A further dependency was found concerning the irradiated area on the retina, i.e. increasing the retinal spot from 6.4 to 9.4 mm2 up to 33.7 to 46.8 mm2 resulted in an increase of the blink reflex  percentage from 20 % to 33.3 %.

Pinholetest

The pinhole camera takes in an extremely wide angle. The rays of light, however, take much longer to reach the edges of the negative than the centre, thus the picture is less exposed along the edges and therefore darkens.

The image created by a pinhole camera has certain characteristics which we won't find in classical lens photography. Since the process entails a central projection, the images in the pinhole camera are rendered in ideal perspective.

The energy density (measure of energy per unit of area) of the laser beam increases as the spot size decreases. This means that the energy of a laser beam can be intensified up to 100,000 times by the focusing action of the eye for visible and near infrared wavelengths. If the irradiance entering the eye is 1 mW/cm2, the irradiance at the retina will be 100 W/cm2. Even a 4% reflection off an optic can be a serious eye hazard. Remember a low power laser in the milliwatt range can cause a burn if focused directly onto the retina. A 40 mW laser is capable of producing enough energy (when focused) to instantly burn through paper.

Another special characteristic is the infinite depth of field which, in a single photograph, allows objects to be captured with equal sharpness whether they are very close up or far away.

The subcutaneous tissue is a layer of fat and connective tissue that houses larger blood vessels and nerves. This layer is important is the regulation of temperature of the skin itself and the body. The size of this layer varies throughout the body and from person to person.

The lens focuses light to form images onto the retina. Over time, the lens becomes less pliable, making it more difficult to focus on near objects. With age, the lens also becomes cloudy and eventually opacifies. This is known as a cataract. Every lens develops cataracts eventually.

Pinholecamera Class 6

This study states that even for the larger spot size on the retina the frequency of the blink reflex has been shown to be less than 35 % and the same is true for the maximum of the pupil size, i. e. for low ambient light conditions. The study states 503 volunteers have been irradiated in the lab and 690 in 4 different field trials with laser radiation. Out of these only 15.5 % or 18.26 % respectively have shown a blink reflex. The respective numbers as a function of wavelength are: 15.7 % (670 nm), 17.2 % (635 nm), and 22.4 % (532 nm). An increase from 4.2 % up to 28.1 % in blink reflex frequency was achieved when the ambient illuminance was decreased from 1 700 lx to 1 lx using an LED as a large extended stimulating optical source instead of a collimated laser beam.

Corneal tissue will absorb light with a wavelength longer than 1400 nm. Damage to the cornea results from the absorption of energy by tears and tissue water causing a temperature rise and subsequent denaturation of protein in the corneal surface.

Pinholein ophthalmology

A specular reflection, which is a reflection off a mirror like surface (keeping in mind different surfaces to different wavelengths may or may not be mirror like. Specular reflection will result when the surface roughness is smaller than the wavelength. Specular reflections are generally less than 100%.

Photoablation is the photodissociation or direct breaking of intramolecular bonds in biopolymers, caused by absorption of incident photons and subsequent release of biological material. Molecules of collagen, for example, may dissociate by absorption of single photons in the 5–7 eV energy range. Excimer lasers at several ultraviolet wavelengths (ArF, 193 nm/6.4 eV; KrF, 248 nm/5 eV; XeCl, 308 nm/4 eV) with nanosecond pulses focused on tissue at power densities of about 108 W/cm2 can produce this photoablative effect. Ultraviolet radiation is extremely strongly absorbed by biomolecules, and thus absorption depths are small, of the order of a few micrometers.

Exposure to the invisible carbon dioxide laser beam (10,600 nm) can be detected by a burning pain at the site of exposure on the cornea or sclera.

The epidermis is the outer layer of skin. The thickness of the epidermis varies in different types of skin. It is the thinnest on the eyelids at .05 mm and the thickest on the palms and soles at 1.5 mm.

Constructing a simple pinhole camera is easy. Make a hole in one side of a closable box made of material which doesn't let light in. Place a thin piece of metal or tin can with a tiny hole over the opening. On the outside of the box stick a strip of black tape over the opening which acts as the release. Then, in a dark room, attach a piece of film or photographic paper onto the opposite side and the camera is ready.

Light from an object (such as a tree) enters the eye first through the clear cornea and then through the pupil the circular aperture (opening) in the iris.  Next, the light is converged by the lens to a nodal point immediately behind the lens; at that point, the image becomes inverted.  The light progresses through the gelatinous vitreous humor and, ideally, back to a clear focus on the retina, the central area of which is the macula. In the retina, light impulses are changed into electrical signals and then sent along the optic nerve and back to the occipital (posterior) lobe of the brain, which interprets these electrical signals as visual images

What you experience once you design, construct and try out your own pinhole camera is difficult to describe. You'll find yourself on a whole new plane of imagination, experimentation and creativity. Moreover, the photos themselves have an unusual atmosphere and capture the world in a different way than you are used to. Make your own pinhole camera and take some photographs with it. You'll find out for yourselves.

