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Estimates were made of
the critical exposure time for the retina with the indirect ophthalmoscopy,
slit lamp, operating microscope and overhead surgical lamp.
Ophthalmoscopes
Indirect ophthalmoscopes
can produce levels of retinal irradiance up to five times greater than that
produced by a direct ophthalmoscope, for example the Keeler Indirect produces
nearly 3.6 times more irradiance than the American Optical at maximum voltage
setting. The power of the condensing lens also affects the retinal irradiance,
which increases as the power of the condensing lens decreases. Therefore,
an indirect ophthalmoscope is considered to be unsafe, when compared with the
laser standard, after 23 seconds exposure in a normal patient with clear media
and dilated pupils. The time taken for fundus examination should not,
therefore, be prolonged unnecessarily.
The light sources in
ophthalmoscopes are usually an incandescent bulb (tungsten halogen) and are
composed of one-third visible and UV radiation and two-thirds IR radiation. It
has therefore been suggested that an IR filter should be incorporated in all
ophthalmoscopes to avoid thermal demage from extensive viewing.
The higher the voltage
at which the bulbs are operated, the greater the amount of UV radiation
emitted. As UVA and blue light can cause damage to the retina, especially in
aphakes (no crystalline lens) whose natural filter has been removed, it has
been suggested that a cut-off filter at 450nm should also be incorporated.
Slit lamps
Slit lamp examination of
the retina produces up to three times more irradiance than indirect
ophthalmoscopy. The level of irradiance obviously depends upon the lamp voltage
and can range from 140mW/cm2 to 358mW/cm2 for a 5 and 7.5V lamp, respectively.
The safe durations for retinal examination are also shorter than for indirect
ophthalmoscopy, being as little as 8seconds. It has therefore, been suggested
that medium voltage settings should be used and that short examination times of
10seconds should be employed. This applies particularly to the examination of
patients with macular or retinal degenerations.
Operating microscopes
These can produce up to
ten times more retinal irradiance than indirect ophthalmoscopes, i.e. up to
970mW/cm2. The retinal irradiance and hence safe times are seen to vary with
the patient's refractive error. The safe time varies from 1.8 to 49seconds,
which is still relatively short when considering the time taken during
operation procedures. It has therefore been suggested that corneal occluders
should be used during prolonged procedures with operating microscopes.
Although microscopes do
not produce much UV radiation it is still advisable to incorporate a pale
yellow filter to absorb blue light and UVA. Care should be taken in certain
conditions, such as retinitis pigmentosa, when light exposure may accelerate
the disease. Light sources such as the surgical illuminating lid speculum
provide a safe retinal irradiance. It may provide 900 times less retinal
irradiance than conventional operating microscopes.
To summarize:
- Use IR filters to absorb wavelengths longer than 700nm.
- Absorb wavelengths blow 450nm to eliminate blue light and UVA. This will improve image quality by reducing the light scattering and chromatic aberration.
- Use the minimum amount of light and time necessary for examination.
- In some cases corneal occluders can be used to prevent unnecessary exposure from operating microscopes
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