Photonics for Health & life
Magnification is the process of enlarging something optically. For example, when you use a magnifying glass, you can observe an ant, which appears for example three times larger. Then we can say that the ant has been magnified.
The magnification (M) is the ratio of the image size to the object size, and it is also the ratio of the image distance to the object distance (see figure bellow)
y2/y1 = s2/s1 = M.
What type of lens is a magnifying lens?
A Convex lens is used as magnifying lens as it produces a highly magnified image of an object placed in its path (closer than the focal length). The magnification is seen in below figure that tells that if an object is placed on one side of a convex lens closer to it than the focal point, the image of the object is seen on the same side as the object which appears highly magnified.
What is the concept of the optical microscope?
A microscope is an instrument used to produce enlarged images of small objects. The most common kind of microscope is an optical microscope, which uses lenses to form images from visible light.
An optical microscope with a single lens is known as a simple microscope. Simple microscopes include magnifying glasses and jeweller’s loupes. An optical microscope with two lenses is known as a compound microscope.
The basic parts of a compound microscope are the objective, which holds the lens near the specimen, and the eyepiece, which holds the lens near the observer.
The optical microscope magnifies an object in two steps (see figure below). In both steps optical systems acting like converging lenses are used:
- The first step is to place the object between the single and double focal point. The result is a magnified, real image. This microscope lens (in reality an optical system consisting of several lenses) is called the objective.
- Then a second lens is used to pick up this image exactly in its front focal point. As a result we generate a beam of parallel rays, but not a real image. This optical element is called eyepiece. The human eye is able to handle this parallel beam and generates an image onto its retina.
An optical microscope is mainly composed of the eyepiece and the different objectives. The objective is selected depending on the level of detail that shall be observed. The sample is placed on the table below the objective.
Finally, the illumination is an important part of a microscope. If it is not well regulated, the observation of the object is deteriorated: some details will not be observable.
What is the fluorescence microscope?
We have seen that the microscope allows us to study small objects. However, the quality of observation can be improved especially with the fluorescence microscope.
Fluorescence is the process realised by a substance absorbing light with one wavelength (blue, violet) and re-emitting light with a different wavelength (green, yellow, red). Fluorescence microscopes use this phenomenon to generate images. The specimen is illuminated with a specific wavelength that is absorbed. It then emits light at a different wavelength, which is retrieved using filters.
In the following picture, we can see an example of a result using this technology:
Every day, our sight is helpful. Since our birth, it changes and every year an eye examination is recommended. How do eyes work? Why some people don’t have good sight? What are the similarities with the camera?
What are the concepts of vision?
The human eye is made up of cornea, crystalline lens, retina and aqueous fluids. Each one plays an important role in vision. The cornea and the lens focus the image on the retina. The lens is essential since it is deformable and can perform the adjustment of the eye when the object is closer or further away, this is what we call accommodation.
The iris is the coloured part of the eye, it expands or not as required. It allows you to increase or decrease the brightness at the bottom of our eye.
All the passing light is going to the bottom of the eye, to the retina. The photoreceptor cells of the retina changes the light energy into electrical energy. This electrical signal is then transferred to the brain via the optic nerve. The information processing then takes place in the brain.
In the human eye, there are cone cells in the retina, which enable us to see different colours. In fact there are three types of cells. Each of them detects one colour: green, red and blue; and the combination of these colours creates the observed colour. Moreover, there are animals such as mantis shrimp, which have more type of cone cells and can also see different colours.
What are eye defects?
Vision is a clear mechanism, some people can’t see without glasses due to an eye defect. The latter are due to the fact that the image is not focused on the retina.
There are several types of optical defects: myopia, hyperopia, astigmatism and presbyopia.
Myopia is the consequence of a too long eyeball; the image is focused before the retina. Thus a myopic has a very poor distance vision.
On the other hand, the hyperopia is too short eyeball; the image comes up behind the retina. Thus, the vision is blurred for near objects.
The astigmatic has blurred vision in some directions since its eyeball is deformed. Finally Presbyopia is the loss of elasticity of the lens; it is one of the consequences of old age.
To correct theses defects ophthalmologists usually prescribe optical lenses in order to project the picture exactly onto the retina.
Finally, some people are colour-blind. The following picture can be used to determine if someone is affected by this defect. If the number seems unreadable, the answer is yes.
What is the concept of the camera?
We have seen that the received images exist in our brain through a mechanism present in the eye. This system was useful for scientific inventions, since it has been imitated for creating cameras.
A camera works like an eye: the different elements of the eye and the camera can be matched. Indeed, each main element of a camera has its counterpart in the eye.
Both the camera and the human eye have a lens that focuses light into an inverted image. One major difference between the two lenses, though, is that while a camera lens moves closer or farther from an object in order to bring it into focus, the lens of the human eye stay stationary. To bring an object into focus, muscles in the eye respond to instructions from the brain and change the shape of the lens, thus sharpening the image.
Retina (Film or Sensor)
Additionally, the eye’s retina is like a camera’s film or sensor onto which light is cast. In the eye, light passes through the lens and hits the retina, where rods and cones help transform the received image into electric impulses that are sent along the optic nerve to the brain. While both the retina and a camera’s film or sensor are all highly sensitive to light, the eye is much more so, and performs much better in the dark — even without a flash.