* This is just a short summary of the most important stages of the development of the pinhole camera. One of the sources I used was the exhaustive account of the history of the pinhole camera in science and art presented by Eric Renner in his book Pinhole Photography, Rediscovering a Historic Technique (Focal Press 1995).

The exposure to a visible laser beam can be detected by a bright color flash of the emitted wavelength and an after-image of its complementary color (e.g., a green 532 nm laser light would produce a green flash followed by a red after-image). When the retina is affected, there may be difficulty in detecting blue or green colors secondary to cone damage and pigmentation of the retina may be detected.

Thermal damage occurs because of the conversion of laser energy into heat. With the laser’s ability to focus on points a few micrometers or millimeters in diameter, high power densities can be spatially confined to heat target tissues. Depth of penetration into the tissue varies with wavelength of the incident radiation, determining the amount of tissue removal and bleeding control.

The dermis also varies in thickness depending on the location of the skin. It is .3 mm on the eyelid and 3.0 mm on the back. The dermis is composed of three types of tissue that are present throughout – not in layers. The types of tissue are collagen, elastic tissue, and reticular fibers.

Symptoms of a laser burn in the eye include a headache shortly after exposure, excessive watering of the eyes, and sudden appearance of floaters in your vision. Floaters are those swirling distortions that occur randomly in normal vision most often after a blink or when eyes have been closed for a couple of seconds. Floaters are caused by dead cell tissues that detach from the retina and choroid and float in the vitreous humor. Ophthalmologists often dismiss minor laser injuries as floaters due to the very difficult task of detecting minor retinal injuries. Minor corneal burns cause a gritty feeling, like sand in the eye.

Probably the earliest depiction of the camera obscura, published in the book De radio astronomico et geometrico liber in 1545. Here, Dutch physicist and mathematician Gemma Frisius described his observation of the solar eclipse in 1544. Johannes Kepler observed sun spots in the same way in Prague at the beginning of the 17th century; Kepler was also the first to use the term camera obscura.

A sensation of warmth resulting from the absorption of laser energy normally provides adequate warning to prevent thermal injury to the skin from almost all lasers except for some high-power far-infrared lasers. Any irradiance of 0.1 W/cm2 produces a sensation of warmth at diameters larger than 1 cm. On the other hand, one tenth of this level can be readily sensed if a large portion of the body is exposed. Long-term exposure to UV lasers has been shown to cause long-term delayed effects such as accelerated skin aging and skin cancer.

Pinholeoptics

How to make apinholecamera

The cornea is the transparent layer of tissue covering the eye. Damage to the outer cornea may be uncomfortable (like a gritty feeling) or painful, but will usually heal quickly. Damage to deeper layers of the cornea may cause permanent injury.

If a laser burn occurs on the fovea, most fine (reading and working) vision may be lost. If a laser burn occurs in the peripheral vision, it may produce little or no effect on vision.

Pinholemeaning

The layers of the skin, which are of concern in a discussion of laser hazards to the skin, are the epidermis and the dermis. The epidermis layer lies beneath the stratum corneum and is the outermost living layer of the skin. The dermis mostly consists of connective tissue and lies beneath the epidermis.

This type of damage requires beams of extremely high power density (109–1012 W/cm2) in extremely short pulses (ns) to delivery fluences of about 100 J/cm2 and very high electric fields (106–107 V/cm), comparable to the average atomic or intermolecular electric field. Such a pulse induces dielectric breakdown in tissue, resulting in a microplasma or ionized volume with a very large number of electrons. A localized mechanical rupture of tissue occurs due to the shock wave associated with the plasma expansion. Laser pulses of less than 10 microseconds duration can induce a shock wave in the retinal tissue that causes tissue rupture. This damage is permanent, as with a retinal burn. Acoustic damage is actually more destructive to the retina than a thermal burn. Acoustic damage usually affects a greater area of the retina, and the threshold energy for this effect is substantially lower. The ANSI MPE values are reduced for short laser pulses to protect against this effect.

Pinholelens

Exposure to the Q-switched Nd:YAG laser beam (1064 nm) is especially hazardous and may initially go undetected because the beam is invisible and the retina lacks pain sensory nerves. Photoacoustic retinal damage may be associated with an audible “pop” at the time of exposure. Visual disorientation due to retinal damage may not be apparent to the operator until considerable thermal damage has occurred.

Exposure in the shorter UV-C (0.200 µm-0.280 µm) and the longer UV-A ranges seems less harmful to human skin. The shorter wavelengths are absorbed in the outer dead layers of the epidermis (stratum corium) and the longer wavelengths have an initial pigment-darkening effect followed by erythema if there is exposure to excessive levels.

