Adaptive Optics for Vision Science

Chapter 10.3.7 - Light Detection

10.3.7   Light Detection

The light from the retina is descanned and is focused onto a confocal pinhole.
Choosing the size of the confocal pinhole is important as it plays a role in
governing the axial and lateral resolution of the instrument. The goal is to be
able to make the pinhole as small as possible, until the detected light levels
become the limiting factor. The size of the pinhole has to be selected based
on the geometry of the collection optics and how the pinhole projects into
retinal space. A convenient way to defi ne the size of the pinhole is in coordinates
normalized to the diffraction-limited point spread function, or the Airy
disk. One reason for choosing this scale is because the smallest, practical
pinhole size is equal to the radius of the Airy disk of the collection optics.
Smaller confocal pinholes, even in a diffraction-limited system, will not serve
to improve axial or lateral resolution to any meaningful level but will only
serve to reduce the amount of detected light [27, 33]. Therefore, a confocal
pinhole that has a diameter equal to the radius of the Airy disk of the collection
system is given a normalized size of 1 Airy disk unit. This normalization
facilitates the interpretation of the results and allows a direct comparison
between similar instruments. With the pinhole sizes normalized in this way,
one can easily compute actual pinhole sizes, or the pinhole size projected into
retinal coordinates, using the following equation:

 

where λ0 is the wavelength of light in a vacuum, F is the focal length, n is the
index of refraction of the media in which the light is focused, and d is the
diameter of the beam forming the image.

For example, in the AOSLO described in Chapter 16 (λ = 660 nm), the
collector lens in front of the confocal pinhole (in air) has a focal length (F)
of 100 mm and the beam diameter at this lens is 3.5 mm (d); hence, the radius
of the Airy disk at the confocal pinhole is 23 μm. A 23-mm pinhole projects
into the reduced eye retinal space (6.3-mm pupil diameter, F = 22.2 mm, n =
1.33) as approximately 2.13 μm.

The selection of a detector in an AOSLO is also critical. The amount of
light that can be used to expose the retina is limited by retinal safety factors.
Furthermore, the small amount of light that reflects from the retina is sampled
on such a fine scale (less than 1 μm per pixel) that the number of photons
per pixel can be quite small. Recent measurements from the AOSLO have
determined that after best correcting the wave aberration, the average power
passing through the confocal pinhole when 30 μW of 660-nm light (measured
at the cornea) is used to expose the retina can be as low as 200 pW. With a
pixel integration time of 50 ns, this power converts to less than 33 photons per
pixel, which is even fewer when you consider the quantum efficiency of the
detector. A photomultiplier tube (PMT) or an avalanche photodiode (APD)
is the best choice for such an application. PMTs may be more desirable, even
though their quantum efficiency is less because there is less intrinsic noise
in the detection path, making it suitable for detecting extremely low light
levels.

 

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