The evolution must have optimized the functionality of the visual tract of vertebrates for efficient operation under low light level conditions.
Hubel and Weisel discovered this. They got a Noble price for their work. The duo analyzed the optic tract of many types of vertebrates, including humans.
The sensitivity of the human eye covers a huge range. The visual tract implements several special measures that help to extend that range.
At high dose rates the pupil of the eye acts as a diaphragm that partly closes the lens, and in this way, it increases the sharpness of the picture on the retina. At such dose rates, the cones perform the detection job. The cones are sensitive to colors and offer a quick response. In unaided conditions, the rods take over at low dose rates, and they do not differentiate between colors. In contrast to the cones, the rods apply a significant integration time. This integration diminishes the effects of quantum noise that becomes noticeable at low dose rates.
The sequence of optimizations does not stop at the retina. In the trajectory from the retina to the fourth cortex of the brain, several dedicated decision centers decode the received image by applying masks that trigger on special aspects of the image. For example, a dedicated mask can decide whether the local part of the image is an edge, in which direction this edge orients and in which direction the edge moves. Other masks can discern circular spots. Via such masks, the image encodes before the information reaches the fourth cortex.
Somewhere in the trajectory, the information of the right eye crosses the information that contains in the left eye. The difference is used to construct three-dimensional vision.
Quantum noise can easily disturb the delicate encoding process. That is why the decision centers do not pass their information when its signal to noise ratio is below a given level. The physical and mental condition of the observer influences that level. At low dose rates, this signal to noise ratio barrier prevents a psychotic view. The higher levels of the brain thus do not receive a copy of the image that the retina detected. Instead, that part of the brain receives a set of quite trustworthy encoded image data that together with already stored data, will decipher in an associative way. Obviously for a part other parts of the brain act in a similar noise blocking way.
The evolution of the vertebrates must have installed this delicate visual data processing subsystem in a period in which these vertebrates lived in rather dim circumstances, where the visual perception of low dose rate images was of vital importance.
This explanation indicates that the signal to noise ratio in the image that arrives at the eyes pupil has a significant influence on the perceptibility of the low dose image. At high dose rates, the signal to noise ratio hardly plays a role. In those conditions, the role of the spatial blur is far more important.
It is easy to measure the signal to noise ratio in the visual channel by applying a DC meter and an RMS meter. However, at very low dose rates, the damping of both meters might pose problems.
What quickly becomes apparent is the relation between the signal to noise ratio and the number of the quanta that participate in the signal. The measured relation is typical for stochastic quantum generation processes that classify as Poisson processes.
It is also easy to comprehend that when the signal spreads over a spatial region, the number of quanta that participate per surface unit is diminishing. Thus, spatial blur has two influences. It lowers the local signal, and on the other hand, it increases the integration surface. Lowering the signal decreases the number of quanta. Enlarging the integration surface will increase the number of involved quanta. Thus, these two effects partly compensate each other. An optimum perceptibility condition exists that maximizes the signal to noise ratio in the visual trajectory.
Image intensifier devices extend the range of human low dose rate perceptibility by applying a quantum multiplication process. For each detected quantum at is input the image intensifier device produces a bright spot at its output screen. If at the input no quantum gets lost, the the output screen shows the same signal to noise ratio. Thus, image intensifiers are ideal instruments for observing the quantum nature of the incoming picture. Image intensifiers are used in starlight scopes and in X-ray imaging. In X-ray diagnosis the image intensifiers reduce the X-ray dose that a patient must endure. The observer is still hampered by the noise filtering process of the human visual tract. That process still encounters blocking based on the signal to noise ratio. However, more of the quanta that are arriving at the input of the image intensifier will contribute to the perception process. With Poisson processes the signal to noise ratio will reduce with the number of participating quanta. Science has invented the Detective Quantum Efficiency (DQE) and the Optical Transfer Function (OTF) as objective qualifiers for the behavior of the visual information channel.