Laser radiation injury to the skin is normally considered less serious than injury to the eye, since functional loss of the eye is more debilitating than damage to the skin, although the injury thresholds for both skin and eyes are comparable (except in the retinal hazard region, (400–1,400 nm). In the far-infrared and far-ultraviolet regions of the spectrum, where optical radiation is not focused on the retina, skin injury thresholds are about the same as corneal injury thresholds. Obviously, the possibility of skin exposure is greater than that of eye exposure because of the skin’s greater surface area.

The pinhole camera's simple construction offers a number of ways in which it can be constructed, using various materials. The cameras can be all kinds of shapes and sizes, with various formats and types of light-sensitive material, several holes, curved film planes, for panoramic images etc. There are all sorts of imaginative ways to make these cameras; the most ordinary of objects can unexpectedly become pinhole cameras, for example a matchbox, book, a pepper, travel bag, a delivery van, an old fridge or even a hotel room. You can, of course, turn your ordinary camera into a pinhole camera by simply replacing the lens with a small hole. And to complete the list, there are also a number of commercially produced pinhole cameras in existence, on the whole, highly elaborate models.

The major danger of laser radiation is hazards from beams entering the eye. The eye is the organ most sensitive to light. A laser beam (400-1400 nm) with low divergence entering the eye can be focused down to an area 10 to 20 microns in diameter.

The photothermal process occurs first with the absorption of photon energy, producing a vibrational excited state in molecules, and then in elastic scattering with neighboring molecules, increasing their kinetic energy and creating a temperature rise. Under normal conditions the kinetic energy per molecule (kT) is about 0.025 eV. Heating effects are largely controlled by molecular target absorption such as free water, haemoproteins, melanin, and other macromolecules such as nucleic acids.

As mentioned above, the image in the pinhole camera is created on the basis of the rectilinear propagation of light. Each point on the surface of an illuminated object reflects rays of light in all directions. The hole lets through a certain number of these rays which continue on their course until they meet the projection plane where they produce a reverse image of the object. Thus the point is not reproduced as a point, but as a small disc, resulting in an image which is slightly out of focus. This description would suggest that the smaller the hole, the sharper the image. However, light is essentially a wave phenomenon and so, as soon as the dimensions of the opening are commensurable with the dimensions of the light wavelength, diffraction occurs. In other words, if the hole is too small, the image will also be out of focus. The calculations for the optimal diameter of the hole in order to achieve the sharpest possible image were first proposed by Josef Petzval and later perfected by British Nobel prizewinner Lord Rayleigh (see Making the pinhole). He published the formula in his book Nature in 1891, and it is still valid today.

Fortunately the eye has a self-defense mechanism — the blink and aversion response. Aversion response is the closing of the eyelid, or movement of the head to avoid exposure to bright light. The aversion response is commonly assumed to occur within 0.25 sec and is only applicable to visible laser wavelengths. This response may defend the eye from damage where lower power lasers are involved, but cannot help where higher power lasers are involved. With high power lasers, the damage can occur in less time than a quarter of a second.

The safest reflection is the diffuse reflection, a reflection off a surface that spreads out the laser radiation reducing its irradiance. A diffuse surface will be one where the surface roughness is larger that the wavelength.

The cornea, lens and aqueous humour allow Ultraviolet radiation of these wavelengths and the principal absorber is the lens. Photochemical processes denature proteins in the lens resulting in the formation of cataracts.

A pinhole camera, also known as camera obscura, or "dark chamber", is a simple optical imaging device in the shape of a closed box or chamber. In one of its sides is a small hole which, via the rectilinear propagation of light, creates an image of the outside space on the opposite side of the box.

While the first photograph taken with a pinhole camera was the work of Scottish scientist Sir David Brewster back in 1850, the technique became more established in photography during the late 19th century when it was noted for the soft outlines it produced, as opposed to lenses generating perfect, sharp images. The pinhole camera was later abandoned and it wasn't until the end of the 1960s that several artists began using it in their experiments, thus awakening renewed interest in this simple photographic apparatus which endures to this day.

The study “A Critical Consideration of the Blink Reflex as a Means for Laser Safety Regulations, byH.-D. Reidenbach1,2,3, J. Hofmann1, K. Dollinger1,3, M. Seckler2.

Initially, the camera obscura was, in fact, a room where the image was projected onto one of the walls through an opening in the opposite wall. It was used to observe the solar eclipse and to examine the laws of projection. It later became a portable instrument which was perfected with a converging lens. Instruments of this kind were often used as drawing aids and, at the dawn of photographic history, they formed the basis for the construction of the camera. The pinhole camera was finally also applied in modern science – during the mid-20th century scientists discovered that it could be used to photograph X-ray radiation and gamma rays, which the ordinary lens absorbs. As a result, the pinhole camera then found its way onto spacecraft and into space itself.

