The following scheme is familiar to us - the eye is the lens of a video camera, which transmits the picture to the screen of a small cinema in your head , and in the hall you are sitting in a comfortable armchair with popcorn. Nice diagram, but in the head there is no such screen and besides it is dark there. If not, how?
Modern, generally accepted (by experts) ideas paint a slightly different picture - information from all our senses, including the eyes, enters our brain. Then this information is transformed into images, from which ... EVERYTHING is subsequently formed. This is an extremely extensive and extremely difficult topic to understand and its consideration is not the purpose of this material. All we need in this case for us is to understand that the topic of our interest is the process of image formation. Or, in other words, we are interested in the help of a flashlight in creating the most favorable conditions for the formation of images of the surrounding world, as close as possible to reality.
The process of forming an image can be very conditionally divided into two stages - the first is actually receiving primary information from the sensors, in this case the eye and the second stage is processing this information and obtaining the final result. This is an extremely simplified scheme, but quite suitable for our purposes.
The eye works with light. Light is a fairly narrow part of the range of electromagnetic radiation that is perceived by our eyes. The objects that we encounter usually do not emit, but only reflect the light that hits them.
The color of an object is determined by the ratio of the degree of absorption or reflection of various spectral components of light. That is, if an apple is red, then it absorbs all visible parts of the spectrum, except for red, and reflects this part, and if you shine light on the apple, in which there will be no red component, then the apple will seem black to us - not reflecting light at all.
The next most important parameter for us is contrast, the numerical expression of which is the ratio of the brightness of the object and the background to a higher brightness. Given the necessary contrast, we can define the boundaries and shape of the object.
Let's now take a simplified look at the structure of the eye.
At the level of obtaining primary information, the eye can be compared with a camera, in which the lens plays the role of a lens. Light rays, passing through the lens, are refracted and create a reduced inverted image on the inner wall of the eyeball (retina). The light-sensitive elements of the retina are two types of cells: - rods, and cones. The rods are very sensitive, but do not distinguish between colors and are a night vision apparatus; the cones are less sensitive, but distinguish colors and provide daytime vision. In addition, in the conditional interval between these types of vision, there is a certain transitional state called twilight vision.
Different spectral components of the perceived radiation have different effects on the receptors, or in other words, the receptors have different spectral sensitivity. Radiation fluxes of the same power, but having different wavelengths, cause unequal irritations of the retina and create sensations that are different not only in color, but also in brightness. This is an important point, so let's pay attention to it again. If we take and transform one conventional unit of energy into electromagnetic radiation from a valley wave from the visible part of the spectrum, then, for example, radiation at a wavelength of 555nm will be perceived by the eye 27 times brighter than at a wavelength of 450nm, and so on.
If the total light is small, and we switch to night vision, then the peak of the maximum sensitivity shifts to the left in the direction of decreasing the wavelength and falls on 510 nm, which corresponds to the emerald green color. But since the rods that are responsible for night vision do not distinguish colors, and show the relative brightness of the object, or in other words, the contrast, we will perceive this light as shades of gray, and not as green light.
In addition, rods and cones have different distribution densities on the retina: cones prevail in the central part, and rods in the peripheral (distant from the optical axis of the eye) parts. As a result, such a feature of the structure of the eye leads to the fact that such a landscape in reality with a fixed eye, we will see something like this during the day, and something like this at night:
The central area is in a cone with an angle of about 30 degrees. is optimal for the performance of visual functions. Within this zone, and the closer to the center, it is better there, the color and shape of the object are well distinguished. In the very center, in a cone with an angle of only 2 degrees there is a zone of maximum definition. Further from the center, the image becomes black and white and less clear and completely disappears approximately outside the boundaries of the frontal plane of the face.
The picture becomes familiar to us due to the fact that the eye is usually in constant motion, and these movements are not random, but the brain remembers and processes everything. We may not see with our peripheral vision that the grass is green under our feet, but we remember this.
Just because the picture outside of the central part is black and white and not clear does not mean that it is not important - quite the opposite. The receptors that form it are very sensitive to changes in intensity, or we can say sensitive to movement - the slightest movement and the eye after about 200-300 ms. jumps the signal source into the clear vision zone. To shift the gaze to 20 °, one movement is required, which is completed in 6-7 ms, if it is necessary to shift the gaze to more than 45 °, a movement of the head is required.
The next characteristic that is important for us is the range of brightness perceived by the eye. This range is enormous, from the ability to see a candle flame in the dark for 27 km, to the ability to safely look at very bright objects. But this range is not covered simultaneously across the entire brightness scale, but in parts. The transition from one to another range of perceived brightness does not occur instantly, but over a period of time. It takes about 15 minutes to get used to the dark. Adaptation to bright light is much faster and takes only a few seconds.
When using artificial lighting, in particular a flashlight, in the observer's field of view there will necessarily be areas reflecting light with different intensities. Moreover, if the brightness differences are significantly more than 1 in 30, then this can cause an undesirable state of the eye - blindness.
It follows from the above that in order to create optimal conditions for visual perception, it is necessary not only to provide the required brightness and contrast of signals, but also to avoid critical changes in brightness in the field of view.
Why do we need all this information? On their basis, you can try to create a model of the vision process of an underwater hunter in the dark using a flashlight in a completely transparent environment and try to imagine a flashlight with optimal qualities.
Here's what I got.
First the shape:
The light spot created by the flashlight should be uneven and consist of 3 zones with smooth transitions. The central part created by a 3gr cone. must have the maximum brightness and correspond to the operational field in which the information is processed at once. The second part - the field of constant control should be formed by a cone of 30g. - this is the identification zone for dimensional objects. The third zone is the field of periodic monitoring, limited by the limiting geometric capabilities of the mask-eye system. In my opinion, this part should be formed with a cone of at least 140deg.
The ratio of the brightness of the zones:
The ratio of the brightness of the operational field to the continuous monitoring field is 5 to 1. The ratio of the brightness of the periodic monitoring field to the brightness of the periodic monitoring field should vary from practically 1 to 1 to 1 to 5 depending on the conditions of use.
The operational field and the field of constant control should be illuminated with full-spectrum white light - it is important to obtain a color image in these areas. The field of periodic control - if possible, should be illuminated with monochromatic light with a wavelength providing maximum contrast in specific conditions.
The hunter swims, his eye automatically moves behind the zone with the maximum brightness or the operational field, in which the details are instantly recognized to clarify the identification of more massive objects falling into the field of constant monitoring. As soon as some activity occurs outside these zones, the eye immediately turns in this direction. If the alarm is false, then the eye goes back, if not, the flashlight turns in this direction.