outer retina. In the decade following the guidelines published by the DRS and ETDRS, 75 although visible endpoint DRS/ETDRS laser photocoagulation remains the gold standard for the treatment of PDR, different laser strategies can help reduce ocular side-effects, such as laser burn