Adaptive Optics for Vision Science

Chapter 10.5 - Common Issues For All AO Imaging Systems

10.5.1   Light Budget

In a high-resolution ophthalmic imaging system, the light budget needs careful
consideration as ocular hazards place a hard threshold on the amount of light
that can be safely launched into the eye. In addition, the double-pass nature
of the eye is highly inefficient with light levels emerging from the eye 3 to 5
orders of magnitude below that which entered [9]. Considerable effort, therefore,
should be made to maximize the number of photons that reach the
imaging detector. An important design step is to couple the illumination
beam into the system with a weak reflecting beamsplitter (BS in Fig. 10.1, and
BS1 in Fig. 10.4). Typically 5% or less is used. Light reflected from the retina
is then efficiently transmitted by the splitter (>95%). A disadvantage of this
approach is that it requires a more powerful light source as the double-pass
throughput of the beamsplitter is small (5% × 95% = 4.8%). Another important
step is to use highly reflective mirrors and antireflective coated lenses.
Even though light loss may be small for any single optical element, their
cumulative loss can be significant. The goal is to minimize the amount of light
used for wavefront sensing so that as much light as possible is used to form
the retinal image.

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