Pinholecamera model

One of the deciding factors on how hazardous a laser beam can be is how one is exposed. Is it a direct or intrabeam exposure (where all the energy is directed right at one’s eyes).

Images created via a small opening will be found in the natural environment and in everyday life, and people in various parts of the world have been observing them since ancient times. Probably the earliest surviving description of this kind of observation dates from the 5th century BC, written by Chinese philosopher Mo Ti. In the Western hemisphere, Aristotle in 4 BC was asking, without receiving any satisfactory answer, why sunlight passing through quadrilaterals, for example, one of the holes in wickerwork, does not create an angled image, but a round one instead, and why the image of the solar eclipse passing through a sieve, the leaves of a tree or the gaps between crossed fingers creates a crescent on the ground. In 10 AD the Arabian physicist and mathematician Ibn al-Haitham, known as Alhazen, studied the reverse image formed by a tiny hole and indicated the rectilinear propagation of light. There was another scholar during the Middle Ages who was familiar with the principle of the camera obscura, namely the English monk, philosopher and scientist Roger Bacon. It was not until the manuscript Codex atlanticus (c. 1485) that the first detailed description of the pinhole camera was set down by Italian artist and inventor Leonardo da Vinci, who used it to study perspective.

To the skin, UV-A (0.315 µm-0.400 µm) can cause hyperpigmentation and erythema. UV-B and UV-C, often collectively referred to as “actinic UV,” can cause erythema and blistering, as they are absorbed in the epidermis. UV-B is a component of sunlight that is thought to have carcinogenic effects on the skin. Exposure in the UV-B range is most injurious to skin. In addition to thermal injury caused by ultraviolet energy, there is the possibility of radiation carcinogenesis from UV-B (280 nm – 315 nm) either directly on DNA or from effects on potential carcinogenic intracellular viruses.

The surface of the cornea absorbs all UV of these wavelengths which produce a photokeratitis (welders flash) by a photochemical process, which cause a denaturation of proteins in the cornea. This is a temporary condition because the corneal tissues regenerate very quickly.

Diagram from 1685 showing how rays of light travel through the camera obscura, and how the size of the image depends on its distance from the hole, according to Johann Zahn.

The part of the eye that provides the most acute vision is the fovea centralis (also called the macula lutea). This is a relatively small area of the retina (3 to 4%) that provides the most detailed and acute vision as well as color perception. This explains why eyes move when you read the image has to be focused on the fovea for detailed perception. The balance of the retina can perceive light and movement.

Light below 400 nm does not focus on the retina. The light can be laser output, ultraviolet (UV) from the pump light, or blue light from a target interaction. The effect is cumulative over a period of days. The ANSI standard is designed to account only for exposure to laser light. If UV light from a pump light or blue light from a target interaction is emitted, additional precautions must be taken.

A certain disadvantage of the pinhole camera is the amount of light allowed through (small aperture), which complicates and sometimes prevents entirely the photographing of moving subjects. Exposure time is normally counted in seconds or minutes but, in bad lighting conditions, this could be hours or even days (see Determining exposure times for pinhole cameras).

The chief concern over laser use has always been the possibility of eye injury. While skin presents a greater target, it is injury to one’s eyes that drives laser safety, funding, controls and application. The effect of laser radiation will vary with the wavelength and part of the eye it interacts with. In addition biological effects from direct exposure and diffuse reflection exposure will differ. The anatomy of the eye and skin will be explained and issues associated with biological effects.

There is quite a variation in depth of penetration over the range of wavelengths, with the maximum occurring around 700 to 1200 nm. Injury thresholds resulting from exposure of less than 10 seconds to the skin from far-infrared and far-ultraviolet radiation are superficial and may involve changes to the outer dead layer of the skin. A temporary skin injury may be painful if sufficiently severe, but it will eventually heal, often without any sign of injury. Burns to larger areas of the skin are more serious, as they may lead to serious loss of body fluids. Hazardous exposure of large areas of the skin is unlikely to be encountered in normal laser work.

The cornea, lens and vitreous fluid are transparent to wavelengths. Damage to the retinal tissue occurs by absorption of light and its conversion to heat by the melanin granules in the pigmented epithelium or by photochemical action to the photoreceptor. The focusing effects of the cornea and lens will increase the irradiance on the retina by up to 100,000 times. For visible light 400 to 700 nm the aversion reflex which takes 0.25 seconds may reduce exposure causing the subject to turn away from a bright light source. However, this will not occur if the intensity of the laser is great enough to produce damage in less than 0.25 sec. or when light of 700 – 1400 nm (near infrared) is used, as the human eye is insensitive to these wavelengths.