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Рецензент: д.мед.н., проф. А.М. Петруня
УДК 617.7- 072.1- 7
Киевская городская клиническая офтальмологическая больница "Центр микрохирургии глаза"
Kyiv City Clinical Eye Hospital "Eye Microsurgery Center"
Introduction. Electric ophthalmoscopes are used to study the transparency of the ocular media and the fundus of the eye during direct examination which is an integral part of the eye examination. Electric light source in the ophthalmoscope is an incandescent lamp. Such a lamp is located either directly in the case of the electric ophthalmoscope (e.g. ophthalmoscope OR-2), or the light is transmitted through a flexible optical fibre (ophthalmoscope OFP) .
However, the use of incandescent lamps as light sources causes some difficulties in the fundus examination procedures. First of all, electric bulbs have a low efficiency (about 7%). For the flow of light of sufficient intensity it is necessary to use powerful enough incandescent bulbs. A large amount of heat generated by an incandescent lamp leads to placing the light source in an additional device, as in fixed and portable ophthalmoscope. Portable devices have to be equipped with low-power light bulbs, the light of which is sufficient for viewing the fundus only with good transparency of ocular media .
The stability of the colour temperature of the light source is crucial for proper examination of the fundus, and this is the parameter whose constancy is extremely difficult to achieve when using incandescent lamps. Apart from the stability of the colour temperature of the light source, its spectral characteristics are of considerable importance . For example, for a fairly accurate comparison of colour images it is necessary to maintain the constancy of the spectral characteristics of the light source from study to study, as well as sufficient light source intensity in the short-wave part of the spectrum . The shift of the spectral characteristics to the red zone leads to an increase in the threshold of discrimination, which does not allow capturing the differences in the colours represented by the object under examination.  However, oddly enough, till the present time the lighting conditions at the fundus ophthalmoscopy are not even standardized.
In the course of evolution the human eye adapted to perceive colours in natural sunlight (Diagram 1). The human eye shows the greatest sensitivity in the blue-green and orange areas where the threshold colour discrimination is 1-2 nanometres . In the same area of wavelengths the thresholds of colour depth discrimination are the smallest. The worst colour discrimination is observed in the blue-violet and red areas of the spectrum, where the threshold is greater than 6 nm .
However, incandescent lamps are the least suitable for simulation of solar radiation. For example, the emission spectrum of incandescent lamps with tungsten luminous element even at the nominal supply voltage is artificially enriched with the red component, which creates the worst conditions for distinguishing colours (Diagram 2). With lower supply voltage (both network supply and during running out of energy storage in inbuilt power source), this shift is markedly increased (Diagram 3). Thus, the colour characteristics of the light flux produced by an incandescent lamp is rather far from the solar radiation spectrum, and thus from optimal conditions of colour discrimination .
To a great point conditions of carrying out ophthalmoscopy vary with the age of the patient under examination. The older is the surveyed, the more blue light his crystalline lens absorbs, depleting the flow of light already over-saturated with red rays .
Finally, the durability of incandescent lamps is poor. Not to mention the "aging" of incandescent lamps associated with the gradual evaporation of the tungsten luminous element, lifetime of incandescent lamps is measured in dozens of hours, hundreds of hours at best. Apart from it the cost of a new light can be one tenth of the cost of an ophthalmoscope.
Objective: To improve the electrical properties of ophthalmoscope, and thereby achieve improved quality of ophthalmoscopic examination.
Materials and methods. This goal is achieved by replacing incandescent ophthalmoscope with a different type of a light emitting device - LED.
Replacing incandescent bulbs with LEDs in portable ophthalmoscope is done in the following way. First the glass bulb of the incandescent bulb is broken very carefully. A LED is soldered to the metallic body fulfilling two conditions: a) after soldering the LED emitting area should be in the place where the filament of the bulb was b) the positive terminal of the LED should be soldered to the central terminal of the body of the light bulb, and the negative one to the body itself. After soldering the lens of the LED is cut away and the surface is polished (Fig. 1). The body of the lamp is then inserted into the ophthalmoscope and modifications are completed.
HL-506W LED with the following characteristics, manufactured by Honglitronic Hongli Opto-Electronic (HK) Co. Ltd is used to rework portable ophthalmoscopes:
• Power supply - from 2.1 to 5.0 volts (3.3 volts is the recommended value)
• Recommended power consumption - 20 mA
• The intensity of the luminescence at 20 mA - cd 50-70
If the ophthalmoscope does not have its own container for batteries, such a container is connected via a specially made adapter ring. In most cases, component of the torches are used.
To rework OP-3 and "Zeiss" manual ophthalmoscopes were used more powerful LEDs EWC61DA1-1 from the same manufacturer with the following parameters:
• Power supply - 3.6 to 4.0 V
• Recommended supply current - 350 mA
• Luminous flux at a current of 350 mA - 40 lm.
In this case all the modifications consisted in installation of a container for batteries from a LED lamp. In the same container, the LED was placed in compliance with the above conditions.
Figure 2 shows some ophthalmoscopes of different manufacturers after reworking. Three-year experience with the ophthalmoscope with a LED emitter in clinical practice has proved such reworking to be worthy and allows us to recommend the introduction of such an improvement in the general practice. The proposed improvement can be used in ophthalmic institutions of all levels of accreditation.
Results and discussion. While the bulbs have a luminous efficacy of 10-15 lm / Wt, modern white light LED have an efficiency of 50-70 lumens / watt. LEDs with the light flux of 100-150 lumens have been designed. In combination with other structural features, these parameters of LEDs make them very perspective lighting elements for ophthalmic devices including electric ophthalmoscopes.
LEDs, unlike incandescent lamps, emit significantly less heat. It allows installing a light source with greater intensity in the same device. But taking into consideration the high efficiency of LEDs, there is another possibility for obtaining luminous flux, equal in intensity to the flow generated by an incandescent lamp. The power of the LED emitter can be significantly reduced.
In addition to a significantly higher light output per unit of power consumption, LED illuminators to a greater degree meet the requirements of lighting in which the colour discrimination thresholds will be minimal. The emission spectrum of the white light LED roughly corresponds to the spectrum of solar radiation and, at the same time, the peaks of emission maxima correspond to the peaks of the highest possible colour discrimination thresholds (Diagram 3,4).
Diagram 3. The colour discrimination ability of the eye Diagram 4. The emission spectrum of the white light LED
The technical side of using LEDs as the light source is also quite perspective.
LED provides a full spectrum of emitted light, it is compact and easy to install, it has no glass bulb, which gives a very high mechanical strength and reliability, it is durable (lifetime exceeds 100,000 hours) and energy efficient, allowing the use of tiny autonomous power sources.
Conclusion. Thus, the transition to the use of LEDs as a light source during carrying out ophthalmoscopy can improve the quality the methods of examination as well as improving the performance of ophthalmoscopes.
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