To allow the right amount of light, both the eye and a camera have an aperture. The eye’s version is its iris working together with the pupil, which, just like a camera aperture, widens or narrows depending on the amount of ambient light. Therefore, just the right amount of light hits either the eye’s retina or camera’s film or sensor so as to present a clear, discernible image.
How do image CCD sensors work?
To create a camera, scientists have created a sensor similar to the human sensor named “Retina”. This invention is the CCD, it is the simpler to manufacture and exists since 1970.
Fundamentally, a charge coupled device (CCD) is an integrated circuit etched onto a silicon surface forming light sensitive elements called pixels. They convert the received amount of light into a corresponding number of electrons. The stronger the light, the more electrons are generated. The electrons are converted into voltage and then transformed into numbers by means of an analogue-digital converter. The signal constituted by the numbers is processed by electronic circuits inside the camera.
CCD sensors register the amount of light from bright to dark with no colour information. We can talk about CCD image sensors “colour blind”. To implement a colour in the camera, we place a filter called a “colour filter array” (CFA) in front of the sensor to allow the capitation of primary colours. The most common colour registration method is RGB (Red, Green, and Blue). Red, green, and blue are the primary colours that, mixed in different combinations, can produce most of the colours visible to the human eye.
The most widespread and successful CFA has been the Bayer pattern, which uses alternating rows of red-green and green-blue filters:
Lasers have emerged in the 1960s. They are used in all fields such as industry, physics and also medicine. How does such a light source help surgeons today? What are the effects of the laser on humans?
What are the biological interactions related to laser?
You all know lasers, you have already used them in physics experiments at school. However, you probably do not know laser effects on our bodies when we use them in the medical field.
Biological interactions between laser light and tissue depend on its wavelength, power and time of exposure to the laser beam. Several effects are possible such as electromechanical, photo-ablative, thermal or photochemical effect.
Also, laser radiation delivered via a fibre for photodynamic therapy can treat cancer.
What are the electromechanical effects?
When people decide to remove their tattoos, they pass through a laser procedure. You may have noticed that after a such intervention, the skin is irritated and it is reconstituted with time. This erases the tattoo.
This is the electromechanical effect that occurs when very strong and short laser pulses are applied on the tissue. This irradiance results in the creation of strong electric fields. The tissue then undergoes rupture and plasma formation is observed.
What are the photoablative effects?
You’ve already met people who have had surgery to remove their eye defects. These people with myopia or hyperopia were treated with laser light to change the curvature of their cornea. Ophthalmic laser are used because of their precision and to avoid contact with the tissue.
This method is used in the medical field, to automate the operations. This is called the photo ablative effect. These photons have higher energies than the energy that keeps the molecules intact. Thus, the molecular bonds break during the laser exposure. Hence a breakdown of tissues is obtained.
What are the thermal effects?
In medicine, it’s sometimes important to coagulate the blood of patients to save their life. For this, it is possible to use the laser to induce thermal effects.
This effect is largely used today. This is a three-stage process: a light-heat conversion, a transfer of heat in the skin, and tissue reaction to the temperature.
This effect may cause hyperthermia (increased temperature), volatilization and bleeding of the tissues (necrosis). It is widely used in medicine: ophthalmology, surgery or dermatology for treating skin lesions.
What are the photochemical effects?
Cancer is very complex to treat. To assist in the elimination of cancer cells in a localized area of your body, you need an effective method which might include photochemical effects caused by laser light.
In medicine, photochemical effects are used in two steps:
– Application of a photosensitizer in the lesion to be treated
– Illumination of this lesion with low intensity light
This promotes chemical reactions in the tissues which kill cancer cells
Stars and illuminated planets are out of reach for a hands-on study. The same is true for certain chemicals which are present in very small quantities, toxic or radioactive.
Luckily, many of these objects interact with light in a way that we can get information on them by examining that light.
Spectroscopy is the science of studying the ‘spectral composition’ of light (simply put: the various colours which are present) coming from objects like the sun, the moon or an unknown chemical. This is done by use of so-called spectrometer, see Figure below.
What is a spectrophotometer?
In this figure below you see a spectrometer for examining a sample container with an unknown chemical. The principle is simple: You choose light of one wavelength (colour), shine it onto the sample and see how much light passes through or get scattered. Then, you take the next wavelength and do the same. After doing this for a number of wavelengths you’ll compare the data you got to a database which contains transmission- and scattering data on all kinds of chemical groups. If you find a match in the database you’ll know what’s in your sample container.
In practice, a spectroscope performs those tedious measurements automatically. You have 1 or 2 light sources which generate a beam in the right wavelength range; from there you use filters and a monochromator to select a single wavelength. Then, you split up the beam. One part will go through your unknown sample, the other one through a ‘reference’ sample, which is necessary to get precise results. Photodiodes behind your sample container and the reference each measure the transmitted light intensity. A computer collects the data for each wavelengths, draws a graph and compares it to its database.
What is the theory of spectroscopy in the IR (infra-red) domain?
We have seen how a basic spectroscope works, but there is another spectroscope which works with infrared light.
The IR spectroscopy uses the same mounting as a spectrophotometer but illuminates the sample with a large IR spectrum. As atoms and molecules possess specific resonance frequencies where they absorb best, knowing which IR frequencies have been absorbed typical chemical groups can be spotted.
For example, the OH group, characterizing alcohols and water matches with absorption near 3650-3590 cm